Indian Journal of Chemistry Vol. 43A, November 2004, pp. 2281-2286

NMR studies of styreneln-butyl acrylate copolymers prepared by atom transfer radical polymerization

A S Brar* & Puneeta

Department of Chemistry, Indian Institute of Technology, Delhi, Hauz Khas, New Delhi 110 016, India

Received 18 June 2004; revised 4 August 2004

Styrene (S) and II-butyl acrylate (B) have been copolymerized by atom transfer radical copolymerization (ATRP) catalyzed by CuBr/N,N,N',N',N"-pentamethyldiethylenetriamine. The composition of copolymers has been determined with IH NMR spectroscopy. The copolymer composition data has been used to determine the reactivity ratios by Kelen-Tudos (KT) and nonlinear error in variables methods (EVM). Molecular weights of the copolymers have been determined by gel permeation chromatography (GPC). Molecular weights of the copolymers increase with increase in percentage conversion. The polydispersity of the copolymers is quite low (1.1-1.3). Styrene and It-butyl acrylate centered triads concentrations have been determined from DC { IH} NMR and reactivity ratios and show good agreement with each other.

IPC Code: Int. Cl.7 C08F 120/42; GOIR 33/20

The determination of microstructure of copolymers is of value In establishing structure-properties relationships. NMR spectroscopy, mostly, l3C NMR1

•2

, is an important technique for the investigation of the structures of copolymers, particularly, the monomer sequence determination in the copolymers.

Radical polymerization has been a subject of keen interest. The versatility of the radical polymerization lies in the fact that a large range of the monomers can be polymerized. Nevertheless, free-radical polymerization has been limited by the inevitable, fast and irreversible termination of the growing radicals by the coupling and disproportionation reactions, leading to the poor control of the reaction and high polydispersity of the resulting polymer. Thus, much research has been devoted to develop a controlled radical polymerization to synthesize well-defined polymers with narrow molecular weight distributions and desired complex architectures3

. One of the most successful methods has been atom transfer radical polymerization (A TRP)4.5. This technique is most versatile and has been successfully applied to vinyl monomers such as acrylates and styrene(s) to prepare polymers with controlled molecular weights and well­defined structures6

. ATRP is a catalytic process, where a transition metal complex reversibly activates the dormant polymer chains via a halogen atom transfer reaction. The control of the polymerization

afforded by ATRP is due to a rapid equilibrium between the dormant and active species (Scheme 1).

R·X + M ~ ·YI Ligand. k act

k deact

Monomer Termination

Scheme 1

Radicals are generated through a reversible redox reaction catalyzed by a transition metal (Mt -Y) ligand complex, where Y may be ligand or counter ion, which undergoes one electron oxidation with abstraction of a (pseudo) halogen atom (X) from a dormant species R­X. The process occurs with a rate constant of activation (k.ct)7.8 and deactivation (kde•ct). Polymer chains grow by the addition of radicals to monomers similar to conventional radical polymerization with' the rate constant of propagation (kp). Termination reactions also occur; however, in a well-controlled ATRP not more than a few percent. A successful ATRP will not only have a small contribution of terminated chains, but also a uniform growth of all the chains, which is through fast initiation and rapid reversible deactivation. Tllerefore, the molecular weight distributions are narrow with the polydispersity index9 (MwIMn) ranging from 1.1 to 1.3.

The copolymers of styrene and n-butyl acrylate (SIB)JO was prepared by ATRP and their

2282 INDIAN J CHEM, SEC A, NOVEMBER 2004

microstructure has been determined by IH and I3C NMR spectroscopy. In this paper, we report the reactivity ratio by KTII and EVM 12.13 methods. The triad sequence distribution of SIB copolymers were determined from 13CeH} NMR and from reactivity ratiosl4

.

Materials and Methods n-Butyl acrylate (CDH,99%) was washed with

aqueous NaOH, dried over fused CaCh and distilled under vacuum. Styrene (Merck, 99%) was passed through a column of basic Ah03, and distilled under vacuum. All monomers were stored at O°C and purged with nitrogen gas for 30 min before use. Methyl 2-bromopropionate (Aldrich, 98%) was distilled at reduced pressure. N,N,N',N',N"-pentamethyldiethyl­enetriamine, PMDET A (Aldrich, 99%), Copper(l) bromide(CuBr, Aldrich, 98%) and copper metal powder (Cu(O),CDH, 99.5%) were used as such. Chloroform and methanol were purified by general methods.

Ten boiling tubes fitted with rubber septum were taken and to each tube CuBr (0.8 mmol) and Cu (0) (1.6 mmol) were added. The solids were evacuated and backfilled three times with N2• A solution of styrene (0.04 mol), n-butyl acrylate (0.04 mol), PMDETA (8 mmol) and methyl 2-bromopropionate (2.4 mmol) were added via syringe to each tube. The reaction mixture was heated at 80°C. Tubes were taken out at different intervals of time, to observe the change in molecular weight with conversion, and the reaction was quenched by precipitating in methanol. Precipitated polymers were dissolved in chloroform and passed through the column of neutral alumina to remove the residual copper catalyst (green colour) present in the copolymer. The polymers were dried under vacuum at 78°C for 10 hrs·. Percentage conversion was determined gravimetrically.

Styrene (0.016, 0.024, 0.056, 0.064 mol), n-butyl acrylate (0.064, 0.056, 0.024, 0.016 mol) respectively were taken infeed, then polymerization was carried out as described above.

The molecular weights were determined by GPC using polystyrene as narrow standard and THF as eluent. The eluent flow was 0.3 ml per min. A refractive index detector was used for detection. It was found that the molecular weight increases with increase in conversion.

IH and I3C NMR spectra were recorded in CDCh on a Bruker DPX-300 MHz spectrometer operating at a frequency of 300.13 and 75.5 MHz for IH and l3C nuclei, respectively at 25°C. DEPT experiments were carried in CDCh using the standard pulse with a J modulation time of 3.7 s (JCH = 135 Hz) with a 2s delay time. All the NMR spectra were recorded in CDCl3 at 25°C.

Results and Discussion

Determination of reactivity ratio The copolymer composition of SIB copolymers

was determined from the IH NMR spectrum. The feed mole fraction and copolymer composition data are given in Table 1. The initial estimate of the reactivity ratios was done by the Kelen-Tudos method with the help of copolymer composition data. The values of the terminal reactivity ratios obtained from relevant plots were: rs= 0.87 ± 0.10, rb= 0.25 ± 0.01. These values along with the copolymer data were used to calculate the reactivity ratios using the nonlinear error invariable method. The values of reactivity ratios obtained from this method were rs= 0.87, rb = 0.25 respectively. The values of reactivity ratios obtained from Kelen-Tudos and nonlinear error in variables methods are in complete agreement with each other.

Table l--Copolymer composition data of the SIB copolymers (<10% conversion)

Sample No.

1 2 3 4 5

Styrene mole fraction in feed if,)

0.2 0.3 0.5 0.7 0.8

Styrene mole fraction in copolymer (Fs)

0.29 0.46 0.60 0.79 0.91

Table 2---Percentage conversion and molecular weight of the SIB copolymers by GPC measurements

S.No. Percentage Molecular Polydispersity conversion weight X 10-4 index

(MjMII)

1 25.3 1.3 1.16 2 39.6 2.1 1.15 3 44.3 2.8 1.13 4 58.7 3.7 1.14 5 64.3 4.5 1.15 6 66.0 4.6 1.18 7 73.9 5.4 1.18 8 80.0 6.0 1.24

BRAR & PUNEETA: NMR STUDIES OF STYRENFJn-BUTYL ACRYLATE COPOLYMERS 2283

Molecular weights detennination The molecular weights were determined by gel

permeation chromatography (OPC). It was found that the molecular weight increases with increase in conversion. The increase in molecular weight with conversion is shown in Table 2. Polydispersity of the copolymers is very low i.e. 1.1-1.3. Figure 1 shows the increase of molecular weight with increase in conversion and also OPC chromatograms show the increase in the molecular weight with conversion (Fig. 2).

IH NMR studies

The complete assignment of the 'H NMR spectrum of the SIB copolymer in CDCh is shown in Fig. 3. The composition of the copolymer was calculated from the relative intensities of the phenyl (S,) and -OCH2- (S2) proton resonances of styrene and n-butyl acrylate units respectively, according to the following equation:

70000

60000

50000

1 40000 I c ::E 30000

20000

10000

10 20 30 40 50 60 70 80 90 1 DO

'4 converslon-->

Fig. I--Dependence of molecular weight with conversion for the styreneln-butyl acrylate copolymers by ATRP.

f. " 1\

i \ iii \ ! V I,: \ : 0

! . :\ i i \ I. . • \0 \ 0 : ..0 \

o I \ 0

(----) 13,200 Mwl Mn= 1.16 (_._) 21,500Mwl Mn = 1.15 (------) 23,200Mw/Mn=I.13

C--) 53,800 Mwl Mn = 1-18

j • f \~ \

.. ---------- / .I! .~:---.. ---=-=-~ ~ . ./ ~:-= I t I I

1·00 1.50 2·00 2.50

Elution Tim! X 101 (mi fl.)

Fig. 2--GPC chromatograms of the styreneln-butyl acrylate copolymers by ATRP.

where, F, is the mole fraction of styrene monomer in the copolymer.

13C {IH} NMR studies

The complete assignment of the resonance signals in the l3C {'H} NMR spectrum of the SIB copolymer in CDCl3 is shown in Fig. 4. The carbonyl carbon signals for n-butyl acrylate resonate around 0173.8-176.8 ppm and the quaternary carbon signals of styrene resonate around 0142.2-146.4 ppm and their expanded spectra are shown in Figs 5 and 6. On the basis of variation in the composition of the copolymers, and on comparison with the spectra of

Table 3--Triad fractions calculated from the NMR spectra and reactivity ratios of SIB copolymers «10% conversion)

Sample No.

1

Copolymer composition Fs

0.29

2 0.46

3 0.60

4 0.79

5 0.91

Triads

BBB BBS SBS SSS SSB BSB

BBB BBS SBS SSS SSB BSB

BBB BBS SBS SSS SSB BSB

BBB BBS SBS SSS SSB BSB

BBB BBS SBS SSS SSB BSB

a b

0.03 0.03 0.31 0.29 0.66 0.68 0.26 0.25 0.46 0.50 0.28 0.25

0.11 0.13 0.51 0.46 0.38 0.41 0.05 0.07 0.43 0.40 0.52 0.53

0.03 0.04 0.33 0.31 0.64 0.65 0.19 0.21 0.54 0.50 0.27 0.29

0.00 om 0.11 0.16 0.89 0.83 0.44 0.45 0.46 0.44 0.10 0.11

0.00 0.00 0.15 0.12 0.85 0.88 0.61 0.60 0.35 0.35 0.04 0.05

a--Triad fractions obtained from 13C{ IH} NMR spectra of the quarternary and the carbonyl carbon resonance signals of the S-

o and B- centered monomeric units. b--Triad fractions calculated using rs= 0.87, rb- 0.25.

2284 INDIAN J CHEM, SEC A, NOVEMBER 2004

CHt($+I) + 'CHt t'01, CH,

I f I I I I f I I f I I f I I f I I I I I I I i i I i f i i I I

4.0 3.0 2.0 1.0 0.0 8.0 7.0 6.0 s.o ppm . . (

Fig. 3--IH NMR spectrum of the styreneln-butyl acrylate copolymer (F, = 0.29) in CDCI3 at 25°C.

-Cftt-ln- CH,- ~H-2' ~- 0 , . Is 0

4 I

C=0(8)

r7'\

I 150

,~2

7?: CH,

f 100

i " •. r .,

£ u

I o

Fig.4--13C (IH) NMR spectrum of the styreneln-butyl acrylate (F, = 0.46) in CDCh at 25°C.

BRAR & PUNEETA: NMR STUDIES OF STYRENEIn-BUTYL ACRYLATE COPOLYMERS

ISS

~

~s (~

__ ~_~(a) Iii I I i I

149.0 i I I i

1-48.0 i I I

147.0 I I I I

146.0 I I I i

145.0 1 ... :0 143.0 f I Iii iii f i

142.0 141.0

2285

Fig. 5-Expanded Quaternary carbon of the S unit in the l3C {lH} NMR spectra of the styreneln-butyl acrylate copolymer in CDCI3 at 25°C: a) polystyrene, b) Fs = 0.91, c) F. = 0.79, d) F. = 0.60, e) F, = 0.46, and f)F. = 0.29.

t I i

flU ..... iii 171.0

I I i

171.0 i I I

177.0 i I I 171.0

iii

l7I.G iIi 174.G

iii

In:.o i I I·

172.0 I I i I

171.0

Fig. 6--Expanded carbonyl carbon of the B unit in the l3C {lH} NMR spectra of the styreneln-butyl acrylate copolymer in CDCh at 25°C: a) poly(n-butyl acrylate), b) Fb= 0.71, c) Fb = 0.54, d) Fb= 0.40, e) Fb= 0.21, and f) Fb = 0.09.

2286 INDIAN J CHEM, SEC A, NOVEMBER 2004

ppm

'C-'C(S)

~

I ' tOO

'-Y -OCH,-(S)

1 50

,'-Y CH,(B) 'I--.

'CHr,(S)

o

Fig. 7-DEPT-135 NMR spectrum of the styrene/n-butyl acrylate copolymer (F. = 0.46) in CDCI3 at 25°C.

respective homopolymers, the various triad sequences in the carbonyl and the quaternary carbon resonance signals were assigned. The resonance signals of quaternary carbon around 8146.3-144.8 are assigned to SSS triad, 8144.8-143.6 assigned to SSB triad and around 8143.6-142.4 are assigned to BSB triad. The resonance signals of carbonyl carbon around 8174.6-173.8 ppm are assigned to BBB triad, around 8175.6-174.6 ppm assigned to SBB triad and around 8176.8-175.6 ppm is assigned to SBS triad. The quantitative calculations of these Sand B centered triads were done from the areas of the respective resonance signals in \3C CH} NMR of copolymer and from the reactivity ratios and showed good agreement with each other (Table 3). The spectral region around 828-48 ppm is quite complex, which can be assigned to aliphatic carbon resonances in the main backbone as well as the side chain of SIB copolymer.

Figure 7 shows the DEPT -135 NMR spectrum of the SIB copolymer in CDCI3• The -OCH2- region of the B unit resonate around 863 ppm. The methylene and methine carbon resonances of both Sand B unit of backbone overlap between 835-45 ppm and they appear as multiplet showing their sensitivity towards compositional sequences.

To conclude, the reactivity ratios of monomers have been found to be rs= 0.87 ± 0.10, rb= 0.25 ± 0.01, rs= 0.87, rb= 0.25 by KT and EVM respectively. The carbonyl and the quarternary carbon resonances of B and S unit respectively were assigned to triad

compositional sequences and were used to determine triad concentration.

Acknowledgement One of the authors, Puneeta, wishes to thank the

CSIR, India for providing the financial support.

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