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 (BTP)-B-CMe2Ph-H, t-BuBTP-H, and (BTP)-B-TMCl-H yield mono-adduct [(mu-BTP)ZnEt](2) (1-3), respectively. Bisadduct complex [(t-BuBTP)(2)Zn] (4) results from treatment of ZnEt2 with t-BuBTP-H (2 equiv.) in toluene, but treatment of (BTP)-B-TMCl-H with ZnEt2 in the same stoichiometric proportion in Et2O produces five-coordinated monomeric complex [((BTP)-B-TMCl)(2)Zn(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 epsilon-caprolactone (epsilon-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 epsilon-CL and beta-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 "immortal" character of 3 provides a way to synthesize as much as 16-fold polymer chains of poly(epsilon-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. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: PolymChem49: 4027-4036, 2011