Anionic Polymerization of L-Lactide Effect of Lithium and Potassium as Counterions

The polymerization of L-lactide was carried out with t-BuOK and t-BuOLi in the presence and absence of crown ethers. Each polymerization showed living nature only in the limited sense of having a decreasing amount of active species at high conversion. The initiator efficiency was low in each case because of the deprotonation reaction between the tert-butoxide ion and L-lactide. Although termination did not take place with lithium containing initiators to 90% conversion, intra- and intermolecular transesterification was observed to a significant degree. With potassium containing initiators both termination and transesterification occurred.