Ring-opening polymerization of cyclic esters was studied using catalysts composed of bulky Lewis acids (LA) and Lewis bases (LB). Controlled polymerization of L-lactide (L-Lac) was proceeded by Al(C6F5)(3). THF in combination with trimesitylphosphine (Mes(3)P) or triphenylphosphine (Ph3P) using BnOH as an initiator to produce poly(L-Lac) with narrow molecular weight distribution (MWD; M-w/M-n=1.1). Both the LA and the LB were indispensable to promote the polymerization. The molecular weights of the resulting poly(L-Lac) s were controlled by the feed monomer to initiator ratio. e-Caprolactone (CL) was rapidly polymerized by Al(C6F5)(3). THF with or without Mes(3)P, although the resulting polymer had rather broad MWD (M-w/M-n = 1.7). The CL polymerization by Al(C6F5)(3). THF alone at r.t. gave poly(CL) with relatively narrow MWD (M-w/M-n=1.2). (C) 2016 Wiley Periodicals, Inc.