Chain extending of lactic acid oligomers. 2. Increase of molecular weight with 1,6-hexamethylene diisocyanate and 2,2′-bis(2-oxazoline)

Lactic acid polymers were synthesised from oligomers by the addition of highly reactive 1,6-hexamethylene diisocyanate (HMDI) and 2,2′-bis(2-oxazoline) (BOX) as chain extenders. The effects were studied of adding the extenders simultaneously and sequentially and in different amounts. The reactions were followed and the polymers obtained were structurally characterised by size exclusion chromatography and spectroscopy (NMR and FTIR). High molecular weight poly(lactic acids) were obtained by both simultaneous and sequential addition. The mode of addition and the amounts of extenders had a considerable effect on the branching. The addition of HMDI before BOX in the sequential linking produced more highly branched polymers than did the simultaneous addition. Branching and crosslinking reactions were identified as side reactions of the chain extending reactions. The gel formation was attributed to the reactions of BOX and HMDI with urethane and amide but not with oxamide.