Polysiloxanes equilibration

Fig. 7. Weight fractions of cyclics (R(CH3)SiO)x in high molecular weight (p S1) polysiloxane equilibrates at 383 K (R = CH3, CH3CH2, CF3CH2CH2) and 273 K (R = H) as a function of the volume per cent siloxane in cydo-hexanone (R = CF3CH2CH2 and toluene (R = H, CH3, CH3CH2)...

Linear polysiloxanes containing terminal function groups such as alkoxy groups, chlorine atoms are technically prepared by equilibration of cyclic polysiloxanes with functional silicone compounds291. ... 

Scheme 2. General Synthesis of "End-Blocked" Polysiloxanes via Equilibration Processes.

An important source of experimental and theoretical studies of equilibria in ring formation is represented by the field of so-called macrocyclisation equilibria (Flory, 1969). Interest in this field appears to have been restricted so far to chemists conventionally labelled as polymer chemists. Experimental evidence of cyclic oligomer populations of ring-chain equilibrates such as those obtained in polysiloxanes (Brown and Slusarczuk, 1965) may be delated to the statistical conformation of the corresponding open-chain molecules (Jacobson and Stockmayer, 1950 Flory, 1969). In these studies experimental results are expressed in terms of molar cyclisation equilibrium constants Kx (14) related to the x-meric cyclic species Mx in equilibrium with the... 

A much better agreement between theory and experiment is found in the closely-related field of macrocyclisation equilibria. Investigations of the cyclic populations in ring-chain equilibrates set up in typical polymeric systems such as polyesters, polyethers, polysiloxanes, and polyamides take a major advantage from the relative ease with which the cyclic fraction can be separated from the linear fraction and analysed for the relative abundance of the individual oligomeric rings. This is conveniently done by means of modern analytical techniques such as gas-liquid and gel-permeation chromatography (Semiyen, 1976). 

A solvent at this step is omitted since this may result in a misleading homogeneity at an early stage of equilibration. If a statistical distribution of the functio nalized siloxy-units is not achieved the properties of the resulting polysiloxane are unfavorable. After the reaction the mixture is diluted with 20 volume percent of water and shaken for another hour. The two phases are separated by centrifugation, the polysiloxane is diluted with 1 volume of ether and extracted with water until the test for sulfate is negative. The solvent is removed in vacuo. 

Polysiloxane fluids and resins are obtained by hydrolysis of chlorosilanes such as dichloro-dimethyl-, dichloromethylphenyl-, and dichlorodiphenylsilanes [Brydson, 1999 Hardman and Torkelson, 1989]. The chlorosilane is hydrolyzed with water to a mixture of chlorohy-droxy and dihydroxysilanes (referred to as silanols), which react with each other by dehydration and dehydrochlorination. The product is an equilibrated mixture of approximately equal... 

This method, which is the standard for polysiloxanes, is much less useful for polysilanes. One reason is that polysilane polymers are thermodynamically unstable compared to cyclosilanes. Thus, any method of polymerization leading to equilibration will produce cyclosilanes exclusively the linear polymers can only be kinetic products. 

The synthesis and equilibration reaction kinetics involved in the preparation of aminopropyl-terminated polysiloxanes has been studied most extensively because of the utility of the amino-terminated species as components of a large number of segmented copolymers such as imides, amides, and ureas. 

Significant differences were observed in the rate of incorporation of D4 and l,3-bis(3-aminopropyl)disiloxane for similar concentrations of potassium, tet-ramethylammonium, and tetrabutylphosphonium siloxanolate catalysts. The rate differences affected the reaction times that were required to obtain a completely equilibrated reaction mixture with the desired molecular weight. The potassium catalyst required excessively long reaction times or high concentrations before sufficient incorporation of the aminopropyldisiloxane was realized. The tetramethylammonium and tetrabutylphosphonium catalysts were much more efficient for the preparation of controlled-molecular-weight aminopropyl-terminated polysiloxane oligomers. 

Investigations of the cyclic populations of simple ring-chain equilibrates such as those set up in polysiloxane and polyester systems have at least two major advantages over the study of ring formation in more complex systems, such as kinetically-controlled non-linear polymerisations. 

The well-defined end group functionality also allows the derivatization and manipulation of the mono-silanol functional polymers by the same methods that are suitable for silanol-terminated polysiloxanes (e.g., equilibration with polysiloxanes and NR4OH or PNCI2 catalysis). 

A second method for the synthesis of polysiloxane polyols is the equilibration of cyclic polydimethyl siloxanes with a compound of the following structure (11.7) [6, 7] ... 

III. Cyclic Populations of Polymeric Ring-Chain Equilibrates A. Polysiloxanes... 

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