Ryutaro Wakabayashi

Aqueous Colloidal Mesoporous Nanoparticles with Ethenylene-Bridged Silsesquioxane Frameworks

Aqueous colloidal mesoporous nanoparticles with ethenylene-bridged silsesquioxane frameworks with a uniform diameter of ∼20 nm were prepared from bis(triethoxysilyl)ethenylene in a basic aqueous solution containing cationic surfactants. The nanoparticles, which had higher hydrolysis resistance under aqueous conditions, showed lower hemolytic activity toward bovine red blood cells than colloidal mesoporous silica nanoparticles.

Nonhydrolytic Synthesis of Branched Alkoxysiloxane Oligomers Si[OSiH(OR)2]4 (R=Me, Et)†

Silicon alkoxides are ideal starting materials for the preparation of silica-based materials. The development of synthetic methods toward various alkoxysiloxane oligomers with finely controlled structures and reactivities is important for the synthesis of materials with defined compositions, structures, morphologies, and functionalities. 3] However, examples of the rational synthesis of alkoxysiloxane oligomers are limited. The controlled formation of Si O Si bonds is a key step in the synthesis of alkoxysiloxane oligomers. A conventional synthesis involves hydrolysis of silicon alkoxides or chlorosilanes to form silanol groups and a subsequent condensation reaction. Although the reaction of chloroalkoxysilanes with organosilanols have been reported, 4] the number of molecules synthesized by using this reaction are quite limited because the reaction is restricted to the cases where the resulting organosilanols are stable. The formation of silanol groups during the reaction causes unwanted side reactions, such as self-condensation of silanols and subsequent hydrolysis of terminal alkoxy groups. Therefore, the synthesis of siloxane oligomers that contain alkoxy groups with defined structures is a challenge in circumventing the problem of side reactions. The formation of siloxane bonds without using silanols has attracted much interest. Alkoxysiloxanes can be synthesized by the reaction between chlorosilane and sodium alkoxysilanolates. On the other hand, the use of alkoxysilanes as a precursor with a Lewis acid catalyst is the most promising pathway. The formation of siloxane bonds proceeds with the generation of RX (X = Cl, Br, I, AcO, or H). No competing reagents, such as compounds containing silanol groups, H2O, or HCl, are involved in the overall process. Oligosiloxanes with defined structures that contain more than one alkoxy group are difficult to synthesize, 15] as side reactions, such as ligand exchange, compete with the formation of siloxane bonds. 9, 13,14] Herein we report the nonhydrolytic synthesis, with suppressed side reactions, of branched alkoxysiloxanes Si[OSiH(OR)2]4 (R = Me (1), Et (2)), which possess both reactive SiOR and SiH groups. The reaction proceeds by direct alkoxysilylation of tetraalkoxysilanes with ClSiH(OR)2 in the presence of BiCl3 (Scheme 1). BiCl3, a weak Lewis acid,

Usefulness of alkoxyltitanosiloxane for the preparation of mesoporous silica containing a large amount of isolated titanium.

Mesoporous silica containing a large amount of isolated Ti was prepared from an alkoxytitanosiloxane precursor through a hard template method. Isopropoxytris(tris-tert-butoxysiloxy)titanium (((i)PrO)Ti[OSi(O(t)Bu)(3)](3), TS3) was synthesized and TS3 was mixed with mesoporous carbon (CMK-3), a hard template. The mixture was pyrolyzed at 180 °C to form a composite consisting of titanosilica and the hard template. After calcination at 600 °C for the removal of the carbon template, the titanium species were not transformed to anatase TiO(2), proved by DR-UV-Vis, FTIR, XPS, and XRD, while the ESR results indicated the presence of isolated Ti. The mesoporous structure was verified by SEM, TEM, and N(2) adsorption. The Si/Ti ratio of the product was consistent with that of the precursor. All the results show that the material prepared from the precursor is ordered mesoporous silica containing a large amount of isolated Ti in the frameworks. The use of well-defined alkoxytitanosiloxane precursor leads to the formation of mesoporous silica with exactly controlled composition of titanium with neither loss of Ti nor transformation to anatase.

Synthesis of a multifunctional alkoxysiloxane oligomer

An alkoxysiloxane oligomer (1, SiMe[OSi(CHCH2)(OMe)2][OSi(CH2)3Cl(OMe)2]2), containing vinyl and chloropropyl groups, was synthesized as a precursor for sol–gel synthesis. Di-tert-butoxymethylhydroxysilane (t-BuO)2MeSiOH was reacted with (MeO)2(CH2CH)SiCl to form (t-BuO)2MeSiOSi(CHCH2)(OMe)2 which was further alkoxysilylated with Cl(CH2)3SiCl(OMe)2 to form 1. The 1H, 13C, 29Si NMR and HR-MS data confirmed the formation of 1, indicating the successful synthesis of an alkoxysiloxane oligomer possessing different kinds of functional groups by a chemoselective route. Hydrolysis of 1 under acidic conditions was completed in a few hours. The solution state 29Si NMR spectra of samples hydrolyzed and condensed at various reaction times show no signals due to species generated by the cleavage of the siloxane bonds in 1, indicating the validity of the synthesized substance as a precursor for the formation of hybrids with homogeneously distributed functional groups. Intramolecular condensation of 1 to form cyclic trisiloxane units proceeds more preferentially than intermolecular condensation.

Protecting and Leaving Functions of Trimethylsilyl Groups in Trimethylsilylated Silicates for the Synthesis of Alkoxysiloxane Oligomers

The concept of protecting groups and leaving groups in organic synthesis was applied to the synthesis of siloxane-based molecules. Alkoxy-functionalized siloxane oligomers composed of SiO4 , RSiO3 , or R2 SiO2 units were chosen as targets (R: functional groups, such as Me and Ph). Herein we describe a novel synthesis of alkoxysiloxane oligomers based on the substitution reaction of trimethylsilyl (TMS) groups with alkoxysilyl groups. Oligosiloxanes possessing TMS groups were reacted with alkoxychlorosilane in the presence of BiCl3 as a catalyst. TMS groups were substituted with alkoxysilyl groups, leading to the synthesis of alkoxysiloxane oligomers. Siloxane oligomers composed of RSiO3 and R2 SiO2 units were synthesized more efficiently than those composed of SiO4 units, suggesting that the steric hindrance around the TMS groups of the oligosiloxanes makes a difference in the degree of substitution. This reaction uses TMS groups as both protecting and leaving groups for SiOH/SiO- groups.