A series of copolyesters having a broad range of biodegradable crosslinks were synthesised by FeCl13-catalyzed polyesterification of trimellitic acid and glycerol containing a small mol percent of poly(ethylene glycol) (PEG) of varied molecular weights. The polymer samples designated as I (1.5% PEG 2000),II (4.5% PEG 2000), III (7.5% PEG 2000), IV (1.5% PEG 4000), V (4.5% PEG 4000), VI (7.5% PEG 4000), VII (1.5% PEG 6000), VIII (4.5% PEG 6000), and IX (without PEG) are insoluble and moderately tough-to-elastic solids and were characterized by their swelling values in ethanol, glass transition temperatures (T-g), IR spectra, and X-ray diffractograms. Sample IX (0% PEG) has the lowest equilibrium swelling (12% at 25degreesC) and the highest T-g (155degreesC) and, therefore, the highest crosslink density. The swelling increases and the T-g decreases as the PEG content or PEG molecular weight in a glycerol-PEG combination increases, indicating a corresponding decrease in the crosslink density of the polymers. Further, the equilibrium swelling value increases with increasing temperature. The IR spectra of the polymers indicate the formation of ester bonds at the expense of COOH and OH groups. The X-ray diffractograms show their semicrystalline nature. The percent crystallinity values of 53, 52, 49, and 46 for II, III, V, and VII, respectively, and 54 for IX showed that the percent crystallinity decreases with an increasing PEG content and molecular weight in the same way as do the T-g values. Thus, higher T-g values are associated with a higher percent crystallinity, that is, with structures of higher order. The synthesized polymer samples with varied crosslink (biodegradable) densities are expected to be very suitable as matrices for controlled drug delivery over a varied period of time. (C) 2003 Wiley Periodicals, Inc.