Hydrolysis of methacrylic acid–methyl methacrylate copolymers

Abstract Several copolymers of methacrylic acid and methyl methacrylate have been prepared with acid contents of 28.8, 50, 60, 66, 72, 83, and 86.5%; they have been hydrolyzed in acidic medium at 110°C. at different neutralization ratios r = HA/A‐. Except for those with the highest acid content, all the copolymers show a two‐step hydrolysis, in which the second step is about 1/6 −1 /7 slower than the first. The order of reaction is one with respect to the acid concentration and one with respect to the ester concentration. On the basis of general catalysis, as suggested by Brønsted, two hydrolysis rate constants k HA and k A ‐ may be evaluated; they correspond to the hydrolysis catalyzed by undissociated carboxyl groups and by carboxylate groups. At 110°C., k HA and k A ‐ are equal, respectively, to 1.5 × 10 5 and 1.0 × 10 5 ; consequently, the rates of hydrolysis are practically insensitive to the ratios of neutralization r , except when r = 3. From comparison of the behavior of these acid‐ester copolymers with that of a methyl methacrylic‐ N ‐vinylpyrrolidone copolymer in the presence of an acid‐buffered solution (acetic tic or polymethacrylic acid), it becomes evident that the hydrolysis reaction consists essentially of a functional interaction between an ester function on one side, and a carboxyl and/or a carboxylate directly neighboring function on the other side. Moreover, on the basis of structural probability calculations, the immediate vicinality effect is insufficient to explain the rates and limited degree of conversion of the hydrolysis reaction. For the copolymers with high acid content (C 72 , C 83 , and C 86.6 ) at r = 3, the rate of reaction is triple that at the other r ‐values. This strong increase of the reaction velocity is, as expected, characterized by an appreciable decrease in the energy of activation (16 kcal. instead of 23 kcal.) This behavior is attributed to a “concerted” mechanism which should be particularly enhanced at r = 3. It has been demonstrated that the stereochemical composition of these copolymers exerts a deep influence on the rate of hydrolysis as well as on the degree of conversion in these reactions.