The effect of backbone flexibility on the mesomorphic behavior of side-chain liquid crystalline polymers synthesized by ring-opening metathesis polymerization was investigated. The synthesis of norbornene and cyclobutene monomers containing a p-nitrostilbene moiety as the mesogenic group and polymerization of these monomers to produce side-chain liquid crystalline polymers with low polydispersities and defined molecular weights was accomplished. The relatively rigid poly(norbornene)s displayed enantiotropic nematic mesomorphism with glass transitions from 44 to 64 degrees C and isotropization temperatures between 108 and 121 degrees C, whereas the more flexible poly(butadiene)s showed enantiotropic smectic A mesomorphism with glass transition temperatures from 14 to 31 degrees C and isotropization temperatures between 74 and 111 degrees C. A diblock copolymer containing a 1:1 mixture of the poly(norbornene) and poly(butadiene) also exhibited a smectic A mesophase, clearly demonstrating the dominance of the poly(butadiene) backbone. The dependence of the degree of polymerization and flexible spacer length on the phase transitions of these systems was determined, demonstrating stabilization of the mesophase by both increasing molecular weight and flexible spacer length.