Synthesis of silyl-disubstituted poly(p-phenylenevinylene) membranes and their gas permeability

p-Bis(bromomethyl)benzene with two silyl groups [SiMe2-n-C8H17, SiMe3 (1a), SiMe2-n-C8H17, SiEt3 (1b), SiMe2-n-C8H17, SiMe2-n-C8H17 (1c), SiMe2-n-C8H17, SiMe2-n-C18H37 (1d), SiMe2-n-C18H37, SiMe2-n-C18H37 (1e)] were polymerized by Gilch method to afford silyl-disubstituted poly(p-phenylenevinylene)s (2a-e). The polymers containing SiMe3 groups (2a) showed poor solubility and dissolved in only CHCl3. On the other hand, the other polymers (2b-e) exhibited good solubility in common organic solvents. According to thermogravimetric analysis (TGA), the silyl-disubstituted poly(p-phenylenevinylene)s showed high thermal stability (thermal decomposition temperature: Td ≥ 310 °C). Polymers 2a-e had relatively high molecular weight over 7.6 × 104, and gave free-standing membranes by solution casting method. Except 2e, the oxygen permeability coefficients (PO2) of membranes of the silyl-disubstituted poly(p-phenylenevinylene)s increased as increasing alkyl length of silyl groups (2a: 5.4, 2b: 6.0, 2c: 15.0, 2d: 20.4 barrers). The membrane of 2e having two dimethyl-n-octadecylsilyl groups in the repeating unit exhibited the lowest gas permeability among the present polymers, and its PO2 was 1.8 barrers. This is because polymers 2a-d were amorphous while 2e was crystalline. The PO2 values for amorphous poly(p-phenylenevinylene)s increase in direct proportion to the number of carbon in the side chains.