Phosphoric acids catalyzed asymmetric kinetic resolution polymerization of benzyl substituted ε-caprolactones: Efficient protocol for stereogradient polycaprolactones

An asymmetric kinetic resolution polymerization (AKRP) of 2-benzyl-ε-caprolactone (2-BnCL) and 6-benzyl-ε-caprolactone (6-BnCL) has been developed. Phosphoric acids catalyze the ring-opening polymerizations smoothly, delivering stereogradient polycaprolactones with controlled molecular weights and narrow molecular weight distributions. At 50% conversions, the selectivity factors of this AKRP process are in a range of 1.9–2.5, which indicates clear dynamic differentiations of two enantiomers. Regarding the AKRP of 6-BnCL, a high selectivity factor of 7.2 is obtained at the early stage, and an interesting chiral reduction phenomenon is observed as reaction proceeding. Kinetic studies illustrate the enantiomorphic site control plays dominate role in this AKRP process, and a mismatch of chiral induction and chain steric hindrance is proposed to be responsible for the chiral reduction issue.