Syntheses of poly(cyclic orthoester) bearing an exo-methylene group (4) by a polymer reaction of the corresponding bromomethyl derivative and radical additions of thiols to 4 were carried out. The exo-methylene-containing poly(cyclic orthoester) 4 was prepared in 74 % yield by dehydrobromination with potassium tert-butoxide of bromomethyl-substituted poly(cyclic orthoester) which was synthesized by cationic single ring-opening polymerization of 2-(bromomethyl)-1,4,6-trioxaspiro[4.6]undecane (6) with tin(IV) chloride. 4 contained little bromomethyl group, indicating that the elimination of hydrogen bromide proceeded very efficiently. 4 underwent radical additions of ethanethiol and 1,3-propanedithiol under photoirradiation conditions at room temperature to afford the corresponding ethylthio group-containing poly(cyclic orthoester) (8) and cross-linked polymer in quantitative yields, respectively. The ethanethiol adduct (8) was readily converted to the corresponding monomeric ethanethiol adduct by treatment with an acid catalyst.