Siloxane chains

A further extension of the principle of polymerization through ethylenic unsaturation allows copolymers to be made with the chemically similar organic substances. For example, methylallyldichloro-silane may be hydrolyzed to the corresponding diol and mixed with monomeric methyl methacrylate, and the mixture polymerized to a homogeneous solid by heating with benzoyl peroxide as a catalyst. 

The siloxane chains which form the structural basis for these organosilicon oxides have been treated in the discussion of siloxanes in 

Chapter 1. The general formula of the linear siloxanes may be represented as 

X The siloxane chain is not cleaved by alkalies with the evolution of hydrogen neither do the siloxanes take fire in air as do the silanes. - 

Such organosiloxanes commonly are formed from the corresponding organosilanediols by a process of intermolecular condensation, as has been described in the discussion of the silicols  

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Solvophobicify (hydrophobicify witli respect to water) is most often exemplified as a linear or branched hydrocarbon chain. Fluorocarbon chains and siloxane chains are also hydrophobic. Many commercially important... 

SiHcone mbber has a three-dimensional network stmcture caused by cross-linking of polydimethyl siloxane chains. Three reaction types are predominantiy employed for the formation of siHcone networks (155) peroxide-induced free-radical processes, hydrosdylation addition cure, and condensation cure. SiHcones have also been cross-linked using radiation to produce free radicals or to induce photoinitiated reactions. 

Fig. 4. Data of Auroy et al. [25] on polydimethyl-siloxane chains covalently grafted in the interior of a silica porous medium. L is the measured layer thickness from neutron scattering d is the average spacing per chain from the grafting density. The format of the plot is suggested by Eq. 5. The linearity of that relation is seen in these data...

In our own research, the functional termination of the living siloxanolate with a chlorosilane functional methacrylate leading to siloxane macromonomers with number average molecular weights from 1000 to 20,000 g/mole has been emphasized. Methacrylic and styrenic monomers were then copolymerized with these macromonomers to produce graft copolymers where the styrenic or acrylic monomers comprise the backbone, and the siloxane chains are pendant as grafts as depicted in Scheme 1. Copolymers were prepared with siloxane contents from 5 to 50 weight percent. 

From these results it is evident that there is a definite limiting number of silazane units that can be introduced into a siloxane chain by copolymerization of silazoxane and siloxane rings. The six-membered and eight-membered... 

Figure 10.62 Molecular dimensions of alkane, ether and siloxane chain segments [489]...

Figs. 5 and 6 demonstrate that, contrasting to some NMR based expectations [5] Raman spectroscopy indicates significant structural differences between the overall structures of dissolved silicate molecules when they contain different alkaline metals. Since there have not been siloxane rings associated with the 460 cm 1 centred large Raman shift in Lithisil-25 it is reasonable to assume that this dilute solution contains open siloxane chains. 

Z. Chang, M. C. Kung, and H. H. Kung, Stepwise synthesis of siloxane chains, CherrL Comrrmn. 206 (2004). 

Me3Si)3SiH as a pure material or in solution reacts spontaneously and slowly at ambient temperature with air or molecular oxygen, to form a major product that contains a siloxane chain (Reaction 8.4) [18]. The percentages of... 

The homologous (Me3Si)2Si(H)Me does not react spontaneously with air or molecular oxygen at room temperature. However, a reaction takes place at 80 °C when air or molecular oxygen is bubbled into the pure material or its solutions to form a major product that contains the siloxane chain (Reaction 8.5) [15]. In general, yields of siloxane are about 50%. Also in this case the reaction is accelerated by radical initiation and retarded by common radical inhibitors. 

SNR was prepared by direct chloromethylation of PDS. Chloromethylation of PDS was successfully performed in chloromethylmethylether solution using stannic chloride as a catalyst. The PDS obtained from Petrarch Systems was terminated with OH groups at the end of siloxane chain. A typical... 

Molecular dynamic simulations are performed on a series of siloxane chains of the form polydiaikylsiloxane. Analysis of the trajectories are performed by generating pair-wise probability maps. Using various minimization techniques, a system of equations is evaluated to yield a set of statistical weights for each of the systems. 

Work by Sigwalt, Bischoff and Cypryk22 have used this inter-and intramolecular catalysis to explain the condensation kinetics and cyclic formation processes in siloxane condensations. The kinetics show a very complex dependence on siloxane chain length, complicated by equilibria involving acid, silanol and water. They do indicate that the dominating reaction in the process is condensation and that chain disproportionation and chain scrambling are negligible The kinetics of condensation are influenced by the involvement of triflic acid in several equilibria, i.e. the formation of triflate esters, shown in equation 3, the possible involvement of triflic acid in the reaction of these esters with silanol, shown... 

This disproportionation offers an insight into the mechanism of hydrolytic degradation of PDMS, especially in the presence of traces of acid. Thus, the siloxane chain may undergo a slow scission in the presence of water to give a silanol ended chain, which then depolymerizes by loss of dimethylsiloxanediol. 

Figure 4.2 The poly(di- -propyl-siloxane) chain, showing the conformational variability of the propyl side chains.10 Reproduced by permission of the American Chemical Society.

A pair of vinyl or other unsaturated groups can also be linked by their direct reactions with free radicals. Similar end groups can be placed on siloxane chains by the use of an end blocker during polymerization,73,74 as mentioned earlier. Reactive groups such as vinyl units can, of course, be introduced as side chains by random copolymerizations involving, for example, methylvinylsiloxane trimers or tetramers.11... 

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