Rate retardation trends in RAFT – an emerging monomer classification tool?

Abstract

Rate retardation is a common and unusual feature of reversible addition–fragmentation chain-transfer (RAFT) polymerization, where a decline in polymerization rate occurs with higher concentrations of the chain transfer agent (CTA). The strength of retardation depends on the RAFT equilibrium constant between the RAFT intermediate radical and the propagating radical plus CTA. Rate retardation occurs for both the more activated monomers (MAMs) and the less activated monomers (LAMs). Herein, we exploit the ubiquitous phenomenon of RAFT rate retardation to identify the unique RAFT kinetics of MAMs: methyl methacrylate, styrene, phenyl vinyl ketone, methyl acrylate, and dimethyl acrylamide, as well as LAMs: vinyl acetate and N-vinyl pyrrolidone. Investigating the above monomers indicates that LAMs and acrylic/acrylamide monomers show substantial retardation, even using optimized conditions targeting polymers of over a degree of polymerization near 1000. In contrast, highly activated monomers such as styrene, methyl methacrylate, and phenyl vinyl ketone showed only weak retardation under their optimized conditions at near a target degree of polymerization of 300. The analysis indicates that even within the MAM family, there exist differences in monomer reactivity, with acrylic/acrylamide monomers having lower reactivity and higher propensity for retardation, than methacrylic, vinyl ketone, or styrene monomers. The current study highlights how retardation kinetics in RAFT can be used to extract general trends in monomer reactivity and radical stabilization that can be employed to pre-plan polymerization outcomes.