Synthesis and radical crosslinking reaction of poly (vinylcyclopropanone acetal)s (4), volume change on the crosslinking, and thermal analysis of 4 and the crosslinked polymers were carried out. 4 was prepared by the reaction of 1,1-dichloro-2-vinylcyclopropane with sodium dialkoxides. Radical crosslinking of 4a was carried out in the presence of AIBN, BPO, or DTBP at 60, 80, or 120 degrees C, respectively. 4a afforded no dichloromethane-insoluble part in the crosslinking in chlorobenzene but afforded in 37% yield in bulk at 120 degrees C. On the other hand, 4b afforded dichloromethane-insoluble part in 33% yield in the crosslinking in chlorobenzene at 120 degrees C. Volume shrinkage of 4a and 4b on crosslinking at 120 degrees C was 4.11 and 3.55%, respectively. Glass transition temperatures of the crosslinked polymers obtained from 4a and 4b were 28 and 163 degrees C, respectively, which were 32 similar to 49 degrees C higher than those before crosslinking.