Two unsaturated poly(silyl ester)s that contained innoxious fumaryloxyl units in the main chain were prepared by the polycondensation reaction of 1,5-dichloro1,1,5,5-tetramethyl-3,3-diphenyltrisiloxane or 1,3-dicworotetramethyldisiloxane with di-tert-butyl fumarate under nitrogen at 100 degrees C for 1-3 days. To investigate the crosslinking reaction of the unsaturated poly(silyl ester)s, the two unsaturated poly(silyl ester)s were crosstinked in the presence of 2,2'-azobisisobutronitrile as a radical initiator. After the crosslinking, the unsaturated poly(silyl ester)s, which were viscous liquids, turned into solid products. The characterization of the two poly(silyl ester)s and the crosslinked products included infrared spectroscopy, H-1-NMR spectroscopy, differential scanning calorimetry, and thermogravimetric analysis. Comparisons were made between the linear poly(silyl ester)s and the crosslinked poly(silyl ester)s. After the crosslinking, the important resonance signal for ethenylene (C=C) disappeared, and this showed that the crosslinking reaction was carried out progressively. The glass-transition temperatures of the crosslinked poly(silyl ester)s were higher than those of the uncrosslinked poly(silyl ester)s, and the thermal stability of the crosslinked poly(silyl ester)s was better than that of uncrosslinked poly(silyl ester)s. (c) 2007 Wiley Periodicals, Inc.