The structure and phase transitions of cyclolinear polyorganosiloxane copolymer containing 12-member polysiloxane rings have been studied using synchrotron WAXS, DSC, TEM, variable-temperature AFM and polarized optical microscopy. The primary structure of this polymer can be viewed its a necklace of disk-shaped entities (cyclic groups) connected via flexible linkers. In the mesomorphic state, the presence of two different LC phases has been derived from the analysis of WAXS fiber diffractograms. The morphology of one of the phases shows a conventional hexagonal packing of LC chains where the chain axes are perpendicular to the plane of the 2D hexagonal lattice. The other one, so-called R-phase, has a vertically oriented rectangular 2D lattice formed by inter-chain correlations between the bulky polysiloxane, cycles (disks). To our knowledge, such a disk-necklace mesophase in which the LC lattice is parallel to the backbone direction has not been reported in the literature so far. (c) 2007 Published by Elsevier Ltd.