ABA TYPE COPOLYMERS OF LACTIDE WITH POLY(ETHYLENE GLYCOL) - KINETIC, MECHANISTIC, AND MODEL STUDIES

Block copolymers of lactide and poly(ethylene glycol) with various mol ar ratios were synthesized. Tin(II) bis(2-ethylhexanoate) has been use d as a catalyst. Kinetic measurement and mechanistic studies suggest t hat the reactivity of the initiator, a hydroxyl group bearing reagent, is an important parameter on the polymerization course. In the case o f primary and secondary alcohols, i.e., poly(ethylene glycol) and meth yl lactate, it is found that when the initiator concentration exceeds the catalyst concentration, the number of chains formed exceeds the nu mber of catalyst molecules. The chains are propagated through shifts o f catalysts from one chain to another. In the case of tertiary alcohol s, it appears that the number of chains formed is about the number of catalyst molecules. Therefore, by choosing an appropriate initiator, r eaction course and molecular weight as well as molecular weight distri bution can be designed. A model based on the assumptions of fast initi ation reaction and random propagation reaction is established. Both in itiator and catalyst are found to have an influence on the increase of molecular weight and molecular weight distribution.