Mechanism for the Formation of Chloromethyl Branches in Poly(vinyl chloride)

Starnes, W. H.; Schilling, Frederic C.; Abbås, Kent B.; Cais, R. E.; Bovey, F. A.

doi:10.1021/ma60070a004

Cited by 73 publications

(38 citation statements)

References 8 publications

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“…46 In the latter case, it was shown that signals corresponding to the deuteriumlabeling pattern expected for the methyl branch formed by further propagation of a radical, resulting from a 1-2 hydrogen shift were absent. However, it is worthwhile to note that the polymerization of deuterated monomer 46 was carried out in solution, and the increase of the number of methyl branches begins at ∼85% monomer conversion, far beyond the critical conversion ( Figure 7). As we discuss below, under such conditions, the concentration of polymer chains is extremely high, 48 and the monomer has to diffuse to the polymer-rich phase from the vapor phase and the suspension medium.…”

Section: Scheme 2 Formation Of Defect Structures After Head To Head mentioning

confidence: 97%

New Insight into the Formation of Structural Defects in Poly(Vinyl Chloride)

et al. 2005

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The monomer conversion dependence of the formation of the various types of defect structures in radical suspension polymerization of vinyl chloride was examined via both 1H and 13C NMR spectrometry. The rate coefficients for model propagation and intra- and intermolecular hydrogen abstraction reactions were obtained via high-level ab initio molecular orbital calculations. An enormous increase in the formation of both branched and internal unsaturated structures was observed at conversions above 85%, and this is mirrored by a sudden decrease in stability of the resulting PVC polymer. Above this threshold-conversion, the monomer is depleted from the polymer-rich phase, and the propagation rate is thus substantially reduced, thereby allowing the chain-transfer processes to compete more effectively. In contrast to the other defects, the chloroallylic end groups were found to decrease at high conversions. On the basis of the theoretical and experimental data obtained in this study, this decrease was attributed to copolymerization and abstraction reactions that are expected to be favored at high monomer conversions. Finally, a surprising increase in the concentration of the methyl branches was reported. Although a definitive explanation for this behavior is yet to be obtained, the involvement of transfer reactions of an intra- or intermolecular nature seems likely, and (in the latter case) these could lead to the presence of tertiary chlorine in these defects.

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Section: Scheme 2 Formation Of Defect Structures After Head To Head mentioning

confidence: 97%

New Insight into the Formation of Structural Defects in Poly(Vinyl Chloride)

et al. 2005

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“…[9] The saturated and unsaturated end groups shown in Scheme 1 are the most abundant end groups in PVC. [2,8] The head-to-head addition followed by 1,2-chlorine migration was originally proposed by Rigo et al for the formation of short chain branches in PVC, and later on confirmed by Starnes et al [12,13] There is a consensus in the literature that the Cl-shift reaction proceeds rapidly, so most of the HH linkages formed in the chain by head-to-head additions will immediately disappear and lead to the formation of defect structures as described above. [8,12] Starnes even claims that the head-to-head radical does not add to monomer but always rearranges by a 1,2-chlorine shift of the b-chloro substituent.…”

Section: Introductionmentioning

confidence: 89%

“…[2,8] The head-to-head addition followed by 1,2-chlorine migration was originally proposed by Rigo et al for the formation of short chain branches in PVC, and later on confirmed by Starnes et al [12,13] There is a consensus in the literature that the Cl-shift reaction proceeds rapidly, so most of the HH linkages formed in the chain by head-to-head additions will immediately disappear and lead to the formation of defect structures as described above. [8,12] Starnes even claims that the head-to-head radical does not add to monomer but always rearranges by a 1,2-chlorine shift of the b-chloro substituent. [7,9] The internal HH defect structures in PVC are believed to be formed mainly by another mechanism, that is, recombination of two propagating radical chains with a chlorine atom attached to the radical carbon (i.e.…”

Section: Introductionmentioning

confidence: 89%

“…[8][9][10][11] It is accepted that this well-known type of defect structure is mainly introduced in the chain by the mechanism described above. [9,12] The methyl branch is not thermally unstable since there are no chlorine atoms bonded to the tertiary carbon. [3] Instead of further propagation after the Cl-shift, the chlorine attached to the third carbon atom can be abstracted by a monomer unit, producing an 1-chloro-2-alkene end group (EA, end allylic structure).…”

Section: Introductionmentioning

confidence: 99%

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Ab Initio Study of Poly(vinyl chloride) Propagation Kinetics: Head‐to‐Head versus Head‐to‐Tail Additions

2007

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The relative importance of head-to-head versus head-to-tail additions during the free-radical polymerization of vinyl chloride is determined by ab initio methods for different chain lengths of the polymer. First, a level of theory study is performed to determine cost-effective methods for the ab initio description of the propagation kinetics of vinyl chloride. The study includes the following DFT-based methods: B3LYP, B3PW91, BHandH, BHandHLYP, BLYP, BP86, MPW1K and MPW1PW91, in combination with double or triple zeta basis sets 6-31G(d) and 6-311G(d,p). Also, the more recently developed BMK and MPW1K functionals are included. The influence of diffuse functions is tested by comparison with the basis sets 6-31+G(d) and 6-311++G(3df,2p). The best-performing methods are B3LYP, B3PW91 and MPW1 K combined with the 6-31+G(d) basis set. The converged probability of head-to-head propagation (2 per 1000 monomer units) is put into relation with the experimental concentrations of defect structures. A comparison is made with the head-to-head (HH) content of fluorine-substituted polymers and poly(vinyl acetate). The ab initio calculations correctly predict the relative sequence of HH content among the various polymers.

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“…Moreover, no H-shifts have been reported in solution. 29 From Figure 2 it can be seen that the formation of methyl branches involves the presence of different types of radicals in the reaction mixture, which are depicted as R (s) (s = 1, ..., 7). Therefore, when modeling the formation of structural defects in general and of chloromethyl branches in particular, it is necessary to account for the presence of these different radical types, as will be discussed in section 2.3.…”

Section: Kinetic Modelmentioning

confidence: 99%

Microkinetic Modeling of Structural Properties of Poly(vinyl chloride)

2009

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The free radical suspension polymerization of vinyl chloride is modeled at the elementary reaction level, systematically taking into account diffusion limitations. By deriving balances for structurally distinct polymer and radical species the formation of structural defects can be followed: monomer conversion, averages of the molar mass distribution and the structural defects content can be calculated as a function of polymerization time for industrially relevant conditions. A good agreement between literature reported and calculated values for these properties is obtained for a polymerization temperature range of 325-340 K, a dicetyl peroxydicarbonate and dimyristyl peroxydicarbonate initiator concentration range of 0.00069-1.0 wt %, and a monomer conversion range of 12-96%. The increase of the branching content and the decrease of the terminal double bond content at high monomer conversions can be explained in terms of a favoring of monomolecular reactions and the increasing importance of addition reactions to unsaturations at these conditions.

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Mechanism for the Formation of Chloromethyl Branches in Poly(vinyl chloride)

Cited by 73 publications

References 8 publications

New Insight into the Formation of Structural Defects in Poly(Vinyl Chloride)

New Insight into the Formation of Structural Defects in Poly(Vinyl Chloride)

Ab Initio Study of Poly(vinyl chloride) Propagation Kinetics: Head‐to‐Head versus Head‐to‐Tail Additions

Microkinetic Modeling of Structural Properties of Poly(vinyl chloride)

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