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SYNOPSISThe hydrophilicity of poly(viny1 alcohol) (PVA) was lowered by acetalization with propionic aldehyde. After UV irradiation of pendent styrylpyridinium groups, these prepolymers form networks of different hydrophobicity depending on the degree of acetalization. PVA films containing different amounts of propionic acetals were doped with 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl. Then, the ESR spectra of the spin probe entrapped in the polymeric network were monitored while allowing different solvents to diffuse into the polymer. The rate of spin probe mobilization depends on the hydrophilicity/hydrophobicity of both the polymer network and the solvent. However, the modified polymers, even the more hydrophobic ones, are hydrophilic in comparison to "really" hydrophobic polymers. This is probably caused by the character of the parent polymer (PVA), unreacted hydroxy groups. Simulations of the ESR spectra have shown that in polymers with a higher degree of acetalization domains of different hydrophobicity are built in which the spin probes have a different mobility. 0 1995 John Wiley & Sons, Inc. INTRODUCTIONThe described polymers are to be used for the entrapment of enzymes by photochemical crosslinking. The formed network containing the entrapped enzyme is employed for enzymatic reactions in organic media. Starting with poly (vinyl alcohol) (PVA), the hydrophilicity of the polymer should be lowered, guaranteeing a higher diffusivity of hydrophobic substrates and products. For that purpose, certain amounts of the polymer's hydroxy groups are blocked by acetalization with propionic aldehyde. Therefore, the synthesized polymers are expected to have a varied hydrophobicity that depends on the degree of hydroxy group conversion with the aldehyde.An interesting subject is to monitor these differences in hydrophobicity of the polymers. Integral methods for the determination of bulk hydrophilicity / hydrophobicity are, e.g., swelling experiments or sorption kinetics. Contact angle measurements CCC OOZl-8995/95/091567-10provide information about the hydrophilicity / hydrophobicity of the outer surface. In this article, we report on ESR spectroscopy using the spin probe technique to determine differences in the hydrophobicity of the synthesized polymers. This technique has several advantages, e.g., the sensitivity and selectivity of the spin probe to changes in its microenvironment, providing the possibility to detect local differences in solvent absorption as well as a high time resolution during the measurements.The spin probe technique has found many applications in polymer research. The mainly used spin probes are stable organic nitroxides. They show an easily identifiable three-line spectrum that sensitively reflects the molecular motion of the radical. In a solid amorphous matrix (e.g., at temperature below the Tg of a polymer), the ESR spectrum corresponds to randomly distributed radicals with slow motion. If the motion increases (e.g., by rising temperature or solvent penetration into the polymer), the s...

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