Absorbable copolyesters entailing hexamethylene oxalate repeat units were synthesized and studied to access the effect of incorporating isomorphic sequences on the properties of these absorbable systems. The chemical identity and physical propertieswere determined using infrared spectroscopy (IR), nuclear magnetic resonance (NMR), X-ray diffraction, and differential scanning calorimetry (DSC). The effect of compositional changes on thermal transitions, percent crystallinity, and absorption profiles was emphasized. Copolymers with non--isomorphic sequences with aromatic moieties were studied in the same manner. It was noted that the isomorphic systems maintained a relatively high crystallinity throughout the entire range of composition. Furthermore, hydrolytic stability increased with changes in composition, particularly with the increase in cyclic content and presence of aromatic moieties. Histological evaluations of 3 mm diameter transcortical pins at 4 and 8 weeks post-operatively did not reveal any significant differences (p=0.10) between the oxalate-based polymers and poly(p-dioxanone) (PDS) controls. None of the implants elicited a toxic or severe hard or soft tissue reaction as implanted.The adverse tissue reactions and uncontrollable absorption rates associated with natural absorbable polymers, such as collagen, stimulated the search for synthetic systems. Limitations identified with the lactone systems that are commercially available, including the presence of impurities (i.e. monomers), narrow windows of absorption profiles for structurally useful materials, and sterilization constraints, have lead to investigations of different compositions (1,2). Among those are oxalate-based polymers which are a relatively new system of absorbable polymers. They are synthesized by step-growth polymerization, and therefore, avoid monomer complications associated with ring-opening polymerization. Oxalates were developed by Shalaby and coworkers in the 1970s, who initially focused on the simplest form of alkylene oxalate polymers for suture coating purposes (3). The hydrolytic stability of these systems was reported to decrease primarily with an increase in methylene fraction in the repeat unit, and the melting temperature was found to approach that of polyethylene. To increase the Tm of alkylene oxalate polymers, copolymers of cyclic and aromatic diols were synthesized. In particular, polyesters based on lA-trans-

Effect of Structure on Properties of Absorbable Oxalate Polymers

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