Synthesis of polybutadiene-polylactide diblock copolymers using aluminum alkoxide macroinitiators. Kinetics and mechanism

Polybutadiene-polylactide block copolymers were prepared using well-defined hydroxy-terminated polybutadiene (PBD-OH). PBD-OH samples were reacted wit h 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 gr ows linearly with lactide conversion at [PBD-OH](0)/[AlEt3](0) ratios betwe en 1 and 6 and polymerization temperatures between 70 and 120 degrees C. Na rrow molecular weight distribution block copolymers were obtained under all conditions investigated if the conversion of lactide was kept below 90%. T he kinetics of lactide polymerization were investigated at various [PBD-OH] (0)/[AlEt3](0) ratios. At fixed [PBD-OH](0) using less AlEt3 resulted in si gnificant enhancement in the lactide polymerization rate. We attributed thi s behavior to the aggregation of active species in the polymerization. Whil e the absolute propagation rate constants for the lactide polymerization us ing monoalkylaluminum bisalkoxide and the aluminum trisalkoxide were simila r, increased aggregation of the former lowered the instantaneous concentrat ion of active species and thus lowered polymerization rates.