ChemInform Abstract A wide range of reactions of the type Ph3C+X-+ R3SiH is studied kinetically, e.g., with X: SbF6, AsF6, BF4, or FeCl4, and R: -Me, -Et, -Pr, -Bu or with varying silanes, e.g., Ph3C+ + R-SiMe2-H with R: -Et, -Ph, -CH2-Ph; tropenylium (C7H7)+ salts and Et3GeH are also included.

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.

Cleavage of R-Si bonds has been found to occur unexpectedly rapidly in solutions of RSiMe,OMe (R = m-CIC,H,CH, or PhCiC) in 5 vol-% H,O-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 RSiMe OH and hence RSiMe,O-, and it is suggested that unimolecular dissociation of the silanolate anion RSible,Oto give Rand the silianone Me,Si=O (both of which react rapidly with the solvent) plays a significant role. The silanols RSiMe,OH are roughly estimated t o have pK, values of 11 (R = m-CIC,H,CH,) and 9.8 (R = PhCiC) in water.The cleavages of R-SiMe, bonds by NaOMe-MeOH have been studied for a wide range of R groups.' While investigating the effects of variation in the other ligands on silicon we examined the cleavage of m-ClC,H,CH,Si(OMe), in NaOMe-MeOH.(Rapid exchange of OMe groups occurs between the substrate and the solvent under these conditions, but this should not cause any complication.) We found that the change of the observed first-order rate constant on variation of the concentration of base showed a most unusual form, which we thought would best be accounted for in terms of the presence of small amounts of water, and the consequent formation of Si-OH bonds and hence silanolate ions, e.g. m-C1C6H,CH,Si(OMe),0 -. To investigate the possible role of such ions we turned to the simpler system involving use of rn-C1C6H4CH,SiMe,0Me, with the results described below. A preliminary account has appeared previously.2 Results and DiscussionObserved first-order rate constants, k, for the cleavage of m-C1C6H4CH2SiMe,0Me by 0.05-2.86~~NaOMe in anhydrous MeOH are shown in Table 1, and plotted against the base concentration in Figure 1. Table 2 lists the values of the specific rate constant, k, (= k/[NaOMe]) and of the ratio kJk,", where k," is the specific rate at low base concentrations (in this case 0 . 0 5 ~) ,

Novel thermally stable polysiloxanes with bulky side chain tris(trimethylsilyl)hexyl and reactive glycidoxypropyl substituents were synthesized via hydrosilylation route and were further cross-linked in the presence of diaryliodonium salt upon exposure to UV light. The cross-linking was necessary to stabilize the shape of the linear tris(trimethylsilyl)hexyl group modified siloxanes which potentially can be used as membranes for fluid separation at high temperatures. Formation of oligoether cross-linking chains was proved by solid state 13 C NMR analyses. Six copolymers of diverse molecular architecture were made based on methylsiloxane and methylsiloxane-co-dimethylsiloxane backbones. The ratio between [tris-(trimethylsilyl)hexyl](methyl)-siloxane and (glycidoxypropyl)(methyl)siloxane monomeric units in polymer chain, ranging from 0.5 to 5.4, was found to be an important factor affecting the thermal properties of cross-linked films, as shown by TGA.

As scientific literature considers polyhedral oligosilsesquioxanes (POSS) as potential drug delivery systems, it is necessary to check their impact on mammalian cells. Toxicity of octaammonium chloride salt of octaaminopropyl polyhedral oligomeric silsesquioxane (oap-POSS) towards two cell lines: mouse neuroblastoma (N2a) and embryonic mouse hippocampal cells (mHippoE-18) was studied. Experiments consisted of analysis of a cell cycle, cell viability, amount of apoptotic and necrotic cells, and generation of reactive oxygen species (ROS). POSS caused a shift in the cell population from the S and M/G2 phases to the G0/G1 phase. However, the changes affected less than 10% of the cell population and were not accompanied by increased cytotoxicity. POSS did not induce either apoptosis or necrosis and did not generate reactive oxygen species. A cytotoxicity profile of POSS makes it a promising starting material as drug carrier.

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A series of novel side chain polymers-silylated polyethylenes were prepared containing 4′-methoxyphenyl-4-(ω-alkenyloxy)benzoate mesogenic groups of the general formula {-CH2-CH[SiMe2-(CH2)mOC6H4C(O)OC6H4OMe]}n-, where m ) 4, 5, 8, 11. The ones having five and eight carbon atoms in the alkane moiety of a flexible spacer proved to be liquid crystalline, independent of molecular weight (DP ) 75, 230). Alternative synthetic pathways have been described starting with anionic polymerization of Me3SiVi or Me2PhSiVi in the presence of BuLi, followed by functionalization of side silyl group to give reactive Si-H bonds and hydrosilylation of terminal, mesogenic alkenes. The properties of liquid crystal polyethylenes were compared with those of side chain polycarbosilanes of similar structure. The advantage of polyethylenes, while preserving all important features of polycarbosilanes, is shown by their broad temperature range of mesophase formation and the economically attractive synthesis.