Microwave-assisted rapid synthesis of poly(butylene succinate): principal effect of microwave irradiation of accelerating the polycondensation reaction

To determine the origin of the reaction rate acceleration in the synthesis of poly(butylene succinate) (PBS) upon microwave irradiation, we herein investigated the effect of the microwave-assisted activation of the reactants, viz. the monomers 1,4-butanediol (BD) and succinic acid (SA), and of the water produced as a by-product during the reaction. The rate of removal of the water by-product from the reaction solution was precisely measured under equivalent reaction conditions (i.e., time and temperature) using both microwave and conventional heating methods. Microwave heating selectively activated and removed the water from the reaction solution more rapidly and to a greater extent than oil bath heating. Measurement of the temperature-dependent dielectric properties of the reactants revealed that the dielectric loss factor (epsilon'') of BD increased with increasing temperature, peaking at 80 degrees C. In contrast, the epsilon'' of the mixture of BD and SA was significantly lower than that of BD alone from 70-140 degrees C. These results indicate that the principal effect of microwave irradiation in the synthesis of PBS was the accelerated removal of the water by-product.