Porous Silicon Used as an Initiator in Polymerization Reactions
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JULIE L. HEINRICH, ALICE LEE, AND MICHAEL J. SAILOR* University of California at San Diego, Department of Chemistry, 9500 Gilman Dr, La Jolla, CA 92093-0358
ABSTRACT Porous silicon is investigated as an initiator for radical polymerization reactions. Surface radicals and photogenerated e-/h+ pairs in porous silicon can be used to initiate polymerization reactions. Porous silicon wafers were placed in ethanolic solutions of methyl methacrylate, N-vinylcarbazole, acrylonitrile, or styrene and either left in the dark or excited by light. Methyl methacrylate polymerized in the absence of light, presumably via silicon-based radical initiator sites at the porous silicon surface. The N-vinylcarbazole polymerized to a small extent in the dark, and to a much greater extent during white light irradiation of the porous silicon surface. Photopolymerization is thought to occur via excited state hole transfer from the porous silicon to the N-vinylcarbazole monomer. Very little to no polymerization of acrylonitrile or styrene could be detected either in the dark or under illumination.
INTRODUCTION Photoluminescent porous silicon is very simply created by anodically etching single crystal silicon in HF. The immediately obvious applications for porous silicon involve a variety of optoeletronic devices. I Also, well represented inthe literature are porous silicon's potential application as a display material or chemical sensor material 2 ,3 . The following experiments investigate yet another area of chemistry available to porous silicon: use as a initiator for radical polymerization reactions. The reactions studied may ultimately be of use in organic synthesis, photodegradation of toxins, or photochemical protection of the porous silicon surface. A number of researchers have demonstrated that electron-hole pairs photogenerated in semiconductor particles can be used to perform chemical transformations. The research groups of Bard and Fox have demonstrated stereoselective organic syntheses as well as the photodegradation of pollutants found in soils using TiC 2 particulates 4 ,5 Additionally, Hoffman and coworkers have used ZnO and CdS particles as photosensitized radical initiators for polymerization reactions 6 ,7 . All of these experiments exploit the photochemical properties of semiconductors. Electrons and holes, once excited by adsorbed light, are free to roam the conduction and valence bands of semiconducting particles. Either one can be transferred from the semiconductor to a molecule in solution, which then initiates the desired reaction. 605
Mat. Res. Soc. Symp. Proc. Vol. 358 01995 Materials Research Society
In the case of porous silicon, two separate sources of radicals are possible. In addition to the photogenerated charge carriers described above, residual surface radicals, which have been observed in porous silicon by electron spin resonance 8, may potentially act as radical initiators. We have employed methyl methacrylate, acrylonitrile, N-vinylcarbazole, and styrene as monomer probes for the accessibility of radi
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