Polymerization of lactide with zinc and magnesium beta-diiminate complexes: Stereocontrol and mechanism

A series of zinc(II) and magnesium(II) alkoxides based upon a beta -diimina te ligand framework has been prepared. [(BDI-1)(ZnOPr)-Pr-i](2) [(BDI-1) = 2-((2,6-diisopropylphenyl)amido)-4-((2,6-diisopropylphenyl)- imino)-2-pente ne] exhibited the highest activity and stereoselectivity of the zinc comple xes studied for the polymerization of rac- and meso-lactide to poly(lactic acid) (PLA). [(BDI-1)(ZnOPr)-Pr-i](2) polymerized (S,S)-lactide to isotacti c PLA without epimerization of the monomer, rac-lactide to heterotactic PLA (P-r = 0.94 at 0 degreesC), and meso-lactide to syndiotactic PLA (P-r = 0. 76 at 0 degreesC). The polymerizations are living, as evidenced by the narr ow polydispersities of the isolated polymers in addition to the linear natu re of number average molecular weight versus conversion plots and monomer-t o-catalyst ratios. The substituents on the beta -diiminate ligand exert a s ignificant influence upon the course of the polymerizations, affecting both the degree of stereoselectivity and the rate of polymerization. Kinetic st udies with [(BDI-1)(ZnOPr)-Pr-i](2) indicate that the polymerizations are f irst order with respect to monomer (rac-lactide) and 1.56 order in catalyst . Polymerization experiments with [(BDI-1)(MgOPr)-Pr-i](2) revealed that th is complex is extremely fast for the polymerization of rac-lactide, polymer izing 500 equiv in 96% yield in less than 5 min at 20 degreesC.