Based on the reaction of trichloro(ch1oromethyl)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 H2PtC1, was employed to prepare the corresponding poly(dipropylsily1enemethylene) (PDPSM, 4) with strictly alternating SiR2/CH2 backbone structure. A high-molecular-weight fraction of the material (weight-average molecular weight a, = 166500 and number-average molecular weight a, = 115 200) obtained by fractionating precipitation was investigated with respect to glass transition and formation of conformationally disordered mesomorphic phases. The glass transition temperature Tg = 232 K of PDPSM evidenced lower backbone flexibility than observed for the analogous poly(dipropylsi1oxane) (PDPS). PDPSM exhibited mesomorphic behavior. In contrast to poly(dipropylsiloxane), PDPSM showed a surprisingly narrow mesomorphic regime between 355 K and 365 K. Based on polarizing microscopy and 29Si-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. some scientific attention so far is poly(dimethylsilylenemethylene), a poly(dimethylsi1oxane) analog, which was characterized to some e ~t e n t ~. ~~ lo). However, even for this polymer no glass transition temperature has been reported in literature so far.