In Vivo Degradation Characteristics of Poly (β-propiolactone) Prepared by Radiation-Induced Polymerization at Low Temperature

The polymerization rate of β-propiolactone (PL) by irradiation at -78°C in vacuo increased markedly with the irradiation dose of up to 50kGy and then gradually increased at an irradiation dose of up to 200kGy. The yields of poly (PL) obtained at irradiation doses of 50 and 200kGy were 22 and 25%, respectively. The polymer was shaped into a cylindrical form with high rigidity and density by pressing at 200 kg/cm2 in a range of 50°C to 75°C. The in Vivo degradation of cylindrical specimens was checked by implanting subcutaneously in the back of male Wistar rats. The degrees of in Vivo degradation of poly (PL) obtained at 5, 10, 50, 100, 160, and 200kGy irradiations were 5, 9, 25, 34, 37, and 40%, respectively. The viscosity (ηsp/c value) of these polymers showed a decrease tendency with an increase in the irradiation dose. From the relationship between the ηsp/c value and the in Vivo degradation, it was found that the degree of in Vivo degradation had a inflection point at a η>sp/c value of 0.4 dl/g (30kGy irradiation). That is, the in Vivo degradation of poly (PL) with η>sp/c values below 0.4 dl/g (above 30kGy irradiation) was markedly accelerated by a slight decrease of viscosity, but such an acceleration was not remarkable at η>sp/c values aboves 0.4 dl/g (below 30kGy irradiation). This may be primarily attributed to the scission of polymer chains during the irradiation polymerization. The data of X-ray diffraction patterns and DSC curves also showed that the in Vivo degradation is influenced by a change in the crystallinity or by a decrease in the melting point of poly (PL).