A series of poly(ethylene oxide) (PEO)/poly(epsilon-caprolactone) (PCL) containing biodegradable poly(ether ester urethane)s, covering a wide range of compositions, were synthesized and characterized. The synthesis consisted of a two-step process. During the first step, the ring-opening reaction of epsilon-caprolactone was carried out, initiated by the hydroxyl terminal groups of the PEO chain. The second step involved the chain extension of these PCL-PEO-PCL trimers with hexamethylene diisocyanate. By varying either the ethylene oxide/epsilon-caprolactone ratio or the length of both segments, we obtained a series of polymers having different morphologies and displaying a broad range of properties.

Low molecular-weight polylactic acid (PLA) was obtained by direct polycondensation of a mixture of 95% L and 5% D-lactic acid isomers, without catalyst, at 195°C. This polymer was used for the synthesis of microspheres by emulsion-solvent evaporation method. Gel Permeation Chromatography (GPC), X-ray Scattering (XRD), Differential Scanning Calorimetry (DSC), and Scanning Electron Microscopy (SEM) techniques were applied to follow morphological and structural changes of particles along in vitro degradation at 37°C. The original microspheres were amorphous but could crystallize partially upon heating. Samples stored in a humid environment exhibited an increase in the crystallization capability upon heating. Initial smoothsurface microspheres were transformed to porous particles at the time of degradation at pH ϭ 7 (37°C). The shape of mass loss vs. time curve supports the presence of a heterogeneous bulk degradation process. After hydrolytic degradation the residual particles showed a molecular weight decrease and a crystallinity increase. After 90 days the crystallinity attained a value of 53%. The X-ray diffraction spectrum indicated the formation of a crystalline oligomeric structure. Crystallization of low molecular weight species will not enable the desired PLA absorption in drug delivery systems.

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