The polymerizabilities of ethyl trans-4-ethoxy-2,4-pentadienoate (EEP) and trans-4-ethoxy-2,4-pentadienonitrile (EPN) initiated by radical or ionic species were investigated. High-molecular-weight polymers were-obtained from both monomers only by radical polymerization. Spectroscopic study showed the polymers to be composed of the 2,5-addition units of the corresponding monomers. The hydrogenation of these polymers gave alternating copolymers of ethyl vinyl ether with ethyl acrylate and of ketene with acrylonitrile, respectively. KEY WORDS 1,3-Disubstituted 1,3-Butadiene I Ethyl trans-4-Ethoxy-2,4pentadienoate I trans-4-Ethoxy-2,4-pentadienonitrile I Radical Polymerization I Polymer Structure I Microtacticity I Hydrogenation I Alternating Copolymer I