Study of ketalization reaction of poly(vinyl alcohol) by ketones. VI. Reaction between poly(vinyl alcohol) and cyclic ketones and behavior of poly(vinyl ketal) in water

The ketalization reaction of poly(vinyl alcohol) (PVAL) by cyclic ketones, with dimethyl sulfoxide (DMSO) as solvent, in the presence of an acidic catalyst in homogeneous system, was carried out. The synthesis of poly(vinyl ketals) was thus successfully performed, except for the case of poly(vinyl ketal) highly ketalized by cyclohexanone; these poly(vinyl ketals) with a ketalization degree of more than 60 mol % were insoluble in DMSO, and thus, the reaction could not be carried out in a homogeneous system. The equilibrium constant at 40°C was ca. 0.50 in the case of cyclohexanone and ca. 0.35 in the case of cyclopentanone, some 10-50 times higher compared with the case of aliphatic ketones. Because the heat of reaction is 7.5 kcal/mol in all ketones, all ketalization reactions are considered to proceed by the same reaction mechanism. Films prepared from the poly(vinyl ketals) were soaked in water, and hydrolysis, degree of swell, and solubility were measured. The dissolution time of films is affected by the kind of the ketones, ketalization degree, and the pH of water, which reveals that deketalization reaction proceeds proportionally to the proton concentration. It is more difficult to dissolve poly(vinyl ketal) obtained by cyclohexanone than that by cyclopentanone. The rate of hydrolysis of poly(vinyl ketal) film obtained by cyclohexanone is nearly equal to that by methyl butyl ketone, and that by cyclopentanone is nearly equal to that by methyl propyl ketone. The contact angle, surface free energy, moisture regain, and water permeability of poly(vinyl ketal) films were measured. All the results show that poly(vinyl ketal) obtained from cyclohexanone is a more hydrophobic polymer than that from cyclopentanone. The hydrophobicity seems to depend upon the kind of the original ketones and the flexibility of the ring.