Polyesters containing thiolate chain end were obtained from the "controlled" ring-opening polymerization (ROP) of epsilon -caprolactone or delta -valerolactone initiated by several novel aluminum thiolates. The initiators, [(MOBT)(2)AlR] (1, R = Me; 2, R = Cl) can be obtained in high yield from the interaction of Me3Al or Et2AlCl with 2 molar equiv of 2-methoxybenzenethiol (MOBT-H). [(mu -TMBM)AlR2](2) (3, R = Et; 4, R = Me; 5, R = Bu-i) were prepared from the reactions of R3Al with 1 molar equiv of 2,4,6-trimethylbenzylmercaptan (TMBM-H) in dichloromethane at ambient temperature. The X-ray structures of 1-3 have been determined and have shown that 1 and 2 are monomeric while 3 is dimeric in the solid state. The "controlled" character of the aluminum complex shown in the polymerization process has enabled us to synthesize a P-delta -VL-b-P-epsilon -CL block copolymer, and the thioester end group may provide a suitable method for the design of PCL conjugates. In addition, ROP of L-lactide initiated by 1 can be achieved under refluxing toluene conditions.