How chain length dependencies interfere with the bulk RAFT polymerization rate and microstructural control

A pseudo-steady state based (PSSA) deterministic solution strategy is presented to calculate the concentrations of single and multi-arm macrospecies types in reversible addition fragmentation chain transfer (RAFT) polymerization, accounting for chain length dependent apparent rate coefficients to fully reflect the impact of diffusional limitations. The simulation time is of minute scale and diffusional limitation on termination are accounted for with the RAFT-chain length dependent-termination technique. Based on an extensive set of experimental data (18 conditions), addition and fragmentation kinetic parameters are determined for RAFT polymerization of styrene initiated by 2,2'-azobis(2-methylpropionitrile) and 2-cyano-2-propyl dodecyl trithiocarbonate. Due to higher chain length dependent apparent termination rates, a rate retardation with respect to the free radical polymerization (FRP) results despite that a simplified degenerative mechanism can be considered. This rate retardation is enhanced for polymerizations exhibiting a stronger gel-effect (e.g. methyl methacrylate). Then even the average RAFT polymerization characteristics cannot be reliably calculated by approximating the macroradical chain length distribution (CLD) through a Poisson or Schulz-Flory distribution. The calculation of the macroradical CLD always requires a full kinetic model taking into account the chain length dependencies for all individual chain lengths. (C) 2017 Elsevier Ltd. All rights reserved.