We report the novel synthesis of thermally degradable polyesters through the postpolymerization modification of an unsaturated aliphatic polyester from maleic acid via Michael addition reaction with beta-amino mercaptans. A high molecular weight polyester (P0) was synthesized by the acyclic diene metathesis (ADMET) polymerization of a diene monomer, di(10-undecenyl) maleate. Postpolymerization modification of PO with three beta-amino mercaptans aminoethanethiol, L-cysteine methyl ester, and 2(butylamino)ethanethiol) and 2-mercaptoethanol via the selective and quantitative thiol-Michael addition reaction with the maleate vinylic double bonds afforded four polyesters (P1-P4). All the polyesters were characterized, by H-1 NMR, GPC, and MALDI-TOF-MS, and their thermal properties were studied by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). The three polymers with pendant amino groups (P1-P3) subjected to thermal-induced degradation in the solid state, the driving force of which is, the intramolecular cyclization at elevated temperatures with the formation, of six-membered lactam derivatives. Degradation kinetics of P1-P3 in the solid states at different temperatures was studied in detail. It was found that increase of the steric hindrance and decrease of the nucleophilicity of the pendent amino groups could increase the degradation temperatures from 0 degrees C (P1) to 50 degrees C (P3). Alternatively, protonation of the amino groups of P1 and P2 with trifluoroacetic acid (TFA) could decrease the nucleophilicity of the primary amino groups, and the polymers P1-Boc and P2-Boc showed enhanced thermal stability up to 80 degrees C. Degradation of these two polymers via intramolecular cyclization occurred only when TFA was removed at higher temperatures. When the pendant group was a hydroxyl group as in polymer P4, no intramolecular cyclization occurred, and the polymer was thermally stable up 130 degrees C. Thus, the results highlight the importance of sequence, functional group, and nucleophilicity of the amino groups in determining the intramolecular cyclization and thermal degradation of these polyesters.