NEW APPROACHES TO RING-OPENING POLYMERIZATION-EVIDENCES OF STEP TRANSFER POLYMERIZATION

The ring-opening polymerisation of lactones, by different initiator-ca talyst systems, has been studied. Tin(II) bis(2-ethylhexanoate) has be en used to catalyse the polymerization of lactide with various types o f alcohol initiators. H-1 NMR spectroscopy was used to monitor the pol ymerization process. Mechanistic studies suggest that the polymerisati on course, to a large extent, depends on the reactivity of the initiat ors. In the case of primary or secondary alcohols, e.g. poly(ethylene glycol) and methyl lactate, it is found that when the initiator concen tration exceeds that of the catalyst, the number of chains formed also exceed the number of catalyst molecules. The chains are propagated th rough a shift of the catalyst from one chain to another. GPC chromatog raphy clearly shows oligomer formation in the first stage of the polym erization. In the case of tertiary alcohols, it appears that the numbe r of chains formed is comparable to the number of catalyst molecules. By choosing an appropriate initiator the reaction course and molecular weight can be controlled. A general polymerization mechanism of the r ing-opening polymerisation of this type is briefly discussed.