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(O iPr)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 L 2MCl210-12 [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 (Mn). 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. © 2013 The Royal Society of Chemistry.