Efficient catalysts for ring-opening polymerization of ε-caprolactone and β-butyrolactone: Synthesis and characterization of zinc complexes based on benzotriazole phenoxide ligands

A series of efficient zinc catalysts supported by sterically bulky benzotriazole phenoxide (BTP) ligands are synthesized and structurally characterized. The reactions of diethyl zinc (ZnEt₂) with ^CMe2PhBTP-H, ^t-BuBTP-H, and ^TMClBTP-H yield monoadduct [(μ-BTP)ZnEt]₂ (1-3), respectively. Bisadduct complex [(^t-BuBTP)₂Zn] (4) results from treatment of ZnEt ₂ with ^t-BuBTP-H (2 equiv.) in toluene, but treatment of ^TMClBTP-H with ZnEt₂ in the same stoichiometric proportion in Et₂O produces five-coordinated monomeric complex [( ^TMClBTP)₂Zn(Et₂O)] (5). The molecular structures of compounds 1, 4, and 5 are characterized by X-ray crystal structure determinations. All complexes 1-5 are efficient catalysts for the ring-opening polymerization of ε-caprolactone (ε-CL) in the presence of 9-anthracenemethanol. Experimental results indicate that complex 3 exhibits the greatest activity with well-controlled character among these complexes. The polymerizations of ε-CL and β-butyrolactone catalyzed by 3 are demonstrated in a "living" character with narrow polydispersity indices (monomer-to-initiator ratio in the range of 25-200, PDIs ≤1.10). The character of 3 provides a way to synthesize as much as 16-fold polymer chains of poly(ε-CL) (PCL) with narrow PDI in the presence of a catalyst in a small proportion. The controlled fashion of complex 3 also enabled preparation of the PCL-b-poly(3-hydroxybutyrate) copolymer.