Wlodzimierz A. Stanczyk

The crystal structure of (HOMe2Si)2O

Abstract X-Ray crystallography shows that in the solid state, molecules of(HOMe 2 Si) 2 O are extensively hydrogen bonded together to form zig-zag double chains with no bonding between the chains.

Side-chain Liquid Crystal Polymers (SCLCP): Methods and Materials. An Overview

This review focuses on recent developments in the chemistry of side chain liquid crystal polymers. It concentrates on current trends in synthetic methods and novel, well defined structures, supramolecular arrangements, properties, and applications. The review covers literature published in this century, apart from some areas, such as dendritic and elastomeric systems, which have been recently reviewed.

Base cleavage of R–Si bonds of silanols RSiMe2OH. A proposed new mechanism of substitution at silicon

Cleavage of R–Si bonds has been found to occur unexpectedly rapidly in solutions of RSiMe2OMe (R =m-ClC6H4CH2 or PhC⋮C) in 5 vol-% H2O–MeOH containing relatively small concentrations of NaOMe, but with a levelling off of the rate at high concentrations of the base. The behaviour is attributed to the formation of RSiMe2OH and hence RSiMe2O–, and it is suggested that unimolecular dissociation of the silanolate anion RSiMe2O– to give R– and the silianone Me2SiO (both of which react rapidly with the solvent) plays a significant role. The silanols RSiMe2OH are roughly estimated to have pKa values of 11 (R =m-ClC6H4CH2) and 9.8 (R = PhC⋮C) in water.

1,3-Migration of a phenyl group via a silicocation

The reaction of (Me3Si)2C(SiMe2Ph)(SiEt2l) with AgBF4 in Et2O has been shown to give a ca. 2 : 3 ratio of the unrearranged (Me3Si)2C(SiMe2Ph)(SiEt2F) and the rearranged (Me3Si)2C(SiMe2F)(SiEt2Ph); the reaction is thought to proceed via a Ph-bridged cation (II).

Organosilicon mesomorphic polymer systems

Abstract This review is devoted to unusual organosilicon polymers exhibiting birefringence due to the formation of mesophase (μ) of the ‘condis’ (conformationally disordered) type. The polymers (cyclolinear and linear polysiloxanes, polysilanes, poly(oxymultisilanes), polycarbosilanes, their copolymers having organic monomeric units and their networks) are able to form thermotropic mesophases, basically of columnar architecture, although the macromolecules do not incorporate any typical ‘rod’- or ‘disc’-like mesogenic moieties. Their structure and morphology (if studied), as well as methods of synthesising them are reviewed with particular stress on the effect of polymer composition, molecular weight, and types of side chain substituents on the stability of mesophases and phase transfer temperatures. Most of these polymers show mesomorphic behaviour over a wide temperature range, even at rather elevated temperatures and the isotropisation occurs often only at the decomposition point. It is believed that they may be of importance as relatively simple structures with potential practical applications.

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 and characterization of liquid crystal polycarbosilanes: poly(1-methyl-1-silaethylene), poly(1-methyl-1-silabutane) and poly(1-silabutane) with pendant mesogenic groups

Abstract The synthetic routes for preparation of side chain liquid crystal polycarbosilanes, with rod type mesogens, pending from every second and every fourth atom in the main chain, forming ‘comb-like’ and ‘double comb-like’ polymers, are described. Structure—property relationships were investigated. X-ray measurements of the smectic materials, oriented in a magnetic field, proved the S A1 phase to be a dominant one for these new mesomorphic polymers; however, generation of S Ad and more ordered S B phases was also detected. Polycarbosilanes present useful, highly thermally and chemically resistant materials, which provide useful models for studies of the influence of the main chain architecture on the properties of polymers with various pendant groups.

Dissociative Pathways in Substitution at Silicon in Solution: Silicon Cations R3Si+, R3Si+ ← Nu, and Silene-Type Species R2Si=X as Intermediates

Publisher Summary This chapter discusses mechanistic pathways involving the heterolytic cleavage of a bond to silicon leading to the transient existence of tri- or tetracoordinate silicon cations or tricoordinate doubly bonded silicon species. The theory surrounding such intermediates in solution has recently drawn the attention of many chemists. The role of the silicon analog of the carbenium ion has been of interest to many organosilicon chemists. Quantum mechanical calculations of silylenium ion structures have been made mostly by ab initio methods. The greatly diminished stabilization of the silylenium compared to the carbenium structure by π substituents is connected with the known low effectiveness of 2p–3p (π) conjugation. Silylenium ions are common in gas-phase organosilicon chemistry, where they may be generated by various techniques, including electron impact, photoionization, chemical ionization, collision-induced dissociation, and chemical–nuclear methods. Electron-impact ionization of trimethylchlorosilane in an ion cyclotron resonance (ICR) mass spectrometer resulted in the formation of the trimethylsilyl cation, which could be detected for as long as 800 mseconds. Mass spectroscopy has been often used to study the chemical behavior of silylenium ions. Knowledge of the dynamic behavior of silylonium complexes is of great importance for understanding the role of these species as intermediates in the processes of substitution at silicon. Many important substitution reactions at silicon are known to be effectively accelerated by uncharged Lewis bases. Nucleophile-induced racemization of chiral halosilanes has been extensively studied in connection with interest in the mechanism of nucleophilic catalysis in substitution at silicon.

Liquid crystal dimers with organosilicon spacers as models for side chain LC polymers

Abstract A series of six mesogenic dimers were prepared containing terminal 4’-methoxyphenyl-4-octenyloxybenzoate groups, of the general formula [MeOC 6 H 4 O(O)CC 6 H 4 O(CH 2 ) 8 SiMe 2 ] 2 R, where R=–CH 2 –, –(CH 2 ) 2 –, –(CH 2 ) 3 –, –SiMe 2 –, –O– and –NH–. Their liquid crystal properties were compared with those of the structurally relevant side chain polymers with the same mesogenic moiety: polycarbosilanes, polyethylene, and polysiloxane. It is shown that the low molecular dimers can be used as models for evaluating and predicting mesogenic characteristics of macromolecular systems with backbones structurally equivalent to spacers of the dimers.

Cleavage of α-halo-substituted alkyl groups from silicon : The effect of halogen in the base-catalysed solvolysis of (α-dihalomethyl)trimethylsilanes

Abstract The kinetics of the solvolysis of Me 3 SiCHCl 2 (I), Me 3 SiCHBr 2 (II), Me 3 SiCHI 2 (III) and Me 3 SICCl 2 Me (IV) in the system n-propanol/water (8/2 v/v) in the presence of an NH 3 /NH 4 Cl buffer have been investigated. The results show that α-haloalkyl groups are removed from silicon by two routes: (1) by nucleophilic substitution as a result of attack of a conjugate base of the solvent on silicon, and (2) by base-catalysed ammonia substitution. The existence of the latter route is connected with the fact that α-haloalkyls are very poor leaving groups in nucleophilic substitutions. The relative reactivities at 25° are as follows, I/II/III/IV = 1/116/246/0.2 for route 1 and I/II/III/IV = 1/24/49/0.05 for route 2. The reactivity increases with increasing stabilization of the negative charge in the separating carbanion, but there is also evidence that the back-strain in the leaving group affects the reactivity.