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 β-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 P0 with three β-amino mercaptans (2-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 1H 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 i...