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 hig hly reactive 1,6-hexamethylene diisocyanate (HMDI) and 2,2'-bis(2-oxazoline ) (BOX) as chain extenders. The effects were studied of adding the extender s simultaneously and sequentially and in different amounts. The reactions w ere followed and the polymers obtained were structurally characterised by s ize exclusion chromatography and spectroscopy (NMR and FTIR). High molecula r weight poly(lactic acids) were obtained by both simultaneous and sequenti al addition. The mode of addition and the amounts of extenders had a consid erable effect on the branching. The addition of HMDI before BOX in the sequ ential linking produced more highly branched polymers than did the simultan eous addition. Branching and crosslinking reactions were identified as side reactions of the chain extending reactions. The gel formation was attribut ed to the reactions of BOX and HMDI with urethane and amide but not with ox amide. (C) 2001 Elsevier Science Ltd. All rights reserved.