Chiral phosphoric acid catalyzed asymmetric kinetic resolution polymerization of 6-aryl-ε-caprolactones†
An asymmetric kinetic resolution polymerization (AKRP) of 6-aryl-ε-caprolactones has been developed. As bifunctional organocatalysts, chiral phosphoric acids promoted the synthesis of stereogradient polycaprolactones (PCLs) with controlled molecular weights and narrow molecular weight distribution. The highest enantioselectivity factor was up to 4.1 for the polymerization of 6-p-Cl-C6H4-CL using CPA-3 as an organocatalyst at 44% conversion, indicating that an enantiomorphic site control mechanism was involved. A phenomenon of chirality reduction was observed during the polymerization process, and a mismatch of chiral induction and chain steric hindrance has been proposed.