Stereoselective nucleophilic substitution of poly(vinyl chloride) with potassium...

Stereoselective Nucleophilic Substitution of Poly(vinylchloride) with Potassium 4-Acetamidothiophenolate

GERARDO MARTINEZ, JOSE LUIS MILLAN

Instituto de Ciencia y Tecnologıa de Polımeros, Consejo Superior de Investigaciones Cientıficas, Juan de la Cierva 3,28006 Madrid, Spain

Received 11 September 2003; accepted 4 December 2003

ABSTRACT: The nucleophilic substitution reaction of poly(vinyl chloride) (PVC) withpotassium 4-acetamidothiophenolate was performed in a cyclohexanone solution. Thequantitative microstructural analysis, as a function of the conversion, was followed by13C NMR spectroscopy. Through a comparison of the microstructural changes with thedegree of substitution, a small fraction of mmr tetrads was found to react occasionallywith the central chlorine of the mr triad instead of the mm, such as for sodiumbenzenethiolate (NaBT). This conclusion was confirmed by Fourier transform infraredresults. However, unlike NaBT, the evolution of the glass-transition temperature (Tg)with the degree of conversion changed with the degree of substitution similarly to theratio of the extents to which mmr and rrmr structures intervened in the substitutionreaction. From these studies, it followed that the specific interactions due to the polarnature of the nucleophile enhanced the molecular-microstructure-based mechanisms,which were responsible for Tg. Such a novel quantitative correlation, compared withmore tentative ones obtained previously, presents valuable insight into the role of thestereochemical microstructure in the glass-transition process in PVC. © 2004 WileyPeriodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1857–1867, 2004Keywords: poly(vinyl chloride) (PVC); modification; microstructure; stereoselectivesubstitution; local configurational mechanisms; glass transition

INTRODUCTION

The chemical modification of poly(vinyl chloride)(PVC) has been the object of numerous studiesbecause of its polyhalogenated molecular struc-ture. To improve the specific properties of thepolymer, researchers have carried out various re-actions that have a large number of applica-tions.1–7 However, the possibility of equality inthe reactivity between all the PVC units for agiven nucleophile has never been considered, andas a result, no discriminated substitution hasbeen achieved.

In recent years, an extensive research pro-gram has been developed in our laboratory thatis designed to demonstrate the prominent roleof the microstructure-dependent local confor-mation in the mechanisms of the modificationreactions of PVC.8 –13 The nucleophilic substitu-tion of chlorine in PVC in dilute solutions hasbeen widely studied to analyze the reaction as afunction of different parameters: the solventtype, temperature, polymer type, and nucleo-phile.11,14 –16 Basically, it has been demon-strated that (1) the reaction proceeds via thelast triad of isotactic sequences, that is, themmr tetrad, or via the heterotactic triad adja-cent to syndiotactic sequences, that is, the rrmrpentad, exclusively and (2) for these structuresto react, the mm and rm triads must adopt the

Correspondence to: G. Martınez (E-mail: [email protected])Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 42, 1857–1867 (2004)© 2004 Wiley Periodicals, Inc.

1857

GTTG� or GTTT conformation, respectively,which are, in fact, very reactive species.

Our findings8–16 have allowed us to proposethe following stereospecific mechanisms, whichare illustrated in Scheme 1. Mechanism A appliesto the reaction by the mm triad of mmr and in-volves the simultaneous disappearance of one iso-tactic triad and one heterotactic triad by an ex-change with one nucleophile-centered triad andone cosyndiotactic triad, respectively. MechanismB applies to substitution by the rm triad of rrmrand involves the elimination of a single heterotac-tic triad. Finally, mechanism C relates to a well-defined fraction of an mmr tetrad, which reactswith the mr triad instead of the mm triad, andinvolves the disappearance of one isotactic triadwithout yielding either the appearance of one syn-diotactic triad (like mechanism A) or the disap-pearance of one heterotactic triad (like mecha-nism B). Thus, mechanism C can be viewed as aspecific case of mechanism B because both may beassumed to depend mainly on the associated syn-diotactic sequence.8–13

These mechanisms disagree with other au-thors’ proposals that some abnormal structures,such as allylic chlorine and tertiary chlorine,which might arise incidentally during the poly-merization process, are unique reactive struc-tures in substitution reactions in PVC.17–19 Thefact that SN2 substitution proceeds throughmmr[GTTG�] is supported by both the NMR as-signments and the extremely high reactivity of

mmr[GTTG�TT], as implied by our work. The 13CNMR measurements refer exclusively to assign-ments of triads and cannot be affected by theevolution of either allylic or tertiary chlorines, asclaimed by these authors. However, mmr-[GTTG�TT] has been shown to be the only struc-ture to satisfy all the SN2 substitution require-ments,8 unlike the other conformations and al-lylic and tertiary chlorines. Moreover, the contentof mmr[GTTG�TT] has been shown to be muchgreater than that of the overall abnormal struc-tures, as calculated by Hjertbeg and Sorvik.17

On the basis of the aforementioned reasoning, wehave endeavored to investigate the effect of the tac-ticity-dependent microstructure on the chemical re-activity and to correlate the changes in the molec-ular microstructure with the evolution of somephysical properties of PVC with the object of pro-viding some basic support for the molecular natureof the mechanisms involved in the physical or phys-icochemical behavior of polymer materials.

In the course of these investigations, we discov-ered that the stereospecific nature of the nucleo-philic substitution reaction in PVC can be en-hanced if we increase the bulk of the nucleophile(sodium 2-mercaptobenzothiazolate10 or potas-sium 2-ethylhexylthioglycolate12), which providesthe appearance of steric hindrance, thereby eitherpreventing the attack by the heterotactic triad mrof the mmr tetrad (mechanism C) or renderingmore difficult the attack by the rm triad in therrmr pentad (mechanism B).

Scheme 1

1858 MARTINEZ AND MILLAN

However, we found that the evolution of theratio of mmr to rrmr with the degree of substitu-tion may be controlled straightforwardly if wechange the nature of the solvent or the additivesfor the reactions carried out in solution16 or in themelt,20 respectively. The basis of such behaviorlies in the fact that the molecular interactionsbetween the reactive structures and a few sol-vents, esters, and polyesters are different in bothnature and strength. These additives are able tospecifically complex the mmr structures and adoptthe GTTG�TT conformation, thereby making it pos-sible to displace the conformational equilibrium GT-GTTT % GTTG�TT, which usually lies on the leftside, toward the GTTG�TT conformation.21

In light of these results, we became interestedin exploring the efficiency of the stereoselectivenature of the reaction with a nucleophile with ahigh capacity to interact with hydrogen atoms. Inthis work, we first investigated the extent towhich a 4-acetamidothiophenolate anion couldproduce stereocontrol of the reaction based on thePVC–nucleophile interaction of a local conforma-tional nature; second, we examined the changesin the glass-transition temperature (Tg) with theconversion, with the aims of discriminating be-tween the contributions of the microstructure andthe nucleophile through the changes in Tg and ofproviding new evidence for the involvement of themolecular microstructure in the physicochemicalprocesses controlling Tg.

EXPERIMENTAL

Materials

The PVC sample was prepared in bulk at 90 °Cwith 2,2�-azoisobutyronitrile (98%; Fluka) as the

initiator system and was taken to a 20% conver-sion. The number-average molecular weight(21,500) was determined at 34 °C in cyclohex-anone (CH; Ferosa) with a Knauer membraneosmometer. CH was purified by fractional distil-lation under nitrogen. Tetrahydrofuran (THF)was distilled under nitrogen with aluminum lith-ium hydride (Aldrich) for the removal of peroxidesimmediately before use. 4-Acetamidothiophenol(AT), provided by Aldrich, was used as received.Equimolecular amounts of AT and potassium car-bonate (K2CO3) were used to form 4-acetamido-thiophenolate anion in situ in CH.

Substitution reaction of PVC with potassium 4-acetamidothiophenolate (KAT)

PVC (2.0 g, 32 mmol, based on the monomericunit) was dissolved in 83 mL of CH, and 4.28 g (31mmol) of K2CO3 and 5.34 g (32 mmol) of AT wereadded to the polymer solution. The mixture wasstirred and heated at 30 °C under an inert atmo-sphere. At the appropriate reaction times, thesamples were precipitated in methanol–watermixtures, purified in THF/methanol–water as asolvent/precipitant system, and dried at 40 °C invacuo. The substituted PVC samples were char-acterized by UV spectroscopy (the 270-nm bandwas followed) and by NMR to determine the de-gree of substitution. 1H NMR spectra were ob-tained on a Varian Inova 300 300-MHz spectrom-eter with dioxane-d8 (10 wt % solutions) at 80 °Cunder standard conditions. The proton solventsignal was used as a chemical-shift marker. Therelative signal intensities of the spectra weremeasured from the integrated peak areas, whichwere calculated with an electronic integrator:

NUCLEOPHILIC SUBSTITUTION OF PVC WITH KAT 1859

13C NMR spectroscopy

The tacticity of both the starting and modifiedpolymers was measured with 13C NMR spectros-copy; the decoupled spectra were obtained at 80°C on a Varian Unity instrument operating at 125MHz with dioxane-d8 as a solvent. The spectralwidth was 2500 MHz, the pulse repetition ratewas 3 s, and 16,000 data points were used. Therelative peak intensities were measured from theintegrated peak areas, which were calculatedwith an electronic integrator.

IR measurements

The samples were prepared via casting from di-lute THF solutions. After drying in air for about 1day, the films were heated at 50 °C until THF wascompletely removed. The removal was monitoredby the disappearance of the IR band at 1065cm�1, which is characteristic of THF. A Nicolet520 Fourier transform infrared (FTIR) spectrom-eter equipped with a deuterated triglycine sulfatedetector was used for the IR measurements. Thir-ty-two scans were signal-averaged at a resolutionof 2 cm�1. Higher resolutions (up to 0.5 cm�1)yielded no significant improvement in the spec-

tra. The peak absorbances were determined froma tangent baseline.

Calorimetric measurements

Powdered samples, in amounts ranging from 6 to8 mg, were encapsulated in aluminum pans. Thecalorimetric measurements were carried out witha PerkinElmer DSC-7 differential scanning calo-rimeter coupled to a thermal analysis data sta-tion. Dry nitrogen was used as the purge gas.Temperature and enthalpy calibration was per-formed with indium. The samples were scannedtwice, and Tg was taken as the midpoint betweenthe intersections from the glassy state to the liq-uid state of the second scan. All measurementswere obtained at a heating rate of 10 °C/min. Thereproducibility of duplicate runs of samples withwell-defined Tg’s was better than �0.2 °C.

RESULTS AND DISCUSSION

The incorporation of 4-acetamidothiophenolategroups into the polymer was proved by UV spec-troscopy at 269 nm, as shown in Figure 1. Thedegree of substitution of the reaction was deter-mined by 1H NMR spectroscopy. The analysis wasperformed through a comparison of the integratedintensities of the signals appearing at 7.4–7.8ppm (corresponding to the aromatic protons) withthe peaks around 4.6 ppm (ascribed to the CHClprotons). The signal at 8.5 ppm was assigned tothe ONHOCO group.

Figure 2 shows the evolution of the degree ofsubstitution of PVC with KAT at a stoichiometric

Figure 1. UV spectra of modified PVC with KAT atdifferent conversions: (a) 2.2, (b) 5.5, (c) 9.1, (d) 16.4,and (e) 21.1% (concentration � 0.04 g/L).

Figure 2. Nucleophilic substitution on PVC withKAT at 40 °C in CH.


concentration as a function of time. The kineticcurve exhibits a steep period followed by a slowerone that merges into a ceiling. This agrees withprevious works using other reagents at similartemperatures,9,10,12 indicating that each period isrelated to the reactivity of well-defined struc-tures. A particularly interesting observation fromFigure 2 is that the reaction produces a maximumdegree of substitution of 23%. Taking this valueinto account, we can consider that the reactivityof KAT is moderate in comparison with the reac-tivity of the different types of nucleophiles stud-ied in our laboratory and, at first sight, is similarto that of both thiobenzoate9 and 2-ethylhexyl-thioglycolate.12 Consequently, it appears to be avery exciting prospect to investigate whether theobserved kinetic behavior of KAT is characteristicof nucleophiles with the capability of interactingwith hydrogen and could affect the proposed sub-stitution reaction mechanisms of PVC.

With respect to the reaction temperature, amild temperature was chosen (e.g., 30 °C) be-cause, as known from the results with sodiumbenzenethiolate (NaBT),16 when the substitution

temperature increases, the reaction on rrmrstructures of higher activation energy competewith mmr structures related to long isotactic se-quences (mechanism B), bringing about a de-crease in the stereoselective nature of the reac-tion, which is contrary to the purpose of this re-search.

As shown in previous works, the evolution ofthe tacticity with conversion can be followed byhigh-resolution 13C NMR spectroscopy. Impor-tant to this research is the ability to differentiatethe bands associated with the modified polymerunits from those of the unreacted polymer; thisallows a reliable measure of the polymer compo-sition. The content of unreacted isotactic, hetero-tactic, and syndiotactic triads at any degree ofconversion can be determined from the set of sig-nals centered around 57.6, 58.6, and 59.4 ppm,respectively, as long as they are separated enoughfor their areas to be measured accurately. How-ever, by a comparison with the spectrum of un-modified PVC (Fig. 3), it may be concluded thatsubstitution brings about the appearance of newbands at 59.9–60.8 ppm, which are very close to

Figure 3. 13C NMR spectra of (a) virgin PVC and (b) PVC after 5.5% substitution.


the syndiotactic triads of unmodified PVC. Also,some minor bands at 58.9 and 58.2 ppm can beobserved. In light of these observations, we as-sume that the bands at 59.9–60.8 ppm are due tothe polymer unit after modification. On the con-trary, the small bands at 58.9 and 58.2 ppm mightbe due to a long-range steric effect on the 13CNMR chemical shifts induced by the changes inthe local microstructure, which occur in the vicin-ity of the heterotactic triad as the result of sub-stitution. The same argument should hold for thebroadening of the syndiotactic triad signals ob-served. One way of confirming the validity of ourassignments is the correlation between the com-position from 1H NMR and the composition from13C NMR. As shown in Figure 4, a straight linewith a slope of unity is obtained. This resultmakes it feasible to tackle the real aim of thiswork.

By a mere inspection of the evolution of the 13CNMR spectrum (Fig. 3) with conversion, we caneasily observe that the isotactic triad content de-creases and the syndiotactic triad content in-creases, this being the first evidence of the stereo-selectivity of the reaction. At the same time, be-cause of the high resolution of the 13C NMRspectrum, an overall decrease of the mmmr con-tent, which is chiefly relevant to the specific dis-appearance of the heptad mmmmrx (x � r or m),is evident. This result reveals that the more ac-centuated the reactivity is of mmr, the longer theassociated isotactic sequence is; this has also been

observed with other nucleophiles already stud-ied.8–13

However, more interesting for the purpose ofthis work is the exploration of the hypotheticalinfluence of the interacting capability of KAT as anucleophile in the aforementioned reaction mech-anisms of PVC.8–13

As found in a previous work8 with differentnucleophiles, important changes in the tacticity ofthe triads of the unreacted parts of the polymerare produced as the result of SN2 substitution. Asthis point, it is important to evaluate thesechanges, which depend on the level of the ste-reospecific nature of the reaction carried out withKAT. With this aim, we studied from a compara-tive point of view the microstructural evolutionwith the degree of substitution of PVC with thetwo nucleophiles NaBT8 and KAT (a similarbulky nucleophile with a much higher capabilityof interacting with hydrogen atoms than NaBT)to determine the relative contributions of mecha-nisms A, B, and C to each nucleophile. Whetherand to what extent this change disturbs the over-all balance of the isotactic and syndiotactic con-figurations will depend on the specific reactivitythat the nucleophiles have in SN2 substitution.

Figure 5 presents the evolution of the tacticityat the triad level. The isotactic triad content de-creases up to a conversion of roughly 7%; then,the rate of decrease slows down. However, theincrease in the syndiotactic triad content occurs

Figure 4. Correlation of the composition from 1HNMR with the composition from 13C NMR.

Figure 5. Evolution of the relative contents of triadswith the conversion.


almost symmetrically with the decrease in theisotactic content, and this accounts for bothchanges being simultaneous. Finally, the hetero-tactic triad elimination takes place from the onsetof the reaction. The curves in Figure 5 are quitesimilar to those obtained for the reaction withNaBT; this is surprising because the reactantsare quite different in reactivity.15

To elucidate the level of stereoselectivity of thereaction, Figure 6 shows the disappearance of themm triads with the degree of substitution. Twowell-differentiated stages are apparent, and this,as we have pointed out, is an indication that dis-crimination between both mmr and rrmr takesplace. However, what is important for the purposeof this work is to determine whether a small frac-tion of the mmr structures reacts with the mrtriad instead of the mm triad (mechanism C), as isthe case for NaBT. If mechanisms A and B werethe only ones effective for KAT, then the evolutionof the heterotactic triad loss with conversion, andthat of the isotactic triad loss with the syndiotac-tic triad increase, should have been linear with aslope of unity. Furthermore, any departure fromthis behavior would indicate that some of themmr tetrads disappeared without yielding thedisappearance of heterotactic triads and unam-biguously confirmed the occurrence of mechanismC. The results are given in Figure 7. Clearly, aslight deviation from the slope of unity can beobserved. This means that, similarly to NaBT, a

few mmr tetrads react with KAT by mechanismC. Surprisingly, these results clearly indicate thatdespite the lower nucleophilicity of KAT com-pared with that of NaBT, this could guarantee

Figure 6. Loss of the isotactic triad content versus the conversion of PVC after thesubstitution reaction with KAT.

Figure 7. Evolution of (F) the loss of the isotactictriad content with an increase in the syndiotactic triadcontent and (E) the loss of the heterotactic triad con-tent with the conversion of PVC after the substitutionreaction with KAT.


higher stereocontrol of the reaction; the two reac-tants yield similar stereoselective nucleophilicsubstitution.

As we have pointed out previously,8,9,11 everysubstitution, derived from the inversion of theconfiguration of the carbon, alters the configura-tion and the related conformation of the adjacenttriads, thereby generating a significant rear-rangement of a specific chain segment. By meansof the variation of these configurational rear-rangements, we have tried to verify that the re-sults obtained are representative of the nucleo-phile nature studied. Figure 8 depicts the varia-tion of the persistence ratio (�), defined byReinmoller and Fox22 as the ratio of the normal-ized intensity of the isotactic dyad to the condi-tional probability of a syndiotactic placement onan isotactic chain end, against the conversion forthe studied samples. It is apparent that � de-creases linearly with the conversion, and this im-plies that the substitution reaction brings about aprogressive deflection from Bernoullian character(� � 1). However, the slope of the straight line issimilar to that found when the reaction is carriedout with NaBT. This plot makes it clear that thestereospecific nature of the substitution reactionis similar with both nucleophiles and that thehypothetical ability of KAT to interact with PVCis not enough to enhance the stereoselectivity ofthe reaction.

As shown in an earlier work, further evidenceof local configuration-based mechanisms of sub-

stitution reactions have been provided by IR spec-troscopy through an analysis of the evolution ofthe � COCl bands of the FTIR spectrum of PVC,particularly those at 615 and 637 cm�1, with thesubstitution reactions studied.

The specific details of the FTIR band assign-ments for PVC are extensively covered else-where.23 In this respect, the evolution of the 550–750-cm�1 regions of the FTIR spectra with nu-cleophilic substitution is shown in Figure 9;Figure 10 illustrates the evolution of the absor-bance ratio, A615cm�1/A637cm�1, with the conver-sion. This ratio increases progressively up to aconversion of around 6%; then, it does so at aslower rate, although slightly higher than thatobserved for NaBT23 over an analogous range ofconversions. Once again, this clearly agrees withthe aforementioned conclusion that mechanism Cis not wholly hindered for the substitution reac-tion with KAT.

Overall, the aforementioned results stronglysuggest that the interaction capacity of KAT withPVC does not affect the mechanisms of nucleo-

Figure 8. Evolution of � with the conversion of PVCafter the substitution reaction with KAT.

Figure 9. Evolution of the IR spectra with the con-version: (a) 0, (b) 2.2, (c) 5.5, and (d) 14.6%.


philic substitution, which is similar to the substi-tution observed with other nucleophiles of inter-mediate reactivity; that is, the reaction is gov-erned by the stereochemical microstructure,although a fraction of mmr tetrads reacts occa-sionally with the central chlorine of the mr triadinstead of the mm triad (mechanism C).

On the basis of the results of nucleophilic sub-stitution reactions, we have endeavored to corre-late some physical properties with the evolutionof the tacticity microstructure with the degree ofsubstitution. We particularly focused on the evo-lution of Tg with the degree of substitution. Fromearlier studies,24,25 it follows that Tg depends ontwo well-defined factors. One factor is the sub-stituent that has been substituted for chlorineand that is variable in character. Clearly, its in-fluence varies with both the degree of hinderingcharacter and the ability to create interchain hy-drogen bonds. The other factor is the microstruc-ture in terms of the isotactic and syndiotacticsequences and especially the local configurationand terminal conformations of those sequences.Interestingly, this effect is permanent in charac-ter and seems to be independent of the substitu-ent.

On the basis of the previous results, we havetried to accomplish the second objective of ourresearch, that is, to examine the evolution of Tgwith the conversion, seeking to separate the con-tributions of the microstructure and nucleophileto the changes in Tg for the purpose of providingnew evidence of the involvement of the molecular

microstructure in the physicochemical processesthat control Tg.

Notably, although the plot in Figure 6, whichrepresents the evolution of mmr to rrmr with thedegree of substitution, consists of two well-de-fined straight lines for any nucleophile, the corre-sponding plot of Tg versus the conversion, asfound previously,24,25 exhibits two straight linesin the absence of mechanism C; that is, the evo-lution of Tg with the degree of substitution isparallel that of the microstructure evolution, con-trary to what happens when mechanism C ispresent, which produces a single linear change inTg.24

Because the behavior of KAT makes mecha-nism C operative, it seemed to us very exciting toinvestigate whether Tg varies with the degree ofsubstitution with a single straight line or, on thecontrary, varies in a similar way to that of themicrostructure.

The results are given in Figure 11, which de-picts the changes in Tg with the degree of substi-tution. It is worth emphasizing here the enhance-ment of Tg as the substitution increases, which isconsistent with the interacting effect of acet-amidothiophenolate groups. Therefore, this over-all increase is but an example of the variableeffect of the substituent on Tg. Yet, what must behighlighted is that KAT, unlike NaBT, producestwo straight lines corresponding to the conversionranges below and above 6%, as occurs with the

Figure 10. Evolution of the IR absorbance ratio,A615cm�1/A637cm�1, with the conversion.

Figure 11. Evolution of Tg with the conversion ofPVC after the substitution reaction with KAT.


evolution of the microstructure for any nucleo-phile (Fig. 6). At this point, we must analyze thecontributions of the different interactions. First,as found in a previous work,24 both the intramo-lecular and intermolecular interactions appear todepend on the tacticity-dependent microstruc-ture. With respect to the former interactions, ithas been shown how the conversion of the mmr atthe end of the isotactic sequences into a more rigidchain segment with lower excess free volume causesTg to increase, probably because the associated iso-tactic sequence becomes fastened by its end. In fact,mmr after substitution appears to be much moreinclined to form H. . .Cl bonds than the same struc-ture before substitution. However, the reverse be-havior applies to the terminal rrmr of syndiotacticsequences. Thus, the fluctuations of the local freevolume due to the conformational changes, whichare characteristic of the substitution reaction, andthe consequential changes in Tg happen to be re-lated to both mmr and rrmr structures.

As for the microstructure dependence of theintermolecular interactions, a significant de-crease in Tg has been proposed when the reactionby rrmr progresses and is accompanied by a pro-gressive decrease in H. . .Cl bond facilities, whichindicates the importance of the sequences thatare capable of forming consecutive H. . .Cl bonds;this can explained by the fact that sequentialinteractions are connected to the length of thesyndiotactic sequences.

The contribution of the nucleophile to the av-erage change in Tg is variable in nature, depend-ing on its bulkiness and polarity. Apart from itsbulkiness, KAT, unlike the benzenethiolategroup, possesses a carbonyl function that is capa-ble of creating H. . .OAC bonds of a local confor-mational nature and of higher stability than thatof H. . .Cl bonds.21 This effect should favor theinterchain interactions; consequently, the trendwith an increasing degree of substitution wouldbe toward an increase in Tg. It is notable that, asalready indicated, this effect is independent of thepermanent effect that changes in the microstruc-ture have on Tg.

The result is that the drawing together of theseeffects provides an enhancement of the micro-structure effect, as defined in our laboratory, anda lessening of the contribution of mechanism C,and this opens new prospects in the field of anyPVC behavior involving molecular interactions.Further results of the work now underway in ourlaboratory are to be published at a later date.

CONCLUSIONS

The nucleophilic substitution of PVC with KAThas been shown to proceed through the local con-figuration-driven mechanism found in a previouswork for other nucleophiles. However, importantfor the purpose of this research is the highly sig-nificant nature of the tacticity-dependent micro-structure on Tg of PVC. The results presentedhere not only show that there is a permanenteffect of the stereochemical microstructure on Tgregardless of the remaining chemical features butalso lead to a better understanding of the pro-cesses that are responsible for Tg, such as thevariation in the local configuration and conforma-tion as a result of substitution (specific fluctua-tions in the excess free volume, rigidity, and se-quential interchain interactions through consec-utive H. . .Cl bonds) and the possible interchaininteractions through polar nucleophiles. Conse-quently, novel insight into the specific sites inwhich the Tg-determining set of molecular mo-tions originate may be gained, thereby openinguseful prospects for more comprehensive studiesin this important field.

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