Unsymmetrical group 4 amine tris(phenolate) complexes as initiators for the controlled polymerisation of racemic-lactide

The controlled polymerisation of the renewable monomer lactide to give the degradable polyester polylactide (PLA) is an attractive area of research as efforts are made to move away from traditional petroleum-based plastics. A C3-symmetric Group 4 amine tris(phenolate) complex has previously been published exhibiting highly stereoselective polymerisation of racemic-lactide to yield heterotactic PLA.1

In this work a series of unsymmetrical amine tris(phenolate) ligands have been synthesised using a step-wise method. Subsequent reaction with Group 4 alkoxides generated a series of dimeric complexes, which have been characterised in the solid state by crystallography and in the solution state by NMR spectroscopy.

We show that all complexes are active for the polymerisation of lactide and data is presented showing polymerisation kinetics and stereoselectivity (probability of racemic enchainment, Pr) for each complex. In the light of this data, we discuss the influence of sterics and complex symmetry on the polymerisation activity and selectivity.