Characterization of synthetic polymers by MALDI-MS

Giorgio Montaudo a,*, Filippo Samperi b, Maurizio S. Montaudo b

a Chemistry Department, University of Catania, Viale A. Doria 6, 95125 Catania, Italyb Institute of Chemistry and Technology of Polymers, CNR, Viale A. Doria 6, 95125 Catania, Italy

Received 23 June 2005; received in revised form 30 September 2005; accepted 20 December 2005

Abstract

In recent years, matrix assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectroscopy has become a

routine analytical tool for the structural analysis of polymers, complementing NMR and other traditional techniques, a noteworthy

change with respect to the past, when mass spectrometry (MS) was seldom used. In this review, we discuss salient aspects of

MALDI. First, we devote a section to fundamentals and practice in MALDI of polymers (such as the laser, ion source, ion optics,

reflectron, detector, ionization efficiency) as well as to some basic concepts of sample preparation (such as the MALDI matrix and

cationization agents). Then, we focus on measurable quantities of polymers: average molar masses, the chemical formula and the

structure of the monomer (actually of the repeat unit), the masses of the chain end groups, etc. In-depth coverage is given of

coupling MALDI with liquid chromatography (LC), since often LC offers valuable help in exploring macromolecules. The final

section is devoted to recent applications, with a detailed discussion of MALDI of addition polymers, condensation polymers,

polymers with heteroatoms in the chain, copolymers and partially degraded polymers.

q 2006 Elsevier Ltd. All rights reserved.

Keywords: MALDI; Polymers; Copolymers; Polymer degradation; LC-MALDI

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278

2. Fundamentals and practice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279

2.1. Ion extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279

2.2. Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279

2.3. Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280

2.4. Resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280

2.5. MALDI matrices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282

2.6. Developments in sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283

2.7. Sample preparation for low molar mass compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286

2.8. Doping agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287

2.9. Ionization efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287

2.10. Measurement of molar mass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290

2.11. Coupling MALDI with devices that separate macromolecules by size . . . . . . . . . . . . . . . . . . . . . . . . . . . 294

2.12. Coupling MALDI with devices that separate macromolecules by functionality or by composition . . . . . . 298

Prog. Polym. Sci. 31 (2006) 277–357

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* Corresponding author. Tel.: C39 95339926.

E-mail address: gmontaudo@dipchi.unict.it (G. Montaudo).

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357278

2.13. Structure determination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300

2.14. End group determination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303

2.15. Tandem mass spectrometry for structure determination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304

2.16. Copolymer characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308

2.17. Bivariate distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309

3. Recent applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309

3.1. Polystyrene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309

3.2. Polymethylmethacrylates and acrylic polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311

3.3. Other polymers with an all-carbon main chain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315

3.4. Polymers with heteroatoms in the main chain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315

3.5. Polysiloxanes, poly(silsesquioxane)s and polysilanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315

3.6. Polyethers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315

3.7. Polyesters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316

3.8. Polycarbonates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317

3.9. Polyamides and polyimides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317

3.10. Polymers with phenyl and other cycles in the main chain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319

3.11. Copolymer studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320

3.12. Polymer degradation studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327

Appendix A. Size exclusion chromatography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 341

Appendix B. Copolymer composition from MS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344

1. Introduction

Polymers display a variety of structures, including

linear, cyclic, and branched chains, copolymers with

various architectures, dendrimers, and star polymers

with different number of arms. The structural charac-

terization of a polymer sample usually involves:

evaluation of the average molar mass (MM) and of

the molar mass distribution (MMD); determination of

the repeat units structure; copolymers sequence

analysis; end group identification; detection and

identification of impurities and additives. Modern

mass spectrometry (MS) offers the opportunity to

explore the finest structural details in polymers [1–10].

Matrix assisted laser desorption/ionization time-of-

flight (MALDI-TOF) has dramatically increased the

mass range of MS; it provides mass-resolved spectra up

to 50–70 kDa and above, allowing the detection of

quite large molecules (106 Da), even in complex

mixtures, at the femtomole level [3]. Peaks in the

spectra originate from ions of intact polymer chains,

and, therefore, allow structural identification of single

oligomers. The last few years have witnessed out-

standing progress in the application of MALDI to open

problems concerning the characterization of polymers.

Initially, MALDI-TOF instruments had poor spec-

tral resolution (M/DM about 500): i.e. mass-resolved

spectra usually did not go beyond 10,000 Da. This

caused structural identification problems, even in the

lowest mass range. Therefore, MALDI-TOF spectral

data on polymers that appeared in earlier papers (up to

about 1998) may need updating. MS yields information

on the masses of individual oligomers, a remarkable

difference with respect to NMR, which is an averaging

method. Therefore, besides providing unequivocal

information on the chemical structure of polymeric

materials, MALDI allows the identification of chain

end groups, including species present in minor amounts

in a polymer sample. End group identification is so

crucial in polymer analysis that its importance cannot

be overemphasized; in fact, it has been one of the most

popular applications of MALDI to polymers. The

determination of the end group structure of intact

polymer samples often has interesting side effects,

namely, the identification of procedures used in the

synthesis of research and industrial polymers, and the

capture of information on the structure of capping

agents and additives. Applications of the MALDI

technique to the characterization of synthetic polymers

have been summarized [3,4]. However, this field has

recently experienced considerable progress, and it is

our purpose to illustrate advances in the fundamental

and practical aspects of the MALDI-MS technique

related to polymer analysis.

Among other topics, our attention is focused on

recent advances in: MALDI sample preparation,

achieving high mass resolution, detailed structure

identification in polymers and copolymers, accuracy

of molar mass determination, functional and end group

identification, copolymer sequence analysis, in situ

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 279

detection of photo and thermal oxidation products of

polymers, monitoring polymerization reactions, and

coupling MALDI with liquid chromatography. We

discuss MALDI literature concerning selected classes

of polymers, and our bibliography includes about 400

references selected from papers that appeared in 2000–

2005.

MALDI applications are classified on the basis of

the polymer backbone: polymers of styrene and its

derivatives [11–63], acrylic and methacrylic polymers

[64–102], other all-carbon polymers [103–127], poly-

mers with heteroatoms in the chain: namely polysilox-

anes, poly(silsesquioxane)s and polysilanes [128–148],

polyethers [149–205], polyesters [206–250], polycar-

bonates [251–261] polyamides and polyimides

[262–283], polymers with an aromatic ring in the

backbone [284–311], and copolymers [312–365].

Studies of partially degraded polymers are discussed

separately [366–393].

2. Fundamentals and practice

2.1. Ion extraction

MALDI makes use of short pulses of laser light to

induce the formation of intact gaseous ions. Analyte

molecules are not directly exposed to laser light, but are

homogeneously embedded in a large excess of ‘matrix’,

which consists of small organic molecules. The matrix

molecules strongly absorb the laser light to allow for

very efficient energy transfer to the analyte (in our case,

the polymer). The high energy density obtained in the

solid or liquid matrices (even at moderate laser

irradiance) induces instantaneous vaporization of a

microvolume (called a ‘plume’), and a mixture of

ionized matrix and analyte molecules is released into

the vacuum of the ion-source. The laser pulse must not

be too long, otherwise analyte molecules do not all

desorb at the same time. On the other hand, there is no

advantage in using ultrashort pulses (fractions of

picoseconds) and there are many disadvantages (for

instance, a laser which generates ultrashort pulses is

expensive and bulky). The nitrogen laser, operating at a

wavelength of 337 nm, has a very compact design, it is

pulsed and its shots last about 3 ns, which is perfect for

the scope of MALDI.

In commonly used instruments (those equipped with

a time-of-flight tube), the laser pulse is followed by a

time delay, lasting 300–800 ns, before the application

of an extraction voltage, which brings the ions out of

the ion source. After this time delay, the packet of ions

generated in the process is accelerated by an electric

potential, ranging from 15 up to 35 kV. Homemade

instruments may use lower voltages, but they require

careful tuning. For instance, the first MALDI instru-

ment (used by Hillenkamp and coworkers in Munster

for their pioneering experiments [1]) had only 3 kV.

Depending on their mass-to-charge ratio m/z, the

ions have different velocities when they leave the

acceleration zone and enter a field-free flight tube

(drift-tube) 1 or 2 m long. After a time-of-flight of the

order of 100 ms, the ions impact onto an ion detector,

often formed by two microchannel plates connected in

series. The detector produces a signal (proportional to

the number of ions arriving at the detector), which is

processed by an ADC converter (using a clock with a

time base of 2 ns or better). The ADC is connected with

a computer, in which the resulting MALDI spectrum

can be stored and processed (e.g. for smoothing, etc.).

2.2. Detector

The detector amplifies the signal by a factor of about

107; and therefore, it is a very-high-gain amplifier. This

gain cannot be achieved without the presence of a

‘pool’ of secondary electrons and, when the amplifier’s

task is too demanding, the ‘pool’ may become empty.

This annoying effect is called detector saturation. The

presence of low molar mass compounds in the

polymeric sample can cause detector saturation and,

in turn, the amplification of the signals due to low and

high molar mass ions becomes uneven, the latter being

much reduced. This effect is particularly evident in

samples difficult to desorb at moderate laser power. The

user has various possibilities in order to avoid (or at

least to reduce) the negative consequences of detector

saturation [4,5].

First, MALDI mass spectrometers are equipped with

an electrostatic device, the deflector, which acts as a

cut-off, since it pulses away low molar mass

compounds and does not allow them to reach the

detector. The efficacy of this device is high and thus the

use of a deflector is very popular. However, it has a

drawback: namely, the on–off switching needs a small

(but not infinitesimal) time, and thus, the trajectories of

ions possessing masses close to the cut-off are not

straight and can appear in the spectrum as artifacts. The

second possibility is to leave the detector off initially

and to turn it on when high-mass ions start hitting the

detector. In this way, the pool of secondary electrons is

fully available for high-mass ions. However, the two

procedures described above ‘neglect’ low molar mass

compounds, assuming that they are unimportant, and

this assumption is not necessarily true. As an

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357280

alternative, one can replace microchannel plates with

another type of detector (for instance a hybrid detector,

in which the signal coming from the first microchannel

plate is processed by another detector, which is also a

high dynamic-range amplifier). In this way, low molar

mass compounds are not neglected.

2.3. Calibration

The MALDI-TOF mass spectrum is then obtained

by recording the detector signal as a function of time.

According to Eq. (1), the square of the flight time is

proportional to the m/z ratio

m=z Z 2Vt2LK2 (1)

where m is the mass of the ion, z is the number of

charges, V is the accelerating voltage, t is the ion flight

time, L is the length of the flight tube. In principle, since

V and L are known, the m/z ratio can be calculated

solely from Eq. (1). In practice, exact values for the

mass scale are obtained using another (empirical)

formula

m=z Z at2 Cb (2)

because of uncertainty in the determination of flight

time due to a short delay in ion formation after the laser

pulse. Hence, the true starting time of the ion is not

identical with the time of the laser pulse (which

provides the starting signal for the measurement of

flight time).

The constants a and b in Eq. (2) are estimated from

the measured flight times of two ions with known

masses. Usually, a preliminary calibration spectrum

Fig. 1. Diagram of the TOF/TOF mass spectrometer showing the location

analyzer, and the second mass analyzer with a curved field reflectron. Reprod

containing the known ions is recorded at the beginning

of the MALDI session to obtain a and b. Thereafter,

other spectra are recorded and it is assumed that a and b

do not change. This procedure is called ‘external

calibration’, since the two peaks do not belong to the

spectrum under analysis.

Another procedure, called ‘internal calibration’, for

time-to mass conversion yields more accurate m/z

values since the two peaks used to determine the

constants a and b are internal (i.e. they belong to the

spectrum under study) [161]. However, internal

calibration is not used frequently, owing to the fact

that the calibrant must be selected carefully to avoid

loss of spectral quality.

2.4. Resolution

Most MALDI-TOF instruments possess a device to

enhance the resolution, a reflectron, which consists of a

series of electrodes placed at the end of the flight tube.

The reflectron must be used in conjunction with an

additional detector (usually called the reflectron

detector) placed at the opposite side with respect to

the other (ordinary) detector. Fig. 1 shows the scheme

of a reflectron MALDI-TOF mass spectrometer [7].

When the electrodes are turned off, the MALDI

spectrum is recorded in the linear mode, whereas, if

they are turned on, the spectrum is recorded in the

reflectron (or reflected) mode. The reflectron causes a

decrease in sensitivity, and therefore, it cannot be used

with polymers which show a recalcitrance to desorb,

such as dimethylsilaferrocenophane [81], polymers of

high mass [4] (above 40 kDa), polymers terminated by

of the collision chamber, the mass selection point for the first mass

uced from Ref. [7] with permission of the American Chemical Society.

Fig. 2. MALDI spectra of a Jeffamine sample (a) and of a Y-shaped

polymer derived from Jeffamine (b). The figure also reports the

structure of the Y-shaped polymer. Reproduced from Ref. [344] with

permission of the American Chemical Society.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 281

bulky end groups, such as terpyridine [182,214,234]

and polymers of, dibutyl-substituted thiophenes [287].

Commercial MALDI-TOF instruments became

available soon after the first MALDI experiments.

Some of them were equipped with a reflectron; but

despite this, they had poor spectral resolution (M/DM

about 500). This means that mass-resolved spectra

usually did not go beyond 10,000 Da; and this caused

structural identification problems, since the resolution

in the 1000–2000 Da range was uncertain. However,

the discovery that a time-delay produces MALDI

spectra with better resolution came later in 1995; and

it is now possible to build MALDI mass spectrometers

with higher resolution by adding a fast high-voltage

electronic switch that allows for a time delay between

the laser shot and the extraction. This discovery was

patented under the name ‘delayed extraction’ (DE) but

it is also referred to as ‘time-lag extraction’ or ‘pulsed

extraction’. A series of papers dealing with DE-MALDI

spectra appeared soon after this discovery. In one of

them, the authors recorded DE-MALDI spectra of

bradikynin, cytocrome C, apomyoglobin and other

peptides [394]. Another paper presented the DE-

MALDI spectrum of a mixture of two oligonucleotides

[395], which were quite similar (the masses were

9471.2 and 9486.2 Da, respectively, and thus the mass

difference was only 15 Da). Researchers in the field of

polymers were immediately aware that a mass

spectrometer with enhanced resolution could be

particularly useful [396–399]. MALDI instruments

built before the availability of ‘delayed extraction’

must be considered obsolete, and spectral data that

appeared in earlier papers (up to about 1997–1998) may

need updating.

Obtaining good resolution with complex samples is

important. Cai et al. [344] synthesized a Y-shaped

copolymer via Michael addition of 2 equiv. of

2-hydroxyethyl acrylate to a commercial monoamine-

capped poly(alkylene oxide), Jeffamine XTJ-507,

followed by esterification using excess 2-bromoisobu-

tyryl bromide. Fig. 2 reports the MALDI spectra of the

initial Jeffamine sample (Fig. 2a) and of the Y-shaped

copolymer (Fig. 2b), along with its structure. The latter

spectrum suffers from poor resolution: peaks do not

‘pop out’, the valley between peaks is shallow and there

is substantial overlap among neighboring peaks. On the

other hand, it is quite apparent that the first spectrum is

perfectly resolved: the valley between peaks is very

deep (it touches the baseline) and neighboring peaks do

not overlap. This is a rare case in which the problem of

poor resolution can be circumvented. By comparing the

spectra; it can be seen that the mass of the second

sample is about 500 Da higher than that of the initial

Jeffamine sample; and this is in line with the proposed

structure. However, in general, spectra with poor

resolution must be discarded.

As an alternative to TOF analyzers, MALDI

instruments can be equipped with Fourier transform

ion cyclotron resonance (FT-ICR) analyzers. They are

limited in molar mass, and give poor results when used

to analyze polymers with masses of 20 kDa or above.

However, for molar masses in the range 1000–8000 Da

FT-ICR analyzers possess a distinct advantage, namely

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357282

the resolution is astounding (one part in 30,000 and

above). There are cases in which such resolution is

needed. Mize et al. [213] recorded the MALDI spectra

of two homopolyesters and two copolyesters, using a

MALDI instrument equipped with FT-ICR. In one of

the samples, two components, denoted E1n and E2n

were present, E1n having a mass only 2 Da higher than

E2n. Clearly, the isotopic patterns of the two were

partially superposed. For example, E2n(0), the E2n

chains with no 13C carbons, were (almost) isobaric with

E1n(2), the E1n chains with two 13C carbons. As a

result of the superposition, one of the two components

(the second one) was hidden, and its presence was

difficult to detect. However, E2n(0) and E1n(2) do not

have exactly the same mass and the FT-ICR MALDI

instrument was able to spot the very small difference in

mass (only about 10 ppm at mass 1800) and two distinct

MS peaks appeared in the spectrum, revealing the

presence of the second species.

In the case of a copolymer with ethylene and CO

units [342], many oligomers are also almost isobaric,

since the two repeat units are almost isobaric at about

28 Da), the difference being 36 mDa. In the mass region

639–640 Da, five ions are expected at masses 639.28,

639.32, 639.36, 639.39, 639.42 Da, respectively. Fig. 3

shows the expansion of the FT-ICR MALDI spectrum

of the copolymer in this region and identifies the peaks.

The resolution of the peak at 639.355 Da can be

estimated directly from Fig. 3. In fact, the full width at

Fig. 3. Expanded view of the MALDI-FT spectrum of an E–CO

copolymer showing resolutions of isobaric 19-mer oligomers. Peak

shoulders arise from 109Ag isotopes of oligomers of the same

monomer number with an additional unsaturation site. Reproduced

from Ref. [342] with permission of the American Chemical

Society.

half-maximum is about 10 mDa, and thus the resolution

is about 1 part in 64,000. Thanks to the high resolving

power of FT-ICR, the five ions are fully resolved.

Baker et al. [334] prepared a copolymer with units of

glycidyl methacrylate (GMA) and butyl methacrylate

(BMA) by using a free-radical initiator Vazo-52 and a

cobalt chain-transfer agent Co(dimethylglyoxime-

BF2)2. Two types of copolymer chains are expected:

A huge number of almost isobaric structures are

possible due to the fact that GMA and BMA have the

same nominal mass but slightly different exact masses,

with BMA 0.036 Da greater than GMA. When the

authors used a TOF instrument to analyze the

copolymer, the MALDI spectrum was characterized

by insufficient resolution.

However, there are reasons to believe that an FT-

ICR instrument would have better chances to detect the

cited isobar structures as separate peaks [365]. GMA–

BMA copolymers obtained using the Vazo initiator

were successfully characterized by electrospray ioniz-

ation (ESI) [365].

2.5. MALDI matrices

In MALDI-MS analysis, a dilute solution of the

analyte polymer is mixed with a more concentrated

matrix solution. The number of molecules nmol formed

in the desorption/ionization process decreases very fast

as the laser irradiance is turned down (often nmol falls as

the irradiance to the eighth power). However, it is well

known that small values of nmol (e.g. 100 or 1000) are

never found. There exists a threshold irradiance,

peculiar to each matrix, below which ionization is not

observed. Above this level, the ion production increases

nonlinearly. The choice of a matrix tailored for a

particular kind of polymer sample is crucial for

successful characterization of the sample and is usually

done in two stages. In the first step, only the backbone

structure is considered and this implies that chain end

groups and the average molar mass (which can be high

and low) are unimportant. The MALDI user searches

through the literature and retrieves a set of three or four

candidate matrices, which are optimal for that kind of

backbone structure. In order to speed up the process,

one can use tabulations of MALDI matrices and sample

C CCOOH

CN

H

OH

N

COOH

N

OH

OH OHO

COOH

N

C CCOOH

H

H

OH

HO

COOH

DHB HABA Dithranol

α -CHCA

all-trans-retinoic acid IAA

Fig. 4. The structure of some common MALDI matrices.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 283

preparation recipes, such as the document which

appears at the NIST website [16] or the listing produced

by Nielen [8]. Then, the user records the MALDI

spectra, using all the candidate matrices, and identifies

the highest-quality spectrum to select the best matrix.

Notably, for polar polymers the ‘optimal matrix’ is

actually a set of two to four matrices, as stated early by

Danis and Karr [241].

Fig. 4 gives the structures of some common

matrices. 3-amino4-hydroxybenzoic acid and POPOP

need high laser power, since they have a high threshold.

a-cyanocinnamic acid is often used for fragmentation

experiments [174,176,299], because it yields ions with

a (slightly) higher internal energy than the others.

Some MALDI matrices, such as all-trans retinoic

acid, are particularly sensitive to impurities whereas for

other matrices (like HABA and Dithranol) the loss of

efficiency is small, and hence, the latter matrices are

preferred when purification is a problem.

Retinoic acid works with polystyrene but it must be

doped, preferably with Ag salts. 5-clorosalicilic acid

gives good MALDI spectra of nonpolar polymers,

whereas nor-harmane [400] and trihydroxyacetophe-

none are general-purpose matrices.

It sometimes happens that common MALDI recipes

fail. The search is still performed by trial-and-error,

since the exact role of the matrix is still not fully

understood. Nevertheless, the search follows some

broad guidelines, as discussed in the following. Three

key functions of the matrix have been suggested, i.e.

incorporation of the analyte into matrix crystals, a

collective absorption and ablation event, and an active

role of the matrix in ionization [401,402]. Until recently

it was generally agreed that incorporation of individual

analyte molecules into the crystalline host matrix is an

important prerequisite for a successful MALDI anal-

ysis. Nowadays, this incorporation is no more seen as

mandatory, and some researchers prepare samples

without such intimate contact between analyte and

matrix. Generally, an ideal matrix should have the

following properties: high electronic absorption at the

employed laser wavelength, good vacuum stability, low

vapor pressure, good solubility in solvents that also

dissolve the analyte, and good miscibility with the

analyte in the solid state.

Recently, Hoteling et al. [42] suggested a method for

finding the optimal matrix. The polymer and the matrix

are injected in a reverse-phase HPLC equipment. The

best MALDI spectra are obtained when matrix and

polymer have retention times that closely match.

As an example of polymer for which published

MALDI recipes (and matrices) are not effective,

Ameduri et al. [115] cited poly(vinylidene fluoride)

(PVDF) and claimed that when sample preparation

involving conventional matrices is used to record its

MALDI spectrum (and, in general, the spectra of

polymers with a high content of fluorine), they fail.

They proposed a new sample preparation based on three

new matrices: 7,7,8,8-tetracyanoquinodimethane, pen-

tafluorobenzoic acid and 2,3,4,5,6-pentafluorocinnamic

acid (PFCA). Fig. 5 shows the MALDI spectrum of

poly(vinylidene fluoride) using the PFCA matrix. The

spectral quality is excellent and the spacing between

MS peaks is 64 Da, the expected value.

2.6. Developments in sample preparation

In most cases, the pristine ‘dried droplet’ method

[1–4], is utilized for sample preparation. Solutions of

matrix, analyte and salts (cationizing agents) are mixed

Fig. 5. MALDI-TOFmass spectrum of a poly(vinylidene fluoride). Reproduced from Ref. [115] with permission of the American Chemical Society.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357284

and the mixture is spotted onto the MALDI target.

Using the dried droplet method, one can prepare about

100 targets per hour.

Meier et al. [46] showed that MALDI can be used to

perform the ‘screening’ of polymers obtained using

combinatorial chemistry (COCHE). This MALDI-

COCHE combination allows finding the optimal

reaction parameters for a given monomer such as the

optimal temperature, the optimal solvent, the optimal

initiator, the optimal concentration of the reactant

species, etc. However, the dried droplet method is too

slow for this purpose. Meier et al. used, instead, a new

automated (robotic) MALDI sample spotting technique

that allows full integration of MALDI sample prep-

aration. MALDI-COCHE is so demanding that other

authors [56] decided to apply ink-jet technology to the

automated preparation of MALDI target plates. They

employed a multiple layering approach where the

matrix, cation and analyte were deposited as separated

layers and they noted that the spectral quality was good;

mass-resolved peaks were observed up to 3500 Da in

the analysis of PEG and PMMA samples.

Under the dried droplet conditions, crystallization is

relatively slow, thereby increasing the risk of segre-

gation phenomena of analyte, matrix or cationization

salt. If segregation occurs, significant variations of

peaks, peak intensity, resolution and mass accuracy are

observed by focusing the laser on different regions of

the same spot. Optimum results are obtained when the

polymer and the matrix are soluble in the same solvent.

The dried droplet method cannot be used for analysis

of the polymer samples that are insoluble or poorly

soluble in organic solvents. Therefore, considerable

efforts have been devoted to the development of new

sample preparation methods. The ‘solvent-free’ method

consists in immersing the polymer sample in liquid

nitrogen, followed by addition of powdered matrix. The

resulting mixture is finely ground in a rotating-ball mill.

The ‘solvent-free’ methodology has been applied to

polymers such as polyetherimide [268,303,309],

aromatic polyamides [270], poly(9,9-diphenylfluorene)

[300], polycyclic aromatic hydrocarbons (PHAs) [304],

etc. [298]. Contrary to the dried-droplet, where the

solvent evaporation allows for very strong adhesion to

the sample holder, in this case, the matrix/analyte

powder must be carefully fixed on the MALDI sample

holder [11].

MALDI optimization is slightly simpler, due to the

absence of the solvent. However, MALDI spectra are

still sensitive to the matrix, to mixing ratios of

matrix/polymer/cationizing agent, and to the sample

preparation procedure. The accuracy, sensitivity and

resolution of the MALDI spectra obtained using

solvent-free sample preparation are very similar to

those obtained with traditional solvent-based method-

ology [11]. In several cases, an improved signal-to-

noise ratio was obtained and also interference from the

matrix was less intense. The major advantage of the

solvent-free sample preparations is in the characteriz-

ation of insoluble polymers.

Recently, Gies et al. obtained MALDI spectra of

wholly aromatic, poorly soluble and insoluble poly-

amides, Nomex and Kevlar oligomers, by using wet

grinding methods [270] where, the matrix/sample

mixtures is initially processed in the usual way (i.e.

with a rotating-ball mill), and then the solvent is added.

The authors found that the spectrum quality improves

when steps are taken to break the hydrogen bonds that

Fig. 6. MALDI-TOF mass spectrum of Nomex oligomers in dithranol

with KTFA using the R–E–G method. Reproduced from Ref. [270]

with permission of the American Chemical Society.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 285

join polyamide chains. Fig. 6 gives the MALDI

spectrum of poly(m-phenylene isophthalamide)

(Nomex) obtained by the so-called ‘resolvated-evapor-

ation-grinding’ method (REG), using Dithranol (see

Fig. 4) as the matrix and potassium trifluoroacetate

(KTFA) as the cationizing agent. The spectrum shows a

series of peaks separated by 238 Da, which were

identified as Nomex cyclic oligomers [270].

Recently, a new sample preparation method has

been reported [168], which can be considered as a

variant of the above, since it involves two steps. The

first step is the spraying of the analyte/matrix mixture

on a substrate, followed by freeze-drying; the second

step is the positioning of the resulting powder on a

second substrate (the target). This method will be

discussed in the section that deals with molar masses.

The ‘dried droplet’ method gives quite irregular

surfaces. This is inconvenient, since one must move the

target, searching for a spot that gives abundant MALDI

ions. This search constitutes a formidable obstacle for

applications that require unattendedMALDI analysis of

hundreds of samples, such as MALDI sequencing of

proteins and nucleic acids.

Electrospray sample deposition (ES) is a sample

preparation method where matrix and analyte solutions

are sprayed on the target surface under the influence of

a high-voltage electric field [33,36]. ES is reported to

yield much better shot-to-shot and spot-to-spot repro-

ducibility than the dried droplet method. The improved

results are ascribed to the small and evenly sized

crystals thus formed, and the consequent improved

homogeneity of the MALDI sample surface [33,36].

Moreover, repeating the ES, the user can deposit two or

more layers of matrix/analyte mixture, with negligible

mixing between layers. This represents a fascinating

example of flexibility. Wetzel et al. [33] prepared a

large number of targets by ES, changing the deposition

voltage (DEVO) over a broad range, using three narrow

MMD polymers: polystyrene (PS), poly(ethylene

glycol) (PEG) and poly(propylene glycol) (PPG).

Peaks due to ion fragments were present in the

MALDI spectra of PEG and PPG for all the matrices

used, whereas PS, which is more thermally stable, did

not show ion fragmentation. Both PEG and PPG

showed increased ion fragmentation with increasing

DEVO. The authors also used the MALDI spectra to

determine the average molar masses and noted that the

MMD values for PEG and PPG tended to decrease,

owing to fragmentation, with increasing DEVO. On the

other hand, the MMD values of PS did not change over

the whole range of DEVO, indicating absence of

fragments [33]. Therefore, the thermal stability of the

polymer and possible fragmentation should be con-

sidered when using the ES deposition method.

Progress was also made in the analysis of

polyethylene (PE), by a substrate-assisted laser deso-

rption/ionization MS method, which uses cobalt,

copper, nickel or iron metal powders as sample

substrates, and silver nitrate as the cationizing reagent.

Intact ions of PE chains up to 5000 Da were

characterized [117].

Sometimes MALDI spectral quality is so bad that

the polymer cannot be characterized. In these cases, one

may be able to chemically modify the polymer and

record the spectrum of the modified polymer. Finding a

successful chemical modification can be an extremely

time-consuming process in MALDI sample prep-

aration. For instance, poly(trimellitic anhydride-co-4,

4 0-methylenedianiline), PI-PAA, gave no MALDI

signals with classical sample preparation [271].

Modification of the polymer by reacting it with

N-methylethanolamine failed to achieve the intended

purpose. The polymer reacted with N-methylethano-

lamine was further reacted with 2-fluoro-1-methylpir-

idinium p-toluensolfonate and eventually the MALDI

spectrum could be recorded. From it, the authors

determined the structure of the modified polymer and

inferred properties of the pristine polymer. In particu-

lar, one of the two trimellitic anhydride rings (the imide

ring) could be found in the open form (i.e. amic acid),

and this implies that PI-PAA is a copolymer with

approximately 50% amic acid. Saturated polyolefins

such as PE and polypropylene (PP) were derivatized

prior to the MALDI analysis to produce intact

macromolecules by MALDI [118]. The authors reacted

Fig. 7. MALDI-TOF MS spectrum of the PE sample SRM 1482.

Reproduced from Ref. [118] with permission of the American

Chemical Society.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357286

the terminal vinyl groups of narrowly dispersion PE

with a large excess of bromine, then reacted the

resulting chain-end brominated polymer with triphenyl-

phosphine, and recorded the MALDI spectrum [118].

Fig. 7 gives the MALDI spectrum of the modified PE

sample coded as SRM 1482 [118]. The spectrum shows

well-resolved mass peaks centered around 9000 Da.

Although the masses of the macromolecules are

sensibly larger than those discussed above (obtained

using cobalt, copper, nickel or iron metal powders), the

average molar masses measured by MALDI were

sensibly lower than those estimated by conventional

osmometry. In another study, the olefin ends of a series

of polyisobutylene samples were sulfonated before

MALDI analysis, and the measured molar masses

agreed with values obtained by laser light scattering

and vapor pressure osmometry, except for one sample,

characterized by a high molar mass and a large

polydispersity index [112].

Copolymers ethylene and CO units possess too

many isobaric structures for their characterization to be

accomplished using MALDI instruments equipped with

TOF analyzers. Cox et al. [342] proposed two solutions

to overcome the problem, the first one is the use of an

FT-ICR machine. As an alternative, the copolymer can

be derivatized. When the copolymer was reacted with a

multifold molar excess of sodium borohydride or

lithium aluminum hydride, so that the CO units were

reduced to HCOH units, the MALDI spectrum of the

resulting copolymer did not show isobar peaks and was

therefore, easy to interpret.

Among sample preparations, the SEC-MALDI

method deserves a mention. In comparing MALDI

spectra of size exclusion chromatography (SEC)

fractions and of unfractionated sample, the former are

found to have better resolution because they are free of

low molar mass impurities. Gallet et al. [373] recorded

MALDI spectrum of a sample obtained by injecting a

PEO–PPO copolymer in an SEC device and collecting

the fractions. The fraction eluting at 29.7 ml showed

mass-resolved peaks in the 2000–3000 Da region. On

the other hand, the MALDI spectrum of the unfractio-

nated PEO–PPO copolymer (in the same mass region)

showed mass-unresolved peaks, indicating a dramatic

worsening of the resolution which is impossible to

estimate, but is certainly larger than a factor four.

2.7. Sample preparation for low molar mass

compounds

For low molar mass compounds, the usual sample

preparations cannot be applied. Even if the analyte

molecules display negligible fragmentation, the

MALDI matrix breaks apart, producing a variety of

matrix-related ions, and thus the low-mass region of the

MALDI spectrum is literally stuffed with peaks due to

the matrix and its fragments. As a consequence, low

molar mass compounds (m/z!500 Da) cannot easily be

analyzed by MALDI, since the peaks due to the analyte

and to the matrix show virtually inextricable overlap.

Considerable efforts have been made and several

alternative approaches have been developed to over-

come this problem. One solution is to deposit the

polymer on the target, without adding the matrix. This

works for alanine and some peptides, but often the

molar extinction coefficient of the analyte is too small,

and it is necessary to switch to an instrument with a

high-irradiance laser. In order to overcome this

obstacle, a modified MALDI technique has been

proposed, called DIOS (desorption/ionization on sili-

con), where the analyte is deposited on porous silicon.

The latter acts as a matrix (in the sense that it adsorbs

UV light and it is able to promote analyte ionization)

with the advantage that it does not produce peaks in the

spectrum. DIOS has been successfully applied to record

the spectra of PEG samples [180,183], the presence of

spurious peaks being rather limited.

Soltzberg and Patel [403] employed as a matrix

poly(3-n-octylthiophene-2-5-diyl) which has some

interesting advantages: namely, it is commercially

available, light-adsorbing and electrically conductive.

They demonstrated that it could be used to analyze by

MALDI some aliphatic and aromatic molecules

possessing a carboxylic acid group. In another study

[404], the surfactant cetrimonium bromide was added

to the a-cyano-4-hydroxycinnamic acid matrix for the

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 287

successful analysis of a variety of low-mass molecules

by MALDI. Low-mass components of polyesters such

as poly(neopentylglycol adipate) were determined by

MALDI analysis using a 10,15,20-tetrakis(pentafluor-

ophenyl)porphyrin (F20TPP) matrix, which does not

give matrix-related ions below m/zZ822 [242].

2.8. Doping agents

The ionization of synthetic polymers often occurs by

metal clustering (cationization) rather than protonation.

Since many polymers have relatively high cation

affinities, they do not necessarily require a high cation

concentration and thus cations present as impurities in

matrix, reagents, solvents, glassware, etc. may suffice.

However, Bahr et al. [1] put forward the following

objection: if one relies on ‘adventious’ cationization,

the yield of cationized species may turn out to be low

or, more simply, the cationization can prove to be less

than homogeneous. In order to avoid such drawbacks,

they added an alkaline salt solution (LiCl, NaCl, KCl)

to the matrix–analyte mixture (for polystyrene analysis,

silver trifluoroacetate was added instead). Their

objection has a sound foundation, and thus the addition

of dopants is now routine in MALDI.

There are some rough agreed-upon rules for selection

of the dopantmost effective for a given class of polymers,

but they are obviously empirical. As an example, Trimpin

et al. suggested the use of sodium for poly(vinyl

pyrrolidone) [109]. Fig. 8 shows MALDI spectra of

poly(vinyl pyrrolidone) using different cationizing agents

with Na, K, Li and Ag. It can be seen that sodium indeed

Fig. 8. MALDI spectrum of poly(vinylpyrrolidone). Reprod

gives a good S/N ratio, whereas the number of counts (the

ion yield) with Ag is unsatisfactory [109].

2.9. Ionization efficiency

A polymer system comprises hundreds of different

analytes, each present in a given abundance: i.e. the

mixture contains chains with different lengths, but also

with different end groups and different backbone

structures. If the MALDI ionization efficiencies of all

these analytes are the same, they will all produce ions in

amounts proportional to the abundance of each analyte

in the polymer sample. Actually, however, the

ionization efficiency changes with the analyte. Ioniz-

ation efficiency and cation attachment are clearly

connected and some authors believe that a cation

attaches itself preferentially to small macromolecules

instead of large ones. The total number of cationization

sites CTOT is the sum of the cationization sites (CCHA)

along the chain plus the cationization sites CEND at the

end groups. Cyclic macromolecules do not have end

groups and, to a first approximation, CCHA increases as

the length of the chain increases, since each repeat unit

represents a cationization site [405]. However, if the

cyclic polymer chain forms a random coil, cationization

sites that lie inside the coil cannot be accessed, and the

effective CCHA is less than the total value for all repeat

units in accordance with experimental evidence.

The case of linear macromolecules is much more

complex since they possess end groups and the cation

affinity for the latter may be much higher than for the

backbone. Ionization efficiency differences among

uced from Ref. [109] with permission from Elsevier.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357288

chains with different lengths are often called ‘mass

discrimination’ [3]. These differences can be estimated

by simply measuring the changes in ionization

efficiency when a low molar mass and a high molar

mass sample are analyzed simultaneously. Mass

discrimination implies that the MALDI response is

not linear with respect to molar mass.

The simplest (and most used) method to study

ionization efficiency is to create a polymer mixture of

known composition and to measure the relative

amounts of ions. In describing these experiments, we

shall differentiate between polymer mixtures with the

same backbone and mixtures with different backbones.

The second kind of mixture is more demanding. A

mixture of two polymers having different end groups

but the same backbone can be produced by mixing

chains of the type G1-AAAAAAA-G2 with chains of

the type G3-AAAAAAA-G4, where G1, G2, G3, G4

are end groups and A is the repeat unit. By recording

the MALDI spectrum of the mixture and measuring

MALDI peak intensities, we can estimate apparent the

relative abundance of the chains. We discuss some

cases of this type. Cox et al. [34] considered a series of

PS samples terminated with hydroxyl, hydrogen,

tertiary amine, and quaternary amine groups. They

observed that the ionization efficiency of PS oligomers

was affected exclusively by the chemical groups at the

PS chain-ends. The PS samples terminated with a

quaternary amine exhibit 10-fold greater ionization

efficiency than the other PS samples studied. The

authors analyzed nine MALDI spectra of 1:1 blends of

these end-functionalized PS samples, finding that the

relative ionization efficiencies of the polymer com-

ponents varies dramatically with the laser power, and

that spectra recorded at the threshold of the laser power

give the most accurate representation of the blend

composition. However, at this usual instrumental

condition, owing to the large difference in ionization

efficiency between the quaternary amine terminated PS

(PSQ) samples and that for all the other functionalities,

accurate quantization of the other component in

mixtures with the PSQ samples is difficult. They also

measured the relative intensity ratio in the MALDI

spectra of the different end terminated PS blends as a

function of the different blend composition, and found a

linear trend for each blend. On the base of these data,

assuming that the ionization efficiency of the PSQ

samples is 100% and comparing mixtures with a

common component, the authors calculated the

ionization efficiency relative to the PSQ of all PS

polymers investigated. They found that the samples

terminated with OH and H end groups have similar

ionization efficiencies, with the tertiary amine slightly

higher, depending on the average molar masses.

Chen et al. [149] considered a mixture of PEGs with

the following structures:

The MALDI spectral quality was good, since the

masses were very low. They plotted the actual

composition versus the composition measured using

MALDI peak intensities. The points lie on a straight

line, with a slope of 45.58, which does not differ much

from 458, which indicates identical ionization efficien-

cies. Okuno et al. [179] analyzed mixtures of linear and

branched polypropyleneglycol (PPG). They noted that

composition (weight percent of branched chains)

measured by MALDI was slightly different from the

actual composition. They noted that the difference

depended on sample preparation and, in particular, on

the concentration of the polymeric solution, and that it

tended to fade away as more concentrated solutions

were used.

Despite the fact that there are some cases in which

MALDI is only semi-quantitative, many studies have

appeared which assume that the ionization efficiency in

mixtures with the same backbone is uniform.

Maziarz et al. [138] considered a mixture of a,u-bis-(t-butylamine-fumaryl-oxy-butyl) poly(dimethyl silox-

ane) (BAF-PDMS) and another polysiloxane (IMPU-

PDMS). The structure of the latter is quite complex,

with two PDMS chains connected by a bridge (for

brevity, we indicate only its empirical formula:

C40H70N2O10Si2 [C2H6OSi]n [C2H6OSi]x).

The MALDI spectrum is a little crowded, since the

mass of IMPU-PDMS is approximately 12 Da greater

than the mass of the corresponding BAF-PDMS.

Taking the ratios of peak intensities (and applying

some corrections), the authors were able to estimate the

abundance ratio, i.e. BAF/IMPUZ67/33. Campbell

et al. [13] analyzed a polystyrene sample, a mixture of

three different types of chains denoted TDB0, TDB1

TDB2. Here, TDB stands for terminal double bonds,

and chains TDB2 are terminated at both ends with a

styrene molecule (and thus two double bonds), whereas

chains TDB0 are terminated by two hydrogens. Using

MALDI peak intensities, the authors were able to

estimate the ratio of abundances TDB0/TDB1/TDB2Z5/90/5. This result yields information about the

polymerization process since it clearly indicates that

TDB0 and TDB2 chains are side-reaction products.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 289

Libiszowski et al. [257] recorded the MALDI

spectrum of PDX, a polymer obtained by cationic

ring-opening of 1,4-dioxan2one, and noted the simul-

taneous presence of peaks due to cyclic and linear PDX

chains. From the MALDI peak intensities and they

constructed a bar graph of the abundance of cyclic

chains ACIC versus that of linear chains, ALIN. Summing

all the bars in the bar graph, they were able to estimate

the relative (total) abundances, which gave ACIC/

ALINZ59/41. This implies that the reaction produced

large amounts of cycles.

Luftmann et al. [159] reacted a commercial PPO

sample with p-nitrobenzoyl chloride and obtained a

diester- and monoester-terminated PPO chains. They

recorded the MALDI spectrum and, using peak

intensities, found that the diester/monoester mole

ratio was 875/125. It is interesting to note that the

NMR spectrum was useless for this purpose. As the

authors state, it yielded diester/monoesterZ100/0;

but this failure is due to the circumstances that the

monoester and diester signals tend to overlap and

that the integration has an error margin of about

10%.

Ring–chain equilibration reactions are character-

ized by a critical concentration B 0, and when the

concentration is above B 0, they yield a mixture of

cyclic and linear macromolecules. Keki et al. [209]

obtained poly(lactic acid) by ring–chain equilibration.

They measured the MALDI spectral intensities due

to cyclic and linear chains and assumed them to have

the same ionization efficiency. From the ratio of

intensities, they were able to estimate B 0 and found

that it varied from 100 to 1000 l/mol, depending on

the temperature.

Recording the MALDI spectrum of a mixture of two

polymers having different backbones, one finds that

MALDI peak intensities reflect in a distorted manner

the abundances of the chains and the composition of the

blend. In some cases, the distortion is small and thus

MALDI is semiquantitative. In the following, we

discuss some examples. Scamporrino et al. [406]

showed that two instrumental parameters could affect

peak intensities, thus falsifying the composition of the

blend. The two parameters are the delay time (DETIM)

and the voltage of a wire electrode (VOWIEL)

measured in percent of grid voltage (the wire electrode

acts on ion trajectories, attracting the ions). The authors

studied an equimolar mixture of PEG and PMMA,

recorded the MALDI spectrum of the mixture and

found, on changing DETIM and VOWIEL, that the

apparent blend composition changed from 100/0 to 50/

50 to 0/100.

Alicata et al. [273,274] considered a blend of

Nylon 6 (Ny6) and poly(butylene terephthalate)

(PBT) oligomers and noticed that their ionization

efficiencies varied greatly, depending on the end

groups. Fig. 9a shows the MALDI spectrum of the

Ny6/PBT mixture. It can be seen that the peaks due

to the Nylon 6 polymers terminated with carboxyl

groups (Ph–Ny6–COOH) predominate over those for

the PBT sample terminated with hydroxyl end groups

(HO–PBT–OH) although the polymers used have

similar molar mass distributions and were present as

an equimolar blend. Similar behavior was observed in

the analysis of an equimolar mixture of Ny6

terminated with amino groups (Ny6–NH2) and

PBT–OH. However, MALDI spectra of blends of

Ny6–COOH and PBT polymers terminated with

carboxyl groups (PBT–COOH) show peaks of

comparable intensity due to both polymers, as can

be observed in Fig. 9b. When an equimolar mixture

of PBT–OH and PBT–COOH was analyzed, MALDI

spectra showed intense peaks due to PBT–COOH

oligomers at lower mass, whereas above 2000 Da

peaks due to PBT–OH became more intense, as the

intensities of peaks due to the two PBT polymers

tended to equalize. Since the number of end groups in

both polymers decreased at higher mass, this result

indicates the preponderant effect of the end groups on

the ionization efficiency of oligomers.

Yan et al. [141] recorded the MALDI spectrum of a

mixture of PMMA and PDMS and found that PMMA

peaks were absent, even when the mole fraction of

PMMA exceeded the mole fraction of PDMS by a

factor of four. The authors proposed an explanation

based on the fact that the PMMA had a high molar mass

and high polydispersity index (MnZ33,000 and MwZ100,000). However, a simple calculation shows that

such a sample should be characterized by a very large

amount of low-molar mass oligomers and thus strong

PMMA MALDI peaks were to be expected. Possible

explanations are that the ionization efficiencies of

PMMA and PDMS are very different or that the

instrumental parameters DETIM and VOWIEL had

values that favor the PDMS ions and suppress the

PMMA ions.

Chapman et al. [210] reported the MALDI

spectrum of an almost equimolar mixture of poly-

(butylene glutarate) (PBuGu) and poly(butylene

adipate) (PBA). Peaks due to PBA dominate the

spectrum, and this clearly indicates that PBA is

preferentially ionized. Measuring peak intensities and

summing them, we found PBuGu/PBAZ30/70, which

indicates a difference in ionization efficiency of a

0

5000

1000

1500C

ount

s

2250 2300 2350 2400 2450 2500 2550 2600 2650 2700Mass (m/z)

NB19 NB20

NB21

NB22

NB’19NB’20 NB’21 NB’22

NA20 NA22 NA23

THC10

NB19 NB20

NB21NB22

NA21

(a)

NB20 NB21 NB22

NB’20

NB’21

NB’22

TCB10TCA10 TCB11

TCA11

500

1000

1500

2000

2500

3000

Cou

nts

2250 2300 2350 2400 2450 2500 2550 2600 2650 2700

Mass (m/z)

NB19

NB’19

(b)

Fig. 9. Enlarged regions of the MALDI-TOF mass spectra (a) of equimolar mixtures of Ny6–COOH (MvZ2500) and PBT–OH (MvZ2300) and (b)

equimolar mixtures of Ny6–COOH (MvZ2500) and PBT–COOH (MvZ2000), with Mv the viscosity-average molecular weight. The labels NA,

NB, THC and TCA indicate the cyclic Ny6 oligomers, the Ny6–COOH oligomers, the PBT–OH and the PBT–COOH oligomers, respectively.

Reproduced from Ref. [273] with permission of the American Chemical Society.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357290

factor 2–3. Murgasova et al. [41] considered

mixtures of PS and poly(a-methylstyrene) (PAMS)

in which the blend composition (i.e. weight fraction

of PS) varied from 0.2 to 0.75. They recorded the

MALDI spectra of the mixtures and found peaks due

to both PS and PAMS. Since the structure of PS is

quite similar to the structure of PAMS, they assumed

that the ionization efficiencies for PS and PAMS are

the same. From the ratio of MALDI intensities,

they were able to estimate the blend composition

correctly.

2.10. Measurement of molar mass

Mass spectrometry can be used to determine the

molar mass (MM) of each polymer chain species and

the molar mass distribution (MMD) of a polymer

sample by measuring the intensity of each mass spectral

peak corresponding to a molar mass Mi. Mass

spectrometers are equipped with a detector that gives

the same response if an ion with mass 1 kDa or 100 Da

(actually any mass) strikes it. The detector measures the

number fraction and this implies that the intensity of the

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 291

ith peak is proportional to Ni, the number of chains with

mass Mi. The number average and weight average

molar masses defined by

Mn ZX

MiNi=X

Ni ZX

Wi=X

MiWi

n oK1

(3)

and

Mw ZX

M2i Ni=

XMiNi Z

XMiWi=

XWi (4)

are readily obtained.

Wallace et al. [50] developed an operator-indepen-

dent approach to mass spectral peak identification and

integration, which is claimed to increase the accuracy

of the summations and thus the accuracy of the Mn and

Mw values obtained. The method is straightforward and

the calculations can be performed independently of

whether the spectrum is mass-resolved. The definitions

given by Eqs. (3) and (4) are quite old, certainly older

than MALDI, and they may be used with other types of

mass spectrometers such as FAB (fast atom bombard-

ment), PD (plasma desorption), or LD (laser deso-

rption). For instance, in 1986, Brown et al. [407]

recorded the LD spectrum of a polymer sample with

masses well beyond 5 kDa and extracted the average

molar mass values (Mn and Mw) embedded in the mass

spectrum.

In order to obtain the MMD of the polymer, mass

spectral data must be processed using a suitable

transformation algorithm, and the quantities Ni (i.e.

the fraction of the chains with mass mi) are transformed

into Wi (the weight of the chains with mass mi). Barrere

et al. [137] applied the algorithm to a PDMS sample.

The open circles in Fig. 10 represent the SEC trace for

the PDMS sample. The figure also shows a series of

Fig. 10. MM distribution for a PDMS sample. The needles represent

the result when the MALDI spectrum is processed to yield the

resulting weight fraction MMD. The SEC trace (open circles) is also

reported. Reprinted from Ref. [137] with permission from Elsevier.

‘needles’ that represent the result when the MALDI

spectrum is processed to yield the resulting weight

fraction MMD. There is good agreement with the SEC

trace.

The ionization process must be ‘soft’. If hard

ionization occurs, chains are no longer intact (frag-

mentation occurs) and the measurement will be affected

by a systematic error toward the bottom (i.e. under-

estimation of Mn and Mw). Since fragmentation is an

annoying concern, some authors developed a protocol

to avoid it. They noted that the extent of fragmentation

decreases when the laser power is lowered and also

when a large excess of matrix is used in sample pre-

paration. Thus, the protocol consists in using low laser

power (close to the threshold) and in using a matrix-to-

analyte ratio of at least 10,000:1. In our opinion, few

polymers undergo fragmentation; the overwhelming

majority remain intact during desorption, with fragmen-

tation close to zero. Fig. 11 shows the MALDI mass

spectrum of a poly(butylene glutarate) (PBG) sample

[211]. In the mass region between 1000 and 5000, there

are no peaks. This implies that the ionization process is

soft and that the bonds forming the PBG backbone are

quite strong. The small number of polymers subject to

fragmentation includes polyethylene [118] and some

hyperbranched polymers [190,218].

Laine et al. [87] studied fragmentation in MALDI

and carefully labeled the fragmentation peaks. They

studied a variety of samples and deliberately switched

to conditions different from those indicated by the

above-cited protocol. They found that results are

biased when one abandons the protocol. The method

described above for extracting the average mass

information (Mn and Mw) embedded in the mass

spectrum of a polymer sample has been used

extensively and a huge number of authors (we counted

at least 100 reports) compared Mn and Mw values for the

polymer sample with Mn and Mw obtained by

traditional methods for MM (molar mass) determi-

nation (SEC, viscometry, light scattering, etc.).

Considering narrow-MMD polymer (with Mw/Mn!1.10–1.20), as those that can be obtained by anionic or

cationic polymerization, most authors found the

agreement within 10–15%, or even better, i.e. it can

be considered excellent [3,4,396–399].

However, researchers at NIST (formerly the

National Bureau of Standards) noted that authors

knew in advance the molar mass averages of their

samples and thus the results could be biased. For this

reason, they sponsored an interlaboratory comparison

among 23 laboratories, based on a polystyrene sample

of which the participants did not know the average

Fig. 11. MALDI-TOF mass spectrum of an almost monodisperse poly(butylene glutarate) sample, using IAA as the matrix. Reproduced from Ref.

[211] with permission of the American Chemical Society.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357292

mass. After receiving the results of the MALDI

analysis, researchers at NIST made public that the

sample was obtained by anionic polymerization and

had a narrow MMD centered on 7 kDa. Fig. 12 reports

one MALDI spectrum [16] of the sample. It can be seen

that the strongest peaks are around 7 kDa and this

implies that MALDI estimates are in accordance with

estimates obtained by traditional methods for MM

determination.

On the other hand, when the MALDI spectrum is

used to estimate Mn and Mw values for a broad-

MMD polymer, a discrepancy is always noticed: i.e.

MALDI underestimates both Mn and Mw [408]. As

already noted, this problem is usually called ‘mass

discrimination’. Some authors [5] believe that its

nature is ‘instrumental’ (i.e. connected with the

ion-source design or with the detector as discussed at

length above) whereas others [34,141,273,274]

believe that its nature is ‘chemical’: the cation

Fig. 12. MALDI-TOF mass spectrum of the PS 7000 sample used in the Int

Reproduced from Ref. [16] with permission of the American Chemical Soc

attaches itself preferentially to small macromolecules

instead of large macromolecules. Regardless of its

causes, mass discrimination must be avoided or t

somehow circumvented. Many authors have proposed

remedies when using MALDI to estimate Mn and

Mw of polymers. First, we consider methods that aim

to obtain a size-independent ionization efficiency of

sample molecules, and then we turn to methods,

which assume that ionization efficiency changes as a

function of the molar mass and try to correct for this

effect.

Some authors [47,65,168] claim that proper MALDI

sample preparation minimizes mass discrimination. A

simple recipe [47] consists in avoiding common

matrices (such as HABA dithranol DHB) and switching

to another matrix, 2-[(2E)-3-(4-tert-butylphenyl)-2-

methylprop-2-enylidene]malononitrile (TEBUMAL).

The reason why this recipe works well is unclear, but

probably it is connected with the fact that TEBUMAL

erlaboratory Comparison, using retinoic acid as a matrix and AgTFA.

iety.

Fig. 13. MALDI-TOF MS spectrum of PEO 100,000. Reproduced from Ref. [168] with permission from Wiley.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 293

possesses a very low (laser) threshold. Unfortunately,

TEBUMAL is not yet commercially available and it

must be synthesized following the route developed by

Ulmer et al. [409].

Recently, a new sample preparation method has

been reported [168] involving flash spraying/freezing

of the analyte/matrix mixture, followed by freeze-

drying. This protocol was used successfully to acquire

correct molar mass distribution (MMD) estimates for

polydisperse samples of poly(vinylpyrrolidone) (PVP),

poly(ethylene oxide) (PEO), dextran, lichenan and

nigeran. Fig. 13 displays the MALDI mass spectrum of

PEO 100,000, obtained by the above freeze-drying

preparation, showing a MMD from 10,000 to above

250,000 Da. The Mn and Mw values calculated from this

spectrum are in good agreement with those obtained by

conventional techniques [168].

Electrospray sample deposition is known to

improve the homogeneity of the MALDI sample

surface and also the signal strength, in comparison

with the dried droplet method, potentially enabling the

use of MALDI for some MMD measurements of

polymers [33].

Some authors have tried to cope with the problem

of mass discrimination in MALDI by performing an

‘off-line’ correction of the detector response, elim-

inating spurious components in the signal and

generating a new spectral baseline from which the

molar mass of the polymer can be calculated [410].

The method utilizes the MALDI spectrum in

continuous extraction to get the full ion yield from

the detector and the estimation of Mn and Mw using

integral forms of Eqs. (3) and (4) as the asymptotic

limits of these parameters obtained on calculation

with increasing upper mass integration limit Mup.

This procedure has been applied with encouraging

results to several widely polydispersed polymers,

such as PDMS, PMMA and a bisphenol-A copo-

lyether sample [410]. A further procedure [411] for

the correction of decreasing detection response in

MALDI-TOF spectra with increasing ion mass is

based on the use of PMMA standards of known MM

to calibrate the detector response.

Mize et al. [213] tried to overcome mass

discrimination in another way. Fig. 14a shows a

MALDI-FT-ICR spectrum of a poly(hexanediol-alt-

azelaic acid) (poly(HEX-AZ)) homopolymer. The

peak intensities were used to compute MM averages

MnZ1083, MwZ1210, Mw/MnZ1.12; but they dis-

play considerable differences from SEC data from

the supplier (1040, 2590 and 2.5, respectively).

Therefore, the mass spectrum of the whole sample

appears to suffer from mass discrimination of high

molar mass oligomers. The authors used SEC to

prepare low polydispersity samples and, in order to

cope with the problem of mass discrimination, they

developed a reconstruction algorithm, thus generating

a ‘projection’ mass spectrum. The intensity of each

peak in the projection spectrum is the sum of 15–20

terms, each term being the product of intensity of

the corresponding peak in a given fraction multiplied

by the area under the SEC fraction. Fig. 14b shows

the projection mass spectrum of the poly(HEX-AZ)

polymer reconstructed from 15 SEC fractions. The

improvement is remarkable, especially for masses

above 4000 Da.

Fig. 14. (a) Single-shot-mass spectrum of poly(HEX-AZ). (b) Mass spectrum reconstructed from the sum of 15 mass spectra, each representing ions

from 1-min fractions of a 15-min capillary SEC separation. Insets show the presence of higher mass species than those detected in the unfractionated

sample. Reproduced from Ref. [213] with permission from IM publications.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357294

2.11. Coupling MALDI with devices that separate

macromolecules by size

Size exclusion chromatography is a very popular

method for polymer characterization, however, it must

be noted that SEC calibration is an error-prone task.

Furthermore, it needs mass calibration by an absolute

method of MM measurement. Appendix A deals in

detail with SEC elution procedures and algorithms. One

method for measuring the calibration constants consists

in preparing a mixture of five or more polymer samples

with the same repeat unit, each having a narrow MM

distribution and known MM (so-called SEC primary

standards). The mixture is injected in the SEC

apparatus and the resulting chromatogram is recorded.

Measuring the elution volumes and plotting them

against the logarithm of the mass, yields the parameters

that identify the calibration line (see Appendix A).

Actually, the slope of the calibration line is relatively

insensitive to the type of polymer injected whereas the

intercept changes from one polymer to another.

Intercept values correspond to the molar mass, and

these changes may be quite large. The reliability of

SEC results strongly depends on the availability of a set

of polymers of known MM and narrow MM distri-

bution (primary standards) with the same structure as

the polymer of interest. However, a set of such

calibration standards is often unavailable for a specific

0 10 20 30 40Ve (ml)

m/z

m/z m/z

m/z

Arb

itrar

y U

nits

310000

190000 49000

2400

Fig. 15. SEC trace of poly(dimethylsiloxane) (PDMS) in THF as

eluent. The insets display the MALDI mass spectra of four selected

fractions. Reproduced from Ref. [131] with permission from Wiley.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 295

polymer. As a rough approximation, some authors

assume that the parameters, which appear in the SEC

calibration equation, are independent of the polymer

structure. Thus, a common procedure is to use a mixture

of polystyrene primary standards to construct an SEC

calibration line supposedly valid for any type of

polymer. Unfortunately this assumption is rarely

valid. For instance, when tetrahydrofuran (THF) is

used as the solvent, polycarbonate gives an SEC mass

value only about half the mass of polystyrene of the

same mass [254]. To solve this difficulty, the SEC-

MALDI method can be used. The polymer is injected in

the SEC apparatus and fractions are collected. Then,

selected fractions are analyzed by MALDI. SEC

calibration constants (see Appendix A) are easily

obtained by combining MALDI mass data with the

elution volumes of the fractions. The major application

of the SEC-MALDI method [3,4] has been to obtain

accurate values Mn and Mw when the sample is a broad-

MMD polymer or copolymer. Several papers have

recently reported applications of SEC-MALDI for the

analysis of synthetic polymers [412– 416], and Murga-

sova and Hercules [413] reviewed the entire SEC-

MALDI field. Different approaches have been proposed

to couple ‘on-line’ MALDI to liquid separation

methods, and it has been shown how SEC sample

collection and subsequent preparation of samples for

MALDI analysis can be automated [128–130,401].

Several interfacing methods have been proposed where

the eluate stream is deposited on the MALDI target by a

spray or drip process. The matrix is co-added to the

eluate stream, or matrix-precoated targets are used

[128,401]. However, some of these methods require an

expensive robotic system with precise XYZmovements

and some do not allow for varying the matrix/analyte

ratio to optimize the MALDI spectral resolution of the

higher molar mass fractions, which need a matrix/

analyte ratio very different from that of lower MM

fractions. On the other hand, in ‘off-line’ SEC-MALDI

the matrix/analyte ratio can be varied, and this

represents a major advantage. A typical SEC-MALDI

trace is illustrated in Fig. 15, which shows the SEC

trace of a polydimethylsiloxane (PDMS) sample

together with the MALDI mass spectra of four fractions

obtained by SEC [131]. These data allowed calibration

of the SEC curve against absolute molar masses and,

thereafter, computation of the MM averages from the

SEC curve according to the standard procedure adopted

in SEC work [131]. MALDI spectra of high mass SEC

fractions in the figure are not mass-resolved, whereas

the MALDI spectrum of the last (low mass) SEC

fraction is mass-resolved. Thus, the MALDI spectra of

the SEC fractions containing the lowest molecular

species allow the identification both of the polymer

structure and the end groups. It is also possible to

identify the presence of cyclic and open chain

oligomers, a recurrent structural problem in polymer

synthesis. Fig. 16 shows the mass spectra correspond-

ing to SEC fractions of the DPMS polymer of very low

mass reported in the previous figure. Peaks in Fig. 16a

correspond only to linear oligomers, whereas in

Fig. 16b, a distribution of peaks corresponding to

cyclic oligomers can be detected besides that of linear

chains [131]. If the SEC sample injected is a mixture of

linear and cyclic macromolecules with the same

backbone, it must be remembered that linear chains r

and cycles have different hydrodynamic volumes (see

Appendix A) since a linear chain occupies a larger

volume than corresponding cycle and, therefore, linear

chains and cycles are eluted at different times [417–

420]. A quantity of interest is the ratio DCLZMcyc/Mlin

at a given elution volume, where Mcyc is the mass of

cycles and Mlin that of linear chains. Theory [417]

predicts that DCL is 1.25 for masses up to 100 kDa, then

starts to increase towards higher values (1.30 and even

higher). Wright and Beevers [417] report on early

experiments in 1978 on the construction a preparative

SEC device to determine SEC calibration curves for

cyclic PDMS and linear PDMS. The speed of the

analysis is very low (also due to the fact that it must be

preceded by solvent extraction in water–methanol

mixtures), and it is limited in mass since it cannot

separate macromolecules above 70 kDa. In a single

0

4000

8000

12000

240

280

320

2000 4000 6000 8000

2200 2300 2400 2500 2600

4000 6000 8000 10000

2243 2317 2391 2465 2539

2257 2331 2405 2479 2553

B30

B31B32

B33B34

A30A28A29 A31 A32

A27A29

A31

A25 A35

A86A87

A85A83A91

OSi

CH3

CH3

....Na+

n

OSi

CH3

CH3

Si(CH3)3....Na+

nA = (CH

3)3SiO

B =

m/z

m/z

a.i.

a.i.

(a)

(b)

Fig. 16. Reflectron MALDI-TOF mass spectra of two PDMS SEC fractions: (a) fraction 70 and (b) fraction 78, eluted at 28.10 and 30.55 ml (see

Fig. 15), respectively. Reproduced from Ref. [131] with permission from Wiley.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357296

SEC-MALDI experiment, the calibration lines for

linear and cyclic PDMS were determined up to

500 kDa, and DCL was estimated with a consistent

increase of the useful range [3].

SEC-MALDI investigations have been used to

explore the MALDI response to molecular association

in poly(bisphenol A carbonate) (PC) [255]. Most PC

samples are mixtures of PC chains with different chain

ends (also cycles are present) but only PC chains

terminated with one or two hydroxyl groups can

undergo self-association by hydrogen bonding. In the

presence of molecular association in a polymer sample

in a particular solvent, the SEC method fails, and the

usual Mn and Mw information cannot be obtained, since

Mn and Mw values are affected by a systematic error.

Chain self-association was observed when a 10 mg/ml

PC solution was injected into SEC columns, using

chloroform (CHC13) or tetrahydrofuran (THF) as the

eluant. The presence of self-association in PC was

revealed by the difficulty of obtaining SEC fractions

with narrow molar mass distributions [255]. The

MALDI spectra of the SEC fractions contained a

number of species much higher than expected, covering

huge mass ranges (10 kDa and even up to 20 kDa)

clearly showing that narrow SEC fractions are not

obtained and that the standard goal in SEC (i.e. to elute

chains of different sizes at different volumes) is not

achieved. Fig. 17a–c show [255] the MALDI spectra of

PC fractions collected at the same elution volume in

three different SEC runs. As is shown in the spectra, a

higher sample dilution, or the addition of a polar

solvent, such as ethanol, to the CHCl3 eluant,

suppressed self-association in the PC samples. Chain

association of PC produced molecular aggregates of

relatively small molecules with high hydrodynamic

volume, which were, therefore, eluted through SEC

columns at the same elution volume as higher molar

mass chains. However, the molecular aggregates were

broken when the SEC fractions, containing a hetero-

geneous mixture of PC chains of different size, were

diluted in the matrix used for the MALDI sample

preparation. The most common MALDI matrices

contain carboxylic acid units, which are able to break

the hydrogen bonds responsible for the formation of the

chain aggregates. The MALDI spectrum of one of these

PC fractions (Fig. 17b) shows a bimodal distribution of

peaks. The low-mass peaks are due to PC chains

terminated with OH groups, whereas the ions at high

mass correspond to PC chains capped at both ends (see

oligomer structures depicted in the upper part of the

figure). Chmelik et al. [139] were able to fractionate a

poly(dimethylsiloxane) by liquid chromatography

using supercritical CO2 as the eluent. Fig. 18 shows

the chromatographic trace. The separation is apparent;

peaks corresponding to chains of length 10, 20, 30, 40,

50 and 60 units are indicated. The authors also collected

CH3

CH3 O

OO COO

n

O

CA= B=CH3

CH3 O

OO COH

nCH3

CH3 O

OO COH

CH3

CH3

OH

n

C=

100

4000 8000 12000 16000 20000 24000

A 5 1

A 5 9

A 6 7

Counts(a)

200

400

600

5000 10000 15000 20000 25000

A 5 1A 5 5

A 4 5

A 4 1B 4 1

B 3 4

B 2 5C 31

C 21B 1 3

(b)

0

40

80

120

5000 10000 15000 20000 25000m / z

A 5 1

A 5 9

A 6 7

B 4 1

(c)

Fig. 17. MALDI-TOF spectra of PC fractions collected at the same elution volume (31.3 ml) in four different SEC runs: (a) sample PC1 injected at a

concentration of 2.5 mg/ml in CHCl3; (b) sample PC1 injected at a concentration of 20 mg/ml in CHCl3; (c) sample PC1 injected at a concentration

of 20 mg/ml in CHCl3/C2H5OH 95/5 v/v. Reproduced from Ref. [255] with permission from Wiley.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 297

the fraction that eluted between 15 and 18 ml and

labeled it Fraction F (see the lower part of the figure).

The figure also shows the MALDI spectrum of fraction

F. The spectrum is very clean, with well-resolved MS

peaks in the mass region 1200–2200, thus confirming

the power of the method.

Kassalainen et al. [39] developed an alternative to

SEC-MALDI; they used a thermal field-flow frac-

tionation (TFFF) device to separate macromolecules

of different length and equipped it with a detector to

measure abundances. They studied two polystyrene

samples whose TFFF traces turned out to have quite

different shapes, as shown in the inset of Fig. 19.

They collected TFFF fractions and analyzed them by

MALDI. Fig. 19 also shows the MALDI spectra of

five selected TFFF fractions. The molar masses of

the fractions increase with elution time giving 8, 12,

20, 45, 60 kDa, respectively. In the MALDI spectra

of the first and the second fraction, ions due to

chains with different size are detected as separate

peaks. At higher masses, some overlap shows up.

This phenomenon is very common. One can

introduce a useful quantity, REGmax, which is the

experimental mass (or the mass region) in which the

MALDI spectrum is still mass-resolved. For homo-

polymers, REGmax may reach values as high as

100,000, whereas for copolymers REGmax is five

times smaller and rarely equals (or exceeds) 12,000.

A possible explanation for these findings is low

resolution. Indeed, the resolution degrades at high

masses. However, the relation between the resolution

associated with a peak and the molar mass in

MALDI is quite involved, and the derivative K of

the full width at half maximum (FWHM) of MALDI

Fig. 18. Chromatographic trace of PDMS in supercritical CO2. The inset displays the MALDI spectrum of the indicated fraction F. Reproduced

from Ref. [139] with permission from Wiley.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357298

peaks with respect to mass, can take both positive

and negative values. Nevertheless, K is always

negative above 5 kDa.

HPLC can be used to separate macromolecules by

size, but above 10–50 kDa, chains with different sizes are

coeluted. Thus, if highmasses are absent from the sample,

one can collect fractions and analyze them by MALDI.

The HPLC calibration line is readily obtained, and the

HPLC trace yields Mn and Mw values for broad-MMD

polymers. For copolymers, great care must be used since

HPLC ordinarily uses a UV detector, and its response

seldom reflects accurately the abundance of copolymer

species [421] (see Appendix B). MALDI can also be

combined with HPLC for the purpose of purity control.

2.12. Coupling MALDI with devices that separate

macromolecules by functionality or by composition

It has been shown that some liquid chromatography

devices are able to separate macromolecules by function-

ality; in particular, macromolecules having the same

backbone but different end groups may elute at different

times. Under suitable conditions, HPLC separates

macromolecules having different end groups. In an

early study, Pasch and Rode [240] showed that it is

possible to collect HPLC fractions of poly(decamethy-

lene adipate) and to identify the oligomers present in the

fractions by recording their MALDI spectra. The HPLC-

MALDImethod is time-consuming and, therefore, its use

is mostly limited to cases where the MALDI spectrum of

the unfractionated sample is too complex to give a

complete picture of the sample properties. Peetz et al.

[285] prepared poly(2,5-diheptyloxy-1,4-divinyl-ben-

zene) by acyclic diene metathesis. In order to isolate

oligomerswith different sizes (the trimer, the tetramer and

the pentamer) they processed their sample by column

chromatography, collected the fractions and then

recorded their MALDI spectra. They noted that peaks

due to impurities are absent from the spectra (each

fraction displayed a single peak). Fig. 20 shows the

MALDI spectrum of an HPLC fraction; it contains a

single peak due to the pentamer. There is a good match

Fig. 19. TFFF traces of two poly(styrene) samples fractionated by TFFF (right) along with MALDI spectra of five TFFF fractions (left). Reproduced

from Ref. [39] with permission of the American Chemical Society.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 299

between the calculated intensity of each isotopic and

corresponding observed spectral peak.

Liquid chromatography at the critical condition

(LCCC) is performed at the elution–adsorption

Fig. 20. MALDI-TOF mass spectrum of an isolated oligomer (the penta

comparison between calculated intensity of each isotopic and experimental

transition. It can be used for a variety of separations;

but here we focus exclusively on two types, namely. To

separate macromolecules with different functionalities

(mostly chain ends) and for block copolymers.

mer) of poly(1,4-diheptyloxy-2,5-divinyl-benzene). On the right, a

peak. Reprinted from Ref. [285] with permission from Wiley.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357300

Keil et al. [170] showed that LCCC is able to separate

macromolecules with different shapes from a sample in

which three poly(propylene oxide) chains are bonded to

a glycerol molecule. The MALDI spectrum of the

unfractionated sample is very complex, whereas the

MALDI spectra of LCCC fraction turn out to be simpler

to interpret. LCCC is particularly well suited for AB

copolymers with long AAAA. and BBBB. blocks,

since these samples exhibit a most peculiar retention

behavior. As a consequence of this behavior, the spectra

of the fractions exhibit many fewer peaks and thus are

much simpler than the spectra of the unfractionated

sample. Park et al. [37] analyzed a styrene–isoprene

copolymer. The polymer was injected in the LCCC

apparatus and fractions were collected. Thereafter,

selected fractions were analyzed by MALDI. Fig. 21

shows MALDI spectra of six fractions: denoted f1, f3,

f5, f7, f9, f11. It can be seen that in the spectrum of

fraction f1 all peaks are absent except those due to

oligomers with three isoprene units, whereas fractions

f3, f5, f7, f9 and f11 show exclusively MS peaks due to

oligomers with 5, 7, 9, 11 and 13 isoprene units,

respectively. These interesting MALDI results imply

that the length of the isoprene block determines

uniquely the retention behavior, which is completely

Fig. 21. LCCC fractionation of a styrene–isoprene block copolymer.

LCCC trace (upper part) along with the MALDI spectra of LCCC

fractions f1, f3, f5, f7, f9, f11. Reproduced from Ref. [37] with

permission from Elsevier.

independent of the length of the styrene block. In other

words, the authors succeeded in performing a perfect

separation and MALDI was used to prove the absence

of spurious copolymer chains in the fractions. This is a

particularly happy case; usually some contamination

occurs. For instance, MALDI analysis combined with

LCCC separation has been used to characterize a

poly(L-lactide)-block-poly(ethylene oxide)-block-

poly(L-lactide) (PLLA-b-PEO-b-PLLA) triblock copo-

lymers [354]. In this case, the authors found that the

molar mass and composition, and the distribution of

two end blocks, affect chromatographic retention.

In thin-layer chromatography (TLC), polymer

chains terminated in different ways yields different

spots. By scraping away the TLC spots and recording

their MALDI spectra, one may be able to identify the

terminal group. When a densitometer is available, one

can estimate the fraction of macromolecules terminated

with a certain end group. Ji et al. [111] analyzed two

poly(butylene) samples by TLC; Fig. 22 gives a picture

of the TLC plates and their MALDI spectra. One of the

samples gives a single TLC spot, which suggests two

hypotheses: that the sample is a complex mixture or that

the TLC apparatus does not have sufficient resolving

power to separate it. The second hypothesis is (more

simply) that all chains are terminated in the same way.

The MALDI spectrum of the TLC spot indicates clearly

that the second hypothesis is correct. The other sample

gives two TLC spots, which can be interpreted as

evidence that the sample is a mixture of chains

terminated in two different ways. The figure also

shows the MALDI spectra of the two spots and, by

inspection, it is seen that this interpretation is correct.

2.13. Structure determination

Polymers display a variety of structures, including

linear and branched chains, copolymers with different

sequences and star polymers with different numbers of

arms. Because of the variety of possible structures, the

process of analyzing a polymer has to answer several

questions and logically proceeds by steps. The first step

consists in the determination of the chemical structure

of the backbone. The second step consists in finding out

if the chains possess branching points and in the

determination of the degree of branching. The third step

consists in finding out what end groups lie at the chain

ends, and therefore, also in detecting cyclic oligomers

that may be present. The first step is quite simple. In

fact, mass spectra of two polymers possessing different

repeat units will produce widely different mass spectra,

since the spacing between peaks is different: e.g. in

Fig. 22. TLC of two poly(butadiene) samples. A photograph of the TLC plate (left) along with MALDI spectra (right) of spots (a)–(c). Reproduced

from Ref. [111] with permission of the American Chemical Society.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 301

Fig. 23. MALDI spectra of complex mixture obtained when tetrakis(p-hydroxy-phenyl)porphyrin is allowed to react with chlorinated poly(ethylene

glycol) methyl ether. The mixture contains a consistent fraction of star polymers with 2, 3, 4 arms. Reproduced from Ref. [184] with permission

from Wiley.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357302

poly(ethylene glycol) the spacing is 44.05 g/mol, in

poly(lactic acid) t 72.1 g/mol, in poly(dimethyl silox-

ane) 74.1 g/mol, in poly(butylene terephthalate)

220.2 g/mol. This feature provides the polymer identi-

fication. In the case of branched polymers, quantities of

interest are the number and the position of branching

points. Their determination is sometimes impossible

using MS, because branched and linear macromol-

ecules often have the same mass (polyolefins are a

typical instance). However, when they have end groups

different from hydrogen atoms, and the spectra are

mass-resolved, the masses of branched polymers and

the corresponding linear ones differ. Trifunctional (or

multifunctional) units may have different masses than

the corresponding linear polymer (this circumstance

often occurs in grafted copolymers). Sometimes one

deals with a complex mixture of macromolecules with

the same repeat unit but with different architectures. For

instance, during the synthesis of star polymers with four

branches, it may happen that stars with three or with

two branches are also formed. It often happens that

macromolecules belonging to such a complex mixture

have different masses and, in these cases, MALDI can

discriminate between among them. In an interesting

example [184], Fig. 23 gives the MALDI mass

spectrum of the products obtained when tetrakis(p-

hydroxyphenyl)porphyrin is allowed to react with

chlorinated poly(ethylene glycol) methyl ether, provid-

ing an extremely complex mixture of macromolecules.

Ions due to chains with one branch appear in the region

800–1200 Da, whereas ions due to chains with two,

three and four branches appear at 900–1500, 1000–

2000 and 1500–2500 Da, respectively. Each type of

architecture generates an envelope of peaks, and a line

is drawn to indicate the envelope. The abundances of

chains with one, two, three or four branches can be

denoted as B1, B2, B3, B4. By measuring individual

MALDI peak intensities, one can estimate the relative

abundances and, in particular, the ratio B1/B2/B3/B4.

Alternatively one can evaluate the ratio by measuring

the area subtended by each envelope.

Using MALDI, one can elucidate even more

complex structures. Im et al. [32] recorded the

MALDI spectrum of a complex mixture where chains

with three, four and five branches, and other complex

architectures, were present. The MALDI spectrum was

Fig. 24. MALDI spectrum of a polyamide sample with a linear/cycle mole ratio close to 5/95. Reproduced from Ref. [267] with permission from

Dekker.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 303

very clean. Peak positions gave information on the

structures present, whereas peak intensities gave a

semiquantitative estimate of their abundances.

In condensation polymers, one often deals with

samples that are mixtures of linear and cyclic

macromolecules. Fan et al. [267] synthesized an

aromatic polyamide and found the sample to have a

massive amount of cycles, the linear/cycle molar ratio

being close to 5/95. The NMR spectrum was not very

useful for quantitation; signals due to end groups were

very weak whereas signals due to cycles were

superposed with signals due to backbone units.

Fig. 24 shows the MALDI spectrum, from which the

chemical structure of the cyclic macromolecules could

be determined. The ions dominating the spectrum,

indicated as C2, C3, C4, are due to the cyclic dimer,

cyclic trimer, and cyclic tetramer, respectively.

2.14. End group determination

The general structure of ions detected by MALDI is

of the type

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357304

G1–AAAAAAA–G2/CC

where G1 and G2 are end groups, CC is a proton or a

cation and A is the repeat unit.

End group determination by MS is done as follows.

One considers the mass number of one of the MS peaks,

subtracts themass of C, and then repeatedly subtracts the

mass of the repeat unit, until one obtains the sum of the

masses of G1CG2. A linear best fit can also be used to

find G1CG2. As a simple example, we recall the

MALDI spectrum of the polystyrene sample with molar

mass around 7 kDa used for the NIST-sponsored

interlaboratory comparison [16]. The individual peaks

are due to molecular ions (MC), and the repeat unit is

C8H8, with a mass of 104.15 Da. If one subtracts an

integral number of monomer units from the observed

molecular ion, a ‘residual’ mass of 58 Da is obtained

(e.g. 6307K60!104.15Z58). This represents the sum

of the end group masses. A mass of 58 Da is consistent

with a butyl group at one end of the chain and a hydrogen

on the other:H–(St)n–C4H9. In this instance, the polymer

was prepared anionically using tert-butyl lithium [16].

One should note that the lowest residual mass (in this

case 58 Da) is not necessarily the correct sum of the end

group masses. In principle, the sum of the end group

masses might 162, 266, 370 Da, etc. (i.e. 104!nC58,

with nZ1, 2,.). One may need additional information

to elucidate the correct end groups: for example, other

spectroscopic or chromatographic data and/or knowl-

edge of the synthetic procedure that was used.

2.15. Tandem mass spectrometry for structure

determination

For structural studies, one may use a mass spec-

trometer equipped for tandem MS (see Fig. 1). Some

ions, once generated, break apart quickly. This can occur

by ‘in source fragmentation’ (INSF) inside the ion

source or by ‘post-source decay’ (PSD) in the field-free

region. In MALDI-TOF instruments equipped with a

reflectron, utilization of PSD is appealing, since it can

give information on the structure of ions. The method of

analyzing PSD fragments bears distinct similarities to

classical tandem MS (MS/MS) on double-sector

instruments [422]. In the latter, one of the sectors is

used to produce a narrow-mass spectral window (a beam

of ions of approximately the samemass, ‘parent-ions’ or

‘precursor-ions’), whereas the other sector is used to

separate and analyze the fragment ions, ‘daughter-ions’,

produced in the collision chamber. The PSD method

consists of three steps: (1) selecting a parent-ion by the

ion-selector, (2) changing the voltages of the reflectron

electrodes, (3) recording the resulting spectrum, called

the ‘PSD-MALDI spectrum of the parent-ion’ at the

specified mass. The ion-selector (see Fig. 1) is an

ultrafast electronic switch capable of pulsing away all

the ions except those in a narrow temporal window

(a couple of nanoseconds) corresponding to a mass

window of 4 Da or less.

Since MALDI is a soft-ionization method, the

number of intact oligomer ions that break apart

spontaneously is small, and it is necessary to increase

it to improve the quality of the PSD-MALDI spectrum:

i.e. increase the signal-to-noise ratio. For this purpose,

MALDI-TOF instruments accommodate a collision

chamber (see Fig. 1), in which the MALDI ions suffer

hundreds of collisions with molecules of an inert

collision gas (usually argon). The effect of the collision

chamber is to increase the number of ions that break

apart. In this way, one obtains collision-induced-

dissociation (CID), and the resulting spectrum is

referred to as a CID-MALDI spectrum [423]. In recent

years, there have appeared tandem time-of-flight (TOF/

TOF) instruments, which are especially important for

the analysis of high mass, singly charged ions

[422–424].

Hoteling and Owens [174], using PEG1000 as a

model analyte, studied the effect of various instru-

mental parameters operating on the resolution and mass

accuracy in MALDI-TOF PSD and CID analysis of

polymers. In another study, Hoteling et al. [176]

investigated the effect of the collision gas (He, Ar,

air) and collision gas partial pressure, on the MALDI-

TOF CID MS/MS spectra of PEG1000.

Fournier et al. used PSD-MALDI to study

fragmentation pathways of three nylon oligomers

desorbed under MALDI conditions [272]. They

found that the end groups and the length of the

methylene units influenced the fragmentation of the

different nylons and the relative abundance of the

product ions. They observed competitive dehydration

and deamination reactions, depending on the nature of

the terminal groups and the repeat units. In all cases,

the PSD spectra were very similar to the CID spectra

recorded under low-energy conditions, indicating that

the selected precursor ions had similar average internal

energies. Fig. 25a shows the PSD spectrum of the

parent ion with m/zZ599 corresponding to the

protonated Ny6 tetramer dicarboxy terminated (Ny6–

COOH), while Fig. 25b reports the MALDI-CID

spectrum of the protonated Ny6–COOH trimer

oligomer (m/zZ486). Peaks present in both spectra

can be interpreted with the scheme in Fig. 25. The

parent ion (at mass 599) eliminates a water molecule,

Fig. 25. Fragmentation of NY6: effect of the collision cell. The upper spectrum is recorded on an instrument without a collision cell. Reprinted from

Ref. [272] with permission from Wiley.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 305

generating the ion at mass 581. The latter has a

C-terminal (actually a COOH group) and an

N-terminal, and therefore, there are two fragmentation

pathways. The parent ion can suffer consecutive losses

of a neutral lactam (with mass of 113 Da). MS peaks at

masses 468, 355, 242 are due to fragment ions that

retain the N-terminal. Alternatively, ions can retain the

C-terminal and peaks at 471, 358, 245 are due to these

fragment ions (the mass difference is always 113 Da).

Ions giving peaks at 454, 341, 228 retain the

C-terminal too, but they are generated by further

scissions of the previous ions. In the scheme, the

C-terminal and the N-terminal of the parent ion are

drawn at the right and at the left, respectively. The ion

undergoes fragmentation, and thus tandem MS allows

determination of the masses of the C-terminal and the

N-terminal separately [272]. In another paper, Murga-

sova et al. [265] suggested from MALDI-CID spectra

of hexamer, octamer and dodecamer linear Ny6

oligomers, that the fragmentation process includes

cleavage of the end groups followed by dissociation of

the m/zZ113 unit (the repeat unit). They also observed

that the cleavage of the oligoamide chain occurs at the

amide linkage, as well as at adjacent bonds. In the

same work, they studied the effect of the matrix and

cationization agent in MALDI-CID analysis of Ny6

and found that the DHB matrix and NaCl salt gave the

best results.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357306

Arnould et al. [347] synthesized a branched

polymer by reacting a mixture of two diacids,

terephthalic acid (T) and adipic acid (A), with a

mixture containing a large amount of a diol, neopentyl

glycol (G), and a small amount of a triol, trimethylol

propane (M). They recorded the MALDI spectrum of

the polymer and they noted the presence of eight mass

series. The first four mass series were assigned to

linear chains with 0, 1, 2, 3 adipic acid residues along

the chain. The absence of chains with more than three

A units is consistent with the fact that adipic acid was

by far less abundant than T in the feed. The other four

mass series were assigned to branched chains with one

or two branching points and different amounts of

adipic acid. Clearly, the presence of branching points

coincides with the presence of type M units. Although

their structural characterization was complete, the

authors also performed some fragmentation experi-

ments. Fig. 26 shows the PSD-MALDI spectrum of the

parent-ion at mass 1297, which corresponds to the

sodiated pentamer H–(GT)5–G–OH. Peaks labeled L

and C are due to linear and cyclic fragment ions,

respectively; and the subscripts indicate the cation (H

means protonated). It can be seen that linear chains

(being flexible) bind NaC more strongly than cyclic

chains and that cycles are more basic than linear

fragments. The spectrum also contains several low-

mass products that do not contain NaC (marked DH)

and are due to consecutive dissociations of larger

fragment ions. Thus, it is evident that structural

information can be recovered from the PSD-MALDI

spectrum.

Fig. 26. PSD-MALDI spectrum of the parent-ion at mass 1297, which

corresponds to the pentamer H–(GT)5–G–OH/NaC. Reproduced

from Ref. [347] with permission from Elsevier.

Laine et al. [87] observed systematic changes in

fragmentation behavior of PMMA with increasing MM

by PSD MALDI-TOF analysis of alkali–metal catio-

nized PMMA 20, 60 and 100-mer. Using lithium,

potassium and cesium salts, they observed that

increasing MM of PMMA required increased cation

size to optimize the intensity and the number of the

fragments in the PSD spectrum. In fact, they obtained

the best PSD spectra when PMMA 20-mer was

cationized with lithium and 100-mer with cesium. In

the last case, the best results were obtained from SEC

fractionated PMMA sample. The authors postulated the

advantages of various cations to be a consequence of

the strength of interaction of the cations with isolated

PMMA molecules and the PSD fragments [87]. An

alkali cation effect has also been observed in the

MALDI-TOF PSD analysis of ethoxylated polymers

that are commonly used as surfactants [193].

Neubert et al. confirmed the dendrimer structures of

a number of polypentylresorcinol samples by PSD

MALDI-TOF MS analysis, using a-cyano-4-hydroxy-cinnamic acid (CHCA) matrix and alkali salts as the

cationizing agent [299]. They observed heavy frag-

mentation using LiC and NaC cations, and on the basis

of the corresponding PSD spectra proposed fragmenta-

tion pathways. The most abundant PSD fragments arose

from loss of the terminal tetrahydropyranyl (THP) ether

groups.

In an interesting work, Gies and Nonidez [270]

determined the lengths of blocks in poly(ethylene

oxide)-b-poly(p-phenylene ethynylene) (PEO-b-PPE)

by PSD MALDI-TOF. The fragment ion mass spectra

revealed that the main fragmentation process involves

cleavage of the ester function that links the two blocks.

They also determined the composition of the PEO-b-

PPE copolymers from their PSD spectra.

Recently, Rizzarelli et al. [362] showed by PSD

MALDI-TOF analysis of poly(esteramide)s from

dimethyl sebacate or sebacic acid and 1,2-ethanolamine

or 1,4-buthanolamine, that the main cleavage of these

polymers proceeded through a b-hydrogen transfer

rearrangement. MALDI-TOF/TOF spectra acquired

using Ar as a collision gas, showed new intense

fragment ion peaks in the low mass range, mixed with

the peaks present in the PSD-MALDI spectra of the

same parent ion. These new fragment ions were

diagnostic in establishing the random sequences of

the ester and amide bonds in the copoly(esteramide)s

[362]. Fig. 27 shows the MALDI-TOF/TOF-MS/MS

spectrum of the sodiated diamino alcohol terminated

oligomers at m/zZ1220 of the poly(esteramide)

derived from sebacic acid and an excess of

700.0 779.6 859.2 938.8 1018.4 1098.0

722.42

747.46

949.55

974.59

725.46707.80

764.49 819.57 992.72 1046.62861.57 1088.73

472.0 514.4 556.8 599.2 641.6 684.0

495.29

520.32

498.30 592.37477.29

536.37 634.42480.33 606.40523.35502.30 648.45541.38 662.48 676.47563.36

260.0 301.2 342.4 383.6 424.8 466.0

268.15

293.18 365.21

309.19

271.18

407.24435.29

449.31421.28379.21297.19

275.17

279.19 322.17 363.22 393.25375.23347.22

54.0 94.8 135.6 176.4 217.2 258.0m/z

210.15

62.05

228.16253.18

55.03250.1488.06

69.05 98.06180.07

84.05

125.04 229.15138.05112.07 194.0895.08 167.05

225.11

208.11152.07 237.11

223.15

336.23336.23264.16

266.19

452.25

65.9

9

Fig. 27. Expansions of MALDI-TOF/TOF-MS/MS spectrum of sodiated diamino alcohol terminated oligomers at m/zZ1220 of a poly(ester amide)

sample synthesized from sebacic acid and an excess of 1,2 ethanol amine. Reproduced from Ref. [362] with permission from Wiley.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 307

NH2CO(CH2)8CONH2

O(CH2)2 NHCO(CH2)8CONH (CH2)2O O (CH2)2NHCO(CH2)8CONH(CH2)2 O

CH2 CHNHCO(CH2)8CONHCH CH2

O (CH2)2NHCO(CH2)8COO (CH2)2NH

CH2 CHNHCO(CH2)8COOH

NH(CH2)2 OCO(CH2)8COO (CH2)2NH

HOCO(CH2)8COOH

(a)

(b)

(c)

M•Na+ 223.15 M•Na+ 275.17

M•Na+ 250.14

M•Na+ 225.11

Scheme 1.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357308

1,2-ethanolamine. As can be seen in Scheme 1, the

fragment ions at m/zZ223 and 275, 250, and 225 are

diagnostic of amide–amide, ester–amide, and ester–

ester sequences, respectively. These data suggest that

the ester and amide groups are distributed randomly in

the copolymer chains [362].

Muscat et al. observed in-source fragmentation by

MALDI-TOF analysis of hyperbranched polyestera-

mide, prepared from hexahydrophthalic anhydride and

diisopropanolamine. The MALDI spectrum did not

show signals due to the oligomers terminated with –OH

groups, whereas peaks corresponding to protonated

oligomers minus water [HPEAKH2OCH]C [425] did

appear. These data provide evidence that the in-source

decay of hydroxyl terminated HPEA chains causes end

group loss.

2.16. Copolymer characterization

Mass spectra of copolymers are significantly more

complex than those of simple homopolymers, and thus,

the task of peak assignment is more demanding.

However, the procedure is the same: i.e. putting

forward a reasonable hypothesis on the chemical

structures that may be present in the sample, computing

the masses of all the possible chains and checking if

expected peaks are actually present in the spectrum. In

AB copolymers, two repeat units with masses m1 and

m2 alternate along the macromolecular backbone.

The case in which m1 is equal (or almost equal) to m2

is extremely rare, and thus, the identification of the

copolymer backbone structure by MALDI is readily

accomplished. For instance, MALDI spectra of

styrene–methyl methacrylate copolymers display

many peaks and the differences in mass between

peaks are 108 and 100, which correspond exactly to

masses m1 and m2.

MS peak intensities can be used to determine

copolymer composition [3,9,316], provided that the

ionization method used to desorb and ionize the

oligomers does not produce significant ion fragmenta-

tion. Appendix B describes in some detail how to use

MS peak intensities to determine copolymer compo-

sition. The application of the MS method is based on

the assumption that the intensities of peaks appearing in

the mass spectrum of a copolymer are directly related to

the relative abundance of oligomers present in the

copolymer [3,9,315,316]. This condition is usually met,

but there are a few exceptions. For instance, in the case

of a random copolymer with units of isobutylene (IBU)

and para-methylstyrene (pMST), the average molar

composition determined from the MALDI spectrum of

the copolymer was found to be skewed to higher

methylstyrene content (36 mol%) as compared to that

obtained by NMR (13%) [315]. The authors believe

that the facile ionization of methylstyrene-rich oligo-

mers caused the composition discrepancy between

MALDI and NMR data.

Fig. 28. Bivariate distribution for ST-maleic anhydride sample.

Reproduced from Ref. [325] with permission of the American

Chemical Society.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 309

2.17. Bivariate distribution

A bivariate distribution, as a function of mass and

composition, is a feature peculiar to addition copoly-

mers synthesized by high-conversion processes (as

many industrial copolymers are) and it is frequently

associated with a drift of the composition with growing

length. The relevance of the bivariate distribution of

chain length and composition has been discussed [3,

336,337]. The shape of the bivariate distribution has a

sensible influence on copolymer properties, but its

measurement has been somewhat laborious.

In fact, the method traditionally used for measuring

the bivariate distribution is ‘chromatographic cross

fractionation’ (also referred to as ‘two-dimensional

chromatography’ or ‘orthogonal chromatography’).

Macromolecules of different compositions are separ-

ated in a silica column, and an SEC column is then used

to elute chains of different sizes [418– 420]. As an

alternative to conventional methods, MS can be used,

and this has shown great advantage [3,9]. For low molar

mass copolymers, MS alone is sufficient to monitor the

compositional drift in AB copolymers, and the change

in the mole fraction of the A units was followed in the

range 2000–11,000 Da [326]. The method was used to

estimate the weight fraction of copolymer chains,

which possess a given composition and to draw the

compositional distribution histogram for copolymers

containing methylmethacrylate, butylacrylate, styrene

or maleic anhydride units [326].

For copolymers of high molar mass some problems

arise, owing to the fact that REGmax (see above) takes

values of 12 kDa with a loss of spectral resolution

above that value. Nevertheless, a variant of the SEC-

MALDI method used to overcome this problem. The

variant consists in fractionating the whole copolymer

by SEC, collecting the fractions and recording both

MALDI and NMR spectra [337]. In fact, because of the

high sensitivity of the MS and NMR methods, the

amount of sample in the narrow fraction provided by an

analytical SEC device is sufficient to run both types of

spectra. Fig. 28 shows the bivariate distribution of a

commercial ST/MAH sample [325]. The sample was

polymerized to high conversion, and it exhibits an

asymmetric bivariate distribution, showing the compo-

sition drift expected for this type of polymer.

3. Recent applications

In addition to the applications discussed above,

MALDI-MS techniques have been used to analyze

a variety of polymers; extensive listings of MALDI

studies on synthetic polymers have been published

[3,4,8]. A survey of the most important MALDI

literature on selected classes of polymers appearing in

the last 5 years follows.

3.1. Polystyrene

Narrow- and wide-dispersion polystyrenes (PS)

synthesized by different methods and having different

end groups have been extensively studied by MALDI-

TOF [12–63].

Bartsch et al. [23] found that chlorine, amine and

acrylate functionalized TEMPO-capped PS, and bis-

TEMPO-capped PS, could be analyzed by MALDI-

TOF MS by utilizing gentle conditions of protonation

with the DHB matrix. Campbell et al. [13] prepared PS

by thermal polymerization of styrene between 260 and

340 8C, and characterized the product by 13C NMR,

preparative SEC and MALDI-TOF MS. They

measured the distribution of terminal unsaturation by

the last technique. The MALDI data showed that the

backbiting reaction, followed by a b-scission dom-

inates the molar mass development by comparison

with either termination or chain transfer processes. In

other work, Schappacher and Deffieux [53] prepared

macrocycles by direct coupling of a-acetal-u-bis(hy-droxymethyl) heterodifunctional polystyrene at high

dilution, and characterized the structure of the linear

and cyclic PS chains by MALDI-TOF-MS [53].

Zettl et al. [29] obtained rhodamine-B-labeled

polystyrene PS (PS-RhB) by reacting a large

excess of the acid chloride of rhodamine-B with

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357310

hydroxyl-terminated polystyrene (PS-OH). The

MALDI spectrum of PS-RhB shows mass-resolved

peaks in the 9000–13,000 Da, region, a good result,

because rhodamine-B is a very complex, quite massive

(479 Da) molecule and because the addition of a silver

salt as a cationization agent to PS-RhB does not yield

silver-cationized peaks.

Francis et al. [27] prepared AA 02-type asymmetric

stars and AB2-type miktoarm star polystyrenes using a

precursor. The latter polymer consisted of a-bromopo-

lystyrene chains (i.e. chains terminated on one side by a

Y-shaped group with two Br atoms) obtained by atom

transfer radical polymerization (ATRP) using ethyl

2-bromoisobutyrate as the initiator. Alberty et al. [25]

prepared a series of polystyrene dianions and reacted

them with dibromomethane (DBM). They obtained

almost pure cycles (99%) and showed that the 1%

impurity was due to linear chains containing a styrenic

chain end.

Cauvin et al. [18] studied cationic polymer-

ization of p-methoxystyrene in a miniemulsion.

They considered MALDI spectra of two polymer

samples, PM2 and PM5, withdrawn after 2 days,

and after 5 days, respectively. Sample PM2 is at

almost 100 wt% conversion and its spectrum shows

that all chains bear one methyl and one hydroxyl

chain end provided by proton initiation and water

termination, respectively. Side reactions do not

occur. For instance, transfer to monomer would

have generated ethylenic or indanyl terminations,

but peaks due to both reactions are absent from the

spectrum. Sample PM5, withdrawn 3 days after

polymerization ceased, is partially degraded. The

MALDI spectrum shows series of new peaks. From

the position and the intensity of the new peaks, the

authors were able to infer that chain-end dehy-

dration, as well as chain scission, is responsible for

the generation of short chains during the acid-

catalyzed degradation reactions.

The formation of disproportionation products was

revealed during the synthesis of telechelic PS by atom

transfer radical polymerization (ATRP) of styrene at

110 8C using various substituted 2-bromoisobutyrates

as initiators and the homogeneous catalyst CuBr/1,1,4,

7,10-hexamethyltriethylenetetramine [30]. In another

study, Deng and Chen used MALDI-TOF MS to

confirm the branched structure of the core of a star

polymer synthesized by ATRP of N-[2,(2-bromoisobu-

tyryloxy)ethyl]maleimide and styrene [31].

Goldbach et al [19] obtained anthracene end-functio-

nalized PS (PS-Ant) and anthracene end-functionalized

PMMA (PS-Ant). Then, they synthesized a diblock

copolymer (PS-AntAnt-PMMA) via UV coupling of PS-

Ant with PMMA-Ant. When they reacted PS with 2-

(bromomethyl)anthracene, the MALDI spectrum

revealed end-functionalizedPSof the expectedmolecular

weight as well as a considerable amount (O20%) of a

product with exactly double the expected molecular

weight. This impurity was assigned to ‘dimeric’ PS

containing an anthracene middle group. The authors

proposed a mechanism for the formation of this reaction

byproduct by three steps: (a) SN2 substitution of bromine

by polystyryllithium; (b) nucleophilic attack on the

anthracene ring by a second PS chain; (c) rearomatization

back to anthracene. Since the impurity level was

unacceptably high in this preparation, the authors decided

to react polystyrene with 1-phenyl-1-(2-anthryl)ethylene

(PHANE). In this case, theMALDI spectrumalso showed

a peak due to the ‘dimeric’ impurity, but the peakwas less

than 5% of the total. In this way, MALDI aided the

synthesis of PS-Ant, since the spectra indicated clearly

that the synthetic route that uses PHANE is the better one.

Tatro et al. [61] synthesized a series of polystyrene

and PMMA samples with narrow MM distribution.

They measured the viscosities and developed a method

that uses MALDI to determine Mark–Houwink–

Sakurada (MHS) parameters. The method requires

samples with narrow MM distribution and may,

therefore, be impractical for polycondensates. For this

reason, Montaudo [62] proposed a modification based

on the universal calibration concept and on the coupling

of size exclusion chromatography with MALDI. The

modified method was applied to two styrene–maleic

anhydride copolymers and to a series of polymers and

copolymers obtained by condensation. The MHS

parameters were measured for all the samples and

compared with values obtained by a method for

predicting MHS parameters from first principles.

Menoret et al. [20] synthesized polystyrene using a

mixture of lithium hydride and triisobutylaluminum

([Al]/[Li]Z0.7). The MALDI spectrum shows a

bimodal distribution, with the maxima centered at

2900 and at 6800 Da. Peaks in the low-mass region are

due to ions of the type Bu–(St)n–H/AgC (BuZCH3CH2CH2) whereas peaks in the high-mass region

are due to H–(St)n–H/AgC ions. In another study

[21], the mixture was slightly changed and triisobuty-

laluminum was replaced by a solution of n-butylmag-

nesium and n-octylmagnesium, the n-butyl/n-octyl ratio

being about 75/25. Various [Mg]/[Li] ratios were used

and the authors used MALDI to monitor the reaction

products. The polystyrene obtained using a concen-

tration ratio [Mg]/[Li]Z3, shows two series of peaks

due to Bu–(St)n–H, AgC and to Oct–(St)n–H/AgC

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 311

(OctZoctyl). The latter are about three times more

intense than the former (intensity ratio about 75/25),

suggesting that the reactivities of n-butylmagnesium

and n-octylmagnesium are comparable [21].

Park et al. characterized two highly branched PS

samples by a combination of reverse-phase tempera-

ture-gradient interaction chromatography (RP-TGIC)

and MALDI-TOF-MS. As expected the molar masses

increase as integral multiples of the PS precursors [40].

They also used normal phase-TGIC-MALDI-TOF-MS

methods to characterize air-terminated polystyryl-

lithium [40].

Kassalainen and Williams [39] successfully com-

bined thermal field-flow fractionation (ThFFF) with

off-line MALDI-TOF MS analyze polydisperse PS and

poly(2-vinylpyridine) homopolymers and their mix-

tures in the MM range from several kiloDaltons to

several hundred kiloDaltons [39]. The data show that

narrow polymer fractions can be obtained by ThFFF

and, therefore, that combined ThFFF/MALDI-TOF MS

technique may be a viable means for preparing

standards from a widely polydisperse polymer sample.

3.2. Polymethylmethacrylates and acrylic polymers

A host of papers on MALDI-TOF-MS of poly-

methylmethacrylates (PMMA) and acrylic polymers

have appeared in the last 5 years [64–102]. For reasons

of space, our comments will not be exhaustive.

Norman et al. [71] synthesized a series of PMMA

samples in which the chains were mainly terminated

with an unsaturated end group (actually an MMA

molecule). These polymers were produced by the

addition of catalytic chain transfer agents at late stages

of an atom-transfer polymerization. The percentage of

unsaturated end groups, PEUN in one of the samples

(denoted PMMA-A) was measured by 1H NMR,

yielding PEUNZ0.76; and it was also calculated

from the percent weight loss at 225–275 8C, by

thermogravimetric analysis (TGA), yielding PEUNZ0.78. The MALDI spectrum revealed only two types of

ions, T1 and T2. T1 is due to unsaturated end groups,

whereas T2 is due to ions with an end group (referred to

as a lactonized end group), which contains a five-

membered heterocycle. T1 ions are three times more

abundant than T2 ions, implying that PEUNZ0.75, in

good agreement with TGA and NMR measurements.

Favier et al. [76] studied the polymerization of an

acylamide derivative, N-acryloylmorpholine, using

azobis(isobutyronitrile) as the initiator and tert-butyl

dithiobenzoate as a RAFT chain transfer agent. Fig. 29

shows MALDI mass spectra of the reaction products as

a function of conversion. The spacing between mass

spectral peaks is quite large, due to the fact that the

acylamide derivative is a massive group (it contains a

phenyl ring). Theory predicts that the average molar

mass increases linearly with the conversion, and that

the proportionality constant is related in a simple

manner to the monomer/initiator ([M]/[I]) ratio. In

order to compare the theoretical prediction with

experiment, the authors plotted the molar mass

estimated using MALDI versus conversion. The

agreement was good. The most intense peaks in the

MALDI spectra are due to chains terminated by a

dithiobenzoate group. The authors showed that, if

suitably treated, they can further react to form longer

chains.

The matrix trans-2[3-(4-tert-butylphenyl)-2-methyl-

2-propenylidene)]malononitrile (DCTB) was used in

experiments to determine the propagation rate coeffi-

cients Kp of PMMA prepared by pulsed-laser photo-

polymerization (PLP) [65]. The Kp determined by

MALDI-TOF MS became constant after the first 100

propagation steps, whereas the values determined by

SEC decreased with increasing chain length. Willemse

et al. proposed that these differences are due to

instrumental effects in SEC [65].

By increasing the laser intensity, using dithranol

matrix and AgTFA as the cationizing agent, Nonaka

et al. [66] demonstrated that partial dehalogenation

occurs during the MALDI-TOF MS analysis of Cl-

terminated PMMA, as well as for Cl-terminated

poly(methyl acrylate) (Cl-PMA). Moreover, they

observed that Cl-terminated PS is unstable under

MALDI-TOF MS conditions.

MALDI analysis of low MM PMMA polymers,

obtained by anionic polymerization of methyl metha-

crylate (MMA) initiated by phenyllithium combined

with MoCl5 or WCl6, has shown that initiation of

MMA occurs by nucleophilic attack of HK on the

monomer [67]. In addition, MALDI-TOF MS analysis

indicates that iBu3Al controls the polymerization by

improving the uniformity of the polymerization with

respect to initiation and termination and by preventing

a backbiting reaction [67]. MALDI-TOF MS analysis

has permitted direct identification of Co–C bonds in

polymethylacrylate (PMA) prepared by chain-transfer

polymerization of methyl acrylate with a Co(II)

complex [80]. MALDI analysis has also confirmed

the total displacement of cobaloxime from PMA

chains when the polymer is reacted with a-methyl-

styrene.

SEC/MALDI-TOF MS has proved useful in eluci-

dating the products, denoted as PBA-RAFT, from

Fig. 29. MALDI spectra of polymerization products of an acrylamide derivative at various conversions. Reproduced from Ref. [76] with permission

of the American Chemical Society.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357312

polymerization of butyl acrylate in the presence of the

reversible addition-fragmentation chain-transfer

(RAFT) agent, cumyl dithiobenzoate [75]. Fig. 30

shows the structures of the reaction products of PBA-

RAFT with Br-terminated PBA initiated with Cu(I) and

Cu(0). In the latter case, SEC/MALDI analysis of the

polymeric material clearly revealed the formation of

three- and four-arm stars. This constitutes the first

example of the synthesis of a four-arm star through

intermediate–intermediate radical polymerization. The

authors postulated that this peculiar reaction pathway is

due to the much slower fragmentation rate in the BA

system than in the styrene system [75]

Jiang et al. fractionated PMMA obtained in RAFT

polymerization by LCCC and then analyzed the fractions

by off-lineMALDI-TOF-MS and electrospray-ionization

quadrupole-TOF-MS (ESI-QTOF-MS) [82]. Labile end

groups of PMMA, such as the dithioester groups, were

lost in MALDI-TOF experiments but were observed

intact in the ESI-QTOF-MS spectra [82].

Schilli et al. [72] monitored by MALDI the

polymerization of N-isopropyl-acrylamide (NIPAAm)

Fig. 30. Structure derived from MALDI-TOF-MS distributions for reaction of poly(butyl acrylate)–S–C–(Ph)aS with poly(butyl acrylate)–Br

initiated with Cu(I) and Cu(0) Reprinted from Ref. [75] with permission of the American Chemical Society.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 313

Fig. 31. Polymerization of acrylamide in the presence of chain transfer agents. Structure of six types of oligomers expected (upper part) together

with MALDI spectra of the poly(acrylamide) sample (a) and an expansion (b) of the spectrum [1900–2020] with the theoretical isotope distribution.

Reproduced from Ref. [72] with permission of the American Chemical Society.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357314

carried out in the presence of a mixture of two

dithiocarbamates, namely benzyl 1-pyrrolecarbo-

dithioate and cumyl 1-pyrrolecarbodithioate, which

act as chain-transfer agents. The polymerization is

expected to give chains terminated in three different

ways, namely: (1) with dithio (dit), (2) with H (H) (3)

with a double bond (doub). Furthermore, chains can

occur with two starting groups, namely (1) with a cumyl

group (cum) (2) with one-half azobis(isobutyronitrile)

(in) and thus there are six possible combinations.

Fig. 31 shows the MALDI spectrum of the NIPAAm

polymer. There are six mass series, corresponding to

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 315

the six expected types of chains. The figure also shows

expansion of the spectrum in the mass region 1900–

2020 Da, along with the simulated isotopic pattern. The

experiment and simulation show some analogies but

also some differences, probably due to the fact that the

simulation was performed under the (incorrect)

assumption that all chain species are equally abundant.

3.3. Other polymers with an all-carbon main chain

MALDI spectra of poly(vinyl acetate) [103],

poly(ethylene) [117–123], poly(butadiene) [125,126]

and polyisoprene [116] have been recorded by various

authors [103–127]. As a rule, MALDI analysis of

polyethylene requires careful sample preparation, as

discussed above; but low molar mass samples represent

an exception [118–120].

Switek et al. [108] obtained polyisoprene using sec-

butyllithium or 1-tert-butyldimethylsiloxypropyl-

lithium as the initiator. They reacted the resulting

polymer with hexafluoropropylene oxide (HFPO) and

obtained a three-arm star polymer, plus some side-

reaction products. They proposed a mechanism, in

which HFPO acts as a multifunctional coupling agent.

The MALDI spectrum of the reaction products displays

three mass series. The first series appears only at high

mass, and peaks are due to the three-arm star polymer.

The second series and the third are present exclusively

in the middle range and in the low molar mass range,

respectively. These peaks are due to two-chain ketone

structures and to poly(isoprene) chains simply termi-

nated by a proton. All MALDI peaks support the

proposed coupling agent mechanism.

3.4. Polymers with heteroatoms in the main chain

Polymers with heteroatoms in the chain have been

analyzed by MALDI [128–283]. We shall consider

polysiloxanes, poly(silsesquioxane)s and polysilanes

[128–148], polyethers [149–205], polyesters,

[206–250], polycarbonates, [251–261] polyamides

and polyimides [262–283].

3.5. Polysiloxanes, poly(silsesquioxane)s

and polysilanes

MALDI spectra of polysiloxanes are commented

upon above in the discussion of: impurity detection,

SEC-MALDI [128–131,138,140], using supercritical

CO2 [139] and comparing MMDs obtained by SEC and

by MALDI [130,131]. Poly(silsesquioxane)s are sili-

con-containing polymers with a peculiar stoichiometry;

their MALDI spectra are quite complex, but also very

rich in structural information [143–148].

3.6. Polyethers

More than 50 reports on MALDI of polyethers have

appeared in the period considered [149–205].

Gobom et al. [161] showed that it is possible to

calibrate a MALDI-TOF spectrum with fantastic

accuracy, namely to 10 ppm. In order to achieve their

goal, they had to introduce an alternate function for

time-to mass conversion and they had to select an

analyte which produces many intense equally spaced

peaks in the mass range 1000–7000 Da. Their choice

was PPG or, more precisely, a mixture of four PPGs,

each with a narrow distribution, with different masses

that covered the entire mass range.

Kricheldorf et al. [192] prepared poly(ether sul-

fone)s by polycondensation of silylated 4-tert-butylca-

techol and 4,4-difluorodiphenylsulfone in

N-methylpyrrolidone, varying the proportions of the

reactants. The MALDI spectrum of the polymer

prepared with nearly exact stoichiometry showed a

single mass series, with peaks up to 19 kDa.

Creaser et al. [175] analyzed PEG 1500 was by

atmospheric pressure MALDI quadrupole ion-trap MS

(AP-MALDI-QIT) and by classical vacuum MALDI-

TOF MS. Contrary to the MALDI-TOF spectra that

presented cationized oligomers, the AP-MALDI-QIT

spectra showed dimetallated matrix–analyte adducts, in

addition to the expected PEG-alkali ions, for all the

matrix/alkali salts used.

To study the interaction of PEG with ions likely to

be found in the human body, Mwelase et al. [150]

prepared complexes of bivalent ions of Mg, Ca, Cu,

Zn and Pt with a PEG5000 at pH 7 and characterized

them by MALDI-TOF MS and other techniques (UV,

FT-IR, TG). From the MALDI spectra, they deduced

that Cu(II), Zn(II) and Pt(II) are directly bound to

PEG without water whereas Mg(II) and Ca(II)

complexes hold four and six water molecules,

respectively [150].

MALDI spectra of poly(3-ethyl-3-hydroxymethy-

loxetane) (poly-EOX) and of samples obtained by

cationic polymerization of EOX in presence of an

analogous polyether such poly(3,3-dimethyloxetane)

(poly-DIOX) that does not contain OH groups, show

that the intramolecular reactions (backbiting) observed

during cationic polymerization are due to OH groups

while the ether moiety does not participate in the

backbiting processes [167]. Therefore, this reaction,

limits the MM of poly-EOX allowing formation of

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357316

cycles. MALDI analysis of SEC fractions of a

hyperbranched polyether, prepared by melt transether-

ification of 1-(2-hydroxyethoxy)-3,5-bis-(methoxy-

methyl)-2,4,6-trimethylbenzene with mesitol in

presence of an acid catalyst, reveal two families of

peaks due to simple branched structures and to

macrocycles [203]. From the relative intensities of the

two peaks, it is apparent that cyclization is favored at

higher conversions in the melt transetherification

process.

Various end groups in oligo(isobutyl vinyl ether)

(iBVE) have been characterized by MALDI-TOF MS

[105,166], which reveals that oligo-iBVE containing

aryl ketone ends are susceptible to elimination of the

ultimate isobutoxy group in the presence of a strong

Lewis acid, while the ketone groups are unaffected

[105].

MALDI-TOF MS mass spectra of poly(propylene

oxide), obtained by anionic polymerization of propy-

lene oxide in the presence of alkali metal alkoxide

initiators and trialkylaluminum, in addition to the

intense peaks belonging to the expected oligomers

terminated with alkoxide initiator and OH groups,

showed weak peaks due to oligomers terminated with

OH and unsaturated allyl groups, which were not

detected by 1H NMR [162].

In the case of polyethers obtained by polymer-

ization of isosorbide with 1,8-dibromo or dimesyl

octane with phase transfer catalysts under microwave

irradiation or by conventional heating, MALDI-TOF

MS showed that the mechanism of chain termination is

different in the two methods [163]. Polyethers

prepared by conventional heating have short chains

with hydroxylated ends, whereas under microwave

irradiation the polymer chains are longer with ethylene

end groups.

MALDI-TOF MS has also been used to character-

ize ethylene oxide (EO) and propylene oxide (PO)

copolymers [178,346]. Using homemade software,

Terrier et al. [178] determined the copolymer

composition of triblock copolyethers EO–PO–EO,

PO–EO–PO, and a random EO/PO copolymer by

MALDI-TOF. They observed that MALDI spectra of

the triblock copolymers depend on experimental

parameters, such as the number of laser shots relative

to the polymer/salt ratio, and on the nature of the

matrix. They detected the side-reaction products with

unsaturated end groups by MALDI analysis of the

crude copolymer and by MALDI analysis of SEC

fractions. These results were confirmed by 1H NMR

[178].

3.7. Polyesters

There are a number of reports of characterization of

polyesters by MALDI-MS [206–250].

Kricheldorf et al. [227] obtained an aliphatic

polyester by condensation of isosorbide and suberoyl

chloride. The MALDI spectrum showed three types of

peaks (La, Lb, Lc) due to linear polyester chains

terminated in three different ways plus peaks (C) due to

cyclic polyester chains. The intensity ratios were C/La/

Lb/LcZ83/7/7/3. Since cyclodextrins and their partly

methylated derivatives are commercially available and

are known for their ability to thread a variety of linear

polymers (depending on ring size and diameter of the

polymer chains), the authors used this property to

separate linear chains from their cyclic analogs. They

added a concentrated solution of polyesters in

dichloromethane dropwise to a refluxing concentrated

solution of methylated b-cyclodextrin in methanol. Part

of the dichloromethane evaporated, and the polyesters

precipitated. The threading of the methylated cyclo-

dextrin on the linear chains increased their solubility in

methanol and accelerated their methanolytic degra-

dation. The MALDI spectrum of the degradation

product displayed the same peaks as the pristine

polymer, but the intensity ratios changed to C/La/Lb/

LcZ96/1/3/0, implying that the content of linear chains

had been significantly reduced.

Pantiru et al. [208] recorded MALDI and NMR

spectra of a poly(3-caprolactone) (PCL) sample

terminated by ethylene glycol vinyl ether. The molar

mass was low enough to allow for NMR estimation of

the average molar mass (using end group signals); the

result was MnZ1380. The MALDI spectrum indicated

a high purity level (no terminal groups present apart

from those from ethylene glycol vinyl ether) and the

average molar mass computation yielded MnZ1400, in

good accord with the preceding value.

Ming et al. [221] recorded the MALDI spectrum of a

poly(butylene adipate) (PBA) sample. Then, they

reacted the PBA with a large excess of perfluoroocta-

noyl chloride and obtained a partially fluorinated

polymer. The MALDI spectrum showed peaks due to

unreacted PBA plus additional peaks, four or five times

less intense, due to partially fluorinated chains.

Assuming that the difference in cationization efficiency

was not too large, this might indicate that modified

chains are less abundant.

Takashima et al. [217] recorded the MALDI

spectrum of a poly(d-valerolactone) (PDVL) termi-

nated with b-cyclodextrin (CD). The spectrum displays

a series of peaks (separated by about 100 Da) due to

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 317

PDVL terminated with CD and also an intense peak at

lower mass due to CD. Apparently, the polymer does

not fragment, although the end group is bulky. This is

further proof that, under appropriate MALDI con-

ditions, the extent of fragmentation is low.

Kricheldorf et al. [220] obtained polylactides using

Bismuth(III) acetate as an initiator and 1,1,1-tri(hy-

droxy methyl)propane (THMP) as a co-initiator. The

resulting macromolecules are three-arm stars having

three CH–OH end groups. The MALDI spectrum

confirmed the structure.

In several studies, cyclic chains have been detected

in hyperbranched polyesters (h-PEs) by MALDI-TOF

MS analysis [218,219,228,231]; and in the case of

aliphatic h-PEs synthesized by reaction of 2,2-

bis(hydroxymethyl)propionic acid with pentaerythrytol

or trimethylolpropane [218], the MALDI spectra,

besides the expected peaks, showed a series of small

peaks corresponding to the formation of one cyclic

branch per molecule by an intramolecular etherification

side reaction and loss of water. The presence of

etherified units (including the –CHaCH–O– moiety)

was confirmed by 13C NMR [218].

To investigate mass discrimination effects in

MALDI-TOF analysis of polydisperse polymers,

Williams et al. optimized sample preparation and

instrumental parameters, obtaining a uniform

response for each component of an equimolar

mixture of four poly(butylene glutarate) (PBG)

oligomers [210,211]. They proposed this oligomer

mixture as a mass calibration standard for MALDI

analysis of polydisperse polymers in the mass range

780–6000 Da.

MALDI-TOF analysis revealed that linear oligomers

are formed during the melt polycondensation of D,L-

lactic acid at 100–120 8C, while both linear and cyclic

products are formed at higher temperature (220 8C)

[209].

MALDI-TOF analysis has been found useful to

explain the mechanism of the reactions occurring

during the synthesis of aliphatic polyesters from

ethylene sulfite and succinic anhydride or its higher

homologues in the presence of such catalysts as

quinoline and a Lewis acid (BF3 or SnCl4). In fact,

signals of sulfur-free polyesters were observed in

the MALDI spectra of the polyesters synthesized

[230].

Recently, low molar mass all-aromatic polyesters,

derived from 6-hydroxy-2-naphthoic acid (HNA),

4-hydroxybenzoic acid (4-HBA) and 3-hydroxybenzoic

acid (3-HBA), were characterized by MALDI-TOF MS

[225]. The spectra of polymers from 3-HBA showed

solely one series of peaks, whereas spectra of polymers

from 4-HBA showed two series of peaks, namely the

expected main series and an ancillary series generated

by depolymerization reactions via a quinomethide

mechanism.

3.8. Polycarbonates

Polycarbonates have been characterized by combin-

ing MALDI-TOF with chromatographic methods

[254–256], and a few papers have reported on classical

MALDI-TOF MS analyses [251–253,257–261].

Coulier et al. [256] used LCCC-MALDI to identify

chemical changes due to hydrolytic degradation in a PC

sample. The LCCC chromatogram in Fig. 32a of a PC

sample aged for 12 weeks shows two very well resolved

peaks. In Fig. 32b, the MALDI spectrum of the first

peak presents two series of peaks corresponding to

undegraded PC chains. The spectra of the second peak

(Fig. 32c) eluted at about 6.50 min, whose intensity

increases with the degradation time, present only

a series of peaks due to degraded PC chains

terminated with one cumyl group and one OH end

group (species C).

Scamporrino et al. synthesized poly(bisphenol-A

carbonate) (PC) copolymers containing Cu-diimine(I)

units with nonlinear optical (NLO) properties and

characterized them by MALDI-TOF MS [258]. On the

basis of the structure of the copolymer products

identified—versus reaction time, the authors proposed

the reaction mechanism depicted in Scheme 2. Fig. 33

shows the MALDI spectrum of the copolymer obtained

by heating commercial PC with the Cu-diimine(I)

complex (10 wt%) at 250 8C for 5 min; the correspond-

ing assignments are reported recorded in Table 1. The

same authors also prepared PC copolymers containing

porphyrin or fullerene units and characterized them by

MALDI-TOF MS [258].

3.9. Polyamides and polyimides

MALDI analysis has been applied to the character-

ization of a number of polyimides and polymides

[262–283].

Gibson et al. [264] recorded the MALDI spectrum of

an aramide formed by condensation of crown ethers and

4,4 0-oxydianiline (ODA). The matrix/analyte mixture

was doped with a silver salt, and thus silver-cationized

ions were expected, but the results were slightly

different, because of adventitious sodium and potass-

ium cationization. Signals were detected up to m/zZ9100 Da with a spacing of 700.27 Da, which

Fig. 32. Identification of peaks observed with LCCC using semi on-line coupling of MALDI-TOF-MS. Reprinted from Ref. [256] with permission

from Elsevier.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357318

corresponds to the repeat unit molecular weight (see

Fig. 34). In the expanded spectrum in Fig. 34, only

minor amounts of linear structures are detected,

consisting of linear amino acids (AA) (m/zZ700nC18), diacids (DAc) (m/zZ700nC536), and diamines

(DAm) (m/zZ700nC200). The weak cluster at m/zZ2858 Da corresponds to the K adduct of the tetrameric

amino acid (AA4), (m/zZ700nC18C39). The cluster

at m/zZ2866 Da corresponds to the silver adduct of the

macrocyclic tetramer (M4) that has lost CO2, m/zZ700nK44C106.95. The remaining signals for the

predominant cyclic species are identified as (MC

H)C, (MCNa)C and (MCK)C adducts, as illustrated

for the tetramers M4; the corresponding silver adducts

(M4CAg)C can also be seen in Fig. 34.

In another work, after optimization of condensation

polymerization conditions, mainly cyclic polyamides

were detected in the MALDI spectra of various samples

(up to 10,000–13,000 Da) [262,263]. Only cyclic Ny6

oligomers were observed when 3-caprolactam and

3-aminocaproic acid were polycondensed to high

conversion at 250 8C [263].

MALDI-TOF analysis of hyperbranched polyamide

(h-PA) obtained from novel carboxyl and amine

O C

O

C

C

C

H

H

3

3

OH

N N

O OCu

HO O

OC

C

C

H

H

3

3

O

N N

O OCu

HO OOC

C

C

H

H

3

3

C

O

C

O

O O C

C

C

H

H

3

3

C

O[ ] N N

O OCu

HO OH

O O C

C

C

H

H

3

3

C

O n [ OHC

C

C

H

H

3

3

N N

O OCu

HO OH

O O C

C

C

H

H

3

3

C

O

C

C

C

H

H

3

3

O O C

C

C

H

H

3

3

C

O

N N

O OCu

HO OH

+

+

+

1)

2)

3)

( )

( )

( )

( )

(+)

(#) and

( ) and

( ) and

[

]

n

n

]

]n

[

[

]n

O O C

C

C

H

H

3

3

C

On [ ] OHC

C

C

H

H

3

3

OH

(#)

Scheme 2.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 319

terminated caprolactams, showed that ring–chain

equilibria lead to the formation of cyclic branches or

end groups. The formation of anhydride, imides,

amidines and secondary amine, due to a number of

intramolecular or intermolecular side reactions, was

also observed [218].

3200

100

75

50

25

4600

I%

2267 #

∗

∗

∗ ∗∗

∗∗

∗ ∗∗

∗

#

# ##

+

+

+

++

4045

531#

##

###

#

#

#+

+

++ +

++ +

+

100

50

I%

Fig. 33. MALDI-TOF mass spectrum of complex mixture obtained by heati

Reproduced from Ref. [258] with permission from Wiley.

3.10. Polymers with phenyl and other cycles

in the main chain

Polymers with phenyl and other cycles in the main

chain are of great interest because they are often

electrically conducting or possess light-emitting

6000 7400 m / z

∗∗

∗∗ ∗

∗∗

∗ ∗ ∗

∗

5

##

## # #

##

#

+ +

2700 29002800 3000 m / z

2718

2702

27752791

27592956

2972

3013

3029

3045

++

# #

ng a commercial PC with a Cu–diimine complex at 250 8C for 5 min.

Table 1

Structures and molecular ions of the species detected in the positive MALDI-TOF mass spectra of PC and heated mixtures of PC and copper-

diimine complex I

Structures m/z Values

O O C

CH3

CH3

C

On

Na+

(ο)

1801, 2055, 2309, 2563, 2817, 3071, 3325,

3579, 3833, 4087, 4341, 4595, 4849, 5103,

5357, 5611, 5865, 6119, 6373, 6627, 6881,

7135, 7389, 7643

Na+

CH3OCH3

O O C

CH3

CnOHC

CH3

(∗)1759, 2013, 2267, 2521, 2775, 3029, 3283,

3537, 3791, 4045, 4299, 4553, 4807, 5061,

5315, 5569, 5823, 6077, 6331, 6585, 6839,

7099, 7347, 7601

O OC

O nC

CH3

CH3

C

CH3

CH3

C

CH3

CH3

O OC

O

Na+

(•)1743, 1997, 2251, 2505, 2759, 3013, 3267,

3521, 3775, 4029, 4283, 4537, 4791, 5045,

5299, 5553, 5807, 6061, 6315, 6569, 6823,

7077, 7331, 7585

O OC

OnOHC

CH3

CH3

C

CH3

CH3

HONa+

(#)1775, 2029, 2283, 2537, 2791, 3045, 3299,

3553, 3807, 4061, 4315, 4569, 4823, 5077,

5331, 5585, 5839, 6093, 6347, 6601, 6855,

7109, 7363, 7617

O C

O

C

CH3

CH3

OH

N N

O OCu

O OHn

Na+

(♦)

1702, 1956, 2210, 2464, 2718, 2972, 3226,

3480, 3734, 3988, 4242, 4496, 4750, 5004,

5258, 5512, 5766, 6020, 6274, 6528, 6782,

7036, 7290, 7544

OO

N N

O OCu

O OHn

OC

CH3

CH3

C

CH3

CH3

C

O

C

O

Na+

(+)

1940, 2194, 2448, 2702, 2956, 3210, 3464,

3718, 3972, 4226, 4480, 4734, 4988, 5242,

5496, 5750, 6004, 6258, 6512, 6766, 7020,

7274, 7528, 7782

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357320

properties. MALDI of poly(thiophene) and its deriva-

tives has been reported in many papers [284–311].

Chen et al. [305] obtained a series of conjugated

polymers by Suzuki polycondensation and analyzed

them by MALDI. The gap between the valence and

conduction bands corresponds to blue light emission.

One of the polymers, denoted P7, has a complex repeat

unit with a mass of 745 Da and the empirical formula:

C46H59N5O4. The MALDI spectrum shows only four

peaks at 1491.0, 2236.4, 2981.8, and 3727.3, which are

due to P7 chains (dimer, trimer, tetramer, and

pentamer).

Poly(1,3-cyclohexadiene) is an interesting polymer

since it can be dehydrogenated to form poly(pheny-

lene), but the control of MM is difficult. Quirk et al.

[292] developed an innovative method to measure the

amount of chain transfer to monomer, ACTM, during the

alkyllithium-initiated polymerization of 1,3-cyclohex-

adiene. ACTM was evaluated by adding ethylene oxide

as a terminating agent and characterizing the resulting

products. Two experimental procedures were investi-

gated to detect chain transfer by ethylene oxide

functionalization: (A) sec-BuLi in benzene at room

temperature with 1,4-diazo-bicyclo[2.2.2]octane

(DABCO), (B) n-BuLi/TMEDA in cyclohexane at

40 8C (TMEDAZN,N,N 0N 0-tetramethylethylenedia-

mine). MALDI results showed that n-BuLi/TMEDA

systems do not have a living character, since no

reinitiation occurs. By contrastt, the sec-BuLi/DABCO

systems possess a living character since reinitiation was

observed. In this way, MALDI spectra were used to

clarify some aspects of a controversial subject.

3.11. Copolymer studies

Viala et al. [324] studied the radical emulsion

copolymerization of methyl methacrylate and

1,1-diphenylethylene (DPE) in the presence of

Fig. 34. Polyamide containing crown-ether units, which can form rotaxanes. Structure (upper part) of the polyamide and expansion (lower part) in

the mass region 2600–3000 of the MALDI spectrum of the polyamide. Reproduced from Ref. [264] with permission of the American Chemical

Society.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 321

ammonia, sodium dodecylsulfate and ammonium

peroxodisulfate. DPE cannot polymerize by itself

and this implies that DPE units will be found

isolated along the chain. Depending on the

reactants, various types of end groups are expected.

Some examples follow. Termination by dispropor-

tionation can give –H terminal groups. Initiation

with sulfate ion radicals can give –SO4 terminal

groups. Initiation with hydroxyl radical or hydroly-

sis of a polymer chain ending in a sulfate end

group can give –OH terminal groups. The same

types of chain ends can also derive from initiation

with a hydroxyl radical formed during peroxodi-

sulfate decomposition. More complex end groups

derive from termination by the disproportionation

reaction of a polymer radical ending with DPE or

by disproportionation of a polymer radical ending

in MMA. The authors recorded the spectrum of the

copolymer and found all the expected products.

Impallomeni et al. [319] heated a blend of poly(4-

hydroxy butyrate) (PHB) and poly(3 caprolactone)

(PCL) using p-toluenesulfonic acid (PTSA) as a

catalyst. The ester–ester exchange produced a copoly-

mer, and chains terminated with two different end-

groups were expected, namely H/COOH and PTSA/

COOH. The authors used SEC, since chains terminated

with PTSA are expected (due to their stiffness) to

display different elution behavior than the other ones.

Fig. 35 shows an expansion of the MALDI spectrum

along with the m/z assignment of MS peaks. It shows

two ion distributions: the first, centered at m/zZ3700, is

due to sodiated ions of the co-oligomers terminated

with tosyl-terminated and carboxyl groups (species 1 in

Fig. 35); while the second, centered at m/z4Z4600, is

due to the sodiated ions corresponding to the co-

oligomers terminated with OH and COOH groups

(species 2 in Fig. 35). These data suggest that these

oligomers have different elution behavior. Fig. 35

shows an expansion of the MALDI spectrum along with

the m/z assignments of several of the MS peaks, along

with the structures of the oligomeric ions giving rise to

these peaks. The two types of chains are present and

chains terminated with PTSA have distinctly higher

mass, suggesting different elution behavior.

Venkatesh et al. [343] investigated the copolymer-

ization of methyl acrylate (MA) with 1-octene (OCT)

using two different synthetic routes, namely atom-

transfer-radical-polymerization (ATRP) and the usual

free-radical (FR) reaction. The MALDI spectra of MA-

OCT copolymers obtained by ATRP and by FR showed

Fig. 35. MALDI-TOF mass spectrum of SEC fraction of P(HB-co-47% mol CL). The structures of the revealed copolymer chains are shown in the

upper part. Reproduced from Ref. [319] with permission of the American Chemical Society.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357322

a single mass series and five mass series, respectively.

The fact that side-reaction products are more abundant

in copolymers obtained by ATRP with respect to FR

and this implies that ATPR is ‘cleaner’. During ATRP,

most of the polymer chains are halide end-capped.

However, during MALDI ionization, it is observed that

a small percentage of the terminal Br undergo

fragments.

Milani et al. [341] used a palladium-complex

catalytic system to obtain terpolymers with units of

CO, styrene (ST) and 4-methyl-styrene (MST), a

synthetic route that produces chains in which two

consecutive identical units (CO–CO, ST–ST MST–

MST) do not occur.

Fig. 36 shows MALDI spectra, in the region 750–

1200 Da, of three CO–ST–MST terpolymers syn-

thesized using three different ST/MST mole ratios,

namely 2/1, 5/1 and 10/1. The MALDI spectra consist

of very well resolved clusters, each due to polymeric

chains formed by the same number of repeat units, i.e.

(mCn). Many authors agree on the fact that, for an AB

copolymer, the most intense peak in the MALDI

spectrum can be assigned to an ApBq oligomer and the

A/B ratio in the oligomer reflects the A/B ratio in the

copolymer and vice versa: if the copolymer is rich in B

units, B-rich peaks will be intense. The application of

this to the CO–ST–MST system is straightforward.

Comparison of the three spectra evidences that, within

each cluster, the relative abundance of the oligomer

containing more styrene residues increases on increas-

ing the initial amount of styrene in the feed. The authors

found also that MST is more reactive than ST [341] The

composition and microstructure of a low-MM ethylene/

carbon monoxide (E–CO) copolymer have been

determined by MALDI-FT-ICR mass spectrometry

[342]. A part of the mass spectrum is reported in the

Fig. 36. MALDI-TOFmass spectra of CO/styrene/4-Me-styrene terpolymers. Variations observed with styrene/4-Me-styrene ratios: (a) 2/1; (b) 5/1;

(c) 10/1. An enlarged region. Reproduced from Ref. [341] with permission of the American Chemical Society.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 323

section, which deals with resolution and MALDI-FT-

ICR. The E/CO mole fraction computed from MS is

0.728, which compares well with the value 0.818

obtained using NMR

Willemse et al. [313] monitored the synthesis of a

block copolymer with styrene (ST) and isoprene (IPR)

using MALDI. The synthetic procedure consisted in the

sequential addition of the initiator, the ST monomer and

then the IPR monomer. Fig. 37 gives the spectra of the

reaction products before the addition of IPR (Fig. 37a),

and those of the block copolymers after approximately

50% (Fig. 37b) and 100% (Fig. 37c) conversion of the

isoprene monomer [313]. The three spectra have

narrow MMDs (a typical feature of anionic synthesis)

centered at 2200, 3000, 4000 Da, respectively. It is

apparent that the MM of the copolymer grows as the

conversion increases. Fig. 37d is an enlargement of

Fig. 37c and shows that each peak has a width (strictly

speaking FWHM) of about 0.4 Da, which implies (by

definition) that the resolution is about one part in

10,000, which is satisfying at these masses. The peaks

tend to form triangular clusters, the base of the triangle

being about 30 Da. The major cause of these

complicated patterns is the fact that, at these masses,

at least nine 13C isotopes are relevant for each structure

(chains containing three 13C isotopes are the most

abundant ones). There are other causes, namely that

silver has two isotopes (nominal masses 107 and 109)

and a difference of only 4 Da between 3 isoprene units

and 2 styrene units. The MALDI spectral intensities

were used to compute the ST/IPR mole ratio (i.e. the

average copolymer composition), which agrees with

that from 1H NMR. In the case of a random copolymer

of isobutylene (IBU) and methylstyrene (pMST), Cox

et al. [315] found that the average molar composition

determined from the MALDI spectrum of the copoly-

mer was skewed to higher methylstyrene content

(36 mol%) as compared to that obtained from NMR

(13%). They proposed that the facile ionization of

methylstyrene-rich oligomers is the cause of the

discrepancy.

Melt mixing by heating together two homopolymers

is a promising route for synthesis of block, segmented,

or random copolymers [317]. The process proceeds

through exchange reactions, such as ester–ester, ester–

carbonate, ester–amide, amide–amide, siloxane–silox-

ane reactions, etc. This reactive polymer-blend tech-

nology currently encounters problems in control of

Fig. 37. MALDI-TOF-MS mass spectra (a–c) of the system polystyrene-block-polyisoprene after 0, 50 and 100% conversion of isoprene monomer

and (d) an enlargement of (c) between 4000 and 4140 Da. Reproduced from Ref. [313] with permission of the American Chemical Society.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357324

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 325

reaction parameters, due partially to lack of adequate

monitoring methods and analytical protocols. However,

another huge obstacle exists: the current practice is still

that of mixing polymers without considering the chain

ends, looking only at the repeat units of the two

components.

MALDI-MS analysis of the structure of copolymers

obtained by reactive blending in the molten state

of their corresponding homopolymers has been

[318,321–323] reported recently. The experiments

were mainly focused in three directions: (1) structure

of the end groups of the reacting polymers; (2)

copolymer yield as a function of melt mixing time;

(3) composition and sequence of the copolymer formed.

Fig. 38 shows the MALDI spectra of an equimolar

mixture of carboxyl terminated Ny6,6 (Ny6,6–COOH)

and high MM Ny6,10, both for the physical blend

(Fig. 38a) and for melt-mixed blend held at 290 8C for

30 min (Fig. 38b). Ny6,6–COOH oligomers predomi-

nate in Fig. 38a, whereas there is a drastic change in the

MALDI spectrum of the heated blend (Fig. 38b),

hinting that the formation of Ny6,6/Ny6,10 copolymers

by exchange reactions has occurred [321]. In fact, the

most intense peaks are due to copolymer oligomers

formed in the process of melt mixing. Theoretical

matching of the experimental peak intensities (inset in

Fig. 38b) was obtained for a Bernoullian distribution in

the copolymer and a 50/50 mole ratio of the

comonomers. As can be seen in the inset of Fig. 38b,

the theoretical MALDI mass spectrum in the mass

region 1050–1890 Da, generated for a random copoly-

mer containing equimolar units of Ny6,6 (A) and

Ny6,10 (B) units, matches well with the spectrum

recorded for the Ny6,6–Ny6,10 melt-mixed for 30 min

at 290 8C. This indicates the copolymer has a random

sequence distribution, and equimolar composition of

Ny6,6 and Ny6,10 units.

Samperi et al. [318] applied chain statistics analysis

to MALDI spectra of Ny6/Ny4,6 and Ny6/Ny6,10

random copolyamides synthesized by melt mixing of

carboxyl terminated nylon-6 (Ny6–COOH) with high

molar mass Ny4,6 or Ny6,10 at 290 8C for different

times under N2 flow. The molar composition, sequence

distributions, average sequence lengths and degree of

randomness calculated by the chain statistics are in

accord with calculations based on a model that uses

the intensities of the carbonyl peaks in the 13C NMR

spectra [318].In another study, MALDI analysis

allowed structural identification of the copolyester-

amide formed during melt mixing of Ny6/PET and

Ny6/PBT blends [322,323]. This work revealed the

essential role of carboxyl groups in the exchange

reaction, and allowed the formulation of a detailed

mechanism for the reaction [322,323]. The compo-

sition, sequence distribution, degree of randomness

and the yield of the copolyesteramide formed as a

function of the melt mixing time were also calculated

[322,323]. Tillier–Lefebre et al. [320] studied melt

mixing of PET and 3-caprolactone. The MALDI

spectrum of the resulting copolymer turned out to be

very complex, and thus, the authors decided to gain

better insight by fractionating the polymer using SEC

and recording the MALDI spectra of the fractions.

Fig. 39 shows the MALDI spectrum of SEC fractions

46, 48, 50. The fractions eluting first have higher

masses, as expected. All three spectra exhibit

two series of peaks. The intensity of the series

corresponding to the highest masses increases with

the increasing fraction number, i.e. with increasing

elution volume. The peaks in the low-mass range are

due to linear oligomers, whereas peaks in the high-

mass range are due to cyclic oligomers. Owing to their

smaller hydrodynamic volumes the cyclic oligomers

are eluted slightly later than their linear homologs, and

in the MALDI spectra of the corresponding SEC

fractions appear together with lower-mass linear

oligomers the same elution volume [3]. Polce et al.

synthesized a copolymer with units of phenylquinoxa-

line (PQ) and ethersulfone (ES) by combining self-

polymerizable quinoxaline monomers with a 1:1 molar

mixture of 4,4 0-dichlorodiphenyl sulfone and bisphe-

nol-A. They noted differences in cationization

efficiencies: oligomers containing at least one PPQ

unit readily protonated in MALDI, whereas PES

homopolymers required alkali metal ion addition to

become detectable. The MALDI mass spectra of the

polymers revealed that the major products up about

15,000 Da homopolymeric or copolymeric macro-

cycles. Linear byproducts are also observed, arising

from nucleophilic ring opening of already-formed

macrocycles. Montaudo et al. proposed a new model

for the bivariate distribution of chain sizes and

composition in copolymers [336]. They compared

predictions of the model with MALDI data for a block

copolymer of pivalolactone and 3-hydroxybutyrate,

and with some published MALDI data on a block

copolymer a-methyl styrene and methyl methacrylate.

The new model considers a sum of two bivariate

distributions; and it replaces an earlier model that

deals only with a single distribution. The new model

gives better results than the previous model because it

fits better with the experimental compositional

distribution histograms of the copolymer samplesKri-

cheldorf et al. obtained a hyperbranched polymer by

Fig. 38. MALDI-TOF mass spectra of an equimolar mixture of Ny6,6–COOH (MvZ7200 Da) and high MM Ny6,10 (MvZ36,100): (a) physical

blend; (b) melt mixed at 290 8C for 30 min. Part (B) shows an enlarged section of the calculated mass spectrum (above) and experimental mass

spectrum (below) of the last sample. Reproduced from Ref. [321] with permission of the American Chemical Society.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357326

condensing 4,4 0-bis[p(acetoxy)phenyl] valeric acid

(also called diphenolic acid, DPA) [325]. Careful

assignment of the peaks in the MALDI spectrum

revealed that the product was a copolymer of regular

DPA units and modified DPA units possessing a

phenol group. The MALDI analysis detected a drift in

composition: the mole fraction of regular DPA units

changed from 0.80 to 0.95 in passing from low to

high mass.

Quirk et al. [12] studied the functionalization of

PSLi with ethylene oxide. The reaction giving PSCH2-

CH2OH chains was rapid and quantitative. However, at

long reaction times with 10 equiv. EO per mole of

PSLi, oligomerization of the end group by further

Fig. 39. MALDI-TOF mass spectra of low molar mass fractions 46, 48 and 50 collected during SEC analysis of PET/CL (50/50 mol/mol)

copolyester 2. The two distributions correspond to linear macromolecules (lower masses) and cyclic macromolecules (higher masses) eluted at the

same elution volume. Reprinted from Ref. [320] with permission from Wiley.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 327

reaction produce compounds PS(CH2CH2O)nH with

nZ2 or 3 (denoted dimers or trimers, respectively),

detected by MALDI mass spectra (see Fig. 40). After

12 h and after 4 weeks, respectively, the dimer

amounted 4 and 34% of the functionalized chains;

after 1 and 4 weeks the amounts of trimer were 1 and

4%, respectively. The MALDI results were in good

agreement with 1H and 13C NMR analyses. Schmalz

et al. [154] synthesized PS-b-PEO di-block copolymers

by sequential anionic polymerization of styrene and EO

in THF. Fig. 41 reports MALDI spectra of samples

reacted at different times along with MALDI spectra of

the homopolymer. It can be observed that, as reaction

time increases, the spectra allow a number of

copolymer compositions up to chains bearing seven

EO units to be resolved while the initial EO end-capped

chains almost disappear. The spectrum of the EO end-

capped PS (PS-OH) also indicates that the end capping

is quantitative since signals due to the PS precursors are

absent [154]. Venkatesh et al. recorded the MALDI

mass spectrum of a copolymer of methyl acrylate (MA)

and allyl butyl ether (ABE) obtained by atom transfer

radical polymerization (ATRP) using ethyl-2-bromoi-

sobutyrate (EBr-ibu) as the ATRP initiator [332]. They

found four mass series and labeled them E1–E4.

The most intense peaks in the mass spectrum belong

to the first series (E1) and are due to chains terminated

with bromine. Kricheldorf et al. reacted dimercap-

toethane with phthaloyl chloride in the presence of

pyridine and obtained a spirophthalide [248]. Then they

used dibutyltin dimethoxide to obtain tin-containing

PEGs and heated them with excess spirophthalide. The

reaction products contain one, two, three, or more

phthalate groups and thus are copolymers, since they

also have variable amounts of ethylene oxide units.

Fortunately, products with one phthalate group fall in a

different mass range than products with two phthalate

groups, making the MALDI spectrum easy to interpret.

3.12. Polymer degradation studies

A number of studies have established MALDI as a

unique technique for analyzing chemical modifications

in the structure of synthetic polymers induced by

degradation processes [366–393]. For example, the

study of polymer degradation by MALDI involves the

collection of MALDI spectra at different times and/or

temperatures to detect structural changes induced by

heat or light under an inert and/or oxidizing atmos-

phere. MALDI-MS has advantages because, unlike

traditional mass spectral techniques such as GC-MS

[422] and DPMS [422], it allows samples subjected to

oxidation to be analyzed without further decompo-

sition. When a polymer sample is partially degraded at

a given temperature, in an inert atmosphere (e.g.

nitrogen, argon) or in air, the MALDI spectrum will

Fig. 40. MALDI spectra of products obtained when poly(styryl lithium) is reacted with EO. Reprinted from Ref. [12] with permission of the

American Chemical Society.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357328

consist of a mixture of peaks from undegraded and

degraded chains.Puglisi et al. [376] studied isothermal

degradation of BPA-polycarbonate (PC) between 300

and 450 8C under a nitrogen stream. Their MALDI-

TOF spectra showed that a rearrangement of the

carbonate group leads to the formation of several

adjacent xanthone units in PC chains of sizeable molar

mass [376]. Xanthones, found among pyrolysis

Fig. 41. MALDI-TOF mass spectra of samples taken during EO polymerization of S19EO38 (SZstyrene). Spectra were measured in reflectron mode

using AgTFA as cationizing agent and dithranol as matrix. Reprinted from Ref. [154] with permission from Wiley.

2800 2900 3000 3100

D

G

B

G

C

D

B

L

QS

E

O

P

S

E

I

T

N

V I

3033

2779

2793

2809

2821

2835

2875

2885

2913

2929

2941

2955

2971

3019

3047

3063

3103

3089

D

G

B

G

C

D

B

L

QS

E

O

P

S

E

I

T

N

V I

3033

2779

2793

2809

2821

2835

2875

2885

2913

2929

2941

2955

2971

3019

3047

3063

3089

m/z

Fig. 42. Enlarged section of MALDI-TOF spectrum of SEC fraction,

collected at 28 ml, of a PC sample oxidized at 300 8C. Reprinted from

Ref. [378] with permission from Elsevier.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 329

products, are believed to be precursors of graphite-like

structures in the char residue that is produced at

temperatures higher than 450 8C under an inert

atmosphere. The structure of the species produced in

the thermal oxidative degradation of PC has been also

analyzed [377,378]. PC samples heated at 300 and

350 8C in air up to 180 min produced a THF insoluble

gel at the longer heating times. The MALDI spectra of

oxidized PC samples and of their SEC fractions,

showed the presence of oligomers containing acet-

ophenone (peak S, Fig. 42), phenyl substituted acetone

(peaks E and I, Fig. 42), phenols (peaks L, O, P and Q,

Fig. 42), benzyl-,alcohol (peak G, Fig. 42), and

biphenyl terminal groups (peaks T and V, Fig. 42)

[378]. The presence of biphenyl units among the

thermal oxidation products confirms the occurrence of

cross-linking processes, which are responsible for the

formation of the insoluble gel fraction [377,378]. It has

been proposed that the mechanisms accounting for the

formations of thermal oxidation products of PC involve

the simultaneous operation of several reactions: (i)

hydrolysis of carbonate groups of PC to form free

2700

B10B11

E9

F10

C10

D10

S9

D11

E10

R9

S10

R10

F11

C11

M8

C’10

P6

Q4

G9

N9

N10

M9 C’11 P7

Q5

G10

m/z2300 2400 2500 2600

B10B11

E9

F10

C10

D10

S9

D11

E10

R9

S10

R10

F11

C11

M8

C’10

P6

Q4

G9

N9

N10

M9 C’11 P7

Q5

G10

Fig. 43. Enlarged section of MALDI-TOF mass spectrum of

hydrolyzed gel formed by heating Ny66 sample in the presence of

TPP at 290 8C for 30 min. Reprinted from Ref. [366] with permission

from Elsevier.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357330

Bisphenol-A end groups (species C and D, Fig. 42); (ii)

oxidation of the isopropenyl groups of PC; (iii)

oxidative coupling of phenol end groups to form

biphenyl groups [377,378]. Lattimer [370] carried out

the thermal degradation of poly(acrylic acid) (PAA).

Negative ion MALDI analysis of the pyrolysis residues

provided direct evidence of dehydration and decarbox-

ylation. Lattimer et al. [372,373] studied the pyrolysis

of a segmented polyurethane consisting of

4,4 0-methylenebis-(phenylisocyanate) (MDI), poly(bu-

tylene adipate) (PBA) and 1,4-butanediol (BDO) by

MALDI-TOF [372]. Several pyrolysis products

appeared in the range 800–10,000 Da. Dissociation of

the urethane linkage to yield products with isocyanate

and hydroxyl end groups occurred at the lowest

temperatures (ca. 250 8C). Linear polyester oligomers

with hydroxyl and/or vinyl end groups were detected,

as were cyclic polyester oligomers. At higher tempera-

tures (O300 8C) nitrogen-containing pyrolysis

products were no longer present in the residue.

Dehydration of the linear and cyclic polyester

pyrolyzates occurs at these temperatures, producing

olefinic end groups [372]. Star-shaped polymers with a

fullerene core and six polystyrene arms [374] are

thermally unstable, and decompose in toluene solution

at around 100 8C. Since both C60 and PS are thermally

stable, it is supposed that the thermal degradation

reaction results from breaking of C–C bonds in the aand b positions to C60, and that the rate constant KD, is a

combination of two contributions Ka, Kb, due to the

indicated ruptures. The MALDI spectrum of (PS)6C60

shows two series of peaks with different intensities (the

intensity ratio being 3:1) due to the ruptures in the a and

b positions, respectively. The data confirm the

proposed degradation route and suggest that Ka is

threefold greater than Kb.

Puglisi et al. subjected Ny66 to thermal degradation

290 and 315 8C in an inert atmosphere [366]. The

formation of a gel fraction was observed after about

15 min of heating, and the addition of a condensing

agent such as triphenyl phosphite (TPP) made the

gelation complete in few minutes. The MALDI-TOF

mass spectra of the soluble fraction showed that

secondary amino groups and cyclopentanone chain

ends were generated in the heating process, as was

confirmed by MALDI analysis of heated Ny66 that had

been terminated with specific amine and carboxyl end

groups [366]. The gel fraction was partially hydrolyzed

to destroy the network structure and the soluble material

was analyzed by MALDI-TOF. The spectra of the

hydrolyzed Ny66 gel revealed the presence of N,N-

substituted amide (species M, N, P and Q, in Fig. 43,

Table 2) as side chains generated by the condensation of

carboxyl end groups with secondary amino groups and

azomethyne structures (species R and S in Fig. 43,

Table 2) originating from the reaction of cyclopentanone

moieties with terminal amino groups. These structures

were most likely responsible for the gel formation on

heating Ny66. Comparison with similar experiments

conducted on Ny6, showed that only secondary amino

groups were formed in Ny6, leading to branched

structures but not to crosslinking [366].

MALDI investigations by Samperi et al. [368,369]

of PET and PBT samples isothermally degraded at

processing temperatures in the broad range 270–370

and 270–350 8C, respectively, under nitrogen, showed

that the butylene unit in PBT is apparently able to

induce sensible differences in the isothermal degra-

dation of PBT in comparison with PET [368,369].

On the basis of MALDI and NMR data, the

authors proposed chemical degradation mechanisms

(Schemes 3 and 4). Terephthalic anhydride-containing

oligomers are clearly detected in the MALDI spectra

of melt processed PET (Scheme 3), whereas unsatu-

rated oligomers are absent [368]. The opposite is true

in the case of PBT [369]. The b-CH hydrogen transfer

reaction is very efficient in PBT (Scheme 4), and

unsaturated oligomers are present in the MALDI

spectra of the heated PBT samples. They appear to

be the only decomposition products, whereas the

formation of terephthalic anhydride-containing oligo-

mers along the PBT chains, actually observed at

400 8C, does not occur at 270–350 8C [369]. The

different thermal degradation behavior of PET and

PBT was attributed to the different reactivities of the

ethylene and butylene units, and to the greater chain

Table 2

Structural assignments of the peaks displayed in the MALDI-TOF mass spectra of the Ny66 samples, reported in figure 43

Species Structure MCKC (n)

BHO H

nNy6,6

2320 (10)

2546 (11)

2772 (12)

2998 (13)

CHO

nCO(CH2)4COOHNy6,6

2448 (10)

2674 (11)

2900 (12)

3126 (13)

C 0

HOn

Ny6,6

O

C

O 2430 (10)

2656 (11)

2882 (12)

3108 (13)

DH2N(CH2)6NH H

nNy6,6

2418 (10)

2644 (11)

2870 (12)

3096 (13)

EDA-CO-(CH2)4-CO OH

nNy6,6

2359 (9)

2584 (10)

FDA

nNy6,6 H

2233 (9)

2459 (10)

2685 (11)

2911 (12)

GDA

nNy6,6 CO(CH2)4CO-DA

2274 (8)

2500 (9)

2726 (10)

2952 (11)

M(CH2)6NH

yNy6,6 CO

O

DAx

Ny6,6 CO(CH2)4CO-NH(CH2)6N

CO(CH2)4CO OHz

Ny6,6

2343 (8)a

2569 (9)a

NH2N(CH2)6NH CO(CH2)4CO-NH(CH2)6N (CH2)6NH

H

yNy6,6 H

xNy6,6

2517 (9)a

2743 (10)a

PCO(CH2)4CONH(CH2)6N (CH2)6NHDA CO(CH2)4CONH(CH2)6N (CH2)6NH

yNy6,6

CO(CH2)4CODAt

Ny6,6 H

zNy6,6 H

xNy6,6

2473 (6)a

2699 (7)a

G.

Mo

nta

ud

oet

al.

/P

rog

.P

olym

.S

ci.3

1(2

00

6)

27

7–

35

7331

Species

Structure

MCKC

(n)

QC

O(C

H2)

4CO

NH

(CH

2)6N

(CH

2)6N

HD

AC

O(C

H2)

4CO

NH

(CH

2)6N

(CH

2)6N

Hy

Ny6

,6

CO

(CH

2)4C

ON

H(C

H2)

6

zN

y6,6

xN

y6,6

H NH

(CH

2)6

mN

y6,6

uN

y6,6

DA

CO

(CH

2)4C

OD

At

Ny6

,6

2485(4)a

2711(5)a

R

CO

N(C

H2)

6NH

Hy

Ny6

,6

HO

xN

y6,6

2302(9)a

2528(10)a

S

CO

N(C

H2)

6NH

Hy

Ny6

,6

H2N

(CH

2)6N

Hx

Ny6

,6

2401(9)a

2627(10)a

aThe(n)values

correspondto

thesum

oftherepetitiveunits;DAZ–NH(CH2) 9CH3.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357332

flexibility of the PBT chains. The unsaturated

oligomers produced in the thermal degradation of

PBT through the b-CH hydrogen transfer reaction can

easily undergo another b-CH transfer process to yield

butadiene. This process competes with the formation

of terephthalic anhydride-containing PBT oligomers,

and the latter reaction is suppressed in the temperature

range explored here. In PET, the vinyl group formed in

the primary degradation step is electronically con-

jugated to the adjacent ester group, and another b-CHhydrogen transfer reaction (to yield acetylene) is quite

unlikely. Therefore, the vinyl group-ended oligomers

may quickly react with carboxyl-ended oligomers, to

yield terephthalic anhydride-containing oligomers.

Alternatively, the Samperi et al. proposed an easy

unimolecular extrusion of ethyleneoxide (acet-

aldehyde) from PET [368].

The formation of unsaturated oligomers in the

thermal degradation of PBT has also been confirmed

by direct bromination of heated PBT. In Fig. 44,

MALDI spectra of unbrominated (Fig. 44a) and

brominated (Fig. 44b) PBT are heated at 300 8C for

60 min. The spectrum of the latter sample (Fig. 44b)

shows new peaks due to brominated species (peaks

E4Br2, E5Br2, E004Br2, E00

5Br2) [369]. In thermal

degradation of PET carried out in the presence of

p-toluene sulfonic acid (pTsOH) (0.5 wt%) at 270 and

285 8C, it was found that pTsOH induces a strong

hydrolytic reaction with consequent increase of

carboxyl-terminated polyester chains [368].

Weidner et al. [379,380] studied the oxidative and

hydrolytic degradation in PET by MALDI-TOF and

characterized the structures of oligomers formed during

hydrolytic degradation. They found that an ester scission

process generates acid-terminated oligomers H–[GT]m–

OH and T–[GT]m–OH and ethylene glycol-terminated

oligomers H–[GT]m–G, where G is an ethylene glycol

unit and T is a terephthalic acid unit. The scission of ester

bonds during the chemical treatment led to a marked

decrease in the number of cyclic oligomers [GT]m. The

presence of diacid-terminated species demonstrated a

high degree of degradation [379,380].

MALDI analysis of a PEG sample (MMZ2000 Da),

heated at low temperature (150–300 8C) in an inert

atmosphere, showed that the initial pyrolysis products,

obtained at 150 8C, have hydroxyl and ethyl–ether end

groups formed via C–O homolytic cleavage followed

by hydrogen abstraction [381]. At higher temperatures,

the abundance of the ether end groups increases as more

C–C cleavage occurs. Vinyl ether end groups increase

at higher temperatures (250–300 8C), owing to dehy-

dration of hydroxyl end groups [381]. The assignment

-CH transfer

CH2 CH OC

O

OH

C

O

C

O

C

O

O

C

O

C

O

O O CH2 OC

O

C

O

OCH2 CH2 CH2

O

n

(CH2)2 C

O

CO O

C

O

C

O

OOO

OO

CH3CHO

++

+

C

O

OH

C

O

CH3CHO+O O CH2 OHC

O

C

O

CH2

1

23

4

6

5

-CHtransfer

Scheme 3.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 333

of the end group structures was aided by tandem mass

spectrometry (CI-MS/MS) and by deuteration of

hydroxyl end groups in the pyrolyzate [381].

Lattimer characterized eleven series of oligomers in

MALDI-MS analysis of poly(tetrahydrofuran) (PTHF)

degraded at 175–350 8C in an inert atmosphere [371].

The pyrolysis products at about 175 8C, all have at least

one hydroxyl end group, retained from the original low

molar mass polymer, and the other end group is ethyl

ether, propyl ether, butyl ether, or aldehyde. MALDI

spectra of the pyrolysis products at higher temperatures

(250–350 8C) show an increasing tendency to form

products with a combination of alkyl ether and/or

aldehyde end groups. The amount of pyrolysis products

containing the hydroxyl end group diminishes at the

higher temperatures, and butenyl ether end groups are

observed to an appreciable extent. The latter function-

ality is apparently formed mainly via dehydration of

oligomers terminated with OH groups. The author

proposed a free radical mechanism to explain the main

degradation products of PTHF [371].

Gallet et al. carried out the thermal oxidative

degradation of a poly(ethylene oxide–propylene

oxide–ethylene oxide) triblock copolymer (Polox-

amer 407) at 80 8C in air for various times [373].

They found by combination of MALDI-TOF-MS for

the analysis of oligomers, solid-phase microextrac-

tion/gas chromatography-MS (SPME/GC-MS) for

the analysis of low molecular weight compounds,

and 1H NMR for chain-end determinations, that the

thermal oxidation proceeds in three steps. After an

induction period depending on the quantity of

antioxidant present in the polymer (21 days for

100 ppm BHT), the degradation started through a

six-ring intramolecular decomposition reaction of

the PPO block of the copolymer. By SPME/GC-MS

they found that the first volatile degradation product

to appear was 1,2-propanediol-1-acetate-2-formate.

This means that the secondary hydroperoxide

formed on the PPO chain plays a major role in

the thermoxidation of poloxamer materials. Finally,

more chain scissions occurred both in the PPO and

PEO blocks of the copolymer, leading to a dramatic

decrease of the molecular weight and the appear-

ance of formates, acetates, aldehydes and acids

[373].

Although applications of MALDI to the study of

polymer photo-oxidation processes are quite recent

[382], results obtained for the systems so far investi-

gated are highly informative as compared with previous

studies based on such conventional techniques as UV

and IR. Molecules formed in the photo-oxidation

ββ-CH transfer

CH2 CH OC

O

OH

C

O

C

O

C

O

O

C

O

C

O

O O (CH2)4 OC

O

C

O

O (CH2)4

O

n

(CH2)4 C

O

CO O

+

C

O

OH

C

O

1

2

3

(CH2)2

+

β-CH transfer

Scheme 4.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357334

processes are often very reactive, do not accumulate,

and are present only in minor amounts among the

reaction products. Nevertheless, MALDI spectra yield

precise information on the size, structure and end groups

of molecules formed in the oxidation process, allowing

discrimination among possible oxidation mechanisms.

In a recent investigation, Ny6 films subjected to

photo-ageing were analyzed by MALDI. The spectra

show the presence of over 40 different types of

oligomers, as compared to only three in a Ny6 blank

sample (Fig. 45). Three photo-oxidation processes are

occurring Ny6, as summarized in Scheme 5. The first

process is a hydrogen abstraction from the methylene

group adjacent to the amide NH, leading then to the

formation of a hydroperoxide intermediate. The

decomposition of this hydroperoxide by radical

rearrangement reactions generates the final products

of Ny6 photo-oxidation (Scheme 5a). Besides the

hydrogen abstraction and subsequent hydroperoxide

formation, which had been established in previous

studies [382], two other major processes appear to be

operating in Ny6, i.e. chain cleavage reactions of

Norrish types I and II (Scheme 5b and c). In Fig. 45,

each peak carries a label. Letter A specifies any end

group generated by the decomposition of the hydro-

peroxides (Scheme 5a); B specifies any end group

generated by the Norrish type I chain cleavage

(Scheme 5b); C specifies any end group generated by

the Norrish type II chain cleavage (Scheme 5c);

whereas E indicates just one of the end groups present

in the original Ny6 sample. Since each oligomer has

two ends, the notation B–A, for instance, means that a

Norrish type I chain cleavage occurred at one end and

that hydroperoxide decomposition occurred at the other

end. There are five oligomers originating exclusively

from Norrish I and four oligomers exclusively from

Norrish II reactions. Furthermore, nine peaks are

exclusively due to Ny6 oligomers originating only

from hydroperoxide decomposition reactions [382].

In a similar study, the MALDI analysis of the Ny6,6

films photoxidized at 60 8C in air, yielded detailed

information on the photodecomposition mechanism of

Ny6,6 [383]. The results confirm previous insights

about the hydrogen abstraction and subsequent

1000

20000

40000

1200 1400 1600

m/z

A5 + E4A6 + E5

A7 + E6

E’4 E’5

E’6

E’’4

E’’5

E’’6

C4C5

C6C’4C’5

C’6

A6

C4

C5C6

E5Br2E4Br2

E’’4Br 2

E’’5Br 2C’6

C’5C’4

Counts

Counts

C C O

OO

O(CH2)4

n

C C O

OO

OOH (CH2)4

n

C C OH

OO

C C O

OO

OOH (CH 2)4

n

C C ONa

OO

C

O

C

O

O (CH 2)4 O

n

C

O

C

O

OHCH (CH2)2OCH2 C

O

C

O

O (CH 2)4 O

n

C

O

C

O

ONaCH(CH2)2OCH2

n

C

O

C

O

O (CH2)4 O C

O

C

O

O(CH2)2CHCH(CH 2)2OCH2CH 2

A= B= B'=

E= E'=

E''=

Fig. 44. Enlarged sections of MALDI-TOF mass spectra of a PBT sample heated at 300 8C for 60 min: (a) unbrominated sample; (b) brominated

sample. Reprinted from Ref. [369] with permission from Elsevier.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 335

formation of a hydroperoxide intermediate and

reveal, as well, that the Ny6 [382], Norrish I and

Norrish II chain cleavage reactions play an important

role in the photo-oxidation of Ny6,6. The MALDI

spectra of Ny6,6 films photoxidized for short times

show only oligomers produced by hydrogen peroxide

decomposition, indicating that the Norrish I and

Norrish II reactions occur at a later stage of irradiation

[383].

An induction period before the occurrence of

Norrish I and Norrish II reactions was also observed

in the photoxidation of Ny6 and of aliphatic polyesters

[382–384]. It is believed that the initial irradiation of

these polymers triggers a-hydrogen abstraction, a low-

energy process, which induces polymer oxidation

through the formation of hydroperoxides. The latter

are thermally unstable at 60 8C and decompose,

forming oligomers with functional end groups that

enhance the light absorption power of the oxidized

polymer, thus allowing the Norrish reactions to take

place [382–384].

MALDI spectra of poly(butylene succinate)

(PBSu) photoxidized at 60 8C in air have revealed

the formation of oxidized PBSu oligomers containing

succinic acid, malonic acid, butyl ester, ethyl ester

and butyl formate end groups, which have not been

detected with other analytical tools. On the basis of

the structure of photo-oxidized products observed,

Carroccio et al. [384] proposed that process involves

several reactions: (i) oxidation of hydroxyl end

groups; (ii) a-H abstraction decomposition; (iii)

Norrish I photocleavage.

Carroccio et al. [385–387] have also investigated the

photo and thermo-oxidation processes, respectively,

Fig. 45. Enlarged sections of MALDI spectra, obtained in reflectron mode, of 40 mm Ny6 film photo-oxidized for (a) 0 and (b) 289 h. In (c) is

shown the deisotoping mass spectrum of sample (b). Reprinted from Ref. [382] with permission of the American Chemical Society.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357336

occurring during exposure of the polyetherimide

ULTEM in a QUV panel at 60 8C in air and during

heating at 350 8C in air [385–387]. The photo-oxidative

degradation produces a significant reduction of the

molar mass of the ULTEM samples [385,386], whereas

this is not observed in the thermal oxidation at 350 8C

[387]. Most likely this difference is due to the abundant

crosslinked insoluble residue formed during thermal

oxidation, whereas no insoluble residue is formed in

photo-oxidation process at 60 8C. The authors found

that thermal oxidation produces charring only after only

15 h and that the formation of insoluble residue

amounts to 50% after 180 h at 350 8C. Therefore, in

the case of thermally oxidized ULTEM products, only

the MALDI spectra of the fractions soluble in CHCL3

were recorded. MALDI spectra of both the photo-

oxidized and thermo-oxidized samples, recorded using

HABA as a matrix, CHCl3 solvent and sodium

trifluoroacetate salt as cationizing agent, showed the

presence of polymer chains containing acetophenone,

phenyl acetic acid, phenolic, benzoic acid and phthalic

anhydride end groups [385–387]. Oligomers terminated

with phthalic acid groups were observed in the spectra

of photo-oxidized samples [385,386], whereas oligo-

mers terminated with bisphenol A and phthalimide

groups were observed in the spectra of thermo-oxidized

samples [387].

According to the structure of the major oxidation

products detected by MALDI, the authors postulated

four photo-oxidation processes: (Scheme 6): (i) photo-

oxidation of phthalimide units to phthalic anhydride

end groups (P1 in Scheme 6); (ii) photo-cleavage of

methyl groups of the N-methyl phthalimide terminal

units (P2 in Scheme 6); (iii) oxidative degradation of the

CH

H

NH COCO (CH2)4 CH2 CH2 CH2 CH2 CH2 NH CO

H abstraction

NorrishI Norrish II

CO (CH2)4NH CHO

CO (CH2)4NH COOH

CO (CH2)4NH CH3

(B)

(CH2)5CO NH2

(CH2)5CO NH CHO

CO (CH2)3NH CH CH2

CO (CH2)3NH CH3

CO (CH2)2NH CH CH2

(CH2)5CO NH CO CH3

(C)

(A)

CH NH

O OH

CO (CH2)4

(CH2)4CO CHO

(CH2)4CO COOH

NH CO NH2(CH2)5

NH(CH2)4CO CO CO

h

hhA

B

C

CH

H

NH COCO (CH2)4 CH2 CH2 CH2 CH2 CH2 NH CO

H abstraction

NorrishI Norrish II

CO (CH2)4NH CHO

CO (CH2)4NH COOH

CO (CH2)4NH CH3

(B)

(CH2)5CO NH2

(CH2)5CO NH CHO

CO (CH2)3NH CH CH2

CO (CH2)3NH CH3

CO (CH2)2NH CH CH2

(CH2)5CO NH CO CH3

(C)

(A)

CH NH

O OH

CO (CH2)4

(CH2)4CO CHO

(CH2)4CO COOH

NH CO NH2(CH2)5

NH(CH2)4CO CO CO

h

hhA

B

C

Scheme 5.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 337

isopropylidene bridge of BPA units (P3 in Scheme 6);

(iiii) a photo-oxidation reaction introducing an oxygen

atom in several isopropylidene bridges along the main

chain (P4 in Scheme 6) [385,386]. In the case of thermal

oxidation, they proposed a mechanism that involves

three processes (Scheme 7): (1) thermal cleavage of

diphenyl ether units (routes T1 and T2); (2) oxidative

degradation of the isopropylidene bridge of BPA units

(route T3); (3) thermal cleavage of phenyl-phthalaimide

units (route T4).

By comparison of the photo- and thermo-oxidation

mechanisms (Scheme 6 and Scheme 7, respectively),

it emerges that only process T3 corresponds to a pure

thermo-oxidation reaction of ULTEM, whereas the

degradation pathways T1, T2 and T4 (Scheme 7) are

pure thermal scissions [387]. Both photo- and thermo-

oxidative degradation of the isopropylidene bridge,

are initiated by the extraction of a methyl hydrogen

atom [386,387], yielding a methylene radical, which

reacts with oxygen to form the hydroperoxide; and

the decomposition of this group 350 8C leads

directly to the same oligomers listed in pathway P3of Scheme 6. However, when the photo-oxidation is

performed at 60 8C, the formation of further oxidation

products such as B1–B3 can be detected (pathway P4,

Scheme 6). Another apparent difference between the

two processes is that the scission of the diphenyl

ether units is not observed in photo-oxidation at 60 8C

(Scheme 6), whereas the cleavage of the diphenyl

ether units is detected in thermal oxidation at 350 8C

[387]. The structures of the ULTEM thermal

oxidation products were also confirmed by MS/MS

analysis. Fig. 46 shows the CID MALDI-TOF/TOF

spectrum of the parent ions at m/zZ1015.6 together

with the structure of the fragment ions generated.

These correspond to those expected for an oligomer

having two hydroxyl groups attached to phthalimide

units [387].

There is a large class of compounds, called

generically ‘hindered amine light stabilizers’ (HALS),

which are derivatives of 2,2,6,6-tetramethyl piperidine

(TEMPIR). They do not absorb UV radiation, but act to

CH3

CH3

O

N

O

O

NO

O

O

CH3

P1

COCH3O

COOHOCH2COCH3O

OHO

CH2OHO CH2COOHO

NH

O

O

O

O

O

O

O

OHOH

H2O

h ,O2

P2

P3

P4

OH

CH3

OO CH2

h ,O2

P2

Scheme 6.

Scheme 7.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357338

Fig. 46. CID MALDI-TOF mass spectrum of peaks at m/zZ1015.6 from ULTEM sample oxidized for 2 h at 350 8C. Reprinted from Ref. [387].

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 339

inhibit photo-degradation of polymers by slowing

photochemically initiated degradation reactions.

The advantage of using HALS is that no large

thickness or concentration lower limit needs to be

reached to guarantee good results. Significant levels of

stabilization are achieved at very low concentration

(70–100 ppm). HALS high efficiency and longevity are

due to a cyclic process wherein the HALS are

regenerated rather than consumed during the stabiliz-

ation process.

Ohtani et al. [390] studied Adekastab LA-68LD, a

commercial oligomeric HALS in which the repeat

unit has two TEMPIR moieties. They recorded the

MALDI spectrum of the pure HALS and noted peaks

up to 7 kDa. Inspection of the spectrum reveals three

mass series, due to HALS chains terminated in three

different ways, which the authors labelled an, bn, cn.

After these preliminary measurements, they used

MALDI to study the photostabilizing action of

HALS in polypropylene (PP). They recorded the

MALDI spectrum, shown at the bottom of Fig. 47, of

a mixture of HALS and PP after UV irradiation for

700 h. Peaks due to PP are absent, probably because

there is an enormous polarity difference between

HALS and PP (actually, PP is virtually apolar).

The spectrum shows peaks (an, bn, cn) already present

in the MALDI spectrum of pure HALS, along with

additional peaks due to hydrolytic decomposition (dn),

peaks due to oxidation ða0n;b

0n;c

0nÞ and peaks labeled

with a double prime (e.g. b00n) due to double oxidation.

Fig. 47 shows the structures of d1 and b01 ions.

Unfortunately, in the case of b01, it is difficult to say

which of the six TEMPIR moieties is oxidized. The

presence of the dn compounds indicates that HALS is

subject to hydrolysis induced by atmospheric moist-

ure in the PP sample during irradiation. The figure

also shows partial spectra in the mass range 1400–

1800 Da of the HALSCPP mixture before and after

UV irradiation for 200 and 700 h, respectively. As

expected, the intensities of the peaks due to

decomposed and oxidized HALS chains increase as

the irradiation time is increased. This confirms peak

assignments and, at the same time, underlines the

power of the MALDI method.

Sato et al. report the application of MALDI for

characterization of the products obtained by enzymatic

degradation of polymer materials [388,389]. They treated

PCL terminated by a-benzyloxy groups with cholesterol

esterase at 37 8C in phosphate buffer at pH 7.0 for various

times. Two additional mass series appear in the spectrum

Fig. 47. MALDI mass spectrum of HALS in UV-irradiated PP composites for 700 h obtained by the solid sampling method and partial spectra in n

region observed for related PP composite samples: (a) before UV irradiation, (b) after UV irradiation for 200 h, and (c) after UV irradiation for

700 h. The figure displays structures of the original, oxidized and decomposed HALS sample. Reprinted from Ref. [391] with permission of the

American Chemical Society.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357340

of the PCL sample recovered after 36 h of enzymatic

degradation (Fig. 48). These new series were assigned to

ions without benzyloxy units (see Fig. 48 for assignment

details), indicating that the enzymatic degradation of the

PCL might proceed mainly in exo-cleavage mode

from a-benzyloxy terminal groups [388]. In another

example [389], the MALDI spectra of low molar mass

octyl phenol polyethoxylate (OPEO) biodegraded by a

pure culture of Pseudomonas under aerobic condition,

showed the formationofOPEOoligomerswith a carboxyl

terminal of ethylene oxide (EO) chains with molar

mass less than 600 Da [389]. From these data, the

biodegradation of OPEO would proceed by exo-scission

of EO chain accompanied by oxidation of the hydroxyl

end groups [389].

Random styrene-butadiene copolymers were dis-

tinguished from ABA block styrene-butadiene copo-

lymers, by MALDI-TOF analysis of ozonolysis

degradation products [391]. Several acrylonitrile-

butadiene copolymers were also characterized using

the same method [391]. The composition calculated

from the oligomer distributions detected by MALDI-

TOF, was close to the reported composition for these

copolymers (typically within 5 wt%). The discrepancy

Fig. 48. Typical MALDI-MS spectra of (a) original PCL sample and (b) PCL sample recovered after enzymatic degradation for 36 h. Mass numbers

in the expanded spectra indicate monoisotopic mass. Reprinted from Ref. [388] with permission from Elsevier.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 341

in the values was explained, in part, by a

compositional bias resulting from the ozonolysis

process [391].

Appendix A. Size exclusion chromatography

This appendix deals with SEC and SEC calibration.

In SEC, it is generally observed that the molar mass M

of macromolecules eluted at a given elution volume Ve

decreases as Ve increases and the data usually conform

to a relation of the type

log M Z b0Kb1Ve (A1)

where b0 and b1 are constants. The chromatogram of a

standard polymer sample (say polystyrene) serves to

calibrate the column for other samples of the same or

similar polymer. It is found experimentally that b0 and

b1 depend on the column and the solvent.

Another approach to SEC calibration is based on the

hydrodynamic volume Rh of the polymer molecule, a

quantity that is of particular relevance since it can be

taken to determine Ve. Theory shows that Rh is

proportional to the cube root of M[h] where [h] is the

intrinsic viscosity (recommended IUPAC name: limit-

ing viscosity number) of a polymer in solution. It turns

out that Eq. (A1) can be recast in a form including the

intrinsic viscosity of the sample

log M ZQ0 CQ1VeKlogð½h�Þ (A2)

where Q0, Q1 are ‘universal’ constants. It is observed

experimentally that Q0 and Q1 vary when the columnist

changed, but remain unchanged from one polymer type

to another. The intrinsic viscosity is related to M by the

Mark–Houwink–Sakurada (MHS) equation

½h�ZKMa (A3)

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357342

A double logarithmic plot of [h] versus M is linear,

and the MHS parameters K and a are obtained from the

slope and intercept. The MHS parameters depend on

the polymer, solvent, and temperature. With Eq. (A3)

the universal calibration equation becomes

log M ZQ0 CQ1VeKlog KKa log M (A4)

SEC devices are equipped with a refractive index

detector, which gives a signal proportional to the

weight of the chains eluted at a given volume, dW/

(dVe). This quantity must be converted into dW/(dM)

using the chain-rule in differentiation to give

dW=ðdVeÞZCfacdW=ðdMÞ (A5)

where the conversion factor, CfacZdM/dVe, is related to

the calibration equation. The number fraction Ni of each

chain species is Wi/Mi.

The MMD is readily obtained using the SEC trace.

The number- and weight-average molar masses defined

above by Eqs. (3) and (4) in Section 2.10 may also be

computed using the SEC trace, using the version in

terms of Wi, which is proportional to the concentration

measure in SEC; in some cases, the integral form of

these expressions is applied, testing for an asymptotic

limit to the parameters Mn and Mw as the upper bound

on the integration is increased.

The calibration of SEC traces of copolymers is a

more complex problem, and requires additional effort

than needed with homopolymers since copolymer

chains having the same MM may differ in comonomer

composition, and thus in overall chain dimensions.

Consequently, isobaric molecules may have different

hydrodynamic volumes and thus different elution

volumes that depend on copolymer composition. This

poses a serious problem for calibration of SEC traces.

Runyon et al. proposed a method based on

calibration lines obtained for homopolymer A and

homopolymer B to compute Mn and Mw of an AB

copolymer [355]. First, after constructing the cali-

bration lines for the homopolymers, one records the

SEC trace of the copolymer using an RI detector in

series with a UV detector. Comparing the two detector

responses, one obtains wA and wB, the weight fractions

of A and B units at any point of the chromatogram. In

the second step, the molar mass MC of the copolymer at

any elution volume is assumed to obey the relation

log MC ZwA log MA CwB log MB (A6)

where MA and MB are the molar masses of the two

homopolymers eluted at the same Ve.

When the weight fractions of the two units in the

copolymer are comparable (wAz0.5), the copolymer

line falls in the middle and one can draw it directly on

the graph of the two-homopolymer lines. Unfortu-

nately, the average molar mass averages of a copolymer

sample obtained by the method of Runyon et al. are not

reliable since the simple assumption of Eq. (A6) is not

correct [426]. However, SEC-MALDI can be used to

overcome this limitation [426].

Appendix B. Copolymer composition from MS

This appendix describes use of MS peak intensities

to determine copolymer composition. The method,

based on chain statistics, has been widely applied to

intensities derived from model sequence distributions.

The relative abundance of all the oligomers of a defined

chain length (dimers, trimers or higher oligomers)

reflects the composition and monomer sequence in the

copolymer [9]. Thus, the estimate of a sequence might

be done restricting the analysis only to one group of

oligomers; but of course, it is good practice to take the

average of the single estimates (see below), and to keep

in mind that higher oligomers are much more sensitive

to subtle sequence differences [9]. When using a

spectroscopic technique to obtain the copolymer

sequence and composition, the essential step is to

generate a theoretical spectrum, to be compared with

the experimental one.

The chain statistics approach allows discrimination

among different sequence distribution models [9]. The

process can be described as follows. For each

copolymer composition, an arrangement of comonomer

units along the chain is generated, according to a

predefined model. Starting from any sequence, a

theoretical spectrum can be generated, based on the

assignment of each mass peak to a set of sequential

arrangements of monomers. The quantity to be

minimized is the agreement factor AF

AFZ ðH1=H2Þ1=2 (B1)

where H1ZP

U21 and H2Z

PU2

2 . The quantities U1

and U2 are defined as U1Z ½IðAmBnÞKIexp�2 U2Z I2exp,

where Iexp and I(AmBn) are, respectively, the exper-

imental and theoretical mole fractions of copolymer

chains of type AmBn. The sums span all mass spectral

peaks considered.

The best-fit minimization procedure follows the

scheme described (the parameters of the model are

varied iteratively until convergence occurs), and

yields the copolymer composition and sequence.

Following this iteration, one records the spectrum,

selects a model, compares the experimental and

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 343

theoretical intensities and performs a best-fit mini-

mization [9], finds the minimum, and records the

result. Then one selects a different model, and again

finds a minimum. Finally, one selects the model that

gives the best result.

A problem frequently encountered in copolymer

analysis is that the MS peaks can be assigned to two or

more isobaric structures. In this case, the experimental

peak intensity may come from several contributions.

An automated procedure to find composition and

sequence of a copolymer has been developed to cope

with this problem of determining the sequence when a

mass spectroscopic peak has a multiple structural

assignment [9].

Bernoulli statistics predict [9] that the mole fraction,

I(AmBn), of the oligomer AmBn is given by

IðAmBnÞZ fðmCng!=½m!n!�gcmAcn

B (B2)

where cA and cB are the mole fractions of A and B units.

The above equation is the well-known Newton

formula; it predicts that the most abundant oligomer is

that with x units of the type A and y units of the type B,

i.e. the oligomer AxBy, where xZ(mCn)cA and yZ(mCn)cB.

Copolymers with three and four components contain

oligomers of the type AmBnCp, AmBnCpDq, respect-

ively, where m, n, p, q are the numbers of the units of

each kind in the molecule.

The Bernoulli model [9] predicts that the mole

fraction, I(AmBnCp), of oligomer AmBnCp is given by

IðAmBnCpÞZ gABCcmAcn

BcpCc

qD (B3)

whereas the mole fraction, I(AmBnCpDq), of oligomer

AmBnCpDq is given by

IðAmBnCpDqÞZ gABCDcmAcn

BcpCc

qD (B4)

where cA, cB, cC, cD, are the mole fractions of A, B, C,

D units in the copolymers and gABCZ ðmCnCpÞ!=½m!n!p!�; gABCDZ ðmCnCpCqÞ!=½m!n!p!q!�. These are referred to as the Liebniz formulas.

The above three equations allow one to generate

theoretical mass spectra, with the remarkable result that

the peak intensity patterns for any random copolymer

of a given composition will be identical.

Since each series of oligomers (dimers, trimers, etc.)

allows an independent calculation of the copolymer

composition and sequence distribution, the MS method

provides an excellent way to evaluate the precision of

these measurements [9].

The number-average length of sequences of like

monomers hnAi is given by:

hnAiZ 1=ð1KcAÞ (B5)

A random copolymer produced according to the

Bernoulli model is compositionally homogeneous, i.e.

the composition of the copolymer does not vary with

the chain length.

The sequence distribution followed by copolymers

produced by conventional free radical processes at low

conversion is the first-order Markoff distribution [9],

which has an associated P-matrix. This model predicts

[9] that the mole fraction of dimers is given by:

IðA2ÞZ cAPAA (B6)

IðABÞZ 2cAPAB (B7)

IðB2ÞZ cBPBB (B8)

The corresponding equations for trimers and

tetramers can be found elsewhere, along with the

predicted for the number-average sequence lengths of

like monomers [9].

The Markoff model also predicts that the resulting

copolymer is compositionally homogeneous, i.e. that

the composition of the copolymer does not vary with

chain lengths.

The number-average length of like monomers, hnAi,

is given by:

hnAiZ 1=ð1KPAAÞ (B9)

The chain statistics method is of particular value

when it is necessary to discriminate between a pure

copolymer sample and a sample made from a physical

mixture of two copolymers. This is a frequent case,

since commercial copolymers are often obtained by

mixing two copolymer batches.

Let us consider a mixture of two random copolymers

of the same chemical structure. The quantities of

interest are dA and eA, the mole fractions of A units in

the first and second copolymers, respectively, and X the

mole fraction of the first copolymer in the mixture.

The theory shows that the copolymer sequence

distribution followed by mixtures of two copolymers is

peculiar (sequences due to both components of the

mixture are present) [9].

The overall composition of the mixed copolymer cAwill be intermediate between the compositions of the

two components

cA ZXeA C ð1KXÞdA (B10)

The model gives the mole fraction I(AmBn) of the

oligomer AmBn as [9]

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357344

IðAmBnÞZ fðmCng!=½m!n!�gG3 (B11)

where G3ZXðeAÞmð1KeAÞ

nC ½1KX�ðdAÞmð1KdAÞ

n.

Explicit expressions for each oligomer may be derived

from this compact formula [9].

The MMD of copolymers obtained by anionic

synthesis is usually narrow, even narrower than the

polymeric precursor produced before the addition of the

second monomer. Their mass spectra show negligible

mass discrimination. Thus, one can record the mass

spectrum, derive Mn, Mw and cA (see formulas above)

and then use the formulas that yield the number-

average lengths of like monomers hnAi and hnBi: namely

hnAiZcAMn/k1 and hnBiZcBMn/k1, where k1 is the

mean mass of repeat units A and B.

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