A new 1,6-diene, alpha-(2-phenylallyloxy)methylstyrene (1), was synthesized and its radical cyclopolymerizations were studied, since 1 is expected to yield highly cyclized polymers with thermal stability and high glass transition temperatures. A low homopolymerization tendency of the monofunctional counterpart of 1 can be assumed reasonably, because it is a derivative of alpha-methylstyrene with a low ceiling temperature. This means that intermolecular propagation leading to pendant unsaturations is hard to occur during the polymerization of 1, which results in the formation of highly cyclized polymers. In fact, the degree of cyclization of poly(l) obtained at 180 degreesC attained the value 99%. Structural studies using a monomeric cyclic compound obtained by the telomerization of 1 permitted to assign main repeating cyclic units of poly(l) to a six-membered ring. The poly(l)s with a higher degree of cyclization were found to be stable up to 300 degreesC on thermogravimetric analyses and their glass transition temperatures were detected at temperatures over 250 degreesC. (0 2004 Elsevier Ltd. All rights reserved.