Synthesis of PVC-graft-PMMA Through Stereoselective Nucleophilic Substitution on PVC

Macromol. Chem. Phys. 2001, 202, 2377–2386 2377

Synthesis of PVC-graft-PMMA Through StereoselectiveNucleophilic Substitution on PVC

Gerardo Martínez, Elena de Santos, JosØ Luis Millµn*

Instituto de Ciencia y Tecnología de Polímeros (CSIC), Juan de la Cierva 3, 28006 Madrid, SpainE-mail: [email protected]

IntroductionNucleophilic substitution of chlorine in PVC by a seriesof functional groups has been viewed by a number ofauthors as an appropriate method of improving the prop-erties, in particular the thermal stability, through thereplacement of labile chlorines, and the mechanical prop-erties by grafting functions that are capable of provokingcross-linking.[1] The most common reactives used in thereaction with PVC are different types of organic sulfurcompounds.[2–14] Some of these have been found to be aneffective initiator for radical thermal or photopolymeriza-tion which can be used to prepare graft copolymers.

Grafting is a two-step procedure. In the first step, thechlorine atoms of PVC react with thio compounds, suchas potassium mercaptobenzothiazolate and potassiumdiethyldithio carbamate[2, 10] to form macroinitiators withactive thio functionality on the chain. In the second step,UV light decomposes the macroinitiators into macroradi-cals and primary thio radicals which can further reactwith various monomers to form graft copolymers orhomopolymers. In this sense, Huskic and Sevenik[15] havegrafted methyl methacrylate using UV light onto PVCmacroinitiators synthesized from PVC and potassiumpropyl xanthate.

On the other hand, it is known that the mercapto groupreacts with oxidizing agents being susceptible to forma-tion of free-radicals under mild conditions.[16–18] Follow-ing this chemistry Deb and Sankholkar[19, 20] have carriedout the thiolation of PVC and the subsequent grafting ofmethyl methacrylate in the presence of dimethyl sulfox-ide (DMSO). The authors enhance the advantage of thismethod relative to the absence of the homopolymer dur-ing the graft copolymerization.

During recent years an extensive research programusing some of the aforementioned nucleophiles andothers, experiments have been performed in our labora-tory in order to emphasize the prominent role of the tacti-city-governed microstructure on the chemical reactivity,in particular the mechanisms of, on the one hand, a num-ber of analogous reactions[21, 22] and, on the other, the ther-mal and photochemical degradation of PVC.[23–25]

The so-designated microstructure refers to: (i) the aver-age number and length of tactic sequences whether iso-tactic or syndiotactic; (ii) the local configurations locatedat the end of isotactic and syndiotactic sequences, namelythe mmr and the rrmr, respectively; and (iii) the localconformations relevant to the latter structures, in particu-lar those connected with long tactic sequences.

Full Paper: The substitution reaction of poly(vinyl chlor-ide) (PVC) with potassium ethylxanthogenate was per-formed in cyclohexanone. The evolution of unreactediso-, hetero- and syndiotactic triad contents with degree ofsubstitution has been followed by 13C NMR spectroscopy.By comparing quantitatively the microstructure changeswith degree of substitution and taking into account thatthe reaction is of SN2 type, the substitution is proved toproceed exclusively through the mm triad of mmr tetrad,which enhances the stereoselective nature of the reaction.This conclusion was confirmed on the grounds of the FT-IR results. From this stereospecific chemical modificationof PVC the thiolation reaction through hydrolysis underbasic conditions and, the subsequent reaction with methylmethacrylate in the presence of dimethyl sulfoxide offer amethod for the preparation of stereoselective graft copoly-mers. After grafting no variation of the microstructure of

the chain has been proved. This novel contribution is anoriginal approach to the prominent role of the microstruc-ture-dependent local conformation in the mechanisms ofreactions of PVC. These results have been used to studythe effect of the aforementioned structures on the thermalstability of the graft copolymers and therefore providenew approaches to better comprehension at molecularlevel of the structure-property relationships.

Macromol. Chem. Phys. 2001, 202, No. 11 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 2001 1022-1352/2001/1107–2377$17.50+.50/0

2378 G. Martínez, E. de Santos, J. L. Millµn

It is worth noting that the terminal configurations oftactic sequences, likewise the corresponding conforma-tions, are local discontinuities along the polymer chainregardless of the type of polymer. In addition, the confor-mations of higher potential energy that are likely at mmrand rrmr configurations often exhibit higher volume andlocal motion facilities.[21, 26] As to the associated tacticsequences, in particular those of a length above a certainthreshold, these are chain segments of a more regular nat-ure; consequently, they are liable to exhibit enhancedinterchain interactions relative to the atactic parts.[21, 26]

As a consequence, the so-understood microstructure is tobe assumed to create strong changes in the space arrange-ment of the chain thereby strongly influencing the inter-chain and the intrachain interactions, the local freevolume, the local and cooperative motions, etc. whichdetermine, on average, the physical properties of a poly-mer. Also, these local discontinuities, especially whenrelated to specific conformations cause the reactivity of achemical group to be radically altered.[21, 26]

Basically, it has been demonstrated that: i) the reactionproceeds exclusively by the central CHCl group of eitherthe last isotactic triad (mm) of isotactic sequences or theheterotactic triad (mr) at the end of syndiotacticsequences. Thus, as extensively argued, the very reactivesites are necessarily the local configurations mmr andrrmr at the end of isotactic and syndiotactic sequences,respectively;[26] ii) in order for these structures to react,the mm and rm triads have to take the GTTG– or GTTTconformation respectively, which involves the occurrenceof conformational changes between the likely conforma-tions in each structure, so as to ensure the availability ofthe above reactive conformations throughout the reactionprocess[21, 26, 27] and iii) although the GTTG– isotactic triadconformation is extremely reactive compared to theGTTT heterotactic triad conformation, the evolution ofthe ratio of mmr to rrmr with the degree of substitutionmay be easily controlled by changing the nature of thesolvent or of additives, for the reactions carried out insolution [28] or in the melt,[29] respectively.

It is worth noting that every substitution not onlyinvolves the disappearance of one mmr or rrmr struc-ture, but owing to the inversion of the configuration ofthe carbon characteristic of the SN2 mechanism, substan-tially alters the configuration and the related conforma-tion of the adjacent triads thereby bringing about a signif-icant rearrangement of a definite chain segment. Thisconsideration coupled with the findings provided fromprevious work allowed us to propose the followingstereospecific mechanisms: mechanism A applies to thereaction by the mm triad of mmr and involves the simul-taneous disappearance of one isotactic triad and one het-erotactic triad by exchanging for one nucleophile cen-tered triad and one co-syndiotactic triad, respectively.Mechanism B is that of substitution by the rm triad and

involves the elimination of a single heterotactic triad.Finally, mechanism C relates to a well-defined fraction ofmmr tetrad which reacts by the mr triad instead of themm triad. It involves the disappearance of one isotactictriad without yielding either the appearance of one syn-diotactic triad (as in mechanism A) or the disappearanceof one heterotactic triad (as mechanism B). Thus,mechanism C is to be viewed as a specific case ofmechanism B because both may be assumed to dependmainly on the associated syndiotactic sequence.[26]

One implication of these results is the ability to applythe aforementioned results to prepare graft copolymers ofwell-defined structure, based on PVC. The method ofproduction of thiols from halides via xanthates throughhydrolysis under basic conditions is well known.[30] Con-sequently, if, as expected, this function was substitutedselectively for the chlorine atoms that are responsible forthe instability of PVC, the subsequent grafting reactionwith a monomer would not only make the preparation ofimproved materials possible but could enhance the ther-mal stability of the PVC chain. In a similar manner, thedistribution of grafts along the PVC chain might be con-trolled by choosing a starting polymer of appropriate tac-ticity distribution. In line with these ideas, we should suc-ceed in grafting monomers onto PVC exclusively throughthe above quoted fraction of highly reactive isotactictriads, which were demonstrated to be both thermally[23]

and photochemically[24, 25] labile.The subject of this work is first to demonstrate that the

reaction of PVC with potassium ethyl xanthogenate(KXANT) proceeds, likewise with other nucleophiles asstudied in earlier work, through a stereospecific mechan-ism, which makes it possible to specifically substitute thehighly reactive thiol function for some thermally labilechlorine atoms in PVC. Secondly, the graft copolymeriza-tion of methyl methacrylate, specifically on the xantho-genate modified PVC units without changing the micro-structure of the PVC parts that arose from substitution, isoutlined as a useful way to prepare PVC-based materialswith improved properties.

Experimental Part

Materials

The PVC sample used was prepared at 908C in bulk using2,29-azoisobutyronitrile (AIBN) (98%, Fluka) as the initiatorsystem and taken to 20% conversion. The number-averagemolecular weight (M

—n = 21500) was determined at 348C in

cyclohexanone (CH, Ferosa) using a Knauer membrane os-mometer. CH was purified by fractional distillation undernitrogen. Tetrahydrofuran (THF) was distilled under nitrogenwith aluminum lithium hydride (Aldrich) to remove perox-ides immediately before use. DMSO (99%, Aldrich) and CHwere purified by distillation under vacuum and then dried fora few days with 4-� molecular sieves. Methyl methacrylate

Synthesis of PVC-graft-PMMA Through Stereoselective Nucleophilic Substitution on PVC 2379

(99% MMA, Fluka) was distilled in a stream of nitrogenunder vacuum and stored over 4 �-molecular sieves in arefrigerator. Reagent grade KXANT (98%, Fluka) was usedwithout further purification.

Substitution Reaction of PVC with KXANT

3.20 g (51.2 mmol, based on monomeric unit (VC)) of PVCwas dissolved in 180 ml of CH. Then, 9.82 g (61.4 mmol) ofKXANT in 140 ml of CH was added to the polymer solution.The mixture was stirred and heated at 408C under an inertatmosphere. At appropriate reaction times, samples were pre-cipitated with methanol, the products were purified fromTHF into methanol, filtered and then dried under vacuum at408C. The samples were characterized by UV spectroscopyto determine the degree of substitution from the intensity ofthe xanthogenate 284-nm band. A calibration curve was pre-viously obtained[31] from the absorbance at 284 nm for sam-ples modified to well-defined extents as measured by micro-analysis of the content of Cl and S. Some of these valueswere checked by 1H NMR spectroscopy at 200 MHz usingdioxane-d8 under standard conditions, which accounts for theaccuracy of the calibration curve.[31]

IR (film): 1010 cm–1 (1O1CS1S1).1H NMR (200 MHz): d = 2.5 (CH2), 3.7 ppm

(1O1CH21), 4.6 (CHCl).

Functionalization of PVC

The functionalization of PVC was carried out from the mod-ified PVC with xanthogenate groups (PVC-XANT) underaqueous alkaline hydrolysis. The modified samples were dis-solved in THF (0.7 g/70 ml), then, an aqueous solution ofNH4OH (2% w/v) was added under stirring and nitrogenatmosphere. The hydrolisis experiments were followed byUV spectroscopy from the intensity evolution of the xantho-genate 284 nm band. The reaction time was about 6 h at258C.

Grafting PMMA on Functionalized PVC

1.80 g of functionalized PVC was dissolved in 180 ml ofCH. Then, 17.9 ml (0.93 mol N l–1) of purified MMA and7.1 ml (0.56 mol N l–1) of DMSO were added to the polymersolution. The mixture was stirred and heated at 708C under astream of oxygen-free nitrogen. At appropriate reaction timesamples were precipitated into cold methanol, the productswere purified from THF into methanol, washed and dried toconstant weight at 408C under reduced pressure. The separa-tion of homopolymer PMMA and the grafted copolymer wascarried out at 258C using a 1:3 v/v mixture of THF–acetoneas solvent and a 4:1 v/v mixture of methanol with water asprecipitant.[32]

Characterization of Samples

Films of functionalized PVC and graft copolymers were pre-pared by casting from dilute THF solution (40 mg N cm–3).After slow evaporation at room temperature the films weredried in vacuum. All IR spectra were recorded on a Nicolet520 FT-IR spectrometer at 2 cm–1 resolution using 160 scans.

The overall composition of graft copolymers was determinedby 1H NMR. The proton of 1CH3 in MMA group appears atd = 1.0 ppm and that of 1COOCH3 at d = 3.5 ppm. The tacti-cities of PVC, of the modified polymers and of the graftcopolymers were determined by means of 13C NMRdecoupled spectra obtained at 908C on an XL-300 Varianinstrument, operating at 75.5 MHz using dioxane-d8 as sol-vent. The spectral width was 2500 Hz, a pulse repetition rateof 3 s, and 16 k data points were used. The relative peakintensities were measured from the integral peak areas, cal-culated by means of an electronic integrator.

Thermal Degradation

The modified samples and the graft copolymers weredegraded to 0.3% in the solid state at 1608C using a thermo-stat with a silicone heating liquid. The HCl evolved duringthe experiments was swept out by means of a nitrogen flow(9 l N h–1) and trapped in ultra-pure water (Millipore Milli-Qpurification) in a conductimetric cell so that the degradationcould be continuously followed. The electrical conductivityof the solution was measured using an electrode cell con-nected to a Radiometer Copenhagen CDM 83 conducto-meter. The plots of evolved HCl against time are straightlines after an initial period. The slope of the linear part of theisotherm is taken as the degradation rate.

Ultraviolet-Visible Spectra

UV-visible absorption spectra of modified PVC were deter-mined with 0.6 g N l–1 solutions in THF and those correspond-ing to the degraded samples were measured with solutions of4.0 g N l–1 in hexamethylphosphorous triamide using4.0 g N l–1 solutions of the corresponding non-degraded poly-mers as a reference. They were recorded in a Perkin ElmerUV-Vis 554 spectrophotometer at room temperature with a1-cm–1 path length cell.

Results and DiscussionThe incorporation of ethyl xanthogenate groups into thepolymer is proved by UV spectroscopy as shown in Fig-

Scheme 1.


ure 1. The degree of substitution was determined bymeasuring the absorbance at 284 nm in the UV spectrum.Figure 2 shows the evolution of the degree of substitutionof PVC with ethyl xanthogenate as a function of time. Ascan be seen, the kinetic curve exhibits a steep period fol-lowed by a slower one in accordance with what wasfound with other nucleophiles. However, in this case thevery low maximum degree of substitution reached,around 2%, is indicative of the low nucleophility of theethyl xanthogenate group compared to other nucleophilesstudied in preceding work.[22, 26, 33, 34] Nevertheless, thislimitation does not affect the present work. On the con-

trary, it can guarantee of a high stereoselectivity of thereaction, because only the most reactive chlorines areengaged in the reaction.

The modified PVC samples were reacted under basicconditions so as to replace the xanthogenate by mercap-tan groups in order to functionalize the PVC. The reactionwas followed by UV spectroscopy as shown in Figure 3.As can easily be observed, the absorption of the 284-nmband drastically decreases with time and the end of thereaction can be considered to be reached after 60 h.

Graft copolymerization of MMA onto some functiona-lized-PVC samples with mercaptan groups were carriedout in CH solution in the presence of DMSO as a co-initiator of free-radical polymerization.

After a comprehensive separation of the graft products(isolation of PMMA homopolymer), the conversion ofMMA and the total content of PMMA in the graft copoly-mer were determined from the weight increment of thereaction and 1H NMR spectra. We may determine thegrafted PMMA in wt.-% as:

grafted PMMAð%Þ ¼ Weight of grafted PMMAWeight of copolymer

N 100

From the data listed in Table 1, we can see that theMMA monomer yield in all reactions is high, but a lowgrafting efficiency is observed as a consequence of thevery low degree of substitution in the preliminary func-tionalized PVC reaction. Assuming the same reactivityfor all mercaptan groups in the PVC-SH samples, and

Figure 1. UV spectra of modified samples. (a) 1.2%; (b) 1.6%;(c) 1.9%; (d) 2.5%; conc = 0.6 g N l–1.

Figure 2. Degree of substitution as a function of time in thenucleophilic substitution of PVC with KXANT in CH at 40 8C.

Figure 3. Variation of degree of substitution vs. time of PVC-XANT under aqueous alkaline conditions. Initial modification:(H) 0.6%; (9) 1.6%.


consequently, an optimum grafting efficiency, we canobtain the number of MMA units in the copolymer, andthe average number of MMA monomeric units pergrafted chain, from the increase in weight of the copoly-mer, and the degree of substitution in the functionalizedPVC.

In spite of the low grafting efficiency achieved withthe above experimental conditions, the most importantaim of the present work is the analyze of the microstruc-tural variations in the graft copolymers with respect tovirgin PVC, and their comparison with those obtained inthe samples modified with KXANT.

The evolution of tacticity of the PVC parts, both in themodified polymer and in the corresponding graft copoly-mers, with substitution was followed by high resolution13C NMR spectroscopy. From the comparison with theunmodified PVC it may be concluded that the substitu-tion involves the appearance of new signals at 59.8 ppmvery close to the syndiotactic triads of unmodified PVC.No appreciable change in position of the bands of purePVC is observed though some broadening of those ofsyndiotactic triads is noticed (Figure 4). With respect tothe spectra of the PVC-graft-PMMA copolymer nochange in the resolution of the methinic carbons isobserved. On the other hand, the signals of the methyl-enic (d = 56.8–54.5 ppm) and the methyl carbons ofO1CH3 (d = 52.7 ppm) due to MMA are apparent. Thetacticity results are included in Table 2.

First of all, both the heterotactic and isotactic triad con-tents decrease as the substitution increases in both sys-tems but the latter triad does it at a much higher rate thanthe former. On the other hand, the r-to-m ratio is drasti-cally higher as the degree of substitution increases cer-tainly because of the progressive reduction of m. Thesepoints are better illustrated by Figure 5 which refers tothe evolution of the content of each triad, as obtained bysumming the areas of the peaks in each triad and express-ing it as a fraction of the total area (rr) + (mr) + (mm),for the modified and graft copolymer samples, with thedegree of substitution. As can be seen, two well differen-tiated behaviors are evident. Firstly, the disappearance ofmm triads is linear throughout the interval of substitution

studied (around 3%) which is understandable in the lightof the mechanisms proposed in prior work.[22, 26, 33, 34] Actu-ally, the decrease of isotactic triad content was found tooccur steeply up to conversion of roughly 7%; then, itbecomes slower. The slope in each reaction period wasdemonstrated to be a measure of the superiority of themmr tetrad in reacting, relative to the rrmr pentad.[26]

Interestingly in the case of Figure 5, likewise in the reac-tions with those nucleophiles which react exclusivelythrough mechanism A,[26] the slope is unity both for thedecrease of mm and the increase of rr. As extensivelyargued,[22, 26, 34] this means that the substitution throughrrmr (mechanism B) and through mr in mmr (mechan-

Table 1. Experimental values of the grafting reaction ontopoly(vinyl chloride).

Degree Subst:mol-%

a) Conversion%

b) Graft MMAwt:-% %-unit

MMA monomericunit=chain

0.9 79.4 12.2 7.9 9.51.0 81.3 15.7 10.3 11.61.2 85.4 19.3 12.9 12.41.8 89.7 37.1 26.7 20.2

a) Degree of substitution reaction of PVC with KXANT prior tothe grafting.

b) Yield of MMA monomer into polymer (PMMA + graftedPVC).

Figure 4. 13C NMR spectra of (a) virgin PVC; (b) PVC after1.8% substitution; (c) PVC-graft-PMMA (37.1 wt.-% MMA).


ism C) should both be discarded and mechanism A wouldbe the only operative mechanism. Further support of thisappreciation is given by the results discussed later.

The heterotactic triad content decreases with a uniqueslope (Figure 5) as is the general case[26] because everyact of substitution, whether through the mmr or throughthe rrmr reactive species, involves the disappearance ofone heterotactic triad, except in the case where mmrreacts by the mr triad (mechanism C).[26]

The second indication of Figure 5, is that the abovequoted changes in triad content are the same both for thefunctionalized samples and for the graft copolymers,which further proves that the substitution reaction is theonly process able to change the chain microstructure.Interestingly, the behaviors depicted by Figure 5 aresomewhat more marked than those identified for the reac-tions carried out with other nucleophiles. As will be seenlater, this relates to the lower effectiveness of the xantho-

genate as a nucleophile, which probably makes themechanism C of substitution inoperative, thus causing thestereospecificity of the reaction to be enhanced.

Figure 6 refers to the changes in rmmmrx andmmmmrx (x = m or r) heptad content. As can be seen, thedecrease of the 57.51-ppm signal corresponding tommmmrx (x = m or r) is more pronounced than that of57.58 ppm for rmmmrx (x = r or m). Thus, the reactivityof mmr proves to be much more accentuated as the lengthof the associated isotactic sequence increases. This agreeswith the stereospecific mechanism of reaction as conveyedin earlier work and is more conclusive as the superiority inreacting of mmmmrx, relative to rmmmrx, is quiteapparent at conversions lower than 3%. Consequently, inthe light of the foregoing studies with different nucleo-philes, we can assume that the reaction of PVC withKXANT is of greater stereoselectivity. This may furnishsome additional information for the effect of the tacticity-driven microstructure on the polymer properties asstemmed from previous work, to be completely identified.

Table 2. Probabilities of presence of triad and diad of modified and graft copolymers samples.

Degree Subst:%

a) Graft MMAwt:-%

Prrb) Prm + Pmr

b) Pmmb) Pr

c) Pmc) Pr/Pm

– – 0.303 0.494 0.203 0.550 0.450 1.220.9 – 0.312 0.489 0.190 0.557 0.434 1.281.4 – 0.318 0.479 0.189 0.557 0.428 1.301.8 – 0.319 0.478 0.185 0.558 0.424 1.322.2 – 0.323 0.476 0.179 0.561 0.417 1.350.9 13.8 0.314 0.482 0.195 0.555 0.436 1.271.2 19.6 0.315 0.484 0.189 0.557 0.431 1.291.8 37.1 0.324 0.475 0.183 0.561 0.420 1.34

a) Degree of substitution reaction of PVC with KXANT prior to the grafting.b) Probability of presence of syndio (Prr), hetero (Prm + Pmr) and isotactic (Pmm) triads.c) Probability of presence of syndio (Pr) and isotactic (Pm) diads.

Figure 5. Evolution of triads with the degree of substitution.PVC-XANT (open symbols); PVC-graft-PMMA (filled sym-bols).

Figure 6. Evolution of 13C NMR spectra of PVC with thedegree of substitution (PVC-XANT) in the methine carbonregion (mm triads): (a) 0%; (b) 1.4%; (c) 2.2%.


From the results cited above we can conclude that thesubstitution with xanthogenate, unlike other nucleophiles,takes place only through the mmr tetrad, but no conclu-sion can be drawn as to whether all the mmr structuresreact by mechanism A or whether a small fraction ofthem is able to react by mechanism C, as is the case forother nucleophiles. The reason for that is that mechan-isms A and C both involve the same change in content ofmm triads. Now, if mechanism A were the only operativea straight line would be obtained when plotting the evolu-tion of the isotactic triad loss versus the syndiotactic triadincrease.[34] Also, since the occurrence of mechanism B inCH as solvent is nil in the 0–3% conversion range,[26] thedisappearance of heterotactic triad with conversion wouldoccur linearly with a slope of unity.[34] As can be seenfrom Figure 7, the trend of these plots is towards linearitywithin the experimental uncertainties, both for KXANTand for PVC-graft-PMMA series. Consequently, anymechanism but mechanism A is to be discarded contraryto what happened with most of the nucleophiles studiedpreviously.[26, 33] In fact, the incidental occurrence ofmechanism C has been recently shown to be linked withthe bulkiness of the nucleophile,[34] and, as widely con-veyed, mechanism B in CH solution is operative only atconversions higher than 7%.[26] It is worth noting that theresults of Figure 7 as those of Figure 5, hold for eitherpolymer series.

The above conclusion that substitution with KXANTproceeds exclusively by mechanism A is given furthersupport by Figure 8 depicting the variation of the persist-ence ratio (q) (defined by Reinmöller[35] as the ratiobetween the normalized intensity of isotactic diads and

the conditional probability of a syndiotactic placement onan isotactic chain end), against the conversion for thestudied samples. It appears evident that q decreases line-arly with conversion, which means that as with othernucleophiles,[22, 26, 33, 34] the substitution reaction bringsabout a progressive deflection from Bernoullian character(around q = 1) towards non-Bernoullian statistics. How-ever, what is important for the purpose of the presentresearch is to notice that first the slope is similar to thatfound for those nucleophiles which exhibit enhancedstereoselectivity as the result of the absence of mechan-isms B and C, and secondly that such behavior producessome tacticity rearrangement.[34]

The FT-IR results obtained for the KXANT modifiedsamples lend support to the above conclusion that thisnucleophile is highly stereospecific in character. Figure 9shows the evolution of the mC1Cl vibration bands of thePVC in particular those at 615 cm–1 and 637 cm–1 withthe substitution reactions studied. By inspection, the fol-lowing conclusions may be drawn: i) no change in posi-tion of the 615 cm–1 and 637 cm–1 bands is noted and ii)the intensity of the band at 615 cm–1 compared to that ofthe 637 cm–1 increases; furthermore, it becomes narrower

Figure 7. Loss of isotactic (9, H) and heterotactic triads (0, H)vs. degree of substitution of PVC and loss of isotactic triads (F,f). PVC-XANT: open symbols, PVC-graft-PMMA: filled sym-bols.

Figure 8. Evolution of the persistence ratio (q) with the degreeof substitution of PVC. PVC-XANT (open symbol); PVC-graft-PMMA (filled symbol).

Figure 9. IR spectra of PVC-XANT at different degrees ofsubstitution (regions 560–740 cm–1 and 1400–1500 cm–1): (a)0%; (b) 1.4%; (c) 1.8%; (d) 2.2%.


and more symmetrical. These peculiarities agree withthose found for other nucleophiles, and, as extensivelyargued elsewhere,[33, 34, 36] are consistent with the occur-rence of mechanism A, exclusively or to the greatestextent. These results are more significant as they relate toa very short range of conversion (0–2%).

The above results lead to a better understanding of theprevailing role of the tacticity-driven microstructure inthe nucleophilic substitution mechanisms on PVC soopening new avenues in the field of chemical modifica-tion of polymers. In addition, owing to the chain arrange-ment changes and to the ability to graft appropriate func-tions on the chain in a controlled way, both issued fromspecific substitution through mechanism A, the resultsare a useful path for experimentally modeling polymerswith improved properties.

Based on the results of nucleophilic substitution reac-tions, we have endeavored to correlate some physicalproperties with the evolution of the microstructure withdegree of substitution. Special effort has been focused onthe evolution of the degradation rate with the substitutionextent. The most important approach to this problem thatarises from previous work is that the thermal stability ofthe substituted polymers proves to be higher as the substi-tution extent increases up to a definite value, no higherthan 1%, which depends on the fraction of isotactic triadin the starting material. Since, as indicated before, thebehavior of KXANT is highly stereoselective in the mod-ification reaction and it seemed to us very interesting toinvestigate whether the maximum stabilization with thisnucleophile takes place around the aforementioned con-version so further identifying the GTTG– isotactic triadconformation as being a very labile structure in PVC.[23]

In this case it does and supplementary evidence of thecrucial role of such local conformational defects, asinferred from earlier work,[23] on the degradation and sta-bilization mechanisms of PVC, will be furnished.

Degradation data in Table 3 show that the selectivesubstitution brings about a strong progressive stabiliza-tion until a substitution extent of 0.9% is attained; after-wards the stability decreases with increasing substitution.Figure 10 depicts these changes in degradation rate withdegree of substitution. Clearly, the degradation ratepasses through a minimum at a definite degree of substi-tution which, as widely conveyed, agrees with the contentof mm triad under GTTG– conformation[37] in the poly-mer. This behavior is common to both the modified sam-ples and the graft copolymers synthesized from them. Itis worth emphasizing the strong stabilizing effect thateither series of polymers exhibits. The degree of stabiliza-tion of the former series attains 54.7%, a value substan-tially higher than that found for other nucleophiles.[23, 37]

The inferior value obtained for the graft copolymers, i.e.,42.8, is probably due to some simultaneous eliminationprocesses.

In fact, Guyot et al.[38] studied the thermal degradationof graft copolymers of PVC with methacrylates and haveshowed from thermogravimetric and thermal volatiliza-tion results that products other than HCl are produced assoon as HCl appears.

The extent of the decrease in stability with substitutionafter the initial stabilizing effect (Figure 10) is thatobserved both for other nucleophiles and for a series ofunmodified PVC samples of decreasing isotactic con-tent.[23] It was shown to be due to the fact that in bothcases the polymer tends gradually towards non-Bernoul-

Table 3. Initial dehydrochlorination rates of modified and graftcopolymers samples.

Degree Subst:%

a) Graft MMAwt:-%

Degradation rate6103

½HCl�=½HCl�0 Nminÿ1

– – 4.20.6 – 2.30.9 – 1.91.0 – 2.01.1 – 2.81.3 – 4.21.6 – 6.80.9 13.8 2.41.0 21.8 2.71.2 19.6 3.01.8 37.1 4.9

a) Degree of substitution reaction of PVC with KXANT prior tothe grafting.

Figure 10. Dependence of degradation rate of PVC after modi-fication on the degree of substitution. (9) PVC-XANT; (H) graftcopolymers.


lian syndiotacticity either because of substitution on iso-tactic triads as the selective removal of isotactic triadsunder GTTG– conformation vanishes or because of theevolution of stereochemical composition as the competeisotactic content decreases. In any case the all-trans syn-diotactic sequences in the absence of GTTG– isotactictriad conformations happened to favor the propagationstep of degradation so giving rise to an increase in degra-dation rate.[23] Therefore, the results presented in Fig-ure 10 are consistent with our earlier proposal that thespecific isotactic triads under GTTG– conformation andthe tactic sequences, in particular the syndiotactic, bothgovern the thermal degradation process of PVC in thatthey are mainly responsible for the initiation and propa-gation steps, respectively.[23] Given that the former struc-ture is associated with an isotactic sequence[21, 22, 26–29, 33, 34]

its progressive and selective vanishing is accompanied bya strong drop in initiation and propagation steps. The lat-ter effect for the polymers studied in this work is shownin Figure 11, indicating that the stabilization produced bysubstitution with KXANT is paralleled by a strong reduc-tion of the content of long polyenes sequences comparedwith the unmodified sample.

From the results obtained in the present work it isinferred in the first instance that the stereospecific natureof the nucleophilic substitution reaction in PVC can beenhanced when the reactivity of the nucleophile is verylow. Consequently, the substitution reaction would onlytake place through the most preferential points in the chainpractically giving rise to a single mechanism (mechanismA). In the second instance, making use of these findings asapplied to KXANT as the nucleophile we have succeededin grafting methyl methacrylate onto PVC exclusivelythrough the above quoted fraction of highly reactive iso-tactic triads (GTTG– conformation). As stated in previouswork and confirmed herein, they are the main structuresresponsible for the thermal instability of PVC in terms ofdegradation rate and polyene build-up.

Finally, given that the grafting of methyl methacrylateto produce new properties in the starting PVC occurswithout affecting the properties of the PVC parts, asimproved by the substitution reaction, the results pre-sented are not only a further approach to the mechanismsof both nucleophile substitution and thermal degradationof PVC, stated in earlier work, but also furnish a valuableway to design PVC-based materials through chemicalmodification. These materials exhibit well-defined prop-erties both in the unmodified and in the modified parts ofthe starting PVC.

Acknowledgement: We are grateful to the Dirección Generalde Investigación Científica y TØcnica (DGICYT) for financialsupport (PB 93-1250).

Received: July 12, 2000Revised: January 23, 2001

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Synthesis of PVC-graft-PMMA Through Stereoselective Nucleophilic Substitution on PVC

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Transcript of Synthesis of PVC-graft-PMMA Through Stereoselective Nucleophilic Substitution on PVC