Organic-Inorganic Hybrid Materials From Surface Initiated Polymerization
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45 Mat. Res. Soc. Symp. Proc. Vol. 576 ©1999 Materials Research Society
Organic-inorganic hybrids must overcome the thermodynamic tendency to phase separate. This is an especially urgent issue during processing when it is desirable to disperse nanoscale particles of one component (inorganic) phase throughout a second (organic) phase. Silane coupling agents are one means to prepare a silica surface and we have extended this approach by polymerizing from the distal end of such a coupling agent. End functional coupling agents have been used to attach ruthenium alkylidene catalysts to fumed silica for ring opening metathesis polymerization of cyclooctadiene and norbornenes. Polymerization of cyclooctadiene monomers yields 1,4 butadiene chains on silica surfaces that are candidates for dielectric strength enhancers in high voltage applications. EXPERIMENTAL Fumed Silica was obtained from Cabot (Cab-O-Sil, grade M-5). It was heated at 160 TC for 2 days under vacuum to remove physisorbed water. Silicon tetrachloride, trimethylchloro
silane and I -octenyldimethylchlorosilane were purchased from Gelest and used as received. Toluene, hexane, and tetrahydrofuran were refluxed and distilled over Na/benzophenone. Dichloromethane was distilled and stored over 4A molecular sieves. All solvents and reagents were degassed prior to use. Proton NMR spectra were obtained using Bruker AF-500 NMR and FT-IR were obtained on Magna 560 FTIR spectrometer. Silica surface preparation:Several methods were used to attach catalyst moieties to the surface. One standard method was to chlorosilanate the silica surface by stirring the fumed silica with SiC14 which formed a surface with an abundance of Si-Cl bonds. Excess SiC14 and the HC1 produced by the reaction were removed under vacuum. This reactive surface could be further elaborated with alcohols to place a variety of functionalities on the surface that would facilitate catalyst attachment or, for the discussion below, the catalyst was modified so that the alkylidene ligand would have a terminal alcohol suitable for reacting with the surface.
Cy3
/1RyC
)CY 3 +
OH
Ho
--
R
CI PCY3
Scheme 1: Modification of Ruthenium alkylidene for surface attachment The alcohol functional catalyst was attached to the surface by stirring the chlorosilanated silica in a dichloromethane solution in nitrogen atmosphere for 1.5 hours, followed by a cold methanol wash on a nylon filter membrane, and then drying overnight under vacuum. This catalyst coated surface was then used in subsequent polymerizations of norbornene diacyl chloride and cyclooctadiene. The polymerizations were carried out in neat monomer or in THF solutions of monomer. The reaction mixtures were stirred at room temperature in a capped scintillation vial on the benchtop and became viscous within 1 to 2 hours. Termination occurred upon addition of vinyl acetate. The polymer coated silica was recovered by precipitation in methanol followed by vacuum drying. Some polymerization products were isolated without a termination step. RESULTS AND
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