Chain-Length-Dependent Termination of Vinyl Acetate and Vinyl Pivalate Bulk Homopolymerizations Studied by SP-PLP-EPR

Bulk homopolymerizations of vinyl acetate and vinyl pivalate are studied by EPR experiments between -65 degrees C and 60 degrees C with dicumyl peroxide acting as the photoinitiator. No midchain radicals are seen, which demonstrates that backbiting plays no role. The chain-length dependence of the termination rate coefficients measured up to 13% monomer conversion is adequately represented by the composite model. The power-law exponents alpha(s) and alpha(l) for short-chain and long-chain radicals are: alpha(s)(VAc) = 0.57 +/- 0.05, alpha(s)(VPi) = 0.67 +/- 0.15, alpha(l) (VAc) = 0.16 +/- 0.07, and alpha(l) (VPi) = 0.16 +/- 0.07. The crossover chain lengths differ largely: i(c) (VAc) = 20 +/- 10 and i(c) (VPi) = 110 +/- 30. The rate coefficient for termination of two radicals of chain length unity, k(t)(1,1), which is the fourth composite-model parameter, depends on temperature, as does the monomer fluidity.