Major Advances in the Synthesis of POSS Monomers
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Major Advances in the Synthesis of POSS Monomers Frank J. Feher, Raquel Terroba, Ren-Zhi Jin, Sabine Lücke, Frank Nguyen, Richard Brutchey and Kevin D Wyndham Department of Chemistry, University of California Irvine, CA 92697-2025 U.S.A. ABSTRACT Several new methods have been developed for the synthesis of POSS monomers from fully condensed [RSiO3/2]n and [ROSiO3/2]n (n = 6 or 8) frameworks. These methods, which all rely on highly selective base-catalyzed reactions of Si/O frameworks, provide unprecedented access to new POSS monomers and the means for manufacturing useful POSS monomers on a large scale from readily available silane monomers. INTRODUCTION Polyhedral oligosilsesquioxanes (POSS) are an interesting class of three-dimensional Si/O clusters derived from the hydrolytic condensation of trifunctional organosilicon monomers (i.e., RSiX3) [1]. Since their discovery in 1946 [2], many stoichiometrically well-defined POSS frameworks have been reported, but it was only recently that POSS frameworks suitable for use as mono- or di-functional monomers have emerged [3]. These monomers, originally developed by the Lichtenhan group at Edwards Air Force Base [4], have attracted widespread interest as precursors to nanoreinforced, hybrid inorganic/organic materials. Early methods [5] for producing POSS monomers relied extensively on “corner-capping” reactions of trisilanols 1a and 1b [6,7,8]. These trisilanols are versatile precursors to a wide range of POSS monomers, but they are relatively expensive to produce by early methods and their cycloalkyl pendant groups are less than desirable for many potential applications. R Si
OH
Si
OH O OH O Si O Si R Si O Si R O OO R O Si O Si
O
Si
O O O R Si O O Si R Si O Si R OO OO R Si O Si
R
R
Z
R
R
R
1a R = c-C5H9 c-C6 H11 b
R
Z = polymerizable pendant group
CC2.1.1
Over the past four years we have been exploring the fundamental reaction chemistry of fully condensed [RSiO3/2]n (n = 6, 8, 10, 12) frameworks with an eye toward developing practical, low-cost methods for manufacturing POSS monomers on a large scale [9-14]. In this paper we briefly describe the results from our very successful recent work, which provide unprecedented access to discrete Si/O frameworks with polymerizable pendant groups. RESULTS AND DISCUSSION Fully condensed R6Si6O9 and R8Si8O12 frameworks can be prepared by the hydrolytic condensation of many readily available trifunctional organosilicon monomers. Reactions performed under acidic conditions are kinetically controlled and usually produce R6Si6O9 or R8Si8O12 frameworks in yields lower than 50%. The use of base-catalyzed hydrolytic condensation reactions for POSS synthesis is less well developed because it often leads to intractable resins rather than discrete molecular clusters, but under favorable circumstances (e.g., R = Ph, Me, c-C5H9) it is possible to prepare R8Si8O12 frameworks in practically quantitative yield [15,16]. Spherosilicates, such as (TMSO)6Si6O9 and (TMSO)8Si8O12, can be prepared in good yield by reacting chlorosilanes (
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