A polymer with a hydrolytically labile backbone and having structure (VIII) wherein R9 is an alkyl, aryl or alkyl aryl group with up to 18 carbon atoms having a pendent carboxylic acid group or the benzyl ester thereof； R12 is an alkyl, aryl or alkyl aryl group with up to 18 carbon atoms having a pendent carboxylic acid ester group selected from straight and branched alkyl and alkyl aryl esters containing up to 18 carbon atoms and ester derivatives of biologically and pharmaceutically active compounds covalently bonded thereto, provided that the ester group is not a benzyl group or a group that is removed by hydrogenolysis； each R7 is independently an alkylene group containing up to four carbon atoms； A is selected from: (a) and (b) wherein R8 is selected from saturated and unsaturated, substituted and unsubstituted alkyl, aryl and alkyl aryl groups containing up to 18 carbon atoms； k is between about 5 and about 3,000； and x and f independently range from zero to less than one. <；IMAGE>；
