Anna Bozena Kowalewska

Photoacid catalyzed sol–gel process

The hydrolysis of compounds containing alkoxysilyl moieties catalysed by UV-generated protic acids, followed by condensation of in situ generated silanol groups, is proposed as an effective approach to formation of hard, silicate-type materials by a sol–gel process. It gives highly thermally stable materials which can be of value as precursors to silicon oxycarbide films. The effectiveness of the photocatalyst used in the process depends on the type of anion and seems to be limited, in some cases, by Si–F bond formation.

Novel polymer systems with very bulky organosilicon side chain substituents

Abstract Synthetic methods leading to novel polysiloxanes and polystyrene copolymers are described. Both types of polymer could be modified by incorporation of the highly sterically demanding tris(trimethylsilyl)methyl substituent (Me3Si)3C. The modification increases the rigidity of polymers as shown by d.s.c. analyses. The effect is especially dramatic in the case of silyl substituted polysiloxane copolymer. In that case the steric bulk of the tris(trimethylsilyl)methyl group renders 8% of Si-H bonds in poly(methylhydrosiloxane) (PMHS) unreactive towards the hydrosilylation process. The Si-H bonds of the resultant copolymer can be utilised in crosslinking of the linear polysiloxane.

Siloxane and silsesquioxane molecules—Precursors for silicate materials

Preparation of ceramics by sol-gel method has been known for many years, but recently it has been developed as a method for the synthesis of nanostructural ceramic materials. Hydrolytic polycondensation of simple molecules [e.g. most widely used tetraethoxysilane (TEOS)] leads to xerogel materials that can contain macromolecules of distinct random, ladder and cage or partial cage structure. In order to obtain well-defined silsesquioxanes it is preferred to start the process with more complex molecules, bringing in a specific framework that can govern the structure of the product. In the presented work alkoxy derivatives of cyclosiloxanes and polysiloxanes as well as a hydride derivative of octahedral silsequioxane (T(8)(H)) were applied as precursors in the process of hydrolytic polycondensation. Depending on the reaction conditions, silsesquioxane macromolecules or silica material of ordered structure were obtained. We have prepared mesoporous organiosilica materials without using any template or surfactant whatsoever. The meso-pores are created due to the unique structure of initial oligosiloxane or silsequioxane molecules and the specific interactions in the used catalyst/solvent system. In the case of octasilsesquioxane precursor, the condensation process gives directly mesoporous silica material. Dried polysilsesquioxanes were heated at the temperature of 600°C in argon or air atmosphere (pyrolysis or ceramization). In the atmosphere of argon SiC(x)O(y) glass materials were obtained.

Iodo(methoxydimethylsilyl)bis(trimethylsilyl)methane: a reagent for the preparation of novel organometallic compounds. Crystal structures of Mg{C(SiMe3)2(SiMe2OMe)}2 and MgI2(OEt2)2

The alkyl chloride R2(HMe2Si)CCl (R = SiMe3) reacted with ICl to give the iodide R2(ClMe2Si)CI, which with MeOH gave R2(MeOMe2Si)CI. This reacted with Mg in Et2O to give MgI2(OEt2)2 (which was isolated and structurally characterised) and the chelated dialkylmagnesium Mg{C(SiMe3)2(SiMe2OMe)}2, apparently following initial formation of the Grignard reagent MeOSiMe2CR2MgI. The latter compound was isolated from the products of the reaction between the iodide R2(MeOMe2Si)CI and Mg in toluene. The lithium compound MeOSiMe2CR2Li was made by treatment of the chloride R2(MeOMe2Si)2CCl with Buli in THF and the chelated structure was confirmed by an X-ray study.

Synthesis of Ladder Silsesquioxanes by in situ Polycondensation of Cyclic Tetravinylsiloxanetetraols

Functionalized tetrahydroxy-cyclotetrasiloxanes are very attractive precursors of tetrasiloxane ring systems in linear silsesquioxanes (LPSQs). Vinyl groups are especially important since they can be employed for various chemical transformations (e.g. hydrosilylation, metathesis reactions, thiol-ene addition). However, isolation of such highly reactive species bearing small side substituents at silicon atoms with high yield is challenging. We overcame the problem by in situ condensation of 2,4,6,8-tetrahydroxy-2,4,6,8-tetravinyl-cyclotetrasiloxanes, derived from potassium organosiloxanolate {(K+)4[ViSi(O)O−] 4} nL (L = EtOH, H2O). Oligomeric LPSQ materials having a siloxane backbone with ladder structure and functionalized with side vinyl groups were thus prepared. Hexamethyldisilazane was employed for chain termination. The liquid materials show remarkably good solubility in organic solvents. They were isolated and characterized using spectroscopic methods. Information on their structure and molecular masses was derived from MALDI-TOF and MALLS measurements. Size exclusion chromatography (RI detection, volume of elution measurements) was also used. The products were characterized by wide-angle X-Ray diffraction measurements (WAXS) and their thermal characteristics (TGA and DSC) were obtained.

Alkali-Metal-Directed Hydrolytic Condensation of 3-Mercaptopropyltrimethoxysilane

A series of solid sodium and potassium cyclotetrasiloxanolates was synthetized with high yields by hydrolytic condensation of 3-mercaptopropyltrimethoxysilane in the presence of potassium or sodium hydroxide. The obtained materials were characterized using solid state 29Si and 13C NMR, wide angle X-ray scattering (WAXS), elemental analysis, Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The influence of reaction conditions on the structure and physiochemical properties of the obtained products was studied.

Polymer Nano-Materials Through Self-Assembly of Polymeric POSS Systems

The self-organization of polyhedral silsesquioxane (POSS) moieties tethered as side groups onto flexible backbones of amorphous siloxanes and poly(1,2-butadiene) was investigated. For comparison, linear oligosiloxanes substituted with POSS units at the chain end (α- or α,ω-functionalization) were examined. The properties of POSS-functionalized materials were studied (SAXS, WAXD, DSC, TGA, and optical microscopy). The formation of organized POSS assemblies was monitored as a function of the structure of the polymer chain, as well as the POSS concentration in the polymer matrix. It was confirmed that the rubbery polymers allow the pendant POSS units to assemble without the competition of the main chain crystallization. Materials with a larger number of POSS groups tend to form lamellar structures. It was found that the process of disintegration of POSS assemblies in siloxanes is thermally driven. The temperature of melting depends on the structure of a POSS assembly, and the time needed for the organized structure recovery from the melt depends on the type of arrangement of POSS-moieties that is to be formed. At a low POSS content besides the POSS assemblies, which are crystallites of nanometric size, larger polycrystalline objects of micrometric sizes are formed. They consist of a greater number of POSS crystallites embedded in the siloxane polymer. Their shapes and sizes are dependent on the polymer structure and the history of the polymer sample.

A novel organometallic route to phenylethenyl-modified polysiloxanes

We have synthesized a series of cyclic and linear siloxane materials with phenylethenyl substituents via transition metal complex-catalyzed coupling of the respective vinylsiloxane systems with styrene and α-methylstyrene. It has been shown that the non-carbene metal catalysts [RuCl(H)(CO)(PPh3)3] and [RuCl(SiMe3)(CO)(PPh3)2] are the most effective ones, pointing to a silylative coupling pathway as the most plausible mechanistic route. The process was studied in the presence of a series of catalysts and styrene polymerization inhibitors under different reaction conditions, leading to useful silicone materials characterized by high refractive index values ranging from 1.51 to 1.59 due to strong π-conjugation in side chain substituents.

Reactions of the SiH bonds in tetrakis(dimethylsilyl)methane and silane

Abstract (Me 2 HSi) 4 M (M=C or Si) were used as: (i) branching substrates in the synthesis of hybrid single atom liquid crystal (lc) materials (low-molecular-weight liquid crystal compounds with mesogens attached via flexible spacers to a central single atom) and (ii) free radical reducing agents for organic bromides in processes initiated by UV radiation and ultrasound (US). In the former application, hydrosilylation of the mesogenic terminal alkenes 4′-methoxy-phenyl-4-(alkenyloxy)benzoates and 4′cyano-4-(10-undecenyloxy)stilbene led to synthesis of a novel class of hybrid liquid crystals with Si or carbon as a tetrahedral centre. The respective silane and methane were found to be efficient reducing agents. For model substrates — benzyl bromide and hexadecyl bromide — all four SiH bonds in both branched molecules are equally effective in the UV-initiated reactions. The inefficiency of the ultrasound-promoted reductions is ascribed to low volatility of tetrakis(dimethylsilyl)silane and methane.

Anchimeric assistance by γ-substituents Z, Z=MeO, PhO, MeS or PhS, in reactions of the bromides (Me3Si)2(ZMe2Si)CSiMe2Br with AgBF4

Abstract The new organosilicon bromides (Me 3 Si) 2 (ZMe 2 Si)CSiMe 2 Br with Z=PhO or MeS have been prepared and new spectroscopic data obtained for the previously reported compounds with Z=H, F, Br, Me, Ph, MeO or PhS. Competitions between pairs of bromides for a deficiency of AgBF 4 in Et 2 O, with the determination of the ratio of the fluoride products by 19 F-NMR spectroscopy, have led to the following approximate relative reactivities of the bromides and so to the relative abilities of the γ-Z groups to provide anchimeric assistance to the leaving of Br − in this reaction: Me, 1; Ph, 40; PhO, 3400; PhS, 5000; MeS, 7000; MeO, 54 000. In methanolysis in CH 2 Cl 2 , (Me 3 Si) 2 (MeOMe 2 Si)CSiMe 2 Cl has been found to be roughly 120 times as reactive as (Me 3 Si) 2 (PhOMe 2 Si)CSiMe 2 Cl. Combination of the results with previously available information suggests the following approximate order of ability of γ-groups Z to provide anchimeric assistance in reactions at the SiX bonds in compounds (Me 3 Si) 2 (ZMe 2 Si)CSiMe 2 X: OCOMe>OMe>OCOCF 3 >MeS>PhS, PhO>N 3 , Cl>NCS>Ph>CHCH 2 >Me.