(671f) Control of Molecular Weight Distribution of Poly(2- Hydroxyethyl Methacrylate) Using Ultrasonic Irradiation

Poly (2- hydroxyethyl methacrylate) (pHEMA) was synthesized using ultrasonic irradiation without chemical initiator. Effects of ultrasound intensity on time course of conversion to polymer, number average molecular weight, and polydispersity were investigated in order to synthesize the polymer with low molecular weight distribution (i.e., low polydispersity).

The solvent was the mixture of water and ethanol (50 v/v%) and 2-hydroxyethyl methacrylate (HEMA) was used as a monomer. The solution containing 0.4 mol/dm3 HEMA was prepared and was deoxygenated by nitrogen. Horn type ultrasonic generator was operated at fixed frequency of 20 kHz. The reaction temperature was maintained at constant throughout the reaction. Conversion to polymer was obtained by measuring the weight of polymer generated, and number average molecular weight and polydispersity was measured by using GPC.

The conversion to polymer increased with time. This result suggested that radical species were generated to initiate the polymerization by ultrasonic irradiation without chemical initiator. The higher ultrasonic intensity resulted in larger conversion to polymer, i.e. faster reaction rate. Number average molecular weight increased during the early stage of the reaction, and then gradually decreased with time. This result suggested that the polymer degradation as well as polymerization occurs under the ultrasonic irradiation. The higher ultrasonic intensity resulted in faster degradation rate of polymer. Polydispersity decreased with time. This is because the degradation rate of the polymer with larger molecular weight was faster than that with lower molecular weight. Polydispersity below 1.3 was obtained under ultrasonic irradiation.

According to the experimental results, the kinetic model for synthesis of pHEMA under ultrasonic irradiation was constructed considering both polymerization and polymer degradation. The rate of initiation reaction was proportional to the ultrasonic intensity. The propagation and termination reactions were same as common polymerization mechanism. The products of the polymer degradation are two polymer radicals and the rate of the polymer degradation was proportional to ultrasonic intensity and the square of the molecular weight. The kinetic model was fitted to the experimental results of time courses of conversion to polymer, number average molecular weight, and polydispersity for various ultrasound intensities. The fitted results were in good agreement with the experimental results.

The number average molecular weight, and polydispersity were simulated by using the kinetic model for various conditions. The higher ultrasonic irradiation results in lower number average molecular weight. However, ultrasonic intensity did not affect the polydispersity. Decreasing ultrasonic intensity from 780 W/dm3 to 190 W/dm3 during the reaction increased the number average molecular weight with keeping low polydispersity below 1.3.