Group iv complexes containing the benzotriazole phenoxide ligand as catalysts for the ring-opening polymerization of lactides, epoxides and as precatalysts for the polymerization of ethylene

Abstract

A series of Ti(IV), Zr(IV) and Hf(IV) benzotriazole phenoxide (BTP) complexes were synthesized and characterized by various spectroscopic techniques, elemental analysis and X-ray crystallography. The monosubstituted Zr(IV) BTP complexes [(μ-L)Zr(OⁱPr)₃]₂1–3 [L = ^C1BTP-H (1), ^TClBTP-H (2), ^pentBTP-H (3)] and tetrasubstituted Zr(IV), Hf(IV) complexes ZrL₄4–6 [L = ^C1BTP-H (4), ^TClBTP-H (5), ^pentBTP-H (6)] and HfL₄7–9 [L = ^C1BTP-H (7), ^TClBTP-H (8), ^pentBTP-H (9)] were prepared by the reaction of Zr(OⁱPr)₄·(ⁱPrOH) and Hf(O^tBu)₄ in toluene with the respective ligands in different stoichiometric proportions. The reaction between BTP and TiCl₄ and ZrCl₄ and HfCl₄ in a 2 : 1 stoichiometric reaction resulted in the formation of disubstituted group IV chloride complexes L₂MCl₂10–12 [L = ^C1BTP-H, M = Ti, Zr and Hf]. The molecular structures of complexes 1, 4, 7, 10, 11, and 12 were determined by single-crystal X-ray studies. The X-ray structure of 1 reveals a dimeric Zr(IV) complex containing a Zr₂O₂ core bridged through the oxygen atoms of the phenoxide groups. Each Zr atom is distorted from an octahedral symmetry. These complexes were found to be active towards the ring-opening polymerization (ROP) of L-lactide (L-LA) and rac-lactide (rac-LA). Complex 1 produced highly heterotactic poly(lactic acid) (PLA) from rac-LA under melt conditions with narrow molecular weight distributions (MWDs) and well controlled number average molecular weights (M_n). Additionally, epoxide polymerizations using rac-cyclohexene oxide (CHO), rac-propylene oxide (PO), and rac-styrene oxide (SO) were also carried out with these complexes. The yield and molecular weight of the polymer was found to increase with the extension of reaction time. Compounds 1–12 were activated by methylaluminoxane (MAO) and show good activity for ethylene polymerization and produced high molecular weight polyethylene.