MECHANISM AND MICROSTRUCTURE IN THE FREE-RADICAL POLYMERIZATION OF VINYL-CHLORIDE - HEAD-TO-HEAD ADDITION REVISITED

The recent realization that 1,2-dichloroethyl branches occur in poly(vinyl chloride) (PVC) has now led to a full description of the chemical consequences of head-to-head addition during the free-radical polymerization of vinyl chloride (VC). A kinetic analysis of the relevant reaction scheme is developed here and then verified with the aid of literature values for the branch concentrations of PVC samples prepared by polymerization at subsaturation VC pressures. The analysis allows the following conclusions to be drawn: (1) Kinetically free chlorine atoms are not formed from polymeric -CHC1-CH-CH2Cl or -CH2-CH-CHC1-CH2Cl radicals. Loss of chlorine from these radicals results exclusively, instead, from beta scission reactions that transfer chlorine directly to the monomer and are kinetically second-order, overall. (2) The monomer transfer constant that pertains to the transfer occurring after head-to-head addition is, indeed, a true constant that is independent of the concentration of VC. (3) In conventional bulk or suspension VC polymerizations, the rates of the propagation reactions are not controlled by diffusion, even at conversions of ca. 90%. (4) A second type of monomer transfer process probably is responsible for the reductions in polymer molecular weight that result from polymerization at subsaturation VC pressures. This transfer pathway involves the inter- and/or intramolecular abstraction of methylene hydrogen from the polymer by the ordinary head-to-tail macroradicals. The resultant beta-chloroalkyl radicals then transfer chlorine to VC by beta scission.