Siloxane unit

The behavior of oligosiloxanediols in the presence of strong bases is different. The contribution to the overall process of the disproportionation reaction, involving a migration of the ultimate siloxane unit between siloxane molecules, is much greater and may even completely dominate the polycondensation reaction (80). The reactivity enhancement of the siloxane bond adjacent to the sHanolate anion can be understood in terms of n(0) (7 (SiO) conjugation. 

The position of the equiUbrium depends on a number of factors, such as concentration of siloxane units and the nature of substituents on the sihcon, but is independent of the starting siloxane composition and the polymerization conditions (81,82). For a hulk polymerization of dimethyl siloxane, the equihbrium concentration of cycHc oligomers is approximately 18 wt % (83). The equiHbrium mixture of cyclosiloxanes is composed of a continuous population to at least but D, D, and make over 95 wt % of the total cycHc fraction (84). 

Vulcanization. Generally this is carried out by the action of peroxides, which can cross-link the chains by abstracting hydrogen atoms from the methyl groups and allowing the resulting free radicals to couple into a cross-link. Some varieties of polysdoxanes contain some vinylmethyl siloxane units, which permit sulfur vulcanization at the double bonds. Some Hquid (short-chain) siHcones can form networks at room temperature by interaction between thek active end groups. 

The polyoxyalkylene units in the copolymer have a molecular weight below 500, and the polysiloxane units have 3 to 50 silicon atoms. The resin has a phenol/aldehyde ratio of 2 1 to 1 5 and an average molecular weight of 500 to 20,000 Dalton. The composition shows synergistic demulsification activity when compared with the individual components. The siloxane units can be either in blocks [979,980] of the polyoxyalkylene-polysiloxane copolymer or randomly distributed [728,729]. 

Polysilanes bearing triflate groups are very reactive and form an insoluble gel with a trace of moisture. The triflate group is hydrolized to silanol which rapidly condenses with the remaining triflate groups to a siloxane unit, linking intra or intermolecularly silicon atoms ... 

Polysiloxanes in this category typically contain cyclics of -Si(CH3)2-0- siloxane units of various sizes, or such siloxane units mixed with some carbosiloxanes (with additional -CH2- sequences) [163-165]. The cyclic portions can add considerable stiffness, resulting in isotropization temperatures above the polymers decomposition temperatures. 

Alkalimetal derivatives of stable functionalized silanols are very important in stepwise formation of siloxane units of almost any size. Thus, a detailed structural analysis is important for assisting understanding of the mechanism of their reactions. The dilithiated derivative of di-ten-butyl si landiol... 

Silicone rubbers contain the following dimethyl siloxane unit ... 

The millable gums, which will be the only type considered here, would generally contain 5,000-9,000 of the dimethyl siloxane unit. 

Chojnowski, J. Kazmierski, K. Cypryk, M. Fortuniak, W. Cationic Ring Opening Polymerization of Cyclotrisiloxanes with Mixed Siloxane Units. In Silicones and Silicone-Modified Materials-, Clarson, S. J., Fitzgerald, J. J., Owen, M. J., Smith, S. D., Eds. ACS Symposium Series 729 American Chemical Society Washington, DC, 2000 pp 20-37. 

Condensation of diethyl difluorosilane with 100% hydrogen peroxide in the presence of ammonia results in the formation of ethoxysilane, whereas an analogous condensation with diethyl dichlorosilane produces polymeric peroxides containing ethoxy and siloxane units. The initially formed straight chain or cyclic peroxides probably rearrange to products 3 or 4 containing alkoxy and siloxane moieties (equation 5) . 

It is believed that the transparency of the poly(siloxane)/PC copolymer is related to the size and the distribution of the siloxane units within the copolymer. The distribution may be controlled by the method of preparation. 

Table 1 lists the glass transition temperatures for the pertinent siloxane oligomers as a function of TFP and DP contents. The percent of each comonomer is recorded with reference to the siloxane units as well as the entire oligomer. One notes the difference that this creates between the two nominally 100% TFP siloxanes of different molecular weight. Note also the higher Tg values for the DP series at equal weight percents, a factor which limits their ease of synthesis and may affect their mobility during cure. 

In Table 2 are tabulated the Tg values for resins modified with the siloxane copolymers described in Table 1. Transition temperatures for samples modified with oligomers containing primarily dimethyl siloxane units give little indication of intimate mixing beween epoxy and rubber. Evidence for partial miscibility with increasing... 

Chojnowski and coworkers have examined the acid-catalysed disproportionation of oligomeric dimethylsiloxanols in dioxane in the presence of water2115. The results are interpreted in terms of a rate-determining cleavage of the terminal siloxane unit by water, followed by condensation of the resultant dimethylsilanediol (10) with the remaining dimethylsiloxanols (equations 7 and 8). 

Fig. 9.10 Proposed mechanism for the synthesis of cyclopentyl silsesqui-oxane a7h3. The silsesquioxane structures are represented schematically. Each circle symbolises a siloxane unit [(c-C5H9)SiC>3] silicon atoms are represented by the circles and the oxygen atoms by the lines non-bridging lines represent -OH groups cyclopentyl groups not shown.

Sequencing of siloxane units was also investigated for anionic ring-opening polymerisation of l,l-diphenyl-2,2,3,3-tetramethylcyclotetrasiloxane by 29Si-NMR [120]. 

MDTQ notation - the eight siloxane unit building blocks... 

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