Block copolymers of lactide and poly(ethylene glycol) with various mclar ratios were synthesized. Tin(II) bis(2-ethylhexanoate) has been used as a catalyst. Kinetic measurement and mechanistic studies suggest that the reactivity of the initiator, a hydroxyl group bearing reagent, is an important parameter on the polymerization course. In the case of primary and secondary alcohols, i.e., poly(ethylene glycol) and methyl lactate, it is found that when the initiator concentration exceeds the catalyst concentration, the number of chains formed exceeds the number of catalyst molecules. The chains are propagated through shifts of catalysts from one chain to another. In the case of tertiary alcohols, it appears that the number of chains formed is about the number of catalyst molecules. Therefore, by choosing an appropriate initiator, reaction course and molecular weight as well as molecular weight distribution can be designed. A model based on the assumptions of fast initiation reaction and random propagation reaction is established. Both initiator and catalyst are found to have an influence on the increase of molecular weight and molecular weight distribution.