Single-site salen and salan aluminium complexes for stereoselective polymerization of lactides

Abstract

The development of synthetic pathways and catalyst/initiator systems to produce PLAs with narrow molecular weight distribution and controlled molecular weight has been a continuous topic in polymer chemistry. Especially, stereochemistry will be generated when rac-LA or meso-LA are applied in these polymerizations. The aims of this research described in this thesis have been established to develop new single-site five-coordinated Schiff base (salen) and phenoxyamine (salan) aluminum complexes that could exert stereocontrol over lactide polymerization A pilot study on a series of achiral bis(salicylidene) Schiff base aluminum ethyls and three representative aluminum isopropoxides were synthesized (chapter 3). The activity and the stereoselectivity of this series of (in situ-formed) bis(salicylidene) aluminum isopropoxides toward the ROP of rac-LA are described in chapter 4. A series of aluminum ethyl and isopropoxide complexes ligated by N,N,N,N-tetradentate bis(pyrrolidene) Schiff base ligands that act as single-site catalysts for the polymerization of (S,S)-LA to isotactic PLA, rac-LA to predominant isotactic PLA, and meso-LA to atactic PLA were reported in chapter 5. A series of (R,R) enantiomerically pure chiral salan aluminum ethyl complexes 4–6 and their racemic complexes 7–9 were reported in chapter 6. The structures of two different diastereoisomers a and b for these aluminum ethyls were revealed by 1H NMR and X-ray analysis. Further NMR measurements demonstrated a slow exchange between a sqp and a tbp geometry for diastereoisomers b at low temperatures. In the presence of 2-propanol, these aluminum ethyl complexes acted as efficient initiators for the ROP of rac-LA and meso-LA.