A new variation of poly(ortho ester)s prepared by the addition of diols to the diketene acetal 3,9-diethylidene-2,4,8,10-tetraoxaspiro[5.5]undecane is described. In this variation, a short segment of a glycolic acid dimer is built into the polymer backbone. Such a material does not require the addition of acidic excipients to control polymer hydrolysis rate, as was necessary in previous systems, and excellent control over erosion rate without attendant problems of loss of excipient by diffusion has been achieved.