The random copolymers poly(LA-ran-CL) have improved properties of degradability, mechanical strength, elasticity, and permeability over the PLA and PCL homopolymers. However, the synthesis of such copolymers is still a great challenge in polymer chemistry. In this contribution, we develop a simple but well-designed phenoxyimine Al complex (4) with bulky Ph2CH groups, which achieves controlled random copolymerization of rac-LA and -CL in a living manner (D=1.06-1.09). The reactivity ratios of rac-LA and -CL (r(LA) and r(CL)) are 1.09 and 1.05 and the average sequence lengths of the lactidyl unit (L-LA) and the caproyl unit (L-CL) are in the range of 1.9-2.0 at different stages of conversion. In marked contrast, Al complexes (1-3) having less bulky substituents on the ligands only produce gradient copolymers. Furthermore, this strategy of catalyst design would be readily extended to other catalytic systems including -ketiminato Al complex (5). (c) 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 611-617