We report a facile strategy for the synthesis, functionalization, and controlled degradation of high molecular weight polyesters based on itaconic acid (IA) and 10-undecenol. A diene monomer, di(10-undecenyl) itaconate (DUI), was synthesized from IA and 10-undecenol by esterification reaction. The acyclic diene metathesis (ADMET) polymerization of DUI was conducted with Grubbs first-generation catalyst (G-I) to afford high molecular weight polyesters (PEIA: P1 and P2). Modification of P1 with three mercaptans (3-mercapto-1,2-propanediol (MP), benzyl mercaptan (BM), and dodecyl mercaptan (DM)) and piperidine (PI) via Michael addition reaction with the itaconate vinyl groups was performed. Quantitative postpolymerization modifications with no degradation of the polyesters were achieved for all the three mercaptans and PI as revealed by NMR and GPC characterizations. One-pot sequential modification of P1 with MP and PI was also successfully conducted. Thermal properties of the modified polyesters were studied by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). All the modified polyesters are semicrystalline with variable glass transition and melting temperatures which are affected by the modified side groups. Based on a detailed model reaction study, controlled modification of the itaconate vinyl groups with primary amines (sec-butylamine and 3-pentanamine) without intramolecular cyclization was achieved. Thus, sec-butylamine was used to modify P1 to generate a secondary amine functionalized polyester (P1-BA/DM) which was stable at low temperature but undergone self-degradation in solid or solution via intramolecular cyclization with the formation of a five-membered lactam at elevated temperatures. This is a new kind of self-degradable polyester with controllable degradation rates.