Polydiorganosiloxanes

Additive Approach. As mentioned previously, dimethylsilicone compounds respond poorly to microwave heating because of the low "loss index". To make the dimethylsilicone compounds respond to microwave, one can change the structure of polydiorganosiloxane as shown above, or introduce additives which have a high "loss index" such as carbon black. The latter was demonstrated by applying microwave (2450 M Hz) to a mixture of 10 g of base... 

DOW CORNING Softener SSF is a medium-viscosity polydiorganosiloxane manufactured to yield a substantially linear poly-dimethylsiloxane with reactive aminoalkyl sites. It contains aminoalkyl groups affixed to a predominantly polydimethylsil-oxane structure. 

Keywords polyorganosiloxanes, polyphenylsilsesquioxane-polydiorganosiloxane block copolymers, heat-resistant composites. 

The combination of monofunctional units, M, with difunctional units, D, leads to straight chain polyorgano-siloxanes terminated by M-units. Combination of only difunctional units produces cyclic polyorgano-siloxancs or open-chain polydiorganosiloxanes with, for example, a hydroxy or alkoxy end group. The incorporation of T- and optionally also Q-units leads to branched polyorgano-siloxanes. 

Equilibrium polymerization, which can be anionic or cationic, is utilized to convert cyclic organosiloxanes into polydiorganosiloxane polymer chains. In the chemical industry octamethylcyclotetrasiloxane is preferred as such, or as a mixture with other siloxanes for chain termination and/or production of copolymers for specific applications. Particularly indu.strially important is anionic polymerization with basic catalysts such as alkali hydroxides, whereby the activity falls off in the order Cs > Rb > K > Na > Li. KOH is most frequently used e.g. as a suspension in octamethylcyclotetrasiloxane at 140°C, the catalyst being active from a concentration of several ppm. According to the assumed mechanism of this catalytic process, potassium siloxanolate is initially formed, which leads to cleavage of the Si-O-Si bonds and chain formation ... 

Linear short-chain dimethylsiloxanes (oligomers) from hydrolysis or methanolysis processes are utilized as the starting material in the manufacture of polydiorganosiloxanes by polycondensation. If linear siloxane polymers are required, dimethyldichlorosilane with a high purity is necessary, since distillalive processing of siloxane diols is not possible due to their tendency to condensation (with cleavage of water). 

Variation of structural and compositional parameters allow modification of the physical properties to tailor performance to specific application needs. For example, the effect of ingredients added to control the modulus of two UV curable silicone polymers is presented in Figure 1. Both Polymer A and Polymer B were polydiorganosiloxanes which contained terminal reactive unsaturation. Admixture with a photosensitization system and subsequent cure afforded soft, elastomeric products. Modifier A was incorporated into Polymer A to soften the system further. As the concentration of Modifier A increased, the modulus decreased, and the resultant composition became more gel-like. In a contrary fashion, Modifier C was added to Polymer B to provide reinforcement. As the concentration of Modifier C was increased, the modulus of the resultant cured film increased, the elongation decreased, and the tensile strength went through a maximum. 

Finally, stereoisomerism of polydiorganosiloxanes (RR SiO), may be expected as a consequence of the pseudo-asymmetry of the silicon atoms. These polysiloxanes could be considered as configurational copolymers where the two different siloxane units adopt two different stereochemical configurations147. 

Polydiorganosiloxanes such as polydimethylsiloxanes, have established a very respectable position as high performance polymers. 

The polymers used in RTV-2 silicone rubbers are mostly linear, i.e. non-branched long-chain polydiorganosiloxanes formed by alternating silicon and oxygen atoms. Methyl groups saturate both remaining bonds at the silicon. Reactive groups terminate the chains (Fig. 1). 

Hydrosilylation Cure. The addition of polyfunctional silicon hydrides to poly(vinyl)organosiloxane provides cure for silicone rubber. The commonest activated curing method is based on hydrosilylation of polydiorganosiloxanes containing small numbers of vinyl groups and Si—H groups (usually end-blocked by them) (7) (see Scheme 33). 

Silicone Network Formation. Silicone rubber has a three-dimensional network structure consisting of cross-linked polydiorganosiloxane chains. Three reaction types are generally employed for the formation of silicone networks (207) peroxide-induced free-radical processes, hydrosilylation addition reactions, and... 

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