Benzotriazole Phenoxide Hafnium Complexes as Efficient Catalysts for the Ring‐Opening Polymerization of Lactide: Synthesis, Characterization, and Kinetics of Polymerization Catalysis

Abstract

AbstractHafnium complexes bearing bis‐ or tetra‐BTP ligands (BTP = N,O‐bidentate benzotriazole phenoxide) have been synthesized and structurally characterized. The reactions of [Hf(OiPr)4(iPrOH)] with 2‐(2H‐benzotriazol‐2‐yl)‐4,6‐bis(1‐methyl‐1‐phenylethyl)phenol (CMe2PhBTP‐H), 2‐(2H‐benzotriazol‐2‐yl)‐4,6‐di‐tert‐butylphenol (tBuBTP‐H), and 2‐tert‐butyl‐6‐(5‐chloro‐2H‐benzotriazol‐2‐yl)‐4‐methylphenol (TMClBTP‐H) (2.0 mol‐equiv.) in hexane at 30 °C afforded the monomeric hafnium complexes [(CMe2PhBTP)2Hf(OiPr)2] (1), [(tBuBTP)2Hf(OiPr)2] (2), and [(TMClBTP)2Hf(OiPr)2] (3). The bis‐adduct hafnium analogue [(C8BTP)2Hf(OiPr)2] (4) was obtained by similar treatment of the sterically less bulky 2‐(2H‐benzotriazol‐2‐yl)‐4‐(2,4,4‐trimethylpentan‐2‐yl)phenol (C8BTP‐H) as ligand at 0 °C and with a ligand/metal precursor ratio of 2:1, and treatment of [Hf(OiPr)4(iPrOH)] with C8BTP‐H (4.0 mol equiv.) at 30 °C gave the hafnium tetra‐adduct [(C8BTP)4Hf] (5). Single‐site hafnium 4 proved to be an active catalyst in the polymerization of lactide (LA) in solution with “living” and “immortal” characteristics, producing polymers with the expected molecular weights and narrow molecular weight distributions (PDI ≤ 1.30). BTP‐containing Hf complex 5 efficiently catalyzed the polymerization in a controlled fashion even though 160 equiv. of IPA was added to the Hf catalyst present at a low concentration. Kinetic tests for the polymerization of LA were conducted to compare the activities of hafnium alkoxides 1–4; a kinetic study of 4 revealed the polymerizations to be first order in both complex 4 and L‐LA concentrations.