Nonmesogenic cyclic alkylidene terephthalate is directly connected to flexible polyethylene main chain at every second carbon atom via the phenyl ring to obtain a novel vinyl polymer, poly(alkylidene vinylterephthalate) (PAVT-n), where n (= 10−14 and 16) denotes the number of methylene groups in the side chain. When the molecular weight is large enough, stable hexagonal columnar phases are generated from melt by the PAVT-n polymers with n = 10−13, but not by those containing 14 or 16 methylene groups. Compared with its open-ring counterpart, i.e., poly(dialkyl vinylterephthalate) (PDAVT-n) with linear side chains, PAVT-n has a glass transition temperature (T g ) over 100 °C higher, a much larger persistence length, and a much lower critical molecular weight to form a mesophase. In addition, PDAVT-n enters into isotropic liquid at T g and becomes ordered at temperatures well above T g , whereas PAVT-n forms mesophases immediately above T g . The strong steric interaction between cyclic pendants and the flexible main chain, which causes PAVT-n to take an extended chain conformation, is considered as the main driving force for PAVT-n to generate mesophases. In a sharp contrast, the mesophase formation of PDAVT-n is known to be an entropy-driven process.