A series of branched mesogen-jacketed liquid crystal polymers were synthesized via atom transfer radical copolymerization of mesogenic 2,5-bis[(4'-methoxyphenyl)oxycarbonyl] styrene (MPCS) and nonmesogenic 4-chloromethyl styrene (CMS) catalyzed by the CuCl/CuCl2/bipyridine complex in anisole solution at 110 degrees C. During the early period of polymerization, CMS acted mainly as an initiator to induce the polymerization of MPCS. The molecular weights (MWs) of the resultant polymers increased linearly with monomer conversion and showed a symmetrical and relatively narrow distribution. With the proceeding of polymerization, MWs changed from monomodal to multimodal distribution, indicating the formation of branched structure through the action of vinyl group of CMS. This mechanism was also supported by the increased chain density estimated by a combination of gel permeation chromatography and laser light scattering, end-group analysis, and the reactivity ratios of two monomers (r(MPCS) = 0.71, r(CMS) = 0.10). The thermotropic properties of the copolymers strongly relied on the feed ratio of MPCS to CMS and MW. The feed ratio of MPCS to CMS should be at least 30 with high enough MW. For the copolymer obtained at the feed ratio of 32, the minimum M-n,M-GPC was 1.21 x 10(4) Da to achieve a mesophase. Although linear poly{2,5-bis[(4'-methoxyphenyl)oxycarbonyl]styrene displayed columnar nematic phase (Phi(N)) and hexatic columnar nematic phase (Phi(HN)) depending on MW, the branched polymer showed just a Phi(N) phase, indicating the remarkable depressing effect of the branched structure on the mesomorphic property.