A series of prepolymers were treated with  CF3SO3H to cationically cyclize adjacent diene−styrene/ diene−diene units into main-chain cyclic units. Diene− diphenylethylene alternating copolymers synthesized using anionic copolymerization were first cyclized completely without intermolecular linking reactions. The cyclization  produced much higher Tg values (approximately 200 °C) than those of cyclized random diene−styrene copolymers (approximately 130 °C). To eliminate the number of spacer carbons between cyclic repeating units, poly(1-phenyl-1,3- butadiene) (PPB) was used as a prepolymer in cationic cyclization. The Tg increased substantially from 200 to 271 °C as the 3,4- structure content increased from 8% to 80%. The cyclized PPB with high 3,4-structure content is the highest Tg hydrocarbon polymer yet reported. All results indicated that the backbone rigidity of the cyclic hydrocarbon polymer can be drastically increased by reducing the number of spacer carbons and increasing carbon sharing between cyclic repeating units.