Chain transfer to monomer during the free-radical polymerization of vinyl chloride is shown to occur, in part, by a mechanism that begins with the abstraction of methylene hydrogen from the polymer by a propagating macroradicalThe resultant radical then donates a chlorine atom to the monomer to form an allylic structure that can contribute to the thermal instability of poly(vinyl chloride). Double-bond contents found by NMR spectroscopy and their correlation with mo-
During the preparation of poly(vinyl chloride) (PVC) under
low monomer pressures,
intramolecular hydrogen transfers (“backbites”) lead to the
formation of small amounts of 1,3-bis(2-chloroethyl) branch (DEB) structures, as well as allylic chloride
segments resulting from 1,6 shifts. The
presence of these structural defects was established by the
13C NMR spectra (recorded at 125.77 MHz)
of dechlorinated PVC specimens made by organotin hydride reduction.
At 55−80 °C, the two backbites
leading to DEB groups 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 convincing evidence
was obtained for the presence of a chlorinated
2-ethyl-n-hexyl branch (EHB) segment resulting
from
double backbiting by an alternative route. Backbiting effects on
dichlorobutyl branch concentrations
and on chain transfer to monomer are discussed and quantified, and the
synthesis of models for
dechlorinated DEB and EHB structures is described.
NMR examination of reductively dechlorinated poly(vinyl chloride) at a field strength of 125.77 MHz reveals that the ethyl branches in the unreduced polymer are 1,2-dichloroethyl rather than 2-chloroethyl when the polymerization is carried out in an aqueous suspension at 40 or 82 °C. The 1,2dichloroethyl moieties occur in a -CHClCHiCHiCHClC^CbCHaCHClarrangement that results from a head-to-head emplacement of monomer and two successive 1,2 chlorine shifts. These shifts comprise a "billiard-ball" pathway for free-radical polymerization that apparently is without precedent.Reductive dechlorination of poly(vinyl chloride) (PVC)ß a br a ß ß a br a ß Bu Bu-d I P' o' I -ch2-ch2-ch-ch2-ch2--chd-ch2-ch-chd-ch2
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