Polynuclear titanium complexes for the ring-opening polymerisation of lactide

Abstract

The research presented herein explores the synthesis and characterisation of polynuclear titanium complexes developed as initiators or catalysts for the production of the renewable and green polymer, polylactide. A series of polynuclear titanium isopropoxide initiators, existing as dynamic aggregates held by bridging carboxylate groups tethered to the ligand, were effective initiators in the ring-opening polymerisation of rac-lactide. The nuclearity of these aggregates in solution was related to the steric bulk on the ligand periphery and correlated with increasing activity. Controlled hydrolysis of these aggregated initiators led to the formation of air- and water-stable, tetrametallic, titanium-oxo pre-catalysts that were activated in situ under melt polymerisation conditions. In this case, activity was qualitatively linked to the solubility of the catalysts in molten lactide. These pre-catalysts could be further hydrolysed in the presence of 4-dimethylaminopyridine to form cyclic and linear tetrametallic titanium-oxo species, with Ti4O4 and Ti4O3 cores respectively, which were also active towards the ring-opening polymerisation of lactide. An alternative synthetic route to these species, a one-pot solvothermal synthesis, favoured the formation of cyclic titanium-oxo compounds, where the titanium-oxo nuclearity of the isolated species was found to decrease with increasing steric bulk around the ligand periphery. Overall, the homogeneity of the titanium compounds was key to studying their behaviour in detail, ultimately allowing for meaningful conclusions to be drawn between catalyst design and performance.