(743e) Controlling Molecular Weight of Poly(2- Hydroxyethyl Methacrylate) with Keeping Low Dispersity

Introduction

When ultrasound is irradiated to a solution containing vinyl monomer, thermal decomposition of monomers in or around the local high temperature spot takes place to generate the radical species, which act as an initiator for radical polymerization. Ultrasonic irradiation also causes an extremely high shear flow. Polymer chains are fractured by the stress of the high share flow. In the polymer synthesis process under ultrasonic irradiation, both polymerization and polymer degradation occurs simultaneously. The higher ultrasonic power intensity resulted in the lower molecular weight and the lower molecular weight distribution (i.e., dispersity). It is hard to synthesize the polymer with high molecular weight and low dispersity when the ultrasonic power intensity was constant during the reaction.

In this study, we developed the novel method in which the ultrasonic power intensity was changed during the reaction in order to synthesize the polymer with high molecular weight with keeping low dispersity.

Experimental Procedure

The solvent was the mixture of water and ethanol (50 v/v%). 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 ultrasonic irradiation of 600 W/dm³ was started to initiate the reaction. The ultrasonic power intensity was changed from 600 W/dm³ to certain value at 30 min. The reaction temperature was maintained at constant throughout the reaction.

The number average molecular weight and dispersity was measured by using GPC.

Results and Model Simulation

The ultrasonic power intensity was changed from 600 W/dm³ to 0, 150, or 300 Wdm³ at 30 min. The experimental results of number average molecular weight and dispersity are shown in Figure 1. The plots denote the experimental data. The lines are the fitted results and discussed latter. When the ultrasonic power intensity was constant at 600 W/dm³ during the reaction, the number average molecular weight increased during the early stage of the reaction, and then decreased with time. This result suggested that the polymer degradation as well as polymerization occurred under the ultrasonic irradiation. The dispersity decreased with time. This was because the degradation rate of the polymer with larger molecular weight was faster than that with lower molecular weight.

When the ultrasonic power intensity was changed from 600 W/dm³ to 300 W/dm³ at 30 min, the number average molecular weight and dispersity decreased slower than those of the constant ultrasonic power intensity at 600 W/dm³. When the ultrasonic power intensity was changed to 150 W/dm³, the number average molecular weight and polydispersity decreased much slower. Those results were because the lower ultrasonic power intensity gave slower polymer degradation rate.

When the ultrasonic power intensity was stopped at 30 minutes, the number average molecular weight slightly increased. This result suggested that the radicals were captured by solvent to decrease the polymerization rate.

Figure 1 Effect of changing ultrasonic power intensity on time courses of number average molecurar weight and polydispersity

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 radical species were captured by the solvent. The kinetic model was fitted to the experimental results for various ultrasound intensities. The fitted results shown by lines in Figure 1 were in good agreement with the experimental results.