Linear siloxane polymers

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 major categories of homopolymers and copolymers to be discussed are31 (i) linear siloxane polymers [-SiRR O-] (with various alkyl and aryl R,R side groups),... 

Dimethylpolysiloxane occurs as a clear, colorless, viscous liquid. It is a mixture of fully methylated linear siloxane polymers containing repeating units of the formula (CH3)2SiO that are terminated with trimethylsiloxy end-blocking units of the formula (CH3)3SiO—. It is soluble in most aliphatic and aromatic hydrocarbon solvents, but it is insoluble in water. Note Dimethylpolysiloxane is frequently used in commerce as such, or as a liquid containing silica (usually 4% to 5%), which must be removed by high-speed... 

The PhEur 2005 and USP 28 describe simethicone as a mixture of fully methylated linear siloxane polymers containing repeating units of the formula [-(CH3)2SiO-] , stabilized with trimethylsiloxy end-blocking units of the formula [(CHsls SiO-], and silicon dioxide. It contains not less than 90.5% and not more than 99.0% of the polydimethylsiloxane [-(CH3)2SiO-] , and not less than 4.0% and not more than 7.0% of silicon dioxide. The PhEur 2005 additionally states that the degree of polymerization is between 20-400. 

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). 

The micropyrolytic gas chromatography technique was applied to the identification of siloxane polymers and as an aid to the control of the reproducibility of cross-linked siloxane resins452. In the case of linear polymers the column packing was Apiezon M and a devolatilized linear siloxane polymer (the Hungarian product Hu-Au-120, mol. wt. 59000) supported on Celite. With a detector voltage of 1000 V, column temperature of 100 °C and argon flow rate 20 ml/min, the sample was applied in a solvent-free state to the heated wire-coil. Cross-linked siloxane polymers were analysed as solutions in butyl acetate on a solid support containing 10% linear siloxane polymer as stationary phase under similar conditions. 

Since only OH groups in the hydrolysis products are active toward condensation reaction to form =Si-0-Si= bonds, the stmcture or morphology of the resultant siloxane polymers should depend on the number of OH groups, x, in Si(OH) (OR)i. Namely, when x is 2, the hydrolysis may lead to linear siloxane polymers, and three-dimensional network polymers may be formed when x is 4. This reaction scheme teUs us that the fiber-drawing should be possible if Si(OR)4 is hydrolyzed with an imderstoichiometric amount of water (ideally, when the molar ratio of water to the alkoxysilane or r-value is 2) under the acidic condition. 

T is a constant depending upon the type of polymers, solvent and temperature. The slope, a, of the log[/j] vs. log Mn plot depends on the type of polymer a = 0 for spheric particles, a = 0.5-1.0 for flexible linear polymers and a = 1.0-2.0 for rigid rod-like polymers. As indicated in Figure 12-17, a is 0.75 for the solution with r = 1 and 0.64 for the solution with r = 2, meaning that these two solutions contain flexible linear siloxane polymers. On the other hand, a ranges from 0.2 to 0.5 for the other two solutions with r = 5 and 20. This means these two solutions contain spherically grown particles. Actually, however, those conclusions have to be checked by other techniques, in addition to viscosity measurements. 

Definition Silicone oil consisting of a mixl. of folly meth aled linear siloxane polymers end-blocked with Irimelhylsiloxy units Empirical (C2HgOSi) C4H,2Si... 

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