Soluble, High Molecular Weight Polysilsesquioxanes with Carboxylate Functionalities
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Soluble, High Molecular Weight Polysilsesquioxanes with Carboxylate Functionalities Kamyar Rahimian*1, Douglas A. Loy*2 and David R. Wheeler3 1 Materials Chemistry Department, Sandia National Laboratories, Albuquerque, NM 87185-0888. 2 Catalysis and Chemical Technologies Department, Sandia National Laboratories, Albuquerque, NM 87185-0888, U.S.A. 3 Micro-Total-Analytical Systems Department, Sandia National Laboratories, Albuquerque, NM 87185-0888, U.S.A. ABSTRACT High molecular weight polysilsesquioxanes with carboxylate functionalities were prepared by sol-gel polymerization of organotrialkoxysilanes bearing tert-butyl ester groups. Trialkoxysilyl-containing monomers of the type (RO)3Si(CH2)3C(O)OtBu (R = Me, Et) were prepared by hydrosilation of the corresponding vinylic tert-butyl esters CH3CHCH2C(O)OtBu. Acid- or base-catalyzed polymerization of the monomers leads to very high molecular weight polymers with relatively narrow polydispersities. The polymerization results in complete condensation of the alkoxy groups while the tert-butyl ester functionality remains fully intact. Partial or full deprotection of the tert-butyl group can easily be achieved to yield the corresponding carboxylic acid polymers. The ester and carboxylic acid functionalities of these new materials allow for their potential use in a variety of applications such as scavenging of heavy metals. INTRODUCTION Polysilsesquioxanes, (RSiO1.5)n, have attracted increasing attention in the polymer and materials science fields in recent years because of the ease of their preparation, as well as their excellent physical and chemical properties.1 However, introduction of functionality into these materials has been limited to organic substituents, R, that do not react with the alkoxysilyl and silanol groups or the catalyst itself present under sol-gel polymerization conditions. Thus, reactive functionalities are introduced into polysilsesquioxanes through chemical modification, usually with hydrido, vinyl and allyl substituted polysilsesquioxanes, and a few examples of aminoalkyl, epoxy, isocyanato and thiols are also known.1 Alternatively, the organic group can be modified with a protecting group that is stable to the polymerization conditions and deprotected once the polymer has been formed. Organic carboxylates and carboxylic acids would be useful functionalities to attach to polysilsesquioxanes to provide chemically and thermally robust ion exchange resins for scavenging heavy metals2 and organic amines. There is only one reported case where acid functionality was introduced, in a random fashioon, to an insoluble, bridged silsesquioxane polymer.3 Here, we describe the synthesis and characterization of soluble and processible, high molecular weight polysilsesquioxanes in which every repeat unit bears the tert-butyl ester functionality as a masked or protected carboxylic acid group. The tertbutyl functionality in these polymers can be easily chemically eliminated to give carboxylic acid functionalized polysilsesquioxanes or methyltin carboxylatosilsesqu
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