We report herein the polycondensations of unsaturated cyclic anhydrides with diols at moderate temperatures (60-80 degrees C) using the sublimating organic catalyst bis(nonafluorobutanesulfonyl)imide (Nf(2)NH) to synthesize polyesters containing carbon carbon double bonds (M-n = 5.3-18.1 kDa, M-w/M-n = 1.7-2.2). We initially synthesized isomerization-free unsaturated polyesters containing alternating maleate or itaconate and 3-methyl-1,5-pentanediol units. To avoid gelation during the polycondensation of itaconic anhydride with diols, addition of a radical inhibitor was essential. Using a polyester that contained maleate units, we converted the maleate units to fumarate units by a diethylamine-catalyzed cis-trans isomerization of the maleate double bonds. Additionally, using copolyesters containing maleate and itaconate units, we could selectively photo-cross-link the exo-type double bonds of the itaconate units to afford unsaturated polyester gels. Starting with these gels, we then isomerized the maleate unit cis double bonds to trans double bonds. The swelling properties of the gels were found to depend on the initial maleate to fumarate unit ratios (degree of isomerization) as well as the maleate to itaconate unit ratios.