Based on the reaction of trichloro(chloromethyl)silane (1) with 2 equivalent amounts of the respective Grignard-reagent and subsequent cyclization, 1,1,3,3-tetrapropyldisilacyclobutane (3) has been prepared. Catalytic polymerization with H2PtCl6 was employed to prepare the corresponding poly(dipropylsilylenemethylene) (PDPSM, 4) with strictly alternating SiR(2)/CH2 backbone structure. A high-molecular-weight fraction of the material (weight-average molecular weight ($) over bar M(W) = 166500 and number-average molecular weight ($) over bar M(n) = 115200) obtained by fractionating precipitation was investigated with respect to glass transition and formation of conformationally disordered mesomorphic phases. The glass transition temperature T-g = 232 K of PDPSM evidenced lower backbone flexibility than observed for the analogous poly(dipropylsiloxane) (PDPS). PDPSM exhibited mese morphic behavior. In contrast to poly(dipropylsiloxane), PDPSM showed a surprisingly narrow mesomorphic regime between 355 K and 365 K. Based on polarizing microscopy and Si-29-MAS (magic angle spinning) solid-state NMR the mesophase is described as a conformationally disordered state, which is most probably columnar in analogy to PDPS.