Interplay of Head, Tail, and Mid-Chain Radicals in Bulk Free-Radical and Reversible Degenerative Addition Fragmentation Chain-Transfer Polymerizations of Vinyl Acetate

A detailed kinetic model for isothermal bulk free-radical and degenerative reversible addition–fragmentation chain-transfer (RAFT) polymerizations of vinyl acetate is presented up to monomer conversions of ca. 0.95, considering a distinction between head and tail end-chain radicals and primary and tertiary mid-chain radicals (MCRs). Diffusional limitations are taken into account for conventional initiation and termination, with a reduction of the gel-effect as chain transfer to the monomer lowers the apparent termination reactivities. The tail radicals are essential for the accurate description of backbiting and RAFT deactivation and the primary MCRs for chain transfer to the polymer. The model is based on the application of the method of moments with over 100 macrospecies types and thousands of reactions to obtain the temporal evolution of the monomer conversion, the number and mass average chain length, the number fraction of short- and long-chain branches, the number fraction of unsaturated chains, and the number fraction of chains with head-to-head defects. After a successful model validation of the experimental literature data, the model is applied to understand the complex interplay of the radical types involved and to highlight the control of the branching and unsaturation amounts under RAFT polymerization conditions. Head-to-head defects can however not be avoided for realistic average chain lengths.