research article intramolecular lactonization of poly...

Research ArticleIntramolecular Lactonization of Poly(120572-hydroxyacrylic acid)Kinetics and Reaction Mechanism

Heli Virkki12 Antti Vuori2 and Tapani Vuorinen1

1Department of Forest Products Technology Aalto University PO Box 16300 00076 Aalto Finland2Kemira Oyj Espoo RampD and Technology PO Box 44 02271 Espoo Finland

Correspondence should be addressed to Heli Virkki helivirkkikemiracom

Received 25 May 2015 Revised 5 August 2015 Accepted 12 August 2015

Academic Editor Sanjeeva Murthy

Copyright copy 2015 Heli Virkki et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Poly(120572-hydroxyacrylic acid) PHA is one of the few polymers with biodegradable properties used in mechanical pulp bleaching tostabilize hydrogen peroxide A new method for the in situ follow-up of the lactone ring formation of PHA has been developedThe results have further been applied to describe the reaction kinetics of the lactonization and hydrolysis reactions throughparameter estimation In addition the reactionmechanism is elucidated bymultivariate data analysis Satisfactory identification andsemiquantitative separation of the lactone ring as well as the acyclic (carboxyl and hydroxyl groups) forms have been established by1HNMR in the pH range of 1ndash9The lactonization reaction approaching equilibrium can be described by pseudo-first-order kineticsin the pH range of 1ndash6 The rate constants of the pseudo-first-order kinetic model have been estimated by nonlinear regressionDue to the very low rates of lactonization as well as the weak pH dependency of the reaction an addition-elimination mechanismis proposed Additionally the presence of a transient reaction intermediate during lactonization reaction could be identified bysubjecting the measurement data to multivariate data analysis (PCA principal component analysis) A good correlation was foundbetween the kinetic and the PCA models in terms of model validity

1 Introduction

Poly(120572-hydroxyacrylic acid) PHA (CAS 35326-33-1) is aweakly acidic polyelectrolyte which is used as stabilizingagent for hydrogen peroxide [1] Polyelectrolytes are oftenknown to have a poor biodegradability but the PHAmoleculeis biodegradable to a large extent which makes it attractivealso elsewhere within chemical industry such as detergentapplications as metal and fatty acid removing agent [2 3]

Decomposition of hydrogen peroxide takes place boththrough radical reactions at elevated temperature and alka-line surroundings and catalytically by transition metals ofwhich iron and manganese present the most destabilizingproperties in the bleaching sequence [4 5] Furthermoreaccording to Ni et al the catalytic effect of Mn3+ overridesthat of Mn2+ in alkaline solution [6]

In order to avoid the metal-induced decomposition ofhydrogen peroxide in the industrial applications the con-centration of the transition metals is suppressed as much as

possible Traditionally hydrogen peroxide in bleaching ofmechanical pulp has been stabilized with sodium silicateHowever silicate increases the anionic load in filtrate cir-culations and causes precipitation on piping and equipmentsurfaces PHA is able to replace sodium silicate as stabilizingagent in the bleaching of mechanical pulp

The functional mechanism of PHA as a stabilizing agentfor hydrogen peroxide is so far not completely elucidatedPrevious studies have indicated plausible ionic interactionsbetween PHA and the most detrimental divalent metalsFavorable conditions for its interaction with calcium mag-nesium andmanganese have been defined to exist within thealkaline region pH 6 to 10 [7] Furthermore calcium andmanganese seem to interact with PHA more strongly thanmagnesium

PHA can form intramolecular lactone rings betweenundissociated carboxyl and adjacent 120574-hydroxyl groupaccording to Figure 1 [8] This feature affects its functionalproperties such as dissociation behavior [9 10]

Hindawi Publishing CorporationJournal of PolymersVolume 2015 Article ID 157267 10 pageshttpdxdoiorg1011552015157267

2 Journal of Polymers

O

O

OH

OH

H+ +HCO2

minusminusO2C

Figure 1 The chemical structure of poly(120572-hydroxyacrylic acid)PHA with an intramolecular lactone ring

The build-up of lactone ring structures has an effect onthe affinity of PHA towards metal species in stabilization ofhydrogen peroxide According to our previous study it ishowever likely that nonionic interaction routes betweenPHAand species of for example manganese and iron prevail aswell In addition some differences of preference such as infavor of iron over manganese at acidic pH have been noticed[7] Despite the targeted high alkalinity of the mechanicalpulp bleaching process there are also strong local pH gradi-ents especially in high consistency bleaching due to inefficientmass transfer Depending for example on the chemical dos-ing sequence the stabilizer may have a long residence timeunder process conditions well below the target pHThereforeit is of importance to study the kinetic behavior of the lacto-nization and hydrolysis reactions of PHA in more detail as afunction of pH

The goal of the present research was (1) to investigatethe rate of the lactone ring build-up of poly(120572-hydroxyacrylicacid) as a function of pH by means of 1HNMR spectroscopyas well as other complementary methods of NMR and (2)to estimate the kinetic parameters for the lactonization rateexpression in the following

[PHA]open119896(close)997888997888997888997888997888rarrlarr997888997888997888997888997888

119896(open)[PHA]close (1)

where

[PHA]close + [PHA]open = 1 (2)

So far the only published spectroscopic studies of thechemical structure of PHA consider the static reactionequilibrium results [8 9] Thus no kinetic data consideringthe structural dynamics of the reaction have been presentedutilizing any kind of analytical method The published acid-base titrations indicated extraordinary dissociation behaviorin terms of large variation in the dissociation constantsbut left the reasoning without evidence in context of thecorresponding simultaneous state of the chemical structure[7 10] A kineticmodel covering the lowpHregion is essentialfor obtaining a better understanding of the lactone build-upmechanism (intramolecular versus intermolecular esterifica-tion reaction route) Moreover knowledge of the time scaleof the lactonization reaction provides valuable informationto be utilized especially in the high-consistency bleachingprocesses as well as in gaining more information of thefunctionality of PHA in other chemical applications

Spectroscopic methods (NMR and UV) have successfullybeen applied in many earlier kinetic studies also related to

lactone formation reactions within various compounds [11ndash14] In addition both qualitative and quantitative kineticstudies have been conducted to gain information on com-plex reacting multicomponent mixtures by utilizing NMRspectroscopy [15] However considering the nature of thepolymeric analyte of the current study as well as the chem-ical environment of an electrolyte the spectroscopic datais expected to spread out and have overlap between theobserved components Thus to verify and elucidate the uni-variate approach of classic kinetic modeling by parameterestimation multivariate data analysis has been applied

2 Experimental Section

21 Analytical Methods The propagation of the lactone ringformation reaction was detected and quantified by 1H NMRspectroscopy (Bruker Avance II 400MHz) with D

2O as

solvent and using a 5mm BBO probe Pulse delay was 2 s andwidth was 30∘ The spectral width applied was 8 kHz

22 Experimental Plan The kinetic measurements coveredsamples in the pH range from 1 to 9 with the interval ofone pH unit In addition one additional sample was tested atpH 45 to identify the possible turning point in terms of thereaction mechanism All the measurements were conductedat room temperature (119879 = 247 plusmn 07∘C)

23Reagents The aqueous solution of the sodium salt of poly(120572-hydroxyacrylic acid) (30 NaPHA Mw = 30 000 gmol) wasprovided by Kemira Oyj The hydrochloric acid used was acommercial product of analytical grade and it was used asacquired at a proper dilution by D

2O without further purifi-

cation

24 Experimental Procedure An aqueous stock solutioncontaining 063wt- PHA polymer was prepared by dilutionof the reagent solution (30 NaPHA pH 95) with D

2O

The individual sample solutions were prepared by weighingapproximately 7 g of the stock solution followed by pHadjustment with hydrochloric acid (01 30 dependingon the target pH) after which the 1H NMR measurementwas started immediately The scans were recorded in thebeginning of the measurement every five minutes until 20minutes after which the interval was increased to 10minutesBetween one and four hours of measurement time the scanswere recorded every 30 minutes The pH was measured inparallel with the scans using an identical reference solutionThe pH values given in the present work for all individualmeasurements correspond to the initial values of each sam-ple The measurement was stopped when the reaction hadreached the equilibrium All the measurements fulfilled thecondition 119905Eq gt 10 times 11990512 where 119905Eq denotes the point in timewhen the equilibrium has been reached and 119905

12denotes that

when the conversion of the reaction is at half way [11]

25 Data Analysis The parameter estimation discussed inmore detail later on was conducted by Modest (MOdelESTimation) software utilizing Matlab 61 interface and For-tran 77-based code Modest program has been designed for

Journal of Polymers 3

4

4

3

3

2

2

1

1

F2 (ppm)

F1

(ppm

)

(a) Correlation spectrum of PHA at pH 2

4

4

3

3

2

2

1

1

F2 (ppm)

F1

(ppm

)

(b) Correlation spectrum of PHA at pH 10

Figure 2 1H correlation spectra of PHA (a) at pH 2 and (b) at pH 10The axes1198651and119865

2 represent the two frequency coordinates in chemical

shifts (ppm)

parameter estimation of mathematical models as well as forexperimental design simulation and optimization Explicitalgebraic implicit algebraic (systems of nonlinear equations)and ordinary differential equations can be used to describethe reactions to be estimated [16]

Multivariate data analysis by principal component anal-ysis (PCA) was applied to gain deeper insight into thespectral identification and the reaction mechanism duringthe propagation of the lactonization reaction

PCA is a multivariate projection method designed toextract and display the systematic variation within a datamatrix The data in a multidimensional space is modeled as aplane or hyper plane the axes of which are called principalcomponents This is done in order to reveal trends as wellas relationships between the observations and variables oramong variables themselves Due to different numericalranges of the original spectral data preprocessing by meansof mean-centering and scaling that is normalization to unitvariance is conducted to make the actual variance in the dataattainable If the amount of observations is large (as in thepresent case) averaging is conducted in order to reduce signalnoise The number of principal components needed to builda reliable model is determined by the degree of explanation ofthe model For detailed theory of PCA the reader is referredelsewhere [17]

3 Results and Discussion

31 NMR Analyses The degree of polymerization of thePHA molecule causes the spectral bandwidths in the 1HNMR spectrum to spread out in a significant manner incomparison to the case of monomeric analytes Similar 1Hmeasurements of NaPHA have been conducted in an attemptto assign chemical structures to the broadened chemicalshifts [8] However structural identification was judged asimpossible by Yamazawa et al due to the multiple electroniccircumstances provided by the lactone rings to themethyleneproton The authors obtained a more resolved 1H spectrumat elevated temperature resulting in one central large peak

(racemo) and two small doublets (m1 and m2) They alsostated that this type of spectrum has been observed formethylene protons of poly(methyl methacrylate) (PMMA)and poly(acrylic acid) (PAA) and interpreted as one centralracemo peak and twomeso peaks Our own 1Hmeasurement(conducted at pH 10 and 119879 = 25∘C) supports this conclusionThe m2 peak maximum is located at 18 ppm the racemopeak maximum is located at 20 ppm and the m1 peakmaximum is located at 23 ppm This area corresponds wellwith our 1H measurements presented in Figures 4 5 and 6providing justification for the integration limits of the acyclicPHA form later on in the text Additional verification to theassignment of the chemical shifts is provided by conductingtwo-dimensional 1Hcorrelation spectroscopy (COSY) aswellas 13C measurements (Figures 2 and 3)

The 1H correlation spectroscopy measurement at pH 10(Figure 2(b)) indicates at least three separate spin systemsfor the PHA molecule under alkaline conditions The samenumber of separate signals is also seen in the 13C spectrum atpH 10 (Figure 3(b)) within the methylene group signal Theyoriginate from the possible variations between the orienta-tions of the adjacent n-hydroxyl and carboxyl units All thecoupling constants of approximately 15Hz represent typicalgeminal protons (H-C-H) Because of this no interlinkingbetween methylene groups is seen that would indicate excep-tions from linear chain structure 1H correlation spectrumconducted at pH 2 (Figure 2(a)) presents a coupling betweensignals at 223 and 28 ppm the justification of which isfurther elaborated by applying multivariate analysis of thedata in Section 34 below

The 13C spectrum at pH 2 for lactonized PHA (Fig-ure 3(a)) indicates signals in the range from 85 to 87 ppmwhich supports earlier studies [8] The chemical shift origi-nating from the carboxylate carbon represents the intensitymaximum Additionally the locations of the methylene car-bon signal at approximately 50 ppm aswell as the carbon con-nected to the free carboxylic acid group at ca 78 ppm corre-spond to previous data [8]


0 20 40 60 80 100 120 140 160 180 200Chemical shift (ppm)

(a) 13C NMR of PHA at pH 2


(b) 13C NMR of PHA at pH 10

Figure 3 13C NMR spectra of PHA (a) at pH 2 and (b) at pH 10

0

2

4

6

2 22 24 26 28 3

Rela

tive s

igna

l int

ensit

y

Chemical shift (ppm)

times103

Figure 4 1H NMR spectrum of the lactone ring build-up reactionof PHA at pH 3 after 5 (blue line) 60 (red line) and 1600 (green line)minutes time from the beginning of the measurement

According to the 1H spectra the lactonization reac-tion proceeds extensively at acidic conditions approximatelybelow pH 5 This is depicted clearly in the 1Hmeasurementsin Figure 4 at pH 3 by the decline and finally a substantialdecrease of the original signals of the acyclic carboxylic struc-tures and by the appearance of the new ones related to thelactone ring structures over the reaction time

The decline of the signal intensities of the acyclic PHAat the lowest ppm level in Figure 5 (pH 1 ca 2 ppm at pH45 lt19 ppm) is indicative of the reaction propagation asthere is no overlappingwith the signals arising from the lacto-nization reaction The pH dependency of the signals occur-ring during the reaction is the most apparent and interestingphenomenon This can be seen by studying for example thesplitting of the broad signal of the acyclic PHA in the freeacid regime appearing at 223 ppmat pH 1 (see Figure 5 singlesignal at 223 ppm)

In the measurements conducted at less acidic conditions(pH 3 Figure 4) this signal of acyclic PHA starts to split intotwo separate signals At pH 45 there are two clearly separatesignals at 220 ppm and 215 ppm (Figure 6)

0

2

4

6

8

10

12

14

2 22 24 26 28 3

Rela

tive s

igna

l int

ensit

y


times103


02468

1012141618

2 22 24 26 28 3

Rela

tive s

igna

l int

ensit

y


times103

Figure 6 1HNMRspectrumof the lactone ring build-up reaction ofPHA at pH 45 after 5 (blue line) 60 (red line) and 1600 (green line)minutes time from the beginning of the measurement

The intensity maximum of the lactonized PHA at low pHvalues appears at 28 ppmAs pH increases the intensitymax-imum shifts towards lower ppm values Table 1 depicts the 1Hsignal assignments as well as their average variation (ad) inmeasurements conducted in the pH range from 1 to 9


Table 1 Assignment of the 1H shifts (at intensity maximum) andtheir average deviations (ad) of the acyclic and lactonized structuresof PHA at pH 1ndash9

pH Acyclic PHAppm ad Lactonized PHA

ppm ad

1 218ndash226 004 nandash284 na2 217ndash224 004 nandash279 na3 218ndash224 003 253ndash278 0134 210ndash222 006 247ndash268 01145 207ndash220 007 244ndash267 0125 203ndash213 005 247ndash262 0086 198ndash200 001 242ndash246 0027 197ndash199 001 na-na na8 197ndash198 001 na-na na9 197ndash198 001 na-na na

Thus as the pH value increases the location of theintensity maximum is shifted to lower ppm values This kindof pH dependent behavior is caused by the protonation anddeprotonation of the carboxylic acid groups in the polymerchain at the reaction equilibrium and indicates the presenceof free carboxylic acid structures Similar behavior has beendescribed by Zhang et al for lactone ring formation ofgluconic acid where the displacement of C=O chemicalshifts in 13C NMR spectra towards higher frequencies withincreasing pH values (pH region from 2 to 6) has beenassigned to deprotonation of carboxylic groups [12] Thisphenomenon is consistent alsowith previouswork conductedon proteins [18]

The low numerical values of signal deviation (ad) duringmeasurements conducted at neutral to basic environment(pH 7 to 9 acyclic PHA) indicate the domination of thedeprotonated form and thus minor protonation occurrenceIn this pH region practically no lactone rings are presentwhich is denoted by na in Table 1 Due to the slow lactonering formation reaction at very low pH of 1 and 2 there arealso no lactone rings present at the first point of measure-ments (5min) Correspondingly very low numbers will beshown for themeasured rate constants for lactone ring forma-tion at the same pH values in Table 2 Thus the acyclic form(free hydroxyl and carboxyl groups) of PHA is approximatedto be represented by the 1H shifts in the range of 17ndash24 ppmStudying the acidic solutions the signals occurring in therange of 21ndash24 ppm can be assigned for the most part to freeacid groups Correspondingly contributions to the area of24ndash32 ppmare assumed to arise from the presence of lactonerings

Choosing 24 ppm as the integration limit after evaluatingall the individual spectra gives an acceptable semiquantitativerepresentation of the overall lactonefree acid ratio for thekinetic parameter estimation In addition based on the pHdependent behavior described above it can be concluded thatPHA does not build lactone groups between all the availablehydroxyl and carboxyl groups This conclusion is also sup-ported by the estimated equilibrium constant (119870obs) values

as well as the equilibrium conversion values (119883eq) calculatedon the basis of the 119870obs values for PHA lactonization as afunction of pH (Table 2) However when considering the cal-culated equilibrium conversion values the semiquantitativenature of the spectra integration has to be kept in mind

A careful study of the spectral data reveals also the pres-ence of transient signals in the range of ca 24ndash26 ppm pre-sumably by an unknown reaction intermediate (in Figure 4the transient signal reaching an apparentmaximumat 60minat pH 3) Similar phenomenon can be noticed in all the mea-surements throughout the acidic pH range up to ca pH 45ndash5To get additional support for the assumption of the presenceof a reaction intermediate multivariate data analysis byassigning principal component analysis upon the spectraldata has been applied (Section 34)

The impact of the deprotonation of the carboxylic groupson initiating a modification of the entire molecule conforma-tion is raised in the context of explaining the transient signalEarlier studies have reported this kind of behavior for isosac-charinic acid where the deprotonation resulted in alterationsin the hydrogen bonding environment of the hydroxyl groupsattached to adjacent carbons [19] The effect of hydrogenbonds on conformational changes is also addressed in an ear-lier paper studying the effects of the intramolecular hydrogenbonding between the different functional groups of a struc-turally close-related copolymer of reduced PLAC (poly(1-oxa-2-oxobutane-1433-tetrayl) consisting of allyl alcoholcyclic hemiacetal and unreacted 120572-hydroxyacrylate seg-ments [20 21] Based on the behavior of the different -OHgroups in the reduced PLAC molecule primary hemiacetaland tertiary the conclusion drawn by the authors was thefact that intramolecular hydrogen bonds are formed not onlybetween the allyl alcohol OH and the hemiacetal OH but alsobetween the COOminus and the hemiacetal OH groupsThe latterreaction was further confirmed by accomplishing a stablestructure by ab initio calculation between the hemiacetal OHand the adjacent carboxyl group stabilized by 120572-OH groupsthrough their own hydrogen bonds to the carboxyl oxygenand the hemiacetal one

32 Parameter Estimation The rate profile of the lactone ringformation reaction can be described by the following pseudo-first-order equation [13 22]

119889 [PHA]close (119905)119889119905

= 119896 (close) [PHA]open (119905)

minus 119896 (open) [PHA]close (119905) (3)

The observed equilibrium constant 119870obs at a given pHvalue for the conversion of carboxylate to lactone accordingto Figure 1 and (1) can be written as follows

119870obs =[PHA]close[PHA]open

=

119896close119896open (4)

where [PHA]close and [PHA]open are the equilibrium concen-trations of the ring-closed and ring-opened species respec-tively


Table 2 Effect of pH on the rate constants of lactone ring formation (119896(close)) and opening (119896(open)) and the relative standard error of therate constants as well as the corresponding equilibrium conversion (119883eq) for PHA lactonization at 119879 = 247 plusmn 07∘C

pH 119896(close) Standard error 119896(open) Standard error 119870obs 119883eqsminus1 119896(close) sminus1 119896(open) (119896(close))119896(open))

1 114 times 10minus4 32 454 times 10minus5 52 251 7152 109 times 10minus4 35 446 times 10minus5 68 244 7093 191 times 10minus4 33 543 times 10minus5 48 352 7794 171 times 10minus4 45 813 times 10minus5 66 210 67745 178 times 10minus4 62 838 times 10minus5 95 212 6795 892 times 10minus5 66 836 times 10minus5 87 107 5176 452 times 10minus5 110 791 times 10minus5 134 057 363

0 1 2 3 4 5 6 7pH

k(c

lose

) k

(ope

n)

250E minus 04

200E minus 04

150E minus 04

100E minus 04

500E minus 05

000E + 00

Figure 7 Rate constants of the hydrolysis (119896(open) loz) and lactonering formation (119896(close) ◻) as a function of pH at 119879 = 247 plusmn 07∘C

The reaction rate expression of (3) forms an ordinary dif-ferential equation The rate constants 119896(close) and 119896(open)were both estimated for the lactonization and hydrolysisreactions respectively (see Table 2 and Figure 7) The exper-imental data sets were fitted at individual pH values to therate expression by least squares technique of minimizingthe squared difference between the measured and calculatedconcentrations Due to the unacceptable model validity (seeTable 3) the results from measurements in the range of pH7 to 9 are not shown in Table 2 and are thus excluded fromfurther evaluations

The lactone formation is clearly pH dependent in the pHregion of 3 to 6The lactonization rate is highest at pH 3 afterwhich when moving towards higher pH values the rate isdecreasing The highest calculated equilibrium conversionapproximately 78 for the lactonization is also seen at pH 3At low pH values of 1 to 2 the lactonization rate is decreasedagain and seems to lose its pH dependency (see Figure 7) Inthe lower pH region (pH 1-2) presumably the increasingdistance between the functional groups available for lactoni-zation reaction affects the reaction rate significantly leadingto an inconsistent behavior of the reaction rates in that region

The rate of hydrolysis increases slowly throughout the pHregion of 1 to 3 after which a nonlinear stepwise increase inthe rate is seen between pH 3 and 4 Between pH 4 and 6 thehydrolysis proceeds practically at a constant rateThe hydrol-ysis and lactonization reaction rates coincide approximatelyat pH 53

Table 3The correlation of the kineticmodel to themeasured valuesof the reaction conversion at pH 1ndash9

pH Model correlation1 9942 9843 9974 98645 9725 9706 9397 6528 minus119 699

In the neutral and alkaline pH regions the pseudo-first-order kinetic model loses its validity (see Table 3) most likelydue to new reaction mechanisms like base-induced esterhydrolysis introduced in the system and correspondingly theconsistency of the rate constants is decreased It is howeverevident that the hydrolysis rate ismarkedly increased betweenpH 7 and 9 and that the rate of lactonization approacheszero at high pHvalues Alkaline-catalyzed hydrolysis reactiongenerally takes over at clearly alkaline surroundings Thisreaction is irreversible since once the acid intermediate isformed it is immediately converted to the carboxylate anionwhich is not further attacked by [OHminus] As a result thereaction goes to completion in the direction of hydrolysiswith accelerated rate as alkalinity increases [23]

The conversion of the free acid form to lactone is thusdominating until the pH region of approximately 4 to 5 yetthe rates are very low No evident steric hindrance to explainthe rate behavior (Table 2) can be seen Hence the low reac-tion rates in either direction together with the relatively weakpH dependency suggest an addition-elimination process (seeFigure 8) as the dominating reaction mechanism [23] Thereaction is assumed to take place through a slow protontransition from hydroxyl to carboxylic oxygen via a dipolarintermediate followed by elimination of H

2O giving the final

lactone The simultaneously occurring hydrolysis and lacto-nization reactions hence give rise to an unstable equilibriumstate and variation in the ionization degree locally along thepolymer chain

The rather reversible addition-elimination reaction is con-ventionally assumed to take place in the vicinity of neutral


Table 4 PCA parameters describing the dominating spectral characteristics in the 1H NMR domain of lactone formation of PHA in therange of pH 1 to 5

pHExplained by

PC1Explained by

PC2sumExplainedPC1 + PC2 Chemshift PC1 Chemshift PC2 Time at max

PC1Time at max

PC2 ppm ppm min min

1 885 81 966 2767 2442 1600 902 875 101 976 2761 2416 1720 903 826 124 950 2742 2580 1500 504 779 180 959 2717 2397 1600 6045 794 175 969 2636 2335 1600 605 861 93 954 2561 2297 1600 90

OH

OHOHOHO

O

H

OO

O

HO2C CO2H

+H2O

OH

OH

HO2C CO2H

HO2C OH

OH

OHOH

OH

CO2H

HO2C OH CO2H

Ominus+

HO

Figure 8 Addition-elimination lactonization mechanism suggested for PHA in the pH region 1 to 6

pH region with practically no free OHminus andH+ participatingin the reactions [23] Hence an acid catalyzed reversible lac-tonization would also be possible In the studies consideringcamptothecin both pH dependent as well as pH independentring-closure reactions have been assumed to occur the latterof which taking place at pH gt65 [11 13] The hydroniumion-dependent ring closing was reported to probably occurby acid-catalyzed reaction mechanism of the carboxylic acidat pH lt35 However the reaction was in this case foundstrongly pH dependent which gave support to the acid-catalyzed mechanism Yet the presence of a possible transi-tion state for the lactonizationhydrolysis was also assumedwhere the 120572-hydroxyl group would be involved in one of theproton-transfer steps or stabilizing the transition state throughhydrogen bonding (-O-H-OH) between the 120572-hydroxyl andanother hydroxyl group before the completion of the lactonering formation In the present work no strong pH depen-dency of reaction rates was seen but the rates measured are ingood accordancewith previous studies based on similar cycliclactone structures of gluconic acid or its derivatives thus sup-porting the addition-elimination approach [12 24] Further-more the formation of a transient tetrahedral intermediateresulting from an analogous rate-limiting nucleophilic attackof hydroxide has been discussed in the context of anotherrelated compound D-glucuno-120575-lactone [25]

33 Model Evaluation The plots with measured lactonecontents fitted to the model depict an acceptable fit in the pHregion from 1 to 6 The correlation coefficients for the kineticmodel are given in Table 3 and fits of themeasurements at pH1 and 2 are depicted as examples in Figure 9

34 Multivariate Data Analysis by PCA In the first stageusing principal component analysis on the spectral 1HNMRdata gives indication of the number and the relative contri-bution of the so-called principal components (PCs) neededto build the multivariate PCA model of the reaction studiedIn the following stage the dominating chemical shifts canbe identified thus promoting verification of the spectralassignment shown in Table 1 Incorporation of the timedimension enables also the comparison to the reaction rateparameters gained by parameter estimation

The reliability of the PC analysis is described by theexplanatory values of the principal component planes (seeTable 4) In the cases of pH 1 to 5 PC1 and PC2 planes are ableto capture 95 or more (Explained by PC1 + PC2 in Table 4)of the variance of these data set points which represents ananalysis of high reliability Adding several components wouldnot be meaningful since the predictive ability would notincrease in a significant manner and no further correlationto the chemical behavior could be identified


0 2 4 6 8 10

10

0

20

30

40

50

60

70

80La

cton

e con

tent

(mol

-)

pH 1 10

20

30

40

50

60

70

80

00

2 4 6 8 10 12

Lact

one c

onte

nt (m

ol-

)

pH 2

Time Time (s times104) (s times104)

Figure 9 Experimental lactone conversion fitted to the kinetic model at pH 1 and 2

Table 4 summarizes the essential characteristic values ofthe PCA model of the lactonization reaction

Graphical illustration of the model serves as a tool todisclose the interdependences of the components Out ofthe two components created PC1 component is clearlydominating PC2 in terms of the degree of explanation ofthe model (Table 4 explained by PC1) In other words PC1describes to a large extent the total reaction propagationtowards the evident end product in the spectral data here thearea covering the chemical shifts of lactone ring structuresThus it can well be concluded that PC1 represents the mainchemical reaction of converting free carboxylic acid andhydroxyl groups into lactone ring structures This is stronglysupported by simultaneous inspection of the time stamps(Table 4 time at max PC1) and the dominating chemicalshifts (Table 4 chem shift PC1) of the measurements indi-cating that themaximumvalue of PC1 component is achievedat the end of the measurement with dominating 1H NMRsignals appearing in the range of 256ndash276 ppm dependingon pH

The corresponding evaluation of PC2 component indi-cates a strong linkage to a less dominating reaction takingplace during the main one The data analysis reveals thatchemical shifts appearing at approximately 23ndash26 ppm (seeTable 4 chem shift PC2) are not directly contributed bythe end product lactone ring but are also attributed to atemporary presence of an intermediate chemical structureapproaching lactone ring formation A clear pH dependencycan again be identified to the degree of explanation referringto increasing signal intensities with increasing pH Themaximum in the intermediate reaction is reached in thepH range from 4 to 45 with the corresponding degrees ofexplanation of 180 and 175 respectively The data analysisreinforces the earlier speculations about an intermediatestructure appearance in the chemical shift range of ca 23ndash26 (Table 4 chem shift PC2) where a transient signal couldbe identified (see Section 31 NMR) Additionally the timestamps (Table 4 time at max PC2) for the maximal PC2appearance coincide well with the visual inspections of thesignal maxima in the 1H NMR spectra

Thedominance of PC1 component can be seen to decreaseas a function of pH until 45 while that of PC2 componentincreases This is in good accordance with the earlier visualinterpretations of the presence of a transient intermediatestructure along the main lactone formation reaction

Regardless of the temporary presence of the intermediatesubstance the explanatory degrees of the PCAmodel and thekineticmodel correlation values coincidewell in the pH rangefrom 1 to 5 This is probably due to the fact that the transientreaction intermediate is entirely converted into the end prod-uct and thus the net effect of the intermediate contribution iszero when the overall molar balance is calculatedThe kineticmodel correlation reaches still a satisfactory value at pH 6while the explanatory degree of the PCAmodel drops alreadyunder 90

This study presents thus evidence on the presence of anearlier unknown reaction intermediate during the lactoneformation reaction of PHA Due to the transient and highlypH dependent nature of the polymeric substance appear-ance a complete chemical identification of the substance ischallenging Prior studies on structurally related substancesjustify however the speculation on the structure indicatingthe creation of a temporary stabilized lactone ring intermedi-ate where hydrogen bonding and the vicinity of 120572-hydroxidegroups probably play a crucial role

4 Conclusions

A new method based on in situ 1H NMR spectroscopywas successfully developed for the follow-up of the lactoneformation reaction of PHA Kinetic parameter estimation ofthe experimental data was made by classic data fitting basedon least squares method In addition PCA was successfullyutilized in assigning chemical shifts to the reaction compo-nents and in the further reaction mechanism elucidation

The pseudo-first-order reaction kinetic model suggesteddescribes well the lactonization aswell the hydrolysis reactionin the pH region from 1 to 6 The model validity is decreasedsignificantly above pH 6 as more than one reaction mecha-nism begins to prevail simultaneously Due to very low rates


of lactonization as well as the weak pH dependency of thereaction an addition-elimination mechanism is proposedThe conversion of the free acid to the lactone is dominatinguntil the pH range of approximately 4 to 5 Intermolecularester formation could not be detected in significant extent

The protonation and deprotonation of the carboxylgroups are inducing pH dependency to the chemical shiftsin the 1H NMR spectral domain A possible linkage toaltered molecule conformation and thus increased hydrogenbond creation between carboxylates and hydroxides andorbetween hydroxide groups is raised also due to indicationsof a transient reaction intermediate structure appearance inthe 1HNMR spectraThe presence of a reaction intermediateis further reinforced by PCA indicators confirming also itsmaximal contribution to the reaction around pH 40ndash45Furthermore the maximal rate constant of the lactonizationreaction appearing at pH 3 according to classic modelingcoincides well with the PCA time stamp data The centralparticipation of the 120572-hydroxyl groups in the transition stateformation through hydrogen bonding is addressed in relatedstudies

Reaction models described by classic kinetic modelingand independent multivariate principal component analysiscoincide well in terms of model validity in the pH range from1 to 5

Considering the slow rate of lactone ring formationof PHA in terms of practical stabilization applications ofhydrogen peroxide the adverse effects are mainly viable inlong-term stabilization below neutral pH domain At theconventional alkaline pH range attributed tomechanical pulpbleaching the dominating reaction indicated here appearsto be hydrolysis while the rate of lactonization seemsto approach zero However considering the limited masstransfer during for example high-consistency bleaching theeffect of pH gradients occurring during long residence timesshould not be omitted when evaluating the performance ofPHA in the stabilization of H

2O2

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

References

[1] M Petit-Conil ldquoEfficient silicate-free hydrogen peroxide bleach-ing of mechanical pulpsrdquo Association Technique de LrsquoIndustriePapetiere vol 5 pp 22ndash31 2004

[2] J Mulders and J Gilain ldquoBiodegradation of polymeric build-ersmdashexperiments with a C-14 labelled sodium poly(120572-hydrox-yacrylate)rdquoWater Research vol 11 no 7 pp 571ndash574 1977

[3] M Komaki S K Kim and T Hashimoto ldquoFatty acid soil deter-gency performance of poly(sodium 120572-hydroxyacrylate)rdquo Jour-nal of Surfactants and Detergents vol 5 no 1 pp 25ndash31 2002

[4] B Lonnberg ldquoFiberbearbetning blekningrdquo in TraforadlingensKemiska Processer pp 39ndash40 Abo Akademi University TurkuFinland 3rd edition 1998

[5] J Sundholm ldquoBleachingrdquo in Mechanical Pulping PapermakingScience and Technology J Gullichsen and H Paulapuro Eds p319 Tappi Press Finnish Paper Engineersrsquo Association 1999

[6] Y Ni Y Ju and H Ohi ldquoFurther understanding of the manga-nese-induced decomposition of hydrogen peroxiderdquo Journal ofPulp and Paper Science vol 26 no 3 pp 90ndash94 2000

[7] H Koskinen A SundquistMHahkio and T Vuorinen ldquoChar-acterization of functional properties of poly-(120572-hydroxyacrylicacid) in mechanical pulp bleaching conditionsrdquo Nordic Pulp ampPaper Research Journal vol 24 no 1 pp 12ndash18 2009

[8] K Yamazawa S Kawauchi andM Satoh ldquoLactone ring forma-tion in poly-(120572-hydroxyacrylic acid)rdquo Journal of Polymer Sci-ence Part B Polymer Physics vol 40 no 13 pp 1400ndash14052002

[9] T Tamura S Kawauchi M Satoh and J Komiyama ldquoInfraredspectroscopic study and ab initio calculation for dissociationof poly(120572-hydroxy acrylic acid) in aqueous solutionsrdquo Polymervol 38 no 9 pp 2093ndash2098 1997

[10] T Tamura H Uehara K Ogawara S Kawauchi M Satoh andJ Komiyama ldquoDissociation behavior of poly(120572-hydroxy acrylicacid)rdquo Journal of Polymer Science Part B Polymer Physics vol37 no 13 pp 1523ndash1531 1999

[11] J Fassberg and V J Stella ldquoA kinetic and mechanistic study ofthe hydrolysis of camptothecin and some analoguesrdquo Journal ofPharmaceutical Sciences vol 81 no 7 pp 676ndash684 1992

[12] Z Zhang P Gibson S B Clark G Tian P L Zanonato andL Rao ldquoLactonization and protonation of gluconic acid a ther-modynamic and kinetic study by potentiometry NMR and ESI-MSrdquo Journal of Solution Chemistry vol 36 no 10 pp 1187ndash12002007

[13] B A Hanson R L Schowen and V J Stella ldquoA mechanisticand kinetic study of the E-ring hydrolysis and lactonization ofa novel phosphoryloxymethyl prodrug of camptothecinrdquo Phar-maceutical Research vol 20 no 7 pp 1031ndash1038 2003

[14] M EAmatoGAnsanelli S Fisichella et al ldquoWheat flour enzy-matic amylolysis monitored by in situ 1H NMR spectroscopyrdquoJournal of Agricultural and Food Chemistry vol 52 no 4 pp823ndash831 2004

[15] M Maiwald H H Fischer Y-K Kim K Albert and H HasseldquoQuantitative high-resolution on-line NMR spectroscopy inreaction and process monitoringrdquo Journal of Magnetic Reso-nance vol 166 no 2 pp 135ndash146 2004

[16] HHaarioModestUserrsquos Guide ProfMathOyHelsinki Finland1st edition 2006

[17] L Eriksson E Johansson N Kettaneh-Wold J Trygg C Wik-strom and SWoldMulti- andMegavariate Data Analysis PartI Basic Principles and Applications Umetrics AB Umea Swe-den 2nd edition 2006

[18] D E Anderson J Lu L McIntosh and F W Dahlquist NMRof Proteins CRC Press Boca Raton Fla USA 1st edition 1993

[19] H Cho D Rai N J Hess Y Xia and L Rao ldquoAcidity and struc-ture of isosaccharinate in aqueous solution a nuclear magneticresonance studyrdquo Journal of Solution Chemistry vol 32 no 8pp 691ndash702 2003

[20] K Yamazawa T Sunohara andM Satoh ldquoPreparation of a newhydrophilic copolymer from a polylactone and the structureanalysesrdquo Journal of Molecular Structure vol 690 no 1ndash3 pp121ndash129 2004


[21] T Sunohara and M Satoh ldquoSpecific dissociation behaviorof a polycarboxylate having three kinds of hydroxyl groupsrdquoPolymer International vol 54 no 6 pp 926ndash932 2005

[22] V A Oleınikov O A Ustinova K E Mochalov et al ldquoKineticsof the lactone-carboxylate transition of hybrid camptothecin-netropsin moleculesrdquo Biofizika vol 48 no 3 pp 436ndash4422003

[23] J D Roberts and M C Caserio Basic Principles of OrganicChemistry WA Benjamin Menlo Park Calif USA 2nd edi-tion 1977

[24] C L Combes and G G Birch ldquoInteraction of D-glucono-15-lactone with waterrdquo Food Chemistry vol 27 no 4 pp 283ndash2981988

[25] Y Pocker and E Green ldquoHydrolysis of D-glucono-delta-lac-tone I General acid-base catalysis solvent deuterium isotopeeffects and transition state characterizationrdquo Journal of theAmerican Chemical Society vol 95 no 1 pp 113ndash119 1973

Submit your manuscripts athttpwwwhindawicom

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CorrosionInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Polymer ScienceInternational Journal of



CeramicsJournal of


CompositesJournal of

NanoparticlesJournal of



International Journal of

Biomaterials


NanoscienceJournal of

TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of

NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

CrystallographyJournal of


The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014


CoatingsJournal of

Advances in

Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Smart Materials Research



MetallurgyJournal of


BioMed Research International

MaterialsJournal of


Nano

materials


Journal ofNanomaterials


O

O

OH

OH

H+ +HCO2

minusminusO2C

Figure 1 The chemical structure of poly(120572-hydroxyacrylic acid)PHA with an intramolecular lactone ring

The build-up of lactone ring structures has an effect onthe affinity of PHA towards metal species in stabilization ofhydrogen peroxide According to our previous study it ishowever likely that nonionic interaction routes betweenPHAand species of for example manganese and iron prevail aswell In addition some differences of preference such as infavor of iron over manganese at acidic pH have been noticed[7] Despite the targeted high alkalinity of the mechanicalpulp bleaching process there are also strong local pH gradi-ents especially in high consistency bleaching due to inefficientmass transfer Depending for example on the chemical dos-ing sequence the stabilizer may have a long residence timeunder process conditions well below the target pHThereforeit is of importance to study the kinetic behavior of the lacto-nization and hydrolysis reactions of PHA in more detail as afunction of pH

The goal of the present research was (1) to investigatethe rate of the lactone ring build-up of poly(120572-hydroxyacrylicacid) as a function of pH by means of 1HNMR spectroscopyas well as other complementary methods of NMR and (2)to estimate the kinetic parameters for the lactonization rateexpression in the following

[PHA]open119896(close)997888997888997888997888997888rarrlarr997888997888997888997888997888

119896(open)[PHA]close (1)

where

[PHA]close + [PHA]open = 1 (2)

So far the only published spectroscopic studies of thechemical structure of PHA consider the static reactionequilibrium results [8 9] Thus no kinetic data consideringthe structural dynamics of the reaction have been presentedutilizing any kind of analytical method The published acid-base titrations indicated extraordinary dissociation behaviorin terms of large variation in the dissociation constantsbut left the reasoning without evidence in context of thecorresponding simultaneous state of the chemical structure[7 10] A kineticmodel covering the lowpHregion is essentialfor obtaining a better understanding of the lactone build-upmechanism (intramolecular versus intermolecular esterifica-tion reaction route) Moreover knowledge of the time scaleof the lactonization reaction provides valuable informationto be utilized especially in the high-consistency bleachingprocesses as well as in gaining more information of thefunctionality of PHA in other chemical applications

Spectroscopic methods (NMR and UV) have successfullybeen applied in many earlier kinetic studies also related to

lactone formation reactions within various compounds [11ndash14] In addition both qualitative and quantitative kineticstudies have been conducted to gain information on com-plex reacting multicomponent mixtures by utilizing NMRspectroscopy [15] However considering the nature of thepolymeric analyte of the current study as well as the chem-ical environment of an electrolyte the spectroscopic datais expected to spread out and have overlap between theobserved components Thus to verify and elucidate the uni-variate approach of classic kinetic modeling by parameterestimation multivariate data analysis has been applied

2 Experimental Section

21 Analytical Methods The propagation of the lactone ringformation reaction was detected and quantified by 1H NMRspectroscopy (Bruker Avance II 400MHz) with D

2O as

solvent and using a 5mm BBO probe Pulse delay was 2 s andwidth was 30∘ The spectral width applied was 8 kHz

22 Experimental Plan The kinetic measurements coveredsamples in the pH range from 1 to 9 with the interval ofone pH unit In addition one additional sample was tested atpH 45 to identify the possible turning point in terms of thereaction mechanism All the measurements were conductedat room temperature (119879 = 247 plusmn 07∘C)

23Reagents The aqueous solution of the sodium salt of poly(120572-hydroxyacrylic acid) (30 NaPHA Mw = 30 000 gmol) wasprovided by Kemira Oyj The hydrochloric acid used was acommercial product of analytical grade and it was used asacquired at a proper dilution by D

2O without further purifi-

cation

24 Experimental Procedure An aqueous stock solutioncontaining 063wt- PHA polymer was prepared by dilutionof the reagent solution (30 NaPHA pH 95) with D

2O

The individual sample solutions were prepared by weighingapproximately 7 g of the stock solution followed by pHadjustment with hydrochloric acid (01 30 dependingon the target pH) after which the 1H NMR measurementwas started immediately The scans were recorded in thebeginning of the measurement every five minutes until 20minutes after which the interval was increased to 10minutesBetween one and four hours of measurement time the scanswere recorded every 30 minutes The pH was measured inparallel with the scans using an identical reference solutionThe pH values given in the present work for all individualmeasurements correspond to the initial values of each sam-ple The measurement was stopped when the reaction hadreached the equilibrium All the measurements fulfilled thecondition 119905Eq gt 10 times 11990512 where 119905Eq denotes the point in timewhen the equilibrium has been reached and 119905

12denotes that

when the conversion of the reaction is at half way [11]

25 Data Analysis The parameter estimation discussed inmore detail later on was conducted by Modest (MOdelESTimation) software utilizing Matlab 61 interface and For-tran 77-based code Modest program has been designed for


4

4

3

3

2

2

1

1

F2 (ppm)

F1

(ppm

)


4

4

3

3

2

2

1

1

F2 (ppm)

F1

(ppm

)




shifts (ppm)















0

2

4

6

2 22 24 26 28 3

Rela

tive s

igna

l int

ensit

y


times103





0

2

4

6

8

10

12

14

2 22 24 26 28 3

Rela

tive s

igna

l int

ensit

y


times103


02468

1012141618

2 22 24 26 28 3

Rela

tive s

igna

l int

ensit

y


times103






ppm ad









119889 [PHA]close (119905)119889119905

= 119896 (close) [PHA]open (119905)




=

119896close119896open (4)






0 1 2 3 4 5 6 7pH

k(c

lose

) k

(ope

n)

250E minus 04

200E minus 04

150E minus 04

100E minus 04

500E minus 05

000E + 00









2O giving the final





pHExplained by

PC1Explained by


PC1Time at max

PC2 ppm ppm min min

1 885 81 966 2767 2442 1600 902 875 101 976 2761 2416 1720 903 826 124 950 2742 2580 1500 504 779 180 959 2717 2397 1600 6045 794 175 969 2636 2335 1600 605 861 93 954 2561 2297 1600 90

OH

OHOHOHO

O

H

OO

O

HO2C CO2H

+H2O

OH

OH

HO2C CO2H

HO2C OH

OH

OHOH

OH

CO2H

HO2C OH CO2H

Ominus+

HO







0 2 4 6 8 10

10

0

20

30

40

50

60

70

80La

cton

e con

tent

(mol

-)

pH 1 10

20

30

40

50

60

70

80

00

2 4 6 8 10 12

Lact

one c

onte

nt (m

ol-

)

pH 2









4 Conclusions








2O2



References


































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




4

4

3

3

2

2

1

1

F2 (ppm)

F1

(ppm

)


4

4

3

3

2

2

1

1

F2 (ppm)

F1

(ppm

)




shifts (ppm)















0

2

4

6

2 22 24 26 28 3

Rela

tive s

igna

l int

ensit

y


times103





0

2

4

6

8

10

12

14

2 22 24 26 28 3

Rela

tive s

igna

l int

ensit

y


times103


02468

1012141618

2 22 24 26 28 3

Rela

tive s

igna

l int

ensit

y


times103






ppm ad









119889 [PHA]close (119905)119889119905

= 119896 (close) [PHA]open (119905)




=

119896close119896open (4)






0 1 2 3 4 5 6 7pH

k(c

lose

) k

(ope

n)

250E minus 04

200E minus 04

150E minus 04

100E minus 04

500E minus 05

000E + 00









2O giving the final





pHExplained by

PC1Explained by


PC1Time at max

PC2 ppm ppm min min

1 885 81 966 2767 2442 1600 902 875 101 976 2761 2416 1720 903 826 124 950 2742 2580 1500 504 779 180 959 2717 2397 1600 6045 794 175 969 2636 2335 1600 605 861 93 954 2561 2297 1600 90

OH

OHOHOHO

O

H

OO

O

HO2C CO2H

+H2O

OH

OH

HO2C CO2H

HO2C OH

OH

OHOH

OH

CO2H

HO2C OH CO2H

Ominus+

HO







0 2 4 6 8 10

10

0

20

30

40

50

60

70

80La

cton

e con

tent

(mol

-)

pH 1 10

20

30

40

50

60

70

80

00

2 4 6 8 10 12

Lact

one c

onte

nt (m

ol-

)

pH 2









4 Conclusions








2O2



References


































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials









0

2

4

6

2 22 24 26 28 3

Rela

tive s

igna

l int

ensit

y


times103





0

2

4

6

8

10

12

14

2 22 24 26 28 3

Rela

tive s

igna

l int

ensit

y


times103


02468

1012141618

2 22 24 26 28 3

Rela

tive s

igna

l int

ensit

y


times103






ppm ad









119889 [PHA]close (119905)119889119905

= 119896 (close) [PHA]open (119905)




=

119896close119896open (4)






0 1 2 3 4 5 6 7pH

k(c

lose

) k

(ope

n)

250E minus 04

200E minus 04

150E minus 04

100E minus 04

500E minus 05

000E + 00









2O giving the final





pHExplained by

PC1Explained by


PC1Time at max

PC2 ppm ppm min min

1 885 81 966 2767 2442 1600 902 875 101 976 2761 2416 1720 903 826 124 950 2742 2580 1500 504 779 180 959 2717 2397 1600 6045 794 175 969 2636 2335 1600 605 861 93 954 2561 2297 1600 90

OH

OHOHOHO

O

H

OO

O

HO2C CO2H

+H2O

OH

OH

HO2C CO2H

HO2C OH

OH

OHOH

OH

CO2H

HO2C OH CO2H

Ominus+

HO







0 2 4 6 8 10

10

0

20

30

40

50

60

70

80La

cton

e con

tent

(mol

-)

pH 1 10

20

30

40

50

60

70

80

00

2 4 6 8 10 12

Lact

one c

onte

nt (m

ol-

)

pH 2









4 Conclusions








2O2



References


































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials






ppm ad









119889 [PHA]close (119905)119889119905

= 119896 (close) [PHA]open (119905)




=

119896close119896open (4)






0 1 2 3 4 5 6 7pH

k(c

lose

) k

(ope

n)

250E minus 04

200E minus 04

150E minus 04

100E minus 04

500E minus 05

000E + 00









2O giving the final





pHExplained by

PC1Explained by


PC1Time at max

PC2 ppm ppm min min

1 885 81 966 2767 2442 1600 902 875 101 976 2761 2416 1720 903 826 124 950 2742 2580 1500 504 779 180 959 2717 2397 1600 6045 794 175 969 2636 2335 1600 605 861 93 954 2561 2297 1600 90

OH

OHOHOHO

O

H

OO

O

HO2C CO2H

+H2O

OH

OH

HO2C CO2H

HO2C OH

OH

OHOH

OH

CO2H

HO2C OH CO2H

Ominus+

HO







0 2 4 6 8 10

10

0

20

30

40

50

60

70

80La

cton

e con

tent

(mol

-)

pH 1 10

20

30

40

50

60

70

80

00

2 4 6 8 10 12

Lact

one c

onte

nt (m

ol-

)

pH 2









4 Conclusions








2O2



References


































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials







0 1 2 3 4 5 6 7pH

k(c

lose

) k

(ope

n)

250E minus 04

200E minus 04

150E minus 04

100E minus 04

500E minus 05

000E + 00









2O giving the final





pHExplained by

PC1Explained by


PC1Time at max

PC2 ppm ppm min min

1 885 81 966 2767 2442 1600 902 875 101 976 2761 2416 1720 903 826 124 950 2742 2580 1500 504 779 180 959 2717 2397 1600 6045 794 175 969 2636 2335 1600 605 861 93 954 2561 2297 1600 90

OH

OHOHOHO

O

H

OO

O

HO2C CO2H

+H2O

OH

OH

HO2C CO2H

HO2C OH

OH

OHOH

OH

CO2H

HO2C OH CO2H

Ominus+

HO







0 2 4 6 8 10

10

0

20

30

40

50

60

70

80La

cton

e con

tent

(mol

-)

pH 1 10

20

30

40

50

60

70

80

00

2 4 6 8 10 12

Lact

one c

onte

nt (m

ol-

)

pH 2









4 Conclusions








2O2



References


































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials





pHExplained by

PC1Explained by


PC1Time at max

PC2 ppm ppm min min

1 885 81 966 2767 2442 1600 902 875 101 976 2761 2416 1720 903 826 124 950 2742 2580 1500 504 779 180 959 2717 2397 1600 6045 794 175 969 2636 2335 1600 605 861 93 954 2561 2297 1600 90

OH

OHOHOHO

O

H

OO

O

HO2C CO2H

+H2O

OH

OH

HO2C CO2H

HO2C OH

OH

OHOH

OH

CO2H

HO2C OH CO2H

Ominus+

HO







0 2 4 6 8 10

10

0

20

30

40

50

60

70

80La

cton

e con

tent

(mol

-)

pH 1 10

20

30

40

50

60

70

80

00

2 4 6 8 10 12

Lact

one c

onte

nt (m

ol-

)

pH 2









4 Conclusions








2O2



References


































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




0 2 4 6 8 10

10

0

20

30

40

50

60

70

80La

cton

e con

tent

(mol

-)

pH 1 10

20

30

40

50

60

70

80

00

2 4 6 8 10 12

Lact

one c

onte

nt (m

ol-

)

pH 2









4 Conclusions








2O2



References


































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials








2O2



References


































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials
















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials



research article intramolecular lactonization of poly...

Documents