Recently, new synthetic methods have been reported to prepare novel siloxane nanostructure macromolecules. In particular, novel, cage-like siloxane nanostructures, such as an asymmetric bicyclosiloxane and shell cross-linked nanocages have been prepared (ref. 1,2). The syntheses of these structures, however, are tedious if complete control over the nature of every ligand of Si is desired. Such control of every ligand of the siloxane units is similar to the situation in peptide synthesis, where the order of the amino acids is critical.

In searching for a more efficient method to synthesize well-defined siloxane macromolecules, we have investigated the strategy of solid-phase synthesis. The method is similar to that employed in peptide synthesis with suitable modifications. In this method, a linker molecule is used to anchor the siloxane intermediates to a solid support, such that they can be easily separated from the rest of the reaction mixture and purified for subsequent steps. At the end of the synthesis, the desired product is released and recovered by selectively cleaving the linker unit from the support.

We report here a demonstration of such a solid phase synthesis method. A novel bis-(chlorosiloxy)alkane linker is synthesized and used as the linker unit. It is attached to a silica gel, and the siloxane macromolecular intermediates and products are characterized by 1H, 13C, and 29Si NMR, GC-MS, and ESI-MS techniques; the functionalized silica gel support is characterized by transmission FTIR. The extent of reaction is monitored by the uptake of monomer from solution in each step and the release of condensation byproducts into solution, as observed by 1H NMR. Since the support is transparent to UV-visible light, chromophore-labeled species can be attached and monitored. The yield of the desired product, as a function of surface coverage of intermediates, will be discussed, as well as advantages and potential improvements.

References. 1. “Rational Synthesis of Asymmetric BicycloSiloxane,” W. Xue, M.C. Kung, and H.H. Kung, Chem. Comm. (2005) 2164 - 2166. 2. “Size-Selective Shell Cross-Linked Interior Functionalized Siloxane Nanocages,” Young-Woong Suh, Mayfair C. Kung, Yingmin Wang, Harold H. Kung, J. Amer. Chem. Soc. 128 (2006) 2776.