Polymers are provided that are useful for making biodegradable sustained release agent dispensers and which contain at least one of the following mer units (I) and (II) ##STR1## wherein X is a quadrivalent organic grouping, A and B are hydrogen or lower alkyl, and R is hydrocarbyl or oxyhydrocarbyl of 1 to 14 carbon atoms and, if oxyhydrocarbyl, containing 1 to 4 oxy groups, and may be either aliphatic or aryl, unsubstituted or substituted with one or more lower alkyl, amino, nitro or halogen moieties. Methods of synthesizing the novel polymers are also provided, as biodegradable beneficial agent dispensers prepared using the novel polymers.