Syntheses, structures and catalytic activity of tetranuclear Mg complexes in the ROP of cyclic esters under industrial relevant conditions
Four tetranuclear magnesium complexes were synthesized and fully characterized. They are excellent catalysts for the ring-opening polymerization (ROP) of bulk racemic-lactide (rac-LA) and ɛ-caprolactone (ɛ-CL) even with low catalyst loading under industrially relevant conditions without additional co-initiators, yielding polymers with high molecular weights (Mn) and moderately controlled molecular weight distribution (MWDs). The polymerization activity depends on the steric and electronic properties of the imino(phenolate) ligands. Kinetic studies for the most active catalyst 1 verified that the ROP of rac-LA and ɛ-CL has a first order dependence on the monomer concentration. ROP of rac-LA and ɛ-CL using 1–4 with BnOH ga ve polymers with narrow MWD and a close correlation between MnObs and MnTheo values. Catalyst 1 was also capable of polymerizings technical-grade rac-LA and ɛ-CL even with large monomer-to-catalyst ratios ([M]0/[C]0) of up to 10000:1 and without any additional co-initiator. Very high turnover frequencies (TOF) of 9600 h-1 (rac-LA) and 24000 h-1 (ɛ-CL) prove that 1 belongs to the most active biocompatible, sustainable and nontoxic ROP catalysts under industrially relevant conditions.