Chain Linked Lactic Acid Polymers: Polymerization and Biodegradation Studies

The two-step polymerization method, including polycondensation and chain linking reactions, was conducted to obtain high-molecular-weight lactic acid polymers. The biodegradability of these polymers was subsequently investigated. Three polymerization routes were studied. Hydroxyl-terminated prepolymers were linked with diisocyanates, 1,6-hexamethylene diisocyanate (HMDI) or 1,4-butane diisocyanate (BDI), to produce poly(ester-urethanes) (PEU). Carboxyl-terminated prepolymers were linked with 2,2’-bis(2-oxazoline) (BOX) to produce poly(ester-amides) (PEA). In addition, lactic acid oligomers having both carboxyl and hydroxyl end-groups were linked, with sequential or simultaneous addition of HMDI and BOX, to produce both urethane and oxamide bonds in the lactic acid polymer (PEUA). The structures of novel chain linked lactic acid polymers were identified and the polymerization behavior of carboxyland hydroxyl-reactive chain extenders with prepolymers was carefully evaluated with the use of SEC, NMR, and FTIR. BOX was found to be simultaneously an effective chain coupling agent and acid value reducer for lactic acid based prepolymer, whilst also increasing the thermal stability of PLA polymers. Side-reactions were detected, which can be utilized to obtain branches and crosslinks to PEU and PEUA during chain linking polymerizations. Amide groups, formed in the reaction between HMDI and the COOH group, played an important role in the branching and crosslinking, rather than the oxamide or urethane groups. Also, the mode of addition of chain extenders had a considerable effect on the branching. The biodegradability of lactic acid polymers prepared by chain linking was demonstrated using hydrolysis and a controlled compost test. The quality of the compost after biodegradation was evaluated with biotests. All the polymers biodegraded to over 90% of the positive control in six months, which is the limit set by the CEN standard. Toxicity was detected with the Flash test and plant growth tests in PEU samples, where chain linking of lactic acid oligomers had been carried out with 1,6-hexamethylene diisocyanate. All other polymers showed no toxicological effect. The results clearly showed that 1,6-hexamethylene diisocyanate should not be used as a building block in biodegradable polymers on account of the environmental risk.