Ring-opening polymerisation of rac-lactide mediated by cationic zinc complexes featuring P-stereogenic bisphosphinimine ligands

Abstract

The diastereomerically pure P-stereogenic bis(phosphinimine) ligands 4,6-(ArNPMePh)₂dbf [Ar = 4-isopropylphenyl (Pipp): rac-4, meso-4; Ar = 2,6-diisopropylphenyl (Dipp): rac-4a; dbf = dibenzofuran] were synthesised and complexed to zinc using a protonation-alkane elimination strategy. The cationic alkylzinc complexes thus obtained, RZn[4,6-(ArNPMePh)₂dbf][B(Ar′)₄] [Ar = Pipp, Ar′ = C₆H₃(CF₃)₂: rac-6 (R = Et), meso-6 (R = Et), rac-7 (R = Me) meso-7 (R = Me); Ar = Dipp: rac-6a (R = Et, Ar′ = C₆H₃(CF₃)₂), rac-6b (R = Et, Ar′ = C₆F₅)] were investigated for their competency as initiators for the ring-opening polymerisation of rac-lactide. The formation of polylactide was achieved under relatively mild conditions (40 °C, 2–4 h) and the microstructures of the resulting polymers exhibited a slight heterotactic bias [polymer tacticity (P_r) = 0.51–0.63].