Polybutadiene-polylactide block copolymers were prepared using well-defined hydroxyterminated polybutadiene (PBD-OH). PBD-OH samples were reacted with AlEt3 at various [PBD-OH]0/[AlEt3]0 ratios to form the corresponding aluminum alkoxide macroinitiators. Controlled polymerization of lactide was performed using these macroinitiators. The polylactide molecular weight grows linearly with lactide conversion at [PBD-OH]0/[AlEt3]0 ratios between 1 and 6 and polymerization temperatures between 70 and 120 °C. Narrow molecular weight distribution block copolymers were obtained under all conditions investigated if the conversion of lactide was kept below 90%. The kinetics of lactide polymerization were investigated at various [PBD-OH]0/[AlEt3]0 ratios. At fixed [PBD-OH]0 using less AlEt3 resulted in significant enhancement in the lactide polymerization rate. We attributed this behavior to the aggregation of active species in the polymerization. While the absolute propagation rate constants for the lactide polymerization using monoalkylaluminum bisalkoxide and the aluminum trisalkoxide were similar, increased aggregation of the former lowered the instantaneous concentration of active species and thus lowered polymerization rates.