Stereoselective homo- and co-polymerization of lactides and ε-caprolactone catalysed by highly active racemic zinc(II) pyridyl complexes

Abstract

A series of Zn(II) chlorido complexes (1′–5′) supported by N,N′-bidentate N-(pyridin-2-ylethyl)amine ligands have been synthesized. Zn(II) alkyl and alkoxy complexes 1′-Me–5′-Me and 1′-OBn–5′-OBn were synthesized in situ by first reacting complexes 1′–5′ with methyl lithium and subsequently with benzyl alcohol. Both species 1′-Me–5′-Me and 1′-OBn–5′-OBn showed excellent catalytic activity in ring-opening polymerization (ROP) of cyclic esters with the alkoxyl species performing better. The ROP reactions exhibited pseudo-first-order kinetics with respect to monomer concentration. Polymer molecular weights increased as ligand steric hindrance decreased and they lie between 3096 and 8837 g mol−1 and relatively high molecular weight distributions with dispersity (Ð) values ca. 2 were observed. The poly(rac-lactide) polymers were predominantly heterotactic, while poly(ʟ-lactides) formed were largely isotactic. All polymerization reactions proceeded through coordination insertion mechanism followed by hydrolysis of the metal. Notably, the stereogenic centres of the ligand skeleton influenced control of polymer stereochemistry. Random copolymerization of ε-caprolactone (ε-CL) and lactides (LA) resulted in block gradient copolymers. The sequential addition of lactides after ε-CL gave diblock PCL-b-PLA, and reversing monomer addition did not form any copolymer. Graphical abstract: [Figure not available: see fulltext.]