A mesogen-jacketed liquid crystalline polymer (MJLCP) containing triphenylene (Tp) moieties in the side chains with 12 methylene units as spacers (denoted as PP12V) was synthesized. Its liquid crystalline (LC) phase behavior was studied with a combination of solution H-1 NMR, solid-state NMR, gel permeation chromatography, thermogravimetric analysis, polarized light microscopy, differential scanning calorimetry, and one- and two-dimensional wide-angle X-ray diffraction. By simply varying the temperature, two ordered nanostructures at sub-10-nm length scales originating from two LC building blocks were obtained in one polymer. The low-temperature phase of the polymer is a hexagonal columnar phase (phi(H), a=2.06 nm) self-organized by Tp discotic mesogens. The high-temperature phase is a nematic columnar phase with a larger dimension (a=4.07 nm) developed by the rod-like supramolecular mesogenthe MJLCP chain as a whole. A re-entrant isotropic phase is found in the medium temperature range. Partially homeotropic alignment of the polymer can be achieved when treated with an electric field, with the polymer in the phi(H) phase developed by the Tp moieties. The incorporation of Tp moieties through relatively long spacers (12 methylene units) disrupts the ordered packing of the MJLCP at low temperatures, which is the first case for main-chain/side-chain combined LC polymers with MJLCPs as the main-chain LC building block to the best of our knowledge. The relationship of the molecular structure and the novel phase behavior of PP12V has implications in the design of LC polymers containing nanobuilding blocks toward constructing ordered nanostructures at different length scales. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 295-304