Siloxane, linear

Linear LC siloxanes have been well known since the end of the seventies [4] and an enormous variety of compounds, including networks, have been synthesized in the meantime [5]. These side-chain polymers are usually synthesized by a well-established standard procedure, that is, the coupling of co-alkenyl substituted mesogenic groups to H-siloxanes with the aid of a Pt catalyst. This concept has been extended to various geometric and chemical classes of backbone siloxanes, linear [6], cyclic [7, 8] and cage-like siloxanes [9], or siloxanes with a great variety of substituents [10]. 

A vast majority of the commercially available silicon-containing polymers today are polysiloxanes or silicones. In the same way that aldehydes and alcohols are characterized by the presence of the -CHO-H and = C-OH moieties, respectively, a siloxane is a compound containing the bond =Si-0-Si=, and silicones are materials consisting of alternating silicon and oxygen atoms in which most silicon atoms are bound to at least one monofunctional organic radical. Polymers and copolymers of R, R2, and R3 monomers are termed poly(siloxanes). Linear poly(siloxanes)... 

Figure 2.23. The reduced surface tension as a function of the reduced temperature for a number of polymers (A, poly(vinyl acetate) , polystyrene , polyisobutylene A, poly(dimethyl siloxane) , linear polyethylene o, branched polyethylene), compared with the prediction of a gradient theory using the Poser and Sanchez lattice fluid model and a square gradient term modified to account for loss of polymer configurational entropy near a surface (full line). After Sanchez (1992).

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... 

Figure 9.17 Plot of log [i ]M versus retention volume for various polymers, showing how different systems are represented by a single calibration curve when data are represented in this manner. The polymers used include linear and branched polystyrene, poly(methyl methacrylate), poly(vinyl chloride), poly(phenyl siloxane), polybutadiene, and branched, block, and graft copolymers of styrene and methyl methacrylate. [From Z. Grubisec, P. Rempp, and H. Benoit, Polym. Lett. 5 753 (1967), used with permission of Wiley.]...

The reaction is of practical importance in the vulcanization of siUcone mbbers (see Rubber compounding). Linear hydroxy-terrninated polydimethyl siloxanes are conveniently cross-linked by reaction with methyldiethoxysilane or triethoxysilane [998-30-1]. Catalysts are amines, carboxyflc acid salts of divalent metals such as Zn, Sn, Pb, Fe, Ba, and Ca, and organotin compounds. Hydroxy-terrninated polysiloxanes react with Si—H-containing polysiloxanes to... 

Unlike the linear analogues, cycHc dimethyl siloxanes are low melting soHds and the generally decreases with increasing ring size. Physical... 

Silicone Resins. Sihcone resins are an unusual class of organosdoxane polymers. Unlike linear poly(siloxanes), the typical siUcone resin has a highly branched molecular stmcture. The most unique, and perhaps most usehil, characteristics of these materials are their solubiUty in organic solvents and apparent miscibility in other polymers, including siUcones. The incongmity between solubiUty and three-dimensional stmcture is caused by low molecular weight < 10, 000 g/mol) and broad polydispersivity of most sihcone resins. 

As another consequence of the properties of the siloxane bond, the value of n in the common linear trimethylsiloxy-endblocked-PDMS, (M-D -M) can vary from zero to tens of thousands giving a range of viscosity from 0.65 to 2,500,000 centipoise to the polymeric material. This relationship between viscosity and polymer chain length allows PDMS polymers to vary in form from water-like fluids to a flowable gum, while retaining the same chemical character. 

Viscosities of the siloxanes were predicted over a temperature range of 298-348 K. The semi-log plot of viscosity as a function of temperature was linear for the ring compounds. However, for the chain compounds, the viscosity increased rapidly with an increase in the chain length of the molecule. A simple 2-4-1 neural network architecture was used for the viscosity predictions. The molecular configuration was not considered here because of the direct positive effect of addition of both M and D groups on viscosity. The two input variables, therefore, were the siloxane type and the temperature level. Only one hidden layer with four nodes was used. The predicted variable was the viscosity of the siloxane. 

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...

During the aqueous hydrolysis of dichlorosilanes there is always a very important side reaction. It is the self-condensation of silanols which are formed initially during the hydrolysis. These reactions also give rise to the formation of cyclic siloxanes together with the linear oligomers or polymers (Reaction Scheme III). The amount of cyclic products usually depends on the hydrolysis conditions and the degree of the self-condensation attained as well as concentration considerations. 

The synthetic techniques leading to the formation of siloxane containing linear or graft copolymers will be classified according to the type and nature of the copolymerization reactions as shown below. 

Covalently crosslinked siloxane containing liquid crystalline networks with elastic properties were prepared 349). In all of the networks liquid crystalline phases of the linear precursors were retained. For low degrees of crosslinking the phase transition temperatures remained nearly unchanged, whereas higher degrees of crosslinking reduced the phase transition temperatures. 

Cyclophosphazenes are a fascinating group of inorganic heterocyclic compounds whose chemistry is multi-faceted, well developed and reasonably well understood. They are closely related to the linear poly-phosphazenes this relationship is unlike any other existing between ring-polymer systems. Although cyclic siloxanes and polysiloxanes have a close interrelationship, the number and types of cyclophospha-zene derivatives that are known, together with their exact counterparts in polyphosphazenes, underscore the utility of cyclophosphazenes as models for the more complex polyphosphazenes. The literature on cyclophosphazenes has appeared earlier in the form of books (1,2), chapters of books (3-5), authoritative compilations of data (6,7), and several reviews (8-21). The current literature on this subject is reported annually in the Specialist Periodic Reports published by the Royal Society of chemistry (22). This review deals mostly with chlorocyclo-... 

The procedure described here is not limited to the preparation of polymers such as 2. Starting from the difunctional silane 3 we have synthesized a copolymer, poly(dimethyl-co-isocyanopropylmethyl-siloxane) > as well as a linear homopolymer, poly(isocyanopropyl-methylsiloxane) 8 (Scheme 2). Indeed, preparation of a monofunctional analogue of 2. and h creates the potential for end-capping with an isocyanide function any polymer containing other functional groups, thereby in principle permitting mixed ligand complexes of polymers to be accessed. 

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