Free-radical polymerization behavior of 2-triethoxymethyl-1,3-butadiene, a potential precursor for poly[2-(ethoxycarbonyl)-1,3-butadiene], whose carboxyl group was masked as an ortho ester, was investigated. When polymerization was carried out in bulk at 60 degrees C for 50 h with using 1 mol % of 2,2＇-azobisisobutyronitrile as an initiator, viscous polymer was obtained in 36% yield without forming gel. The resulting polymer containing an ortho ester function was soluble in common organic solvents, including methanol. It can readily be converted to carboxylic ethyl ester by adding small amount of dilute hydrochloric acid. The microstructure of the resulting polymer with ethyl ester group was predominantly a 1,4-E, cis-1,4 structure about the main chain.