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A study of poly(vinyl chloride) microstructure
Zaikov, Vadim Guennadievich
Zaikov, Vadim Guennadievich
Abstract
High-field {dollar}\sp{lcub}13{rcub}{dollar}C and {dollar}\sp1{dollar}H NMR spectroscopies were used to investigate some unusual features of the molecular microstructure of poly(vinyl chloride) (PVC).;Several model monochloroalkenes were synthesized in order to determine {dollar}\sp{lcub}13{rcub}{dollar}C shift increments for the replacement of H by Cl at positions that are near an isolated internal double bond. These increments then were used to predict the {dollar}\sp{lcub}13{rcub}{dollar}C shifts of the internal allylic chloride structure in PVC. The predictions were not satisfactory, because, as expected, the increments were not additive.;It was shown that during conventional VC polymerization, the chloroallylic chain end (-CH{dollar}\sb2{dollar}CH=CHCH{dollar}\sb2{dollar}Cl) does not copolymerize with the monomer and is not destroyed by a mechanism involving allylic rearrangement, macroradical addition, and chlorine-atom {dollar}\beta{dollar}-scission to produce a -CHClCH{dollar}\sb2{dollar}CH=CHCH{dollar}\sb2{dollar}CHCl- structure. Nevertheless, that mechanism was found to operate during the preparation of a special type of PVC (made at 0{dollar}\sp\circ{dollar}C with (t-Bu){dollar}\sb2{dollar}Mg initiation) which contained the rearranged chain end, -CH{dollar}\sb2{dollar}-CHClCH=CH{dollar}\sb2,{dollar} at an abnormally high concentration.;During the preparation of PVC under subsaturation VC pressures, small amounts of a 1,3-di(2-chloroethyl) branch structure were found to be formed by a "double backbiting" mechanism involving two intramolecular H abstractions in succession. The presence of this structural defect was established by the 125.77-MHz {dollar}\sp{lcub}13{rcub}{dollar}C NMR spectra of reductively dechlorinated PVC specimens. at 55-80{dollar}\sp\circ{dollar}C, the two backbites leading to the defect differ substantially in relative rate, in that the backbiting:addition rate ratio is larger for the second backbite by a factor of 15-16, irrespective of temperature. No evidence was obtained for the presence of the 2-ethyl-n-hexyl branch structure that would have resulted from double backbiting by an alternative route. These findings were confirmed by spectral comparisons with the {dollar}\sp{lcub}13{rcub}{dollar}C shifts of two separately synthesized models, 9,11-diethylnonadecane and 9-(2-ethyl-n-hexyl)heptadecane.;Polymerizations of VC were performed in the presence of two potential transfer agents, trans-1-chloro-2-hexene and trans-1,5-dichloro-2-pentene. Preliminary examination of the resulting polymers by high-field NMR provided evidence for the destruction of the -CH{dollar}\sb2{dollar}CH=CHCH{dollar}\sb2{dollar}Cl chain end, during polymerization, by a mechanism involving H abstraction to form the -CH{dollar}\sb2{dollar}CH=CHC{dollar}\sp{lcub}\cdot{rcub}{dollar}HCl radical, followed by the addition of that species to VC in order to give the -CH{dollar}\sb2{dollar}CH=CHCHClCH{dollar}\sb2{dollar}- structure.
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1997
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9820442.pdf
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Applied Science
DOI
https://dx.doi.org/doi:10.21220/s2-6vyk-1f86