Stereochemistry-Induced Discrimination in Reaction Kinetics of Photo-RAFT Initialization

Abstract

Photoinduced reversible addition–fragmentation chain transfer (photo-RAFT) polymerization combines conventional RAFT polymerization with photochemistry, in which the carbon radicals (R radicals) are generated from RAFT agents (except the cases using external radical photoinitiators) by direct photolysis or photoredox catalysis. As one of the key components, RAFT agent is responsible for living control of photo-RAFT polymerization and various reaction kinetics in the processes of activation/deactivation, propagation, and termination. Despite considerable efforts having been devoted to investigating structural effect of RAFT agents on polymerization kinetics, the impact of stereochemistry in RAFT agents is still underexplored. In this article, we presented a significant stereochemical effect of initial RAFT agents on the kinetics of a photo-RAFT polymerization during the initialization period (the period of consumption of initial RAFT agent). Specifically, two diastereomeric RAFT agents were synthesized and employed for photo-RAFT polymerization. Under a condition of low monomer/RAFT agent ratio, we observed distinct consumption rates of two RAFT agents in separated polymerization systems, with one diastereomer faster than the other. A similar result was observed in the mixed polymerization system (a 1:1 mixture of two diastereomeric RAFT agents), demonstrating a discrimination in RAFT agent consumption in spite of the presence of RAFT chain transfer. Additionally, this discrimination was also present in thermally initiated RAFT polymerization, whereas an opposite trend was observed in the separated systems. The kinetics of initialization processes was analyzed with the aid of density functional theory calculations, which provided plausible explanation of such discrimination. The reported stereochemical effect applies to long dormant RAFT polymer chains beyond the initialization period, in which different stereochemical R groups are always present over the course of RAFT polymerization.