Ethyl 2-hydroxy-2-methylpropanoate derivatives of magnesium and zinc. The effect of chelation on the homo- and copolymerization of lactide and ε-caprolactone

Abstract

The preparation, single crystal and molecular structures of the compounds (BDI)Mg(OCMe₂COOEt)(L) are described where BDI = 2-[(2,6-diisopropylphenyl)amino]-4-[(2,6-diisopropylphenyl)imino]pent-2-ene and L = pyridine (py) and 4-dimethylaminopyridine (DMAP). In the solid-state the molecules have five coordinate Mg²⁺ ions that may be considered as derived from a trigonal bipyramid where the chelating BDI and OCMe₂COOEt ligands span equatorial-axial sites. IR spectroscopic data obtained in CH₂Cl₂ indicate that the chelating nature of the OCMe₂COOEt ligand is maintained in solution and variable temperature NMR studies reveal that the ligands py or DMAP undergo a rapid reversible dissociation in toluene-d₈ and especially in THF-d₈. This leads to a facile exchange involving coordinated and free L upon the addition of L and evidence is presented to support the view that this occurs exclusively by a dissociative interchange mechanism. Zinc forms a related complex (BDI)Zn(OCMe₂COOEt) lacking additional coordination of L but maintaining the chelating nature of the OCMe₂COOEt ligand and undergoing reversible coordination of L in the presence of added ligands L. The magnesium and zinc complexes will initiate and sustain the living polymerization of lactide (L- or rac-LA) but not the ring-opening polymerization of ε-caprolactone.