Studies of Coil-Globule Transition and Cross-Linking of Poly(acryloyl-L-proline methyl ester) Prepared by Radiation-Induced Polymerization

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Abstract
Uncross-linked acryloyl-L-proline methyl ester (A-ProOMe) polymer, which was obtained by irradiating up to 30 kGy at 25°C using γ-rays from a 6°Co source, showed a coil-globule transition in water around a lower critical solution temperature (LCST) of approximately 14°C. From results of light-scattering measurements, it was found that the molecular weight (Mw) of the polymer was 964,000 at 4°C, 1,270,000 at 10°C, 56,100,000 at 20°C, 82,200,000 at 35°C, and 98,200,000 at 50°C in water, in contrast to 41, 40, 20, 10, and 8 nm in the above order for the particle radius of gyration (½), in which the pendant ProOMe group with cis-form might occur in big clusters above the LCST temperature. Cross-linked homopolymers were obtained with irradiation at doses more than 80 kGy at 25°C. The results of electron spin resonance (ESR) measurements showed that the formation of cross-linked poly(A-ProOMe) gels is due to the bridging of neighbor ing ProOMe groups. Even after cross-linking, the gels obtained gave characteristic thermo- responses, such as low-temperature swelling and high-temperature deswelling in water.