Functional Hybrid Materials Chapter 3

3 Bridged Polysilsesquioxanes. Molecular-Engineering Nanostructured Hybrid Organic-Inorganic Materials K. J. Shea, J. Moreau, D. A. Loy, R. J. P. Corriu, B. Boury 3.1 Introduction Bridged polysilsesquioxanes are a family of hybrid organic-inorganic materials prepa- red by sol-gel processing of monomers (Fig. 3.1) that contain a variable organic bridging group and two or more trialkoxysilyl groups [1–3]. This arrangement offers exceptional opportunities to combine the important properties from both organic and inorganic realms and to create entirely new compositions with truly unique properties. This molecular versatility, coupled with the mild sol-gel conditions and the ability to prepare bulk, thin films and fibers, makes this group of chemically and thermally robust materials a key resource in advanced materials science and technology. This chapter reviews the syntheses of bridged monomers and their sol- gel polymerization to hyper-cross-linked networks, and how the polymerization con- ditions, methods for post-gelation processing and the nature of the bridging group affect the physical and chemical properties of the resulting materials. The key to this versatility, and much of the unique properties of this class of materials, comes from the bridging organic group that is covalently attached to the polymerizable trialkoxysilyl groups through Si–C bonds. This organic group can be varied in length, rigidity, geometry of substitution, and functionality. Because the organic group remains an integral component of the material, this variability pro- vides an opportunity to engineer bulk properties such as porosity, thermal stabili- ty, refractive index, optical clarity, chemical resistance, hydrophobicity, and dielec- tric constant without the threat of phase segregation at longer length scales. The fine degree of control over bulk chemical and physical properties has made these materials excellent candidates for applications ranging from optical device fabrica- tion [4] to catalyst supports [5–7] and ceramics precursors [8]. A few representatives of bridged polysilsesquioxane monomers are shown in Fig. 3.2. The organic fragments in the building blocks range from rigid arylenic (Fig. 3.2, 1 and 2) [9–13], acetylenic (Fig. 3.2, 3, 4)[12, 14–19], and olefinic (Fig. 3.2, 5–7) [20–23] bridging groups to flexible alkylenes ranging from 1 to 14 methylene groups (Fig. 3.2, 8, 9) in length [11, 24–30]. They also include a variety of functio- 50 Functional Hybrid Materials. Edited by Pedro Gómez-Romero, Clément Sanchez Copyright © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 3-527-30484-3 nalized groups such as amines (Fig. 3.2, 10) [3, 31–36], ethers (Fig. 3.2, 11) [34, 37], sulfides (Fig. 3.2, 12) [33, 38, 39], phosphines (Fig. 3.2, 13) [40–46], amides (Fig. 3.2, 14) [47, 48], ureas (Fig. 3.2, 15) [49–58], carbamates [31, 59–61], carbonates [62], vio- logens (Fig. 3.2, 16) [63, 64] and azobenzenes (Fig. 3.2, 17) [65]. In addition, bridging groups have included organometallics in which the metal atom is part of the bridge as in this ferrocenyl-bridged monomer (Fig. 3.2, 18) [66–68] or pendant to the bridge as in this eta-6-arenechromiumtricarbonyl complex (Fig. 3.2, 19) [69– 71, 72, 73, 74]. At present there are a few commercially available monomers, most- ly for surface modification or coating applications (Fig. 3.2, 3, 5, 8–10, 15) [75–87], but new bridged monomers are beginning to appear for opto-electronic applications [88]. Sol-gel polymerization of these molecular building blocks permits the rapid for- mation of bridged polysilsesquioxanes that irreversibly form gels. The organic group, which comprises approximately 40–60 wt percent of the material, is an integral part of the network architecture. Upon drying, these gels afford amorphous xerogels or aerogels whose surface areas can be tailored, through selection of the organic spacer, to be as high as 1800 m 2 g –1 in supercritical processed aerogels [13, 26, 89, 90] or, at the other extreme, nonporous glassy xerogels [25, 30, 91, 92]. Bulk properties such as pore size may be controlled with a fidelity that is more remini- scent of surfactant-templated mesoporous molecular sieves. Optical properties can be manipulated by incorporating chromophores in the bridging organic component [93–95]. Most recently, functional bridged polysilsesquioxanes have been prepared for use as high-capacity adsorbents [58, 96–102]. 3.1 Introduction 51 X 3 Si SiX 3 H 2 O X 3 Si Si X X OH -H 2 O or HX X 3 Si Si O X X Si X X SiX 3 Hydrolysis X 3 Si Si X X OH Condensation -HX Si X X X Si HO X X Si Si O X Si X OH Si X OSi O X X O Si O OH X Si Si O X X OH X Si X X X Si X X X Si Si O Si O Si O O Si O Si O O Si O O O O Si O Si O Si O O O O Si O O Si O Si O O O Si Si O Si O O Si O O Si Si Si O Si O Si O O Si O O Si O Soluble Oligomers and Polymers Bridged Polysilsesquioxane Gel = arylene, alkylene, etc. Gelation Fig. 3.1 Formation of bridged polysilsesquioxanes by the hydrolysis and condensation of monomers with two or more trialkoxy- silyl groups attached to organic bridging groups 3 Bridged Polysilsesquioxanes 52 Fe Si(OMe) 3 (MeO) 3 Si (RO) 3 Si Si(OR) 3 S (RO) 3 Si Si(OR) 3 (RO) 3 Si Si(OR) 3 Si(OR) 3 (RO) 3 Si (RO) 3 Si Si(OR) 3 (RO) 3 Si Si(OR) 3 (RO) 3 Si Si(OR) 3 N H O N H ARYLENE Si(OR) 3 (RO) 3 Si (RO) 3 Si Si(OR) 3 (RO) 3 Si Si(OR) 3 (RO) 3 Si X Si(OR) 3 Cr(CO) 3 Si(OEt) 3 (EtO) 3 Si n n = 1-4 (4) n = 1-2 (5) n = 1-4 (8) (15) (7) (6) n n n = 5-10, 12, 14 (9) ALKYNYLENE ALKENYLENE ALKYLENE X = NH (10), O (11), S x (12) HETERO-ATOM FUNCTIONALIZED ORGANOMETALLIC (18) (19) n n n = 1-3 N N Si(OMe) 3 (MeO) 3 Si X X N N NH O N H Si(OEt) 3 HN O HN (EtO) 3 Si (1) (2) (3) DIALKYNYLARYLENE ALKADIENYLENE P P Si(OiPr) 3 Si(OiPr) 3 (i-PrO) 3 Si (i-PrO) 3 Si (13) PHOSPHINE O HN O NH(RO) 3 Si Si(OR) 3 AMIDE (14) UREA VIOLOGEN AZOBENZENE (16) (17) Fig. 3.2 Representative bridged polysilsesquioxane monomers 3.2 Historical Background Sol-gel processable monomers containing two or more trichlorosilyl or trialkoxysi- lyl groups (Fig. 3.1) have been known for over 55 years [103]. Prior to the late 1980s, virtually all of these compounds were used as coupling agents, surface modifiers, or coatings, and as components of adhesive formulations. For example, the pheny- lene (Fig. 3.2, 1) and acetylene (Fig. 3.2, 2) [104] bridged monomers were first pre- pared in the 1950s for use as coatings on glass. The dipropyltetrasulfide-bridged monomer or “Si-69” (Fig. 3.2, 12) was developed as a coupling agent for elastomers in the 1970s [33, 38, 39]. More than 30 ¥ 10 6 kg of Si-69 is produced each year; much of this is used in silica-rubber composites. Initial investigations of bridged polysilsesquioxanes with rigid arylene (Fig. 3.2, 1) and acetylene (Fig. 3.2, 3) bridging groups were undertaken to determine if the poro- sity of amorphous hybrid materials could be controlled at the molecular level [9–13]. Subsequently, bridged polysilsesquioxanes were prepared from monomers with tri- methoxysilyl groups [105] and a rapidly growing number of new bridging groups [3, 106]. The primary focus was still control over porosity, based on the nature of the organic group or through its destruction as a template. Since then, the field has bro- adened in scope to include control of the optical [4], thermomechanical [34, 107], and chemical properties. As was mentioned earlier, a bridging organic group has also been reported as an expedient means for securely attaching organic functionalities, such as dyes or metals, to existing porous materials. The ease with which porous bridged polysilsesquioxane gels can be prepared also led to their application as encapsulants for biochemicals [108–111]. Recently, efforts to prepare functionalized materials have evolved into strategies for controlling the long-range order based on surfactant templating, hydrogen bonding, organometallic complexation, and meso- genic interactions. The rest of this chapter will focus first on how various monomers can be prepared, continue with a review of the basics for bridged polysilsesquioxa- ne sol-gel polymerization and processing, and end with a survey of applications. 3.3 Monomer Synthesis One of the attractive features of working with bridged polysilsesquioxanes is the relative ease with which the monomers can be prepared. This means that one can make monomers quickly and begin to study the construction of the hyper-cross-lin- ked materials with little delay. There are a number of useful synthetic methods that permit bridged monomers to be prepared in one or two steps from readily avail- able starting materials. The three most commonly used approaches are by (i) metal- lation of aryl, alkyl, and alkynyl precursors followed by reaction with a tetrafunc- tional silane, (ii) hydrosilylation of dienes (or polyenes) or, less commonly, diynes, and (iii) reaction of a bifunctional organic group with an organotrialkoxysilane bear- ing a reactive functional group. 3.3 Monomer Synthesis 53 3.3.1 Metallation Metallation includes the reactions of arylene Grignards with tetralkoxysilanes (Fig. 3.3a) [12, 105] or from Grignard reagents bearing trialkoxysilyl groups (Fig. 3.3b) [112, 113], lithium-halogen exchange (Fig. 3.3c) [11, 12, 105], and depro- tonation of acetylenes (Fig. 3.3d) [12, 16, 17, 104, 114], or treatment of bis(trialko- xysilyl)methane with a Grignard, organolithium, or metal hydride [115] (Fig. 3.3e). In each case, the resulting organometallic reagent is reacted with a tetraalkoxysila- ne or chlorotrialkoxysilane or other electrophiles to give the final products in mode- rate to good yields. 3.3.2 Hydrosilylation Hydrosilylation (Fig. 3.4) is an efficient reaction for preparing bridged monomers in high yields from chemicals bearing two or more terminal olefins [25, 26, 116, 117]. The reaction has been used to create monomers with alkylene and hetero- 3 Bridged Polysilsesquioxanes 54 Br Br (EtO) 3 Si Si(OEt) 3 (MeO) 3 Si Si(OMe) 3 1) Mg 0 , THF, reflux 2) > 5 Si(OEt) 4 1) 2 BuLi, THF, -78 °C 2) >2 eq (MeO) 3 SiCl (4) (1) (a) (b) Cl 2 P PCl 2 (i-PrO)Si MgCl P P Si(OiPr) 3 Si(OiPr) 3 (i-PrO) 3 Si (i-PrO) 3 Si Br Br Si(OEt) 3 Si(OEt) 3 1) 4 eq.t-BuLi THF, -78 °C 2) >2 eq (EtO) 3 SiCl (c) (d) (e) (i-PrO) 3 Si Cl 1) Mg, THF 2)RBr (i-PrO) 3 Si R (10) Fig. 3.3 Preparation of monomers by metal-halogen exchange or metallation chemistry 3.3 Monomer Synthesis 55 atom-functionalized bridging groups. Addition of the Si–H group in trichlorosila- nes or trialkoxysilanes across carbon-carbon double bonds is most often catalyzed with a noble metal catalyst such as chloroplatinic acid or Karsted’s or Spier’s cata- lyst [118], usually placing the silicon at the terminal position of the double bond (Fig. 3.4a). Hydrosilylation of 1,5-hexadiene (Fig. 3.4b) affords the 1,6-bis(trime- thoxysilyl)hexane (Fig. 3.4, 9) as a clear colorless oil. The 1,4-butylene-bridged monomer (Fig. 3.4, 8) can be prepared from butadiene by palladium-catalyzed hydrosilylation, in situ isomerization to afford the 3-butenyltrichlorosilane, followed by a second hydrosilylation (Fig. 3.4c) [25]. The resulting trichlorosilane is readily converted to trialkoxysilanes with trialkylorthoformates [25] or with alcohols and an amine [12]. 3.3.3 Functionalization of an Organotrialkoxysilane This synthetic route has become increasingly common because it permits a great number of bridging groups to be prepared from readily available starting materials (Fig. 3.5). For example, an electrophilic substituent on the organotrialkoxysilane can be reacted with any organic molecule with two or more nucleophilic groups to cre- ate a bridged monomer. Suitable electrophilic groups available in commercially available silane coupling agents include isocyanates, alkyl or benzyl halides, epoxi- des, and acrylates. Isocyanates, the most frequently used electrophile for preparing new bridged monomers, react readily with amines (Fig. 3.5a) to give urea linkages (Fig. 3.5, 15) [50, 52–56, 119] with alcohols (Fig. 3.5b) in the presence of tin or aci- dic catalysts to give urethanes or carbamates (Fig. 3.5, 19) [31, 61, 120–122], or with carboxylic acids to give, after decarboxylation, an amide linkage. Alkyl halide sub- stituted organotrialkoxysilanes (Fig. 3.5c) have been used with diamines to give bridging groups with amino functionalities (Fig. 3.5, 20) [7, 123–127]. Amines have proven to be one of the most useful starting materials for preparing bridged mono- (MeO) 3 Si Si(OMe) 3 Cl 3 Si SiCl 3 (EtO) 3 Si Si(OEt) 3 (EtO) 3 Si Si(OEt) 3 (EtO) 3 Si Si(OEt) 3 > 2 (MeO) 3 SiH > 2 (EtO) 3 CH Trace Ethanol Reflux > 2 Cl 3 SiH Pd 0 > 2 (EtO) 3 SiH 1 mol% Chloroplatinic Acid Benzene, 50 °C + Major Minor (9) (8) (9) (a) (b) (c) 1 mol% Chloroplatinic Acid Benzene, 50 °C Fig. 3.4 Preparation of monomers by hydrosilylation reactions mers. A number of amide-containing bridges (Fig. 3.5d) have been prepared from precursors bearing two or more sulfonyl chlorides or acid chlorides (Fig. 3.5, 21) [47]. Bridging groups based on Schiff bases (Fig. 3.5, 22) have been prepared by reacting (aminopropyl)trialkoxysilanes with di- or trialdehydes (Fig. 3.5e) [128–130]. 3.3.4 Other Approaches The reaction of the silyl anion of trichlorosilane with allyl or benzyl halides has been used to prepare 2,4-hexadienylene (7) [22], 2-butenylene (Fig. 3.6a&b, 6)[22, 23, 131], and xylylene (Fig. 3.6c, 23) [132–134] bridged monomers. Other approa- ches include ruthenium-catalyzed silylation/desilylation of vinyltriethoxysilane (Fig. 3.6d) to afford a mixture of the E-isomer of the ethenylene-bridged monomer (Fig. 3.6, 5) and the vinylidene isomer [20, 21, 135], photochemical isomerization of the E-isomer to the Z isomer of 5 (Fig. 3.6e) [20, 21], Heck vinylation (Fig. 3.6f) to afford crown- [136, 137] or oligoarylenevinylene-bridged monomers (Fig. 3.6, 24) [18, 127, 138], and the Diels-Alder reaction (Fig. 3.6g) of 1,2-bis(trichlorosilyl)ethe- ne with cyclopentadiene (Fig. 3.6, 25) [139]. A promising new approach for prepa- ring arylene-bridged monomers is the Murai coupling reaction between vinyltrial- koxysilanes and carbonyl functionalized aryl compounds [140]. Another method for forming bridging groups is through the formation of a metal complex (Fig. 3.7) using Lewis base (electron donor) groups such as isonitriles (Fig. 3.7, 26) [141, 142], phosphines (Fig. 3.7, 27) [5, 40–42, 44–46, 57, 143–150], amines [6, 151], thiols [152], or diamines (Fig. 3.7, 28)[126] as metal ligands in an 3 Bridged Polysilsesquioxanes 56 (EtO) 3 Si NCO (EtO) 3 Si NH 2 (EtO) 3 Si N H N H Si(OEt) 3 O a) (15) b) (EtO) 3 Si NCO HO OH (EtO) 3 Si N H O O O N H O Si(OEt) 3 Catalyst (19) NH 2 NH 2 (EtO) 3 Si Cl N H N H Si(OEt) 3 Si(OEt) 3 2 c) (20) (14) d) (EtO) 3 Si NH 2 (EtO) 3 Si N H O N H O Si(OEt) 3 COClClOC 2R 3 N Fig. 3.5 Preparation of bridged monomers from organotrialkoxysilanes 3.3 Monomer Synthesis 57 (b) Cl 3 Si SiCl 3 (EtO) 3 Si Si(OEt) 3 >6 EtOH > Et 3 N, PhH (6) (a) Cl Cl Cl 3 SiH, Et 3 N Et 2 O, CuCl 2 Cl 3 Si SiCl 3 (c) Cl Cl SiCl 3 Cl 3 Si Cl 3 SiH, Et 3 N Et 2 O, CuCl 2 (23) (d) Si(OEt) 3 (Ph 3 P) 3 Ru(CO)H Si(OEt) 3 (EtO) 3 Si (EtO) 3 Si (EtO) 3 Si PhCH 3 , Reflux + (E-5) (e) Si(OEt) 3 (EtO) 3 Si PhC=O hν Si(OEt) 3 (EtO) 3 Si Si(OEt) 3 (EtO) 3 Si + 75% 25% (E-5) (Z-5) (E-5) (g) SiCl 3 Cl 3 Si (25) SiCl 3 SiCl 3 >2 HC(OEt) 3 Si(OEt) 3 Si(OEt) 3 110 °C (f) (EtO) 3 Si cat. Pd(OAc) 2 (EtO) 3 Si Si(OEt) 3 Br Ar Br Ar (24) P(Ph-OMe) 3 Fig. 3.6 Miscellaneous methods of synthesizing monomers organometallic bridging group. Electrochemically active, ferrocenylene-bridged monomers are readily prepared by metallation and reaction with chlorotrialkoxy- silane [66, 67]. Trialkoxysilyl-arene-chromiumtricarbonyl complexes (Fig. 3.5, 19) readily form upon reaction of the corresponding trialkoxysilylaryl compound with chromium hexacarbonyl [69–71, 72, 73, 74]. 3.4 Sol-Gel Processing of Bridged Polysilsesquioxanes Sol-gel processing can be viewed as a series of stages (Fig. 3.8), hydrolysis and con- densation chemistry, gelation, aging, and drying [153], through which an alkoxysi- lane monomer is converted into a hyper-cross-linked siloxane network. 3.4.1 Hydrolysis and Condensation Sol-gel polymerization of bridged trialkoxysilanes proceeds by a series of hydroly- sis and condensation reactions that afford up to three siloxane bonds to each sili- con atom and produce three equivalents of alcohol [1, 154–156]. The dynamic result is a “sol” composed of the hydrolyzed and condensed species that will eventually grow into a percolating network or gel. The reactions are typically performed in the same alcohol generated by the monomer hydrolysis or in tetrahydrofuran. At least three equivalents of water as the co-reactant are added to the polymerization reac- tion. The sol-gel polymerization is generally acid or base catalyzed, although fluo- ride catalysts have also been used by several research groups [105, 157–160]. Hydrochloric acid is typically used as the acidic catalyst. Ammonium hydroxide, sodium hydroxide, and potassium hydroxide have been used as basic catalysts. Hydrolysis and condensation rates of alkyl- and aryltrialkoxysilanes are signifi- cantly faster than tetraalkoxysilanes under acidic conditions and slower under basic conditions [154, 161]. For example, simple alkyltriethoxysilanes hydrolyze 6–10 3 Bridged Polysilsesquioxanes 58 (a) (b) (c) Si(OEt) 3 [RhCl(CO) 2 ] 2 NC Si(OEt) 3 NC(EtO) 3 Si N C Rh Cl OC PhCH 3 Si(OMe) 3 Ph 2 P (COD)PtCl 2 CH 2 Cl 2 Si(OMe) 3 P (MeO) 3 Si P Pt Ph Ph Ph Ph Cl Cl H 2 N H N Si(OEt) 3 N Ni HN N N H N NH Si(OMe) 3 Si(OMe) 3 (MeO) 3 Si Ni(OAc) 2 2 2 2+ (26) (27) (28) Fig. 3.7 Formation of organometallic bridged polysilsesquioxanes 3.4 Sol-Gel Processing of Bridged Polysilsesquioxanes 59 times faster than tetraethoxysilane (TEOS) with acid catalysts. It has been shown for silica sol-gels derived from tetraethoxysilane that only silanol-silanol or water- producing condensation is observed, while tetramethoxysilane leads to both water and methanol producing condensation reactions. While no-one has verified this reactivity trend with silsesquioxanes, it is not unreasonable to suspect that a simi- lar reactivity is observed. The resulting “sol” will contain, depending on the time, reaction conditions, catalyst and other experimental variables, monomeric species, cyclic and acyclic oligomers, polymers and colloids. As would be expected from the sol-gel chemistry of tetraalkoxysilanes, the condensation rates decrease with incre- asing size of the alkoxide substituent (MeO > EtO > n-PrO) [162]. 3.4.2 Gelation The next stage of the sol-gel process is gelation. Gelation is the manifestation of the percolation of colloidal polysilsesquioxanes throughout the liquid. The ease with which bridged polysilsesquioxanes form gels may be their single most distinguis- hing trait. The six reactive alkoxide groups result in rapid gelation times for both Sol Gel Precursor + Solvent + water + Catalyst Xerogel Viscous liquid made