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 °C and 60 °C with dicumyl peroxide acting as the photoinitiator. No mid‐chain 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 αs and αl for short‐chain and long‐chain radicals are: αs(VAc) = 0.57 ± 0.05, αs(VPi) = 0.67 ± 0.15, αl(VAc) = 0.16 ± 0.07, and αl(VPi) = 0.16 ± 0.07. The crossover chain lengths differ largely: ic(VAc) = 20 ± 10 and ic(VPi) = 110 ± 30. The rate coefficient for termination of two radicals of chain length unity, , which is the fourth composite‐model parameter, depends on temperature, as does the monomer fluidity.