Study on ketalization reaction of poly(vinyl alcohol) by ketones .8. Kinetic study on acetalization and ketalization reactions of poly(vinyl alcohol)

The kninetics of acid-catalyzed acetalization and ketalization of poly(vinyl alcohol) (PVA) were systematically studied in completely homogeneous media with carefully selected solvents. Thus the acetalization reaction was run in water with six aldehydes [R1CHO (R-1 = H, CH3, C2H5, n-C3H7, i-C3H7, ClCH2)], whereas the ketalization in dimethylslfoxide with 11 ketones [R2CH3CO (R-2 = CH3, C2H5, n-C3H7, i-C3H7, n-C4H9, i-C4H9, tert-C4H9, C6H5CH2, C6H5CH2CH2), cyclopentanone, and cyclohexanone]. The latter was difficult to proceed in aqueous media. Both reactions were reversible and bimolecular and, despite the use of different solvents, gave similar heats of reaction (7.5 kcal/mol) and activation energies (ca. 15 kcal/mol) except for the case of formaldehyde and chloroacetaldehyde; however the equilibrium constants at 25 degrees C showed that the acetalization is thermodynamically much more favored than the ketalization (ca. 5000 vs. 0.01-0.9), probably because of steric hindrance of the ketone substrate. The rate constants of hydrolysis (reverse reactions) for the poly(vinyl acetal) and poly(vinyl ketal) followed the Hammett-Taft equation to give a single rho* (=3.60) that is very close to that for the hydrolysis of diethyl acetal and ketal. From these and other data, it was concluded that the polymer hydrolysis, as well as PVA acetalization and ketalization, are all electrophilic reaction where the formation of hemiacetal or hemiketal is the rate-determining step. (C) 1996 John Wiley & Sons, Inc.