The influence of intermediate radical termination and fragmentation on controlled polymer synthesis via RAFT polymerization

Abstract

Reversible addition-fragmentation chain transfer (RAFT) polymerization allows the design of tailored polymers by inducing addition and fragmentation reactions via a powerful kinetic mechanism; the process, however, is not well understood. A parametric analysis was conducted in conjunction with experimental data to provide insights into the RAFT mechanism, accounting for the key kinetics events in RAFT polymerization of styrene. The effects of overall, forward and backward fragmentations of intermediate radicals were analysed with experimental data and sensitivity investigations. Our results show that the intermediate radicals undergo relatively fast fragmentation and that the radical cleavage is symmetrical. These factors, critical in design and control, were found to profoundly influence the process kinetics and the final polymer product attributes.

Allylthioketone Mediating Radical Polymerization of Styrene

“…One requisite for this equilibrium reaction is that initiation and cross‐termination of the intermediate radical (I) must be prohibited, so, distinct stability and highly steric hindrance of the intermediate radical (I) are required. However, cross‐termination of the intermediate radicals with the growing chain radicals may occur in the RAFT polymerization, which will remove the active radicals from the polymerization system leading to lowering of the polymerization rate . How can such cross‐termination be prohibited?…”

“…However, cross-termination of the intermediate radicals with the growing chain radicals may occur in the RAFT polymerization, which will remove the active radicals from the polymerization system leading to lowering of the polymerization rate. [14][15][16][17] How can such cross-termination be prohibited? Clearly clarifying this question is not easy owing to the complicated situation that the cross-termination reaction and the fragmentation reaction are intertwined with two overlapping equilibria, [10] therefore, designing an appropriate polymerization system is required for better understanding the mechanism of RAFT polymerization.Besides the extensively studied ATRP, NMP, and RAFT poly merization, there are other controlled radical polymerizations, such as use of 1,1-diphenylethene (DPE) as mediating agent, [18,19] iodine-mediated polymerization, [20,21] among them, the thioketone-mediated radical polymerization received our special attention because the thioketone is a highly efficient radical spin trapping agent.…”

“…However, cross-termination of the intermediate radicals with the growing chain radicals may occur in the RAFT polymerization, which will remove the active radicals from the polymerization system leading to lowering of the polymerization rate. [14][15][16][17] How can such cross-termination be prohibited? Clearly clarifying this question is not easy owing to the complicated situation that the cross-termination reaction and the fragmentation reaction are intertwined with two overlapping equilibria, [10] therefore, designing an appropriate polymerization system is required for better understanding the mechanism of RAFT polymerization.…”