RAFT polymerization of isopropenyl boronate pinacol ester and subsequent terminal olefination: precise synthesis of poly(alkenyl boronate)s and evaluation of their thermal properties

Abstract

In this work, various types of chain-transfer agents (CTAs) were investigated in the reversible addition-fragmentation chain-transfer (RAFT) polymerization of isopropenyl boronate pinacol ester (IPBpin) toward not only precise synthesis of poly(IPBpin) but also elucidation of its feature as a monomer. CTAs suitable for conjugated monomers were found to be more compatible than those for nonconjugated monomers, likely due to the moderate stabilization of the growing radical by boron. This trend was supported by density functional theory-based calculations for energy balance between the growing IPBpin radical and the chain-transfer species to the CTA. The CTA-derived group at the ω-terminus of the obtained poly(IPBpin) was quantitatively converted into olefin under cobalt catalysis leading to improved stability, enabling evaluation of its well-defined structure by MALDI-TOF-MS and its thermal properties by DSC. The precise synthesis of poly(alkenyl boronate)s was achieved through RAFT polymerization of isopropenyl boronate pinacol ester (IPBpin) and subsequent terminal olefination by cobalt catalysis. Experimental and computational investigation on the compatibility of IPBpin with chain transfer agents revealed its conjugated-monomer type behavior that is ascribed to empty p-orbital of boron. Terminal olefination in post-polymerization reaction improved the stability of resulting polymers, enabling the analysis for molecular structure of poly(IPBpin) by MALDI-TOF-MS spectroscopy as well as thermal properties by DSC.