BLOCKY STRUCTURE OF COPOLYMERS OBTAINED BY PD/C-CATALYZED HYDROGENOLYSIS OF BENZYL PROTECTING GROUPS AS SHOWN BY SEQUENCE-SELECTIVE HYDROLYTIC DEGRADATION IN POLY(BETA-MALIC ACID) DERIVATIVES

The pendant benzyl groups of benzyl-protected poly(carboxylic acid) can be cleaved totally or partially by Pd/C-catalysed hydrogenolysis. Partial hydrogenolysis has been suspected to yield copolymers in which acid and acid benzyl ester units are not distributed at random. In order to demonstrate the blocky structure of such copolymers, five samples of poly(benzyl malolactonate), a degradable functional polyester, have been hydrogenolysed under different conditions. It has been found that hydrogenolysis is more complex than suspected, and yields mixtures of homopolymers and/or copolymers depending on the progress of the reaction and on parameters such as the nature of the solvent and catalyst and the macromolecular characteristics of the starting polyester. The ability of these hydrogenolysates to form aggregates has been studied by aqueous size exclusion chromatography. Ageing of aqueous solutions of aggregate-forming copolymers in two different aqueous media yielded non-degradable benzyl ester-rich oligomers, whose degree of polymerization was too large to correspond to the sequences normally present in random copolymers of similar composition. Therefore, it is concluded that malic acid sequences degrade much faster than do malic acid benzyl ester sequences, and that Pd/C-catalysed hydrogenolysates that form aggregates are blocky copolymers. © 1990.