Effect of the modification of the polymer-rich phase composition on the formation of structural defects in radical suspension PVC Purmova, Jindra

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1 University of Groningen Effect of the modification of the polymer-rich phase composition on the formation of structural defects in radical suspension PV Purmova, Jindra IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Publisher's PDF, also known as Version of record Publication date: 2007 Link to publication in University of Groningen/UMG research database itation for published version (APA): Purmova, J. (2007). Effect of the modification of the polymer-rich phase composition on the formation of structural defects in radical suspension PV Groningen: s.n. opyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like reative ommons). Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from the University of Groningen/UMG research database (Pure): For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Download date:

2 hapter 3 Mechanistic implications of the distribution of defect structures in fractions of PV of different molecular weight Purmová, J.; oote, M.L.; Krenske, E..; Pauwels, K.F.D.; Agostini, M.; Bruinsma, M.; Schouwstra, J.R.; Vorenkamp, E.J.; Schouten, A.J. ABSTRAT: The distributions of the various types of defect structures in fractions of PV, made by radical suspension polymerization at different conversions, were analyzed. Low molecular weight fractions contained a higher degree of branching and a lower content of chloroallylic end groups. The difference was more pronounced in the fractions of ~ 96% conversion. This indicates the formation of a greater proportion of the low molecular weight material in the late stages of the polymerization at conditions similar to the so-called subsaturation polymerization. Such PV generally contains an increased number of branches and internal double bonds and a decreased number of chloroallylic end groups. A new mechanism for the formation of low molecular weight PV in suspension polymerization, containing an enhanced number of structural defects, is suggested on the basis of our results. Further, evidence for copolymerization and/or hydrogen abstractions of the chloroallylic end groups is presented. Finally, the number of internal double bonds in PV fractions was studied. The defects were found to exhibit end group-like characteristics: their concentration is largely constant as a function of molecular weight, and is not altered by monomer conversion. The prevalent intramolecular mechanism of formation of the unsaturated structures and their location between carbons 5 6 were confirmed via 13 NMR studies. igh-level ab initio calculations showed that 1 5 backbiting of the 1 2 l shifted head-to-head radical was the most likely origin for these structures. From the experimental analysis and theoretical calculations, it emerged that this backbiting reaction is stereoselective, with the isotactic configuration appearing to be more resistant. The evidence collected in this chapter points to 1 5 backbiting as a reaction for which inhibition should have the most beneficial effect on the thermal stability of PV. 59

3 APTER 3 Introduction In hapter 2, it was shown that inter- and intramolecular transfer reactions compete with propagation from the beginning of the polymerization process, but become considerably more significant at high conversions. In particular, there is a dramatic increase in the formation of branched and internal unsaturated structures above 85% conversion, when the polymer-rich phase becomes much less swollen in monomer and extremely dense. Under such conditions, the propagation reaction becomes diffusion-controlled and its rate decreases dramatically, allowing chain transfer processes to compete more effectively. This is directly reflected in the changes in the molecular weight. In our suspension polymerization system, an increasing trend of 60 M n (number average molecular weight) and M w (weight average molecular weight) with increasing monomer conversion was observed up to 80% of monomer conversion. 1 This is caused by an increasing contribution of polymerization in the polymer-rich phase at the cost of polymerization in the monomer-rich phase. After reaching a monomer conversion of approximately 80%, M w remained almost constant, while M n decreased significantly, indicating formation of short chains. 1 It must be remembered that vinyl chloride polymerization occurs in two phases: monomer- and polymer-rich phase. The reaction takes place in both phases simultaneously up to the critical conversion X f (60% for our polymerization setup 1 ) after which the monomer-rich phase is completely consumed. At conversions below X f, short PV molecules (degree of polymerization lower than V units, i.e M n = kg mol -1 ) 2 are formed in monomer-rich phase. Longer chains precipitate and continue their growth in the polymer-rich phase. After X f the polymerization takes place exclusively in the polymer-rich phase. In this hapter the molecular weight dependence of the defects, such as chloroallylic end groups and branches in the PV polymers, formed at various monomer conversions is examined to study the differences in the mechanism of formation of structural defects in the polymer- and monomer-rich phase. Examination of the internal double- bond content in the PV polymers as a function of molecular weight is the other topic of the present chapter. Allylic chlorides associated with internal double bonds have been shown to be less reactive than tertiary chlorides, but more susceptible to the presence of l. 3 The side-reactions reported in the literature to give rise to internal unsaturations are intra- or intermolecular hydrogen abstraction from the methylene unit, and subsequent β-elimination of a chlorine radical by transfer to monomer (see Scheme 1-3 and 1-4). 4,5 owever, the relative importance of these processes has not, as yet, been revealed. According to earlier studies, the unsaturated defect structures are mostly contained in the low molecular weight fractions of the polymer. 6,7 Understanding this molecular weight dependence and its relationship with other process conditions such as the conversion can help to provide an insight into the mechanism of formation of these defects.

4 Mechanistic implications of the distribution of defects in fractions of different MW To assist in the mechanistic interpretation of the results, high-level ab initio molecular orbital calculations are also used to estimate the rate coefficients for the various possible intramolecular hydrogen transfer reactions. Experimental section Fractionation. The polymers were prepared as described in the experimental sections of hapter 2. The fractionation of the PV samples was performed via the fractional precipitation method. 8 The polymer was first dissolved at 25 in a good solvent (TF), and then successive amounts of non-solvent ( 2 O) were added drop-wise to the solution, under vigorous stirring. After the addition of the non-solvent, the solution was warmed until it became clear again; this usually occurred at 50. The temperature was then slowly decreased until the polymer started to precipitate. The temperature was then set to a constant value and the stirring was stopped. After 6 8 hours the precipitate was isolated. The entire procedure was then repeated several times to obtain the various molecular weight fractions, which were purified by precipitation into ethanol. The lowest molecular weight fractions, which were not possible to obtain by fractionation, were isolated by stirring the PV powder overnight in a mixture of hexane and acetone (75:25). The material insoluble in this mixture of solvents was removed via filtration. The remaining liquid was then evaporated in a rotary evaporator, leaving the low molecular weight PV, which was then dried at room temperature under vacuum. Resulting fractions were characterized by GP and 1 and 13 (performed on reductively dehalogenated material) NMR spectroscopy using the same procedure described in detail in hapter 2. Tacticity was determined by analysis of 13 spectra of unmodified resin. Reductive dehalogenation. Reductive dehalogenation was carried out using a modified one-step variant 9 of the method developed by Starnes et al. 10 as described in hapter 2. omputational procedures. Standard ab initio molecular orbital theory 11 and density functional theory 12 calculations were carried out using the GAUSSIAN and MOLPRO omplete description of the method can be found in the experimental section of hapter 2. Results and Discussion The presence of structural defects in PV polymers was studied as a function of molecular weight for three PV specimens isolated at 23.7%, 87.2 %, and 96.4% conversion. Each of the PV samples was fractionated, with the fractions (and the low molecular weight extracts) uniformly covering the molecular weight range. The molecular weight data for all samples is provided in Table 3-1. In general, the fractions had narrower molecular weight distributions than the original polymer and Mn and Mw decreased for 61

5 APTER 3 every successive fraction. Fractions of each sample were then studied via 1 and 13 NMR, so as to determine the concentration of the chloroallylic end groups (Figure 3-1) and the different types of branches (Figures 3-5 to 3-6). The effects of the molecular weight and conversion on the numbers of these types of defects are discussed in turn. The distribution of the number of internal unsaturations as a function of the molecular weight (Figure 3-8) and the implication for the mechanism of their formation are described in separate sections. Table 3-1. haracteristics of the PV samples of different conversions and additives and their fractions and low molecular weight extracts. onversion [%] M n [kg mol -1 ] Original PV M w [kg mol -1 ] D M n [kg mol -1 ] Fractionated PV M w [kg mol -1 ] D * * Low molecular weight extract *

6 Mechanistic implications of the distribution of defects in fractions of different MW hloroallylic end groups. The content of chloroallylic end groups per 1000 V decreased with increasing M n (Figure 3-1a), while the number of defects per chain remained relatively constant (Figure 3-1b). This behavior is typical for an end group; the concentration of chain ends per 1000 V should clearly decrease with increasing chain length but the number of chain ends per chain should remain constant. In Figure 3-1a, it can be seen that the total concentration of the chloroallylic groups per 1000 V units, and its dependence on molecular weight, appears to be relatively insensitive to monomer conversion. When the number of chloroallylic end groups per chain is examined more closely (Figure 3-1b), some systematic differences emerge which are discussed in the following paragraphs hloroallylic end groups/1000v M n [kg mol 1 ] hloroallylic end groups/chain M n [kg mol 1 ] (a) (b) Figure 3-1. Dependence of the number of chloroallylic end groups on Mn for fractions of different PV samples. a) per 1000 V, b) per chain fractions of PV of different conversions. Monomer conversion: 23.7%; 87.2%; 96.4%. In hapter 2, a reduction of the number of the chloroallylic end groups per chain in the (non-fractioned) PV was reported at monomer conversions above 85%, and the results for the individual molecular weight fractions (Figure 3-1b) are consistent with these previous results. It was also suggested that copolymerization and/or hydrogen abstraction of chloroallylic end groups contributes to this decrease. opolymerization, presumably increasing in frequency at high conversions, would result in the formation of vicinal methyl and long-chain branches (Scheme 3-1). ydrogen abstraction from the 2 l part of a chloroallylic end group would render a very stable allylic radical, which would rather terminate than propagate (Scheme 3-1)

7 APTER l l l 2 l l 2 2 l l 2 l l 2 O l 2 l l Primary radicals 2 Internal double bond ombination l l l Methyl-long branch Propagation Propagation 2 2 l 2 R O O Internal double bond l l 2 Internal double bond 2 l O O R O Scheme 3-1. opolymerization and hydrogen abstraction of chloroallylic end groups. Weak signals, which are in good agreement with the predicted chemical shifts of the methyl-long branch carbons,16 are present in 13 spectra of the lower molecular weight fractions of high conversion PV (Figure 3-2). These signals are lacking in the 13 spectra of low conversion PV fractions and in higher molecular weight fractions of higher conversion PV. Figure 3-2. omparison of the 13 spectra of the fractions of PV with 87.2% conversion; Mn =A) 95.2; B) 60.1; ) 11.4; D) =3.5 kg mol-1. * Peaks indicating the possible chain architecture after the copolymerization of a chloroallylic end group. 64

8 Mechanistic implications of the distribution of defects in fractions of different MW The termination of the allylic radical occurs predominantly by combination. 15 The products of the combinations with other allylic radicals or PV macroradicals would give 1 NMR signals overlapping with the signals of internal unsaturations and l protons of the main chain. Only combination with primary radicals would give a structure with peaks separated well enough from the signals usually present in PV. A small peak at 7.8 ppm found in the spectra of fractions of 96.4% conversion PV with Mn below can be taken as a confirmation of hydrogen abstractions from chloroallylic end groups (Figure 3-3). This signal coincides with the theoretical chemical shift 16 of the protons contained in an O l moiety adjoining the double bond in the product of a combination of primary and allylic radicals. 2 l O O R O Unsaturations: Internal, terminal Termination agent: Aromatic protons ppm Figure 3-3. Partial 1 spectrum of a PV fraction with M n =22.9 kg mol -1 of a sample with 96.4% conversion. The overall 20 % reduction of the number of chloroallylic end groups per chain during the increase of the conversion from 23.7% up to 87 % has to be ascribed to process conditions applicable to all chain lengths. At this moment, it can be assumed that the increase of the viscosity of the polymerizing droplet, which induces a diffusion-dependent propagation rate, might cause the effect; further experiments are needed for a more conclusive explanation. The other effect seen in Figure 3-1b, however, is the remarkable chain length dependent difference between the samples with 96.4%, 87.2% and 23.7% conversion. The low molecular weight fractions in the 96.4% sample contain substantially less chloroallylic end groups per chain than the higher molecular chains. Apparently low molecular weight material of low conversion PV is formed under different conditions compared to high conversion polymer. A mechanism, presumably occurring at very high conversions, is proposed and described more in detail later on in this text. Moreover, this behavior of the fractions of 96.4% PV may indicate that a greater proportion of the low molecular weight chains ( M n ~ 1 to 7 kg mol -1 ), isolated from this 65

9 APTER 3 PV, have been formed at conditions where all kinds of transfer reactions are dominant. E.g. the abstractions converting the chloroallylic end groups into the internal unsaturations (See Scheme 3-1 and 3-6). Support for this idea can be derived from the analysis of the changes in the molecular weight with monomer conversion. The numberaverage molecular weight decreases above 85% conversion, while the weight-average molecular weight remains virtually constant and polydispersity increases (see Figure 3-4). The increased formation of low molecular weight material at high conversion reflects the decline in the rate of propagation and the concurrent relative increase in the rates of side reactions that can lead to the premature termination of the polymer chains MW [kg mol -1 ] M w M n onversion [%] Figure 3-4. Number and weight-average molecular weight, and molecular weight distribution of PV, produced by suspension polymerization at 57.5 with increasing conversion. M n ; M w ; M w Mn Branches. Examining the total number of branches per 1000 V units, we note that this increases with increasing conversion (see Figure 3-5). This trend was reported previously in several studies of non-fractionated PV 17,18,19 including that described in hapter 2, and can be explained by the dramatic increase (compared to propagation) in the frequency of the intra- and intermolecular transfer reactions at high conversions. Focusing on the molecular weight dependence of the total number of branches per 1000 V for a specific PV sample, it can be noted that the total number of branches does not change significantly as a function of molecular weight for the higher molecular weight fractions (see Figure 3-5). This is in contrast with the number of chloroallylic end groups per 1000 V, and is consistent with the fact that at least in a kinetic sense, the branches are not end groups. After closer examination of the distribution of branches in PV of different chain length, higher branching content in fractions of low ( M n <10 kg.mol -1 ) and high ( M n from approximately 90 to 100 kg mol -1 ) molecular weight can be observed 66

10 Mechanistic implications of the distribution of defects in fractions of different MW (Figure 3-5). These values were determined by 1 NMR. When the total number of branches is calculated based on 13 spectra of reductively dehalogenated samples, the number of branches in high molecular weight fractions falls significantly lower than the ones determined from 1 spectrum (Figure 3-6). The explanation for that is most probably the broadening of the 1 NMR signals caused by the high viscosity of the solutions. owever, the viscosity of the solutions of low molecular weight fractions is not high enough to broaden significantly the line widths of the 1 NMR signals. Furthermore, the same tendency was observed in 13 NMR spectra when the line widths are much les sensitive to the solution viscosity. It thus appears that there may be a genuine chain length effect on the concentration of the branches in low molecular weight fractions Total branches/1000v M n [kg mol 1 ] Figure 3-5. Dependence of the total number of branches per 1000 V units on M n in fractions of samples of different monomer conversions. Monomer conversion: 23.7%; 87.2%; 96.4%. Examining next the different types of branches present in the polymer (Figure 3-6), it can be first noted that the branch structures present in the PV samples are methyl, ethyl, butyl, and long branches. Of these, the butyl and some of the long-chain branches have a l atom on the branch point carbon, and their number is therefore most important from a stability point of view, though the concentration of the other types of branches can contribute to our understanding of the polymerization kinetics. Long-chain branches 17,20,21 and some portion of the methyl and ethyl branches (hapter 2) are formed by abstraction of from the polymer chain by the macroradical, followed by propagation. The number of these defects/1000 V was almost unchanged in different fractions with M n > 10 kg mol -1, but was higher in the lower molecular weight chains (Figure 3-6). Butyl branches are known to be formed by a 1 5 backbiting reaction, followed by subsequent propagation, and their number follows the same trend as long-chain branches. The sharp increase in the concentration of these branches in the low molecular weight extracts further confirms the notion that the low molecular weight polymer is formed 67

11 APTER 3 predominantly at conditions favorable for hydrogen transfer reactions. Such conditions arise at late stages of the polymerization (conversion > 85%) at which the polymer-rich phase becomes very dense and the propagation reaction becomes diffusion controlled. Nevertheless, the hydrogen abstractions are not likely to be molecular weight selective. In other words, from a chemical perspective, all chain segments have the same probability to be affected by these transfer reactions Branches/1000 V Branches/1000 V M n [kg mol (a) 1 ] M n [kg mol (b) 1 ] Figure 3-6. Dependence of the number of different types of branches per 1000 V units on M n in fractions of sample of a) 87.2%, b) 96.4% monomer conversion. Total branches determined by 13 NMR; Total branches determined by 1 NMR; Methyl branches, Butyl branches, Long branches, Ethyl branches, all determined by 13 NMR. The suspension polymerization process is essentially a bulk polymerization carried out in VM droplets suspended in a continuous water phase stabilized by a surface-active compound. Owing to the VM solubility in water, 22 some of the material might also be formed on the surface of these monomer droplets, i.e. at the water polymer interface. The poor quality of water as a solvent for PV causes an earlier precipitation of the polymer chains than in the interior of the monomer droplets. Very short oligoradicals are produced. At conversions at which the polymer matrix contains a sufficient amount of VM these oligoradicals migrate into the polymer-rich phase and continue growing (Figure 3-7a). 68

12 Mechanistic implications of the distribution of defects in fractions of different MW PV formed at the interface VM PV formed at the interface VM monomer-rich phase polymer-rich phase water phase polymer-rich phase partially swollen in VM water phase (a) (b) Figure 3-7. Schematic representation of the formation of short PV chains at the polymer water interface. Situation a) below, b) above 85 % monomer conversion. The viscosity of the polymer-rich phase gradually increases and these short chains encounter progressively more difficulties in joining the polymerization in the interior of the particles and therefore stay on the interface (Figure 3-7b). At high monomer conversions, the low monomer concentration at the interface (0.07 mol L -1 ) closely resembles the conditions of the subsaturation experiments reported by jertberg et al. 23,24 Vinyl chloride was polymerized at low monomer pressure using generally a water-soluble initiator and PV grains as seed. A decrease in the molecular weight and an increase in polydispersity were observed. 23,24 Under conditions used by jertberg et al., the reaction is likely to take place almost exclusively at the interface of the monomer-swollen seed polymer and the water phase. The polymer formed under these conditions might be expected to contain different concentrations of unsaturations and branches since, owing to the reduction in monomer concentration, inter- and intramolecular hydrogen abstraction can compete more effectively with propagation. In support of this, it is noted that, an increased number of long-chain and butyl branches with decreasing monomer pressure was reported by jertberg and Sörvik. 17 An increase in the number of internal unsaturations in polymers made by a similar method was observed by Starnes et al. 4 When a monomer soluble initiator was used, the combination of polymerization in the monomer-swollen seed and at the interface might be expected. owever, the same results as for a water-soluble initiator were achieved at lower monomer pressures. 24 In addition, a decline in the number of chloroallylic end groups, 4 an increase in the number of ethyl branches and a decrease in the number of methyl branches were detected. 17 In our PV, the contribution of the methyl branches to the higher branching content in the short chains was indeed lower than that of the other types of defects (Figure 3-6). In low molecular weight extract of PV with 96.4% conversion were only 14% more methyl branches than in higher molecular weight chains. The content of butyl, ethyl, and long-chain branches in low 69

13 APTER 3 molecular weight fractions was higher by 19, 23, and 31% respectively. It seems plausible to assume that, at monomer starvation, the propagation or β-chlorine elimination of the intermediate radical formed by 1 2 chlorine shift after head-to-head addition (resulting in methyl branches and chloroallylic end groups respectively) is less favorable than a second chlorine shift leading to ethyl branches 25 or a 1 5 (with respect to the radical center) backbiting resulting in internal double bonds (see discussion later on in the text). Other evidence for the formation of the low molecular weight material at the interface of the water and organic phases at high conversion are the differences in the distribution of structural defects in fractions of low and high conversion. The relative difference in the total number of branches present in low and high molecular weight fractions was much higher for polymer with 96.4% conversion (about 20%) than for PV with 87.2 % conversion (only about 10%), indicating that the portion of short chains formed in the water phase is increasing with monomer conversion. The fractions of PV with 23.7 % conversion presented a different behavior as in the case of the chloroallylic end groups. No increase in the number of branches was observed for the low molecular weight fractions. Only a slightly increased branching content in fractions of high molecular weight (Figure 3-5) was detected. This can be attributed to the high viscosity of the solution of high molecular weight material used for NMR measurements as was discussed earlier in the text. Effect of chain length and conversion on the number of internal unsaturations. Figure 3-8a shows the concentration of internal unsaturations per chain as a function of chain length for each of the PV samples. In general terms, the concentration per chain is relatively insensitive to chain length, increasing by only 0.1 over the entire molecular weight range. The confidence intervals were 0.03 for both the 23.7% and 96.4% conversion data sets and 0.04 for the 87.2% conversion data set. Expressed as a concentration per 1000 V, the internal unsaturations decrease sharply with increasing molecular weight (Figure 3-8b). These tendencies indicate that the internal unsaturations have characteristics of an end group. That means, just as the number of end groups in a chain does not depend on its length, the number of internal unsaturations does not either. Monomer conversion did not change this tendency significantly (Figure 3-8). owever, the total number of these defects increases with increasing conversion. This is consistent with the findings for non-fractionated polymers, where a steep increase in the number of internal unsaturations after monomer conversion around 85% was observed (hapter 2). Internal allylic functional groups are formed by hydrogen-transfer reactions of an inter- or intramolecular nature. e.g.5,17 In a multiphase system (such as the suspension polymerization of VM), the composition of the reaction phase is the most important factor determining the frequency of these side reactions. At 85 % conversion, the polymerization proceeds exclusively in the polymer-rich phase, where the weight 70

14 Mechanistic implications of the distribution of defects in fractions of different MW fraction of monomer is approximately 6% (calculated according to the model developed by Xie et al. 26 ) and further decreases as the polymerization progresses (see Appendix for more details). The observed increase can therefore be understood in terms of the reduction of monomer concentration and the concurrent increase in polymer concentration in the polymer-rich phase at conversions above 85%, conditions favoring hydrogen-transfer reactions Internal double bonds/chain Internal double bonds/1000v Mn [kg mol 1 ] Mn [kg mol 1 ] (a) (b) Figure 3-8. Dependence of the number of internal double bonds a) per chain or b) 1000 V on M n for fractions of PV samples of different monomer conversions. 23.7%; 87.2%; 96.4%. Mechanistic consequences of the development of the number of internal unsaturations with molecular weight and monomer conversion. It was proposed earlier 5 that the principal mechanism of formation of internal double bonds is an intermolecular transfer reaction abstraction of by the growing macroradical (see Scheme 1-2). Since this intermolecular transfer reaction can occur randomly along the chain, the concentration per 1000 V should have remained relatively constant, and the concentration per chain should have increased sharply with increasing M n. Results reported in this hapter indicate that a small proportion of the internal unsaturations might indeed be formed in intermolecular transfer reactions (Figure 3-8). Some even smaller part of the internal unsaturations is probably formed by the combination reactions of the allylic radicals. owever, since the concentration per 1000 V drops dramatically with chain length, the overwhelming majority of the internal double bonds have to be formed via some other mechanism, in which the internal unsaturation behaves kinetically as an end group. One such mechanism for the formation of internal unsaturated end groups was proposed by Starnes et al. 27 They suggested that internal double bonds could be formed via an intramolecular 1 6 backbiting reaction followed by elimination of the l atom in the β position (see Scheme 3-2). In this case, the internal double bond once formed stops further propagation of the polymer chain, and thus can be considered an end group. Examining Scheme 3-2, it can be seen that, depending on which of the β-chlorine is 71

15 APTER 3 eliminated, the reaction results in a double bond in between the carbons 5 and 6, or between the carbons 6 and 7. 2 l Propagation Pentyl (long) branch a Intramolecular abstraction l l l 2 2 l l or 2 l 2 l l 2 2 l l 2 l l 2 l Internal double bond arbon 5,6 or 6,7 from the chain-end Propagation Propyl branch l l l 2 l l l or 2 2 l l l l Internal double bond arbon 3,4 or 4,5 from the chain-end 2 l l l l l l or Propagation 2 2 l l 2 l Butyl branch l 2 l Internal double bond arbon 4,5 or 5,6 from the chain-end Scheme 3-2. Intramolecular hydrogen transfer reactions possibly occurring during the propagation of radical V polymerization. a Further propagation of the radical formed by 1 6 backbiting would give rise to a pentyl branch. According to jertberg et al. only branches up to 5 carbons can be distinguished using 13 spectra of reductively dehalogenated PV. 5 This structure would give the same NMR peaks as long branches formed by intermolecular transfer. A method for discriminating between an intermolecular mechanism and an intramolecular (backbiting) mechanism for the formation of internal double bonds is the analysis of the 13 spectra of reductively dehalogenated samples. Unsaturated structures undergo 1 5 free radical cyclization upon reaction with Bu 3 Sn, wherein a carbon-centered radical, formed from a l moiety, adds to a double bond to form a five-membered ring. 27 This addition results in various types of cyclopentane moieties, depending upon the location of the double bond in the chain (see Scheme 3-3). The cyclic structures possess very similar chemical shifts, but the monoalkylcyclopentane (MP) structure that is formed by backbiting on a double bond in positions 5 6 (Scheme 3-3) can nevertheless be distinguished unambiguously in 13 spectra. The chemical shifts of carbons α, 3 and 4 or 1 in the MP structure (Scheme 3-3) have been determined (by comparison with 13 spectra of model compounds) 27 not to overlap with those of other structures in reductively dehalogenated PV. 72

16 Mechanistic implications of the distribution of defects in fractions of different MW 4 x y 2 l Bu 3 Sn α 2 3 n α' 2 x y 2 l Bu 3 Sn α IP or EtP 4 MP Position of the expected double bond x=3, y=2, n=1 x=4, y=3 x=5, y=4, n=3 x=7, y=6, n=5 x=6, y=5 x,y =randomly in the chain Expected cyclopentane structure Ethyl cyclopentane (EtP) None Butyl cyclopentane exyl cyclopentane Monoalkyl cyclopentane (MP) Internal cyclopentane (IP) Scheme 3-3. Reductive cyclizations of unsaturated structures present in PV. Figure 3-9 shows the 13 spectrum of a low molecular weight, reductively dehalogenated PV sample. MP structures were most easily detectable in this fraction because of the large content of unsaturations in the non-reduced sample. Even though the signals of MP are very weak (probably due to non-quantitative cyclization), it is clear from the spectrum that the MP structure is present. ence, double bonds in the 5 6 position are present in our PV polymers. This accounts for the end-group-like behavior of the internal unsaturations. Although n-pentyl branches (products of a propagation of the 1 6 intramolecular rearranged radical) have not been found in 13 spectra of PV dehalogenated with Bu 3 SnD, 21 jertberg et al. 20 stated that only branches up to 5 carbons can be distinguished using 13 spectra of reductively dehalogenated PV. At this point, it should be noted that internal unsaturations might be formed via other types of backbiting reactions. For example, the 1 4 backbiting reaction (Scheme 3-2) could also produce internal double bonds, which, upon reduction with Bu3Sn, would give rise to butyl cyclopentane moieties (Scheme 3-3). The 13 signals of such a structure are difficult to distinguish from the other cyclopentane moieties; however, it must be noted that propyl branches which would result from the propagation of the intermediate radical have never been detected in the 13 NMR spectra of PV 31 (although a possible reason might be that the intermediate radical undergoes β-chlorine scission before the propagation can take place)

17 APTER 3 Figure 3-9. Expansions the 13 spectrum of the dehalogenated low molecular weight extract of suspension PV at 87.2 % conversion. Signals of monoalkylcyclopentane (MP), trans ethylcyclopentane (tetp) and trans internal cyclopentane (tip) moieties are shown (For the numbering of the atoms of the cyclopentane structures, see Scheme 3-3). O-β = arbon in position β to O group in the 2 2 (O) 2 2 segments, probably originating from air oxidation during the reductive dehalogenation. In order to discriminate between the various possible mechanisms, the rate coefficient of the 1 4 transfer reactions was included in the ab initio molecular orbital calculations (see below). The rate coefficient of 1 5 backbiting was calculated too in order to have a point of reference. This type of backbiting predominantly occurs in polymerization of many polymers such as ethylene, 32 acrylates, 33 and as well as PV. In the last case, the propagation of the intermediate radical is the origin of chlorobutyl branches, whose presence in PV was previously reported. 20,31 Before examining the ab initio calculations, it is worthwhile to note that backbiting appears to be stereoselective. It preferentially attacks the syndiotactic triads, while the isotactic configuration is more resistant. In Figure 3-10, the lower content of syndiotactic triads and higher content of isotactic triads in low molecular weight extracts of the 87.2 and 96.4% conversion polymers can be clearly seen. The higher content of isotactic triads in low molecular weight extracts is not an artifact of the isolation procedure, 34 as the same preference for isotactic triads was also observed in ether extracts and in some low molecular weight material obtained by fractionation. The differences in the sequence distribution of the low molecular weight part and complete PV (samples of conversion above 87%) indicate that some portion of the syndiotactic triads is consumed at the time when these short chains are formed. PV with 23.7% conversion does not exhibit any difference in tacticity of low molecular weight part and non-fractionated polymer (Figure 3-10). It seems reasonable to suppose that the short chains of high conversion PV are formed under conditions of monomer starvation where backbiting and subsequent l elimination (leading to the conversion of l units to double bonds) are favored. It can therefore be assumed that the observed decrease in the number of syndiotactic triads is caused by intramolecular hydrogen abstraction. Short chains formed in the beginning of the polymerization (when plenty of monomer is available for propagation) undergo backbiting and l elimination less frequently, accounting for the lack of detectable 74

18 Low MW extract Mechanistic implications of the distribution of defects in fractions of different MW differences in the content of iso- and syndiotactic triads in polymer of low monomer conversion (Figure 3-10). The 2 group in a syndiotactic conformation probably provides less steric hindrance to backbiting from the radical chain end than the atoms in an isotactic conformation do. Indeed, ab initio molecular orbital calculations of the transition structures for the various types of backbiting reactions indicate that the l atoms tend to be orientated on alternating sides of the polymer chain in the lowest energy conformations (see Figure 3-11, and also Figure 2-9 in the hapter 2) Low MW extract Low MW extract 0.30 P(mm) Low MW extract P(rr) 0.20 Low MW extract Low MW extract % 87.2 % 96.4 % onversion % 87.2 % 96.4 % onversion (a) (b) Figure omparison of the content of (a) isotactic (mm) and (b) syndiotactic (rr) triads in nonfractionated PV of different monomer conversion, and in the corresponding low molecular weight extracts. White and black columns represent non-fractionated PV, whilst gray columns represent low molecular weight extracts. Ab initio molecular orbital calculations. Rate coefficients (at 57.5 ) were calculated for the model reactions depicted in the Scheme 3-4. To perform the calculations at a high level of theory, the propagating polymeric vinyl chloride radical was modeled as a tetramer. (1) l l l l l 2 l l l (2) l l l l l 2 l l l (3) l l l l l 2 l l l Scheme 3-4. Model reactions used in the calculation of the rate coefficients for (1) the 1 4, (2) 1-5 and (3) 1-6 intramolecular hydrogen abstraction reaction. The key kinetic and thermodynamic parameters for reactions (1) (3) are provided in Table 3-2, while the geometries of the relevant species are shown schematically in Figure

19 APTER 3 Table 3-2. alculated Kinetic and Thermodynamic Parameters for the Intramolecular ydrogen Transfer Reactions of the Vinyl hloride Propagating Radical at 0 K and K a (kj mol 1 ) (kj mol 1 ) S (J mol 1 K 1 ) G (kj mol 1 ) Q k (s 1 ) a alculated at the G3(MP2)-RAD//MPW1K/6-31+G(d,p) level of theory. Rate coefficients (k) were calculated using simple transition state theory and incorporate Eckart tunneling corrections (Q) (see text). The Eckart tunneling corrections for reactions (1) (3) were calculated using imaginary frequencies of , and cm 1, and approximate G3(MP2)-RAD (see text) reverse barriers of 98.9, 68.1 and 79.8 kj mol 1, respectively. Figure MPW1K/6-31+G(d,p) optimized geometries of the transition structures for the intramolecular 1-4 (1), 1-5 (2), and 1-6 (3) hydrogen transfer reactions. Also shown are the geometries of the reactant vinyl chloride propagating radical (4), and the corresponding radical products (5 7) of reactions (1) (3). 76

20 Mechanistic implications of the distribution of defects in fractions of different MW omparing the rate coefficients provided in the Table 3-2, the most favorable intramolecular hydrogen transfer is, as expected, the 1 5 shift. The 1 6 shift is the next most favorable, while the 1 4 shift is the least favorable. This trend reflects the fact that the 6-membered transition structure for the 1 5 shift has the least strained geometry. This preference is also reinforced by enthalpic factors. The 1 4 and 1 6 shifts convert an α-lsubstituted radical to a β-l-substituted radical, and are endothermic by approximately 8 kj mol 1, while the 1 5 shift (which converts an α-l substituted radical to another α-l substituted radical) is almost thermoneutral (endothermic by only 2 kj mol 1 ). The entropies of activation actually favor the 1 4 shift over the 1 5, with the 1 6 being the least favorable. This is because there is a loss of vibrational entropy in the transition structure, which increases as a larger fraction of the reactant radical is constrained in a ring structure. Nonetheless, the differences in the barriers dominate the reactivity preferences, such that the rate coefficient of the 1 5 shift is approximately 1500 times faster than that of the 1 6 shift and times faster than that of the 1 4 shift. Even though the 1 5 shift is the most favorable, this reaction is unlikely to give the unsaturated structures that we have detected. The formation of a 5 6 double bond from the 1 5 intermediate would require α-chlorine elimination, and the carbene involved in this process is a species of high energy, whose formation would be enormously endothermic. Moreover, α-chlorine elimination from head-to-tail radicals would generate l= 2 end groups, and, furthermore, the vinyl chloride/2-chloropropene copolymer would likewise contain a considerable amount of l= 3 termini; these are situations that have previously been experimentally discounted. 35 Non-zero slopes of the dependencies of the number of internal double bonds per chain on molecular weight (Figure 3-8a) might indicate that a small portion of internal unsaturations may be formed by the intermolecular mechanism. When the rates of both types of backbiting and intermolecular transfer are compared (Figure 3-12) it becomes obvious that even at high conversions intramolecular processes are much more favorable than intermolecular reactions. The non-zero slopes are probably caused by line broadening of the 1 NMR peaks caused by the high viscosity of the solutions used for the measurements. At this point, the 1 6 shift might seem the best candidate for the formation of internal unsaturations. owever, this is not supported by the ratios of the defect structures. The concentration of butyl and long-chain branches in non-fractionated polymer is ~1.7/1000 V and ~0.6/1000V respectively, while there are as many as ~0.4 internal double bonds/chain (hapter 2). Such a difference might be attributed to differences in the reaction rates of the radicals formed by above-mentioned side reactions. The tendency of the 1 6 backbitten radical to β-chlorine elimination should be much higher than the propensity of the 1 5 backbitten radical to propagation. 77

21 APTER log (ν bb/ν tr inter) onversion [%] Figure omparison of the ratios between the rate of 1 5 ( ) or 1 6 ( ) backbiting and intermolecular hydrogen abstraction. The tendencies observed for the propagation or l elimination of a rearranged head-tohead radical (which structure closely resembles that of 1 6 backbitten radical, See Scheme 1-3 and 1-5) indicate an opposite trend. Propagation of such a radical leads to the formation of methyl branches, whilst the β-l elimination to chloroallylic ends. 36 The number of methyl branches observed in the PV, fractions of which are studied in this paper is around 5 per 1000 V, whereas the number of chloroallylic end groups per chain is approximately 0.7 per 1000 V and 0.4 per chain respectively (hapter 2). Even if the consumption of chloroallylic end groups by copolymerization (hapter 2) and hydrogen abstraction 37 are taken into account as the cause of the low number of these unsaturations, it is unlikely that the rate of β-l elimination would be so high as to compensate for the differences in the values of rate constants. Moving away from both 1 5 and 1 6 backbiting of a head-to-tail radical as the initial source of 5 6 unsaturations, several other possibilities have been considered as shown in Scheme 3-5. ere, reaction (4) represents a 1 5 chlorine transfer, which, analogous to a 1 5 hydrogen transfer, produces an intermediate radical that can undergo two alternative reactions. Its propagation would generate butyl branches with hydrogen at the branch point carbon, while a β-hydrogen elimination (reaction 5) would lead to unsaturation in the 5 6 position. Another alternative route to 5 6 unsaturations is that a propagating radical may undergo head-to-head addition with vinyl chloride monomer, followed by a 1 2 chlorine shift and then a 1 5 (numbered starting on the radical center) hydrogen transfer. The first two steps in this sequence are known to occur in the chemistry of PV, while the latter step is modeled in reaction (6). 78

22 Mechanistic implications of the distribution of defects in fractions of different MW (4) l l l l l l l l l (5) l 2 l l (6) l l l l l l l l Scheme 3-5. Model side reactions investigated as alternative sources of 5 6 unsaturations in PV Kinetic parameters for reactions (4) (6) are shown in Table 3-3 and the geometries of the relevant species are shown in Figure The very low rate constants for reactions (4) and (5) indicate that any pathway involving 1 5 chlorine transfer as a key step is unlikely to be of importance. The rate constant for 1 5 chlorine transfer is much lower than that for 1 5 hydrogen transfer, which probably reflects the reduced ability of the transition state for 1 5 chlorine transfer to adopt a chair geometry. On the other hand, the relatively high rate constant for reaction (6) (k = s 1 ) indicates that such a reaction could plausibly be involved in the formation of internal unsaturations. Table 3-3. alculated kinetic parameters for the alternative side reactions leading to 5 6 unsaturation ( K) a Reaction (4) (5) (6) G (kj mol 1 ) Propagation of head-to-head radical after 1 2 l shift k s L mol 1 s s L mol 1 s 1 aalculated at the G3(MP2)-RAD//MPW1K/6-31+G(d,p) level of theory. Rate coefficients (k) were calculated using simple transition state theory and incorporate Eckart tunneling corrections (Q) in the cases of hydrogen transfer reactions. The Eckart tunneling corrections for reactions (5) and (6) were calculated using imaginary frequencies of and cm 1, and reverse barriers of and 65.9 kj mol 1, respectively. The rate of this 1 5 hydrogen transfer process is much higher than that for intermolecular hydrogen transfer from dead polymer chains (Figure 3-14a), and at conversions below approximately 90% it is slightly lower than the rate for propagation of the same radical (Figure 3-14 b). At high conversions (conditions of monomer starvation), the 1 5 hydrogen transfer out-competes propagation, and more internal unsaturations are formed (Figure 3-14b). 79

23 APTER 3 Figure MPW1K/6-31+G(d,p) optimized geometries of the reactants and transition structures for the side reactions investigated as alternative sources of 5 6 unsaturation in PV: 1 5 chlorine shift (8); β- hydrogen elimination following 1 5 chlorine shift (9, 10); and 1 5 hydrogen shift following 1 2 chlorine shift in head-to-head radical (11, 12). When compared to the propagation rate-constant calculated previously for the PV dimer radical (k = L mol 1 s 1 ), the propagation rate-constant for the 1 2 chlorine shifted species shows only minor retardation due to the increased steric hindrance at the secondary radical centre. It seems therefore that a three-step pathway involving head-to-head addition, 1 2 chlorine migration, and 1 5 hydrogen abstraction (Scheme 3-6) is a plausible mechanism to explain the end-group-like characteristics of internal unsaturations in PV. It is worthwhile noting that the radical formed by a second chlorine shift (Scheme 3-6) might undergo 1 5 backbiting too. owever, the formation of internal unsaturations from the resulting tertiary chlorine-containing radical is unlikely for the same reasons as in the case of 1 5 backbiting of the head-to-tail radical (see above). omparison of the proposed intramolecular mechanisms leading to internal unsaturations. Figure 3-15 clearly shows that at conversions lower than 96% the probability that internal double bonds will be formed by the mechanism proposed and described in the previous section (See Scheme 3-6) is much higher than the probability of formation of these defects by 1 6 backbiting. Only beyond 96% conversion does the probability of 1 6 backbiting become slightly higher than that of 1 5 backbiting of