Siloxane polymers Polysiloxanes

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-... 

Two major classes of hydrophobic chemical substances can be applied to glass in ultrathin layers to inhibit surface wetting. Siloxanes or polysiloxanes or silicones are polymers with a backbone of alternating silicon and oxygen atoms. These macromolecules are quite chemically inert, show resistance to water, and exhibit stability at high and low temperatures. The most common siloxane polymer, polydimethylsiloxane, is composed of the monomeric (i.e., repeating) unit illustrated in Fig. 7.5.1. 

Other siloxane polymers are of interest because their very low glass transition temperatures render plasticisers superfluous. However their low polarity and high hydrophobicity means that the simple commercial polydimethyl-siloxanes have high membrane resistances [92], probably due to the low solubility of ions in such media. Plasticisers could aid this but would defeat the object of using polysiloxanes in the first place. Incorporation of more polar moieties as co-monomers in the polymer, such as cyanopropyl [93], lowered membrane resistance by a factor of up to 20, with the incorporation of triflu-oropropyl groups leading to sensors for nitrate and cations [94]. Studies have also been made on using photopolymerisable crosslinkers, for example in the construction of a nitrate sensor [95]. 

The formation of compounds (polysiloxanes or silicones) containing Si—O—Si linkages is an important industrial process, a fundamental step of which is the hydrolytic polycondensation of monomeric silicon containing precursors (usually chlorosilanes). The details of siloxane polymer formation will not be dealt with here but can be found in... 

These ferrocene modified polysiloxane polymers were also used to construct glycolate [6,7], lactate [7], acetylcholine [12,81], glutamate [12] and cholesterol [81] sensors. All these electrodes showed that ferrocene containing siloxane polymers efficiently shuttled electrons between redox center(s) of enzyme and the electrode surface. 

This modification is connected with a reversed use of the silane (siloxane) coupling agents. On the other hand, the hydrosilylation reaction is commonly applied as a method of crosslinking organic polymers containing vinyl and allyl groups with siloxanes and polysiloxanes with Si-H functionality (e. g., [27]). 

Polysiloxanes, [-O-Si(RR )-], are linear resins that can be branched or crosslinked into elastomers. They have high compressibility, permeability to gases, low T and viscosity, exceptional weather-ability, low surface tension coefficient and are relatively expensive. Siloxane polymers or copolymers have been incorporated into engineering or specialty resins to improve processability, toughness, HDT, solvent and weather resistance. 

Clarson SJ, Semiyen JA (1986) Cyclic polysiloxanes. 1. Preparation and characterization of poly(phenylmethyl siloxane). Polymer 27 1633-1636... 

ALTERNATIVE NAMES, TRADE NAMES Silphenylenes, silaiylene-siloxane polymers CLASS In-chain modified polysiloxanes... 

Grassie, N. and K.R Francey, Thermal degradation of polysiloxanes. Part 3. Poly(dunethyPmethylphenyl siloxanes), Polym. Degr. Stab., 2 53 (1980). 

Table 6.4 lists the most commonly used liquid phases for both packed and capillary columns. Basically, there are two types of liquid phases in use today. One is siloxane polymers, of which OV-1, SE-30, DB-1 (100% methyl polysiloxane) and OV-17, OV-275, DB-1701, DB-710 (mixtures of methyl, phenyl, and cyano) polysiloxanes are the most popular. The other common liquid phase is a polyethylene glycol (Carbowax 20M, Superox and DB-wax ). 

Silicone polymers that exhibit good surface modification effects are classified as surfactants. Silicone polymers are derived from a polysiloxane backbone, wherein the reactive sites could be attached to this backbone. The reactive sites are typically reacted to organic moieties forming organomodified siloxane polymers. Such derivatives form a different class of surfactants providing unique properties and applications that differ significantly from traditional surfactants. A Surfactant Science Series volume was published in 1999 that covers a wide spectrum of topics related to the chemistry. 

Siloxane polymers possess a number of properties that seem almost contradictory. One example is the tremendous stability and durability shown by polysiloxanes in a wide variety of applications. One reason for this stability is the fact that the chain is already in a high oxidation state, and reduction takes place only at very high temperatures. Most scientists... 

Apart fi-om the peroxide curing method as discussed above, there are several other ways to crosslink siloxane polymers. These rely on coupling reactions between reactive functional groups that have been deliberately introduced either as end-groups or in the side-chain of the pol3nner. Representative examples of such fimctional-group- containing polysiloxanes are shown in Fig. 6.19. 

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