Ring opening polymerization of rac-lactide by group 4 tetracarbamato complexes: activation, propagation and role of the metal

Abstract

A series of group 4 metal tetracarbamates M(O₂CNR₂)₄ (M = Ti, R = Et, 1a; M = Zr, R = Et, 1b; ⁱPr, 1c; M = Hf, R = Et, 1d; R = ⁱPr, 1e) were studied as catalytic precursors in the solution polymerization of rac-lactide. The titanium complex but not the zirconium and hafnium ones increase the activity by addition of ⁱPrOH. The structure of the carbamato ligand markedly influences the molar mass of polymer; the complexes with isopropyl carbamato ligands produced PLA with molar masses up to 94 000 g mol⁻¹. The main mechanistic aspects of the initial stages of the polymerization reactions were outlined by spectroscopic and computational analyses. In the case of zirconium and hafnium complexes, an interaction between a carbamato ligand and the CH unit of one lactide molecule is established at room temperature. This interaction is followed by the high temperature proton transfer from the lactide to the carbamato O-atom, affording a catalytic active alkoxy complex with release of CO₂ and NHR₂. The polymerization mediated by Ti(O₂CNEt₂)₄ involves the release of a radical fragment [O₂CNEt₂]˙, with consequent generation of a Ti(III) center. The propagating chain is an alcoholate ligand coordinated to a Ti(IV) centre and containing a radical mainly localized at the tail of the chain (DFT, EPR).