How does the single unit monomer insertion technique promote kinetic analysis of activation and initiation in photo-RAFT processes?

Abstract

The reversible addition–fragmentation chain transfer (RAFT) agent plays a key role of “photo-iniferter” that is responsible for photo-initiation, chain transfer and termination in most photo-RAFT polymerization systems. With the merging of a visible light initiation method, photo-RAFT polymerization has achieved significant progress in the last decade. However, kinetic analysis is still less explored due to its inherent complexity in RAFT processes and photochemistry. Encouraged by previous works on thermally initiated RAFT polymerization, we herein investigate the kinetics of the early stage of the photo-RAFT process that comprises photo-activation of the initial RAFT agents and addition of RAFT leaving group radicals to the monomers. Apart from the complex kinetics, these reactions are generally too fast to investigate in RAFT polymerization. We employ the single unit monomer insertion (SUMI) technique to simplify and slow down these processes, in which chain propagation is absent. An in-depth look at the kinetics of the initiation process and establishment of kinetic models is thus possible. To this end, three model RAFT agents with the same terminal units and different penultimate units were synthesized and used as photo-iniferters for further single monomer additions. The effect of the penultimate units on the consumption rates of these RAFT agents has been demonstrated to be significant. A kinetic model accounting for various rate coefficients was developed to analyze the distinct rates of RAFT agent consumption. The kinetic rate equation reveals that the apparent RAFT consumption rate coefficient is positively correlated to the rate coefficients of photo-dissociation of the RAFT agents (k_d) and radical-monomer addition (k_p). Density functional theory calculation was performed to study the photo-activation and radical-monomer addition steps, implying that the reaction energies of these two steps are highly influenced by the penultimate units but in the opposite ways. For the contribution to overall RAFT agent consumption rates, the monomer addition is dominant.