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 T-g values (approximately 200 degrees C) than those of cyclized random diene-styrene copolymers (approximately 130 degrees 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 T-g increased substantially from 200 to 271 degrees C as the 3,4-structure content increased from 8% to 80%. The cyclized PPB with high 3,4-structure content is the highest T-g 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.