Linear siloxane polymers formation

Saam, J. C., Formation of Linear Siloxane Polymers, Chap. 3 in Silicon Based Polymer Science, Am. Chem Soc., Adv. Chem. Ser., Vol. 224, Washington, DC, 1989. 

The preparation of poly(m-carborane-siloxane) polymers has also been successfully achieved directly from the carborane monomer.22 The reaction used is shown in scheme 9. Here, the direct salt elimination reaction between dilithiocarborane and a dichlorosiloxane (e.g., 1,5-dichlorohexamethyltrisiloxane) results in the formation of linear polymers with a molecular-weight (M ) typically of 6800 dalton. However, the reported literature detailing this approach is very limited indeed, and the reaction has not found significant use. This is most probably because only relatively low molecular-weight polymers can be produced, ultimately restricting the flexibility to produce materials of controlled mechanical properties. 

Silicone paints are formed by controlled hydrolysis and condensation of alkyl alkox-ysilanes, and may be encountered either alone or in formulations with other synthetic resins. The typical structural unit in the polymer chain is dimethyl siloxane, and pyrolysis of such resins takes place with random chain scission and the extended formation of stable cyclic fragments. In Figure 12.14 the pyrogram of a silicone resin is shown, with cyclic siloxane oligomers eluting at the shorter retention times, followed by the linear siloxane fragments. 

The formation of the linear cyclic polymers is dependent upon the reaction conditions. Hydrolysis with water alone gives rise to 50-80 per cent linear polydimethyl siloxane a, w-diols and 50-20 per cent polydimethyl-cyclosiloxanes. 

The ring-opening polymerization of is controlled by entropy, because thermodynamically all bonds in the monomer and polymer are approximately the same (21). The molar cychzation equihbrium constants of dimethylsiloxane rings have been predicted by the Jacobson-Stockmayer theory (85). The ring—chain equihbrium for siloxane polymers has been studied in detail and is the subject of several reviews (82,83,86—89). The equihbrium constant of the formation of each cychc is approximately equal to the equihbrium concentration of this cychc, [(SiR20) Thus the total concentration of cychc oligomers in the equihbrium is independent of the initial monomer concentration. As a consequence, the amount of linear polymer decreases until the critical dilution point is reached, at which point only cychc products are formed. 

Figure 15 NMR analysis of polydimethylsiloxane (ROMS) elastomer formation by cross-linking of ROMS with methylvinyl siloxane co-unils and a difunctional silane cross-linker (a) reaction scheme, (b) solution spectrum ofthe linear precursor polymer, and (c) MAS spectrum of the bulk elastomer. The insets are vertically magnified by a factor of 100. For comparison, a static spectrum of octane-swollen ROMS is shown as dotted line in panel c.

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. 

Thus, Andrianov et al. (26) attempted to catalyze polymerization of a number of alkyl and alkyl/aryl cyclosilazanes using catalytic amounts of KOH or other strong bases at temperatures of up to 300°C. In general, the reactions proceed with evolution of NHj, hydrocarbons and the formation of intractable, crosslinked, brittle products even at low temperatures. Contrary to what is observed with cyclotri-siloxanes, no evidence was found for the formation of linear poly-silazanes. Copolymerization of mixtures of cyclosilazanes and cyclosiloxanes gave somewhat more tractable polymers with less evolution of hydrocarbons or ammonia, however very little was done to characterize the resulting materials. 

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