Synthesis of comb-type polycarbosilanes via nucleophilic substitution reactions on the main-chain silicon atoms

A series of comb-type polycarbosilanes of the type [Si(CH₃)(OR)CH₂]_n {where R = (CH₂)_mR′, R′ = - O-p-biphenyl - X [X = H (m = 3, 6, 8, or 11) or CN (m = 11)], and R′ = (CF₂)₇CF₃ (m = 4)} were prepared from poly(chloromethylsilylene-methylene) by reactions with the respective hydroxy-terminated side chains in the presence of triethylamine. The product side-chain polymers were typically greater than 90% substituted and, for R′ = - O-p-biphenyl - X derivatives, they exhibited phase transitions between 27 and 150 °C involving both crystalline and liquid-crystalline phases. The introduction of the polar p-CN substituent to the biphenyl mesogen resulted in a substantial increase in both the isotropization temperature and the liquid-crystalline phase range with respect to the corresponding unsubstituted biphe-nyl derivative. For R = (CH₂)₁₁ - O-biphenyl side chains, an analogous side-chain liquid-crystalline (SCLC) polysiloxane derivative of the type [Si(CH₃)(O(CH₂)₁₁ - O-biphenyl)O]_n was prepared by means of a catalytic dehydrogenation reaction. In contrast to the polycarbosilane bearing the same side chain, this polymer did not exhibit any liquid-crystalline phases but melted directly from a crystalline phase to an isotropic liquid at 94 °C. Similar behavior was observed for the polycarbosilane with a fluoro-carbon chain, for which a single transition from a crystalline phase to an isotropic liquid was observed at -0.7 °C. The molecular structures of these polymers were characterized by means of gel permeation chromatography and high-resolution NMR studies, and the crystalline and liquid-crystalline phases of the SCLC polymers were identified by differential scanning calorimetry, polarized optical microscopy, and X-ray diffraction.