Arm-first star-polymer synthesis in one-pot via alkylborane-initiated RAFT

An alkylborane initiated reversible addition-fragmentation chain transfer (AI-RAFT) process was developed for the synthesis of star-polymers using a one-pot approach at room-temperature in the presence of oxygen. Linear poly(tert-butyl acrylate) arms were first polymerized using a latent trialkylborane-amine initiator, which generated trialkylborane, in situ, and subsequently radicals after reaction with oxygen. Polymerizations were optimized to maximize monomer conversion (~70–80%) and minimize arm-dispersity (~1.10) through the oxygen concentration, initiator concentration, and polymerization time. The oxygen concentration was a critical AI-RAFT parameter, providing maximum conversion at a ~0.5:1 molar ratio of oxygen-to-initiator. After arm-polymerization, multifunctional acrylates were injected into the reactor to commence crosslinking without intermediate purification. The impact of the crosslinking time and the crosslinker's functionality, concentration, and injection time were investigated to enhance arm incorporation and diminish star-polymer dispersity, quantified by deconvolution of size-exclusion chromatography data. Crosslinker concentration had the largest influence on arm conversion with optimal concentrations at a 20-25-fold excess to chain transfer agent. Under optimal conditions, arm conversions were maximized to ~75–85% and star-dispersity minimized to ~1.35–1.50. Herein an initial effort is made toward the synthesis of star-polymers with well-defined structures and high-arm conversions, while also striving for oxygen tolerance, minimal purification, low-temperatures, and metal-free conditions.