Chain extending of lactic acid oligomers. Effect of 2,2 '-bis(2-oxazoline)on 1,6-hexamethylene diisocyanate linking reaction

New L-lactic acid polymers were synthesised with the use of highly effectiv e carboxyl and hydroxyl reactive chain extenders. The chain extending behav iour of 2,2'-bis(2-oxazoline) (BOX) and 1,6-hexamethylene diisocyanate (HMD I) with two different oligomers was followed by means of acid value (AV) an d molecular weight increments measured by size exclusion chromatography (SE C). Both chain coupling agents were found to react selectively with end-gro ups, forming oxamide and urethane groups as characterised by spectroscopy ( FTIR and H-1 NMR). Reaction between BOX and carboxyl groups of lactic acid oligomer led to hydroxyl terminated prepolymer with low AV, which provided significant increase of molecular weight in the HMDI linking reaction. In a ddition, an improvement in the thermal stability of the resulting polymers was observed with excess amount of BOX. The material properties of the poly mers were characterised by dynamic mechanical thermal analysis (DMTA), dyna mic rheometry and tensile testing. Introduction of oxamide groups into the polymer structure increased the chain stiffness, which was detected in mech anical properties and increment in glass transition temperature. (C) 2001 E lsevier Science Ltd. All rights reserved.