Alkylborane initiated RAFT polymerization: impact of carboxylic acid deblockers

Abstract

In this report, we explore the role of carboxylic acid deblockers in an oxygen tolerant, room temperature, and alkylborane initiated reversible addition–fragmentation chain transfer (AI-RAFT) polymerization. Our studies employ an air-stable and coordinated trialkylborane–amine complex as the initiator which upon exposure to deblocker liberates free alkylborane that consumes oxygen and generates radicals to drive RAFT polymerization. Kinetic experiments reveal an enhancement in the polymerization rate when using higher concentrations of propionic acid deblocker and that the carboxylic acid has a partial order of one, aligning with the homolytic substitution mechanism for the autoxidation of alkylboranes. Different carboxylic acid deblockers were also investigated by their relative pK_a values, showing that lower pK_a acids can be employed at greatly diminished concentrations while still providing high monomer conversions, predetermined molecular weights, and narrow dispersity polymers. The findings disclosed in this report provide important kinetic parameters, mechanistic insights, and formulation details which are instrumental in understanding and improving AI-RAFT.