Porous inorganic-organic hybrid polymers derived from cyclic siloxane building blocks: Effects of substituting groups on mesoporous structures

Porous inorganic-organic hybrid polymers (PHPs) with cyclic siloxane frameworks are synthesized via Friedel-Crafts alkylation between siloxane molecules and formaldehyde dimethyl acetal. The synthesized polymers with high specific surface areas (up to 1310 m(2) g(-1)) show a combination of type I(b) and type IV(a) isotherms, as determined by argon adsorption-desorption measurements, suggesting that they contain both micropores and mesopores. Interestingly, two different types of hysteresis loops are observed, depending solely on the organic substituting groups of the siloxane building blocks. PHPs synthesized from cyclic siloxane building blocks substituted with one methyl group and one phenyl group per one silicon atom show type H3 hysteresis loop; while those synthesized from cyclic siloxane building blocks substituted with two phenyl groups show type H2(a) hysteresis loop. This indicates that the resulting PHPs possess different porous networks. Si-29 MAS NMR spectra indicate that the Si-phenyl covalent bonds are selectively cleaved during polymerization; while the Si-methyl bonds are not, leading to the formation of different pore structures.