Polysiloxanes linear

Linear polysiloxanes—compared to the equivalent polymer of saturated hydrocarbons—can be fluid even with high molecular weights, thanks to the unusual flexibility of the siloxane chain. Their viscosity depends on their molecular weight. Because of this, it is quite simple to study the effects of radiation by monitoring the molecular weight changes of the irradiated siloxane liquids, as an indication of crosslinking process. 

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Linear polysiloxanes containing terminal function groups such as alkoxy groups, chlorine atoms are technically prepared by equilibration of cyclic polysiloxanes with functional silicone compounds291. ... 

Chojnowski, J. Cypry k, M. Synthesis of Linear Polysiloxanes. In Silicon-Containing Polymers. The Science and Technology of Their Synthesis and Applications, Jones, R. G., Ando, W., Chojnowski, J., Eds. Kluwer Dordrecht, 2000 Chapter 1, pp 3—41. 

Linear polysiloxanes obtained by the hydrolysis of dichlorosilanes are of relatively low molecular weight they can, however, be condensed further through the terminal OH groups by thermal after-treatment. 

DeBolt and Mark [71] reported that a 30-membered cyclic dimethylsiloxane was not threaded by linear polysiloxane via Method 2, but 38-membered rings were threaded to the extent of 5.0% (neat, molar ratio cyclic/linear=43) in a sil-oxane rotaxane. 

Chojnowski J, Cypryk M (2000) Synthesis of Linear Polysiloxanes. In Jones RG, Ando W, Chojnowski J, editors. Silicon-Containing Polymers. Dordrecht Kluwer Academic Publishers, pp. 3 1... 

Beside the synthesis of linear polysiloxanes, hydrosilylation was also nsed to obtain liquid crystalline elastomers. This can be performed in two ways, either a one pot reaction involving a diolefln together with the functionalized mesogen or a two step process involving first the synthesis of a linear polysiloxane, bearing adequate functions for a further crossfinking. For example, a photosensitive group such as benzophenone could be introduced (Fig. 5, x and y are theoretical values). In order to ensure a full conversion in silane, the time of reaction was increased here to 65 h [17]. 

Di)D R2SiO R o sO—jji—Oo 5 R Linear polysiloxanes Silicone fluids, rubbers, elastomers... 

Linear polysiloxanes having reactive terminal groups are often condensed with reactive orgaiuc polymers. For example, hydroxy-terminated siloxanes can be condensed with a carboxy-terminated polyester, as illustrated in equation (28). The resulting polymers behave as thermoplastic elastomers, having some of the desirable properties of both siloxanes and polyesters. Similar condensation reactions have been used to synthesize siloxane copolymers with polyamides, polyethers, polycarbonates, and so on. 

Ml he history OF LINEAR POLYSILOXANES dates back at least 116 years (i), and research activity in this area steadily accelerated during this period as synthetic methods improved and as the fundamental nature of polymers became clear. The industrial prominence of poly(dialkylsiloxane)s was a particularly strong impetus to the development of this field (2). Thus, a large body of literature has accumulated, which has been extensively reviewed. The reviews by Wright (3), Sigwalt (4), and Kendrick et al. (5) are excellent and current, whereas that by Voronkov et al. (6) covers the earlier literature. Siloxane polymerization has now become suflSciently commonplace, so that it is sometimes discussed in general textbooks on polymer chemistry (7). 

In practice, step-growth polymerization in linear polysiloxanes consists of either homocondensations of silanol-ended siloxanes or heterocondensations... 

The equilibrium 20 is important not only in the synthesis of linear polysiloxanes but also in their applications. The effects of water vapor on inducing chain cleavage at high temperature are not only reduced molecular weights but also a dramatic increase in the rates of chemically induced stress relaxation at 250 °C in cross-linked poly(dimethylsiloxane) networks under load (70). Slow hydrolytic bond cleavage in cross-linked networks is seen even in studies of stress relaxation in air at room temperature, and appreciable rates of stress relaxation in the loaded networks are measured at temperatures as low as 70 (7i). The stress relaxation is greatly accelerated... 

The simplest silicones are cyclic or linear polysiloxanes. Their viscosity or volatility characteristics are related to their molecular weight. The higher-molecular-weight siloxanes can carry specific functional groups adapted to their use. 

Table 1. Grafting of linear polysiloxanes onto the silica surface.

The polymerization reaction leads to an equilibrium mixture of linear polysiloxanes and cyclic siloxanes, wheras the equilibrium of the polycondensation reaction can be shifted to the polymer side by removing the condensation water. 

In another form of acid catalysis, the polymerization of cyclic and linear siloxanes is carried out on acidic solids such as ion exchange resins and acid-activated silicates (heterogeneous catalysis). This process also leads to an equilibrium mixture of linear polysiloxanes with a content... 

Linear polysiloxanes. Several studies have been made of the simplest member of this family, 0(SiH3)2. The most reliable estimate of the Si-O-Si bond angle may be compared with the much smaller value in the sulphur analogue... 

Silicone Polymers - Laser flash photolysis studies on poly(silylenes) generates radical cations along with silyl radicals and polysiloxane composities for the space shuttle have been found to be stable to far UV light exposure. Linear polysiloxanes have been found to be more unstable than branched or crosslinked polymers while the transparency of poly(methylphenylsilane) increases with light exposure. Photooxidised polysiloxanes doped with iodine are converted into semiconductors. ... 

The most important silicon-oxygen systems of course, are the linear polysiloxane polymers. These important materials are resistant to heat or oxidation, exhibit little change in properties with temperature, are water repellent and biologically inert. The history of the discovery and development of silicones has been summarized172. 

Another important class of linear polysiloxanes are the siloxane copolymers. The replacement of methyl groups by other organic groups improves the specific physicochemical properties of PDMS, and one such improvement is in the area of resistance to thermal degradation, e.g., phenyl-containing polysiloxanes have superior thermal stability compared to PDMS. 

Studies by Curtis group have offered evidence for the participation of silylene and silanone (R2Si=0) ligands in other catalyzed redistributions. These reactions effect the disproportionation of HMe2SiOSiMe2H into Me2SiH2 and linear polysiloxanes, poss-... 

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