We demonstrate that monomer sequence is important to regulate the vinyl copolymer thermal properties by intrachain cyclization. This is exemplified by the spontaneous intrachain cyclization with the formation of ?-butyrolactone by dihydroxylation of the internal alkenes of a designed periodic copolymer. A structurally symmetric a,?-diene monomer 1 containing two tail-to-tail connected ethyl acrylate units was synthesized via a two-step approach. The acyclic diene metathesis (ADMET) polymerization of monomer 1 was conducted with a HoveydaGrubbs second generation catalyst in the presence of p-benzoquinone to get well-defined polymer (P1) with high molecular weights as revealed by GPC, NMR, and MALDI-TOF-MS characterizations. Dihydroxylation of P1 using hydrogen peroxide as the oxidant was successfully conducted to afford HP1. Characterization of HP1 by NMR and IR spectra indicated that it contained gamma-butyrolactone units in the polymer main chain. Control experiments with four different polymers indicated that the formation of this ring structure was the outcome of the specific sequence of vinyl alcoholethyl acrylate formed by dihydroxylation of P1. The cyclization efficiency of P1 was calculated to be 77%. The T(g)s of all the modified polymers were increased compared to those of the unsaturated samples; significantly, the T-g of HP1 was drastically elevated by 160 degrees C as compared to that of P1.