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 effective carboxyl and hydroxyl reactive chain extenders. The chain extending behaviour of 2,2′-bis(2-oxazoline) (BOX) and 1,6-hexamethylene diisocyanate (HMDI) with two different oligomers was followed by means of acid value (AV) and molecular weight increments measured by size exclusion chromatography (SEC). Both chain coupling agents were found to react selectively with end-groups, forming oxamide and urethane groups as characterised by spectroscopy (FTIR and 1H 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 addition, an improvement in the thermal stability of the resulting polymers was observed with excess amount of BOX. The material properties of the polymers were characterised by dynamic mechanical thermal analysis (DMTA), dynamic rheometry and tensile testing. Introduction of oxamide groups into the polymer structure increased the chain stiffness, which was detected in mechanical properties and increment in glass transition temperature.