From zinc alkyl precursors to a 2,6-bis(imino)pyridine-ligated cationic zinc alkoxide: enabling controlled ring-opening polymerization of ε-caprolactone.

Abstract

Controlled alcoholysis of the cationic alkylzinc complexes [(DiPPBIP)ZnR]+[BArF4]- (ArF = 3,5-C6H3(CF3)2) (1) generates a DiPPBIP-supported cationic zinc alkoxide that acts as an efficient initiator for the ring-opening polymerization (ROP) of ε-caprolactone. Although the alkyl precursors are sufficiently Lewis acidic to form associative adducts with the monomer in solution, they display poor initiating ability owing to the low nucleophilicity of the Zn-C bond. Activation with benzyl alcohol (BnOH) at 80 °C markedly improves the polymerization performance. Optimization of the alcoholysis conditions enabled the cationic zinc benzyloxide [(DiPPBIP)Zn(OBn)]+ (2+) to be identified as the effective initiating species. Variable-temperature NMR experiments showed that 2+ binds one molecule of BnOH in solution to form 2+·BnOH, which undergoes intermolecular exchange with free alcohol and intramolecular alkoxide-alcohol exchange. Attempts to isolate an alcohol-free salt of 2+ instead afforded the trinuclear complex 3, presumably through reaction with trace water. Less nucleophilic fluorinated aryloxide ligands allowed the corresponding monomeric cationic complexes 4 and 5 to be isolated as structural models for 2+. Single-crystal X-ray diffraction revealed coordinated water molecules in both complexes, highlighting the strong Lewis acidity of the Zn(II) centre. Controlled pre-generation of 2+ affords predictable molecular weights and very narrow dispersities, consistent with a well-controlled ROP process under mild conditions.