Microwave radiation in the synthesis of urethane prepolymers

Abstract

This paper describes the use of microwave radiation in the synthesis of urethane prepolymers in bulk (without solvent). The prepolymers were synthesized using 1,6-hexamethylene diisocyanate (HDI) and poly (ε-caprolactone) diol (PCL) at a molar ratio of 4:1. The reaction was carried out without a catalyst in the reactor with a conventional heating system (oil bath) or in the microwave reactor (MW Reactor NOVA 09) at temperatures of 60, 80 and 100°C. The progress of the reaction was monitored based on the degree of HDI and PCL conversion, as assessed by acidimetric titration of non-bonded isocyanate groups (0–120min). The chemical structure of the resulting products was determined via Fourier Transform Infrared Spectroscopy. Size exclusion chromatography (GPC) was applied to determine the molecular weight distribution of products at different time points of prepolymer synthesis. The obtained results indicate that the use of microwave radiation as a heat source during prepolymer synthesis accelerates the reaction in the temperature range from 60 to 80°C. This finding allows a significant reduction in energy required for heating the reaction mixture. The acceleration of prepolymer synthesis by the application of microwave radiation as a heat source also allows for the elimination of catalysts from the system. The aforementioned catalysts are necessary to perform the prepolymer synthesis with the use of aliphatic diisocyanates and polyols, particularly at temperatures below 100°C. In the case of prepolymers that are further used in the synthesis of biomedical polyurethanes, the catalyst elimination is especially advantageous as the presence of residual catalyst may significantly influence the final material’s biocompatibility and disturb host cell viability.