Mechanism and Stereocontrol in Isotactic rac-Lactide Polymerization with Copper(II) Complexes

Abstract

Reaction of N-R,N′-R′-2,5-diiminopyrroles (R = R′ = S-CH(Me)Ph; R = R′ = CH2Ph; R = S-CH(Me)Ph, R′ = H) with Cu(OMe)2 in the presence of chelating alcohols, ROH (R1 = C2H4NMe2, R2 = C2H4Py, R3 = CH2Py, R4 = CMe2Py) yielded the dinuclear, alkoxide-bridged complexes L2Cu2(OR)2. The complexes catalyze the polymerization of rac-lactide at room temperature with catalyst concentrations of 1–3 mM in 4–24 h (v = k[cat][monomer] with k = [2.3(5)] × 102 – [6.5(6)] × 102 M–1 h–1). EPR and mechanistic studies indicate that the complexes remain dinuclear during the polymerization reaction. In complexes with OR1, both alkoxides of the dimer initiate polymerization, with OR2 or OR3 only one alkoxide initiates polymerization, and OR4 is inactive in polymerization. The nature of the bridging ligand in the dinuclear complex determines stereocontrol. Independent of the spectator ligand L, complexes which retain an OR3 or OR4 bridging ligand in the active species show preference for isotactic polymerizations (Pm = 0.60–0.75), while those with only polymeryloxo bridges or OR2 as the bridging ligand provide atactic polymer. Stereocontrol follows a chain-end control mechanism, with the catalytic site likely adapting to the configuration of the chain end.