Polymerization of cyclic siloxanes

Anionic Polymerization of Cyclic Siloxanes. The anionic polymerization of cyclosiloxanes can be performed in the presence of a wide variety of strong bases such as hydroxides, alcoholates, or silanolates of alkaH metals (59,68). Commercially, the most important catalyst is potassium silanolate. The activity of the alkaH metal hydroxides increases in the foUowing sequence LiOH < NaOH < KOH < CsOH, which is also the order in which the degree of ionization of thein hydroxides increases (90). Another important class of catalysts is tetraalkyl ammonium, phosphonium hydroxides, and silanolates (91—93). These catalysts undergo thermal degradation when the polymer is heated above the temperature requited (typically >150°C) to decompose the catalyst, giving volatile products and the neutral, thermally stable polymer. 

L-ascorbic acid and, 25 751 catalytic esterification of, 10 482 in cationic polymerization of cyclic siloxanes, 22 560 cellulose as, 11 266 a-chiral and homologated, 13 669 control methods for, 26 687-690 derived from halogen fluorides,... 

Association of University Technology Managers (AUTM), 24 368, 391 Association phenomenon, in anionic polymerization of cyclic siloxanes, 22 559... 

Protic acids, in cationic polymerization of cyclic siloxanes, 22 560 PROTO (-)-Protoemetine, 2 84, 85 Protonated ozone, 7 7 774-775 Protonated pyridines, 27 100-101 Protonation, 75 653-654... 

Chojnowski and co-workers have studied the polymerization of octamethyltetrasila-l,4-dioxane, a monomer more basic than cyclosiloxanes, which is capable of forming more stable oxonium ions, and thus being a useful model to study the role of silyloxonium ions.150-152 In recent work, these authors used Olah s initiating system and observed the formation of oxonium ion and its transformation to the corresponding tertiary silyloxonium ion at the chain ends.153 The 29Si NMR spectroscopic data and theoretical calculations were consistent with the postulated mechanism. Stannett and co-workers studied an unconventional process of radiation-initiated polymerization of cyclic siloxanes and proposed a mechanism involving the intermediate formation of silicenium ions solvated by the siloxane... 

Kress, J. D. Leung, P. G. Tawa, G. J. Hay, P. J. Calculation of a Reaction Path for KOH-Catalyzed Ring-Opening Polymerization of Cyclic Siloxanes. In Silicones and Silicone-Modified Materials Clarson, S. J., Fitzgerald, J. J., Owen, M. J., Smith, S. D., Eds. AGS Symposium Series 729 American Chemical Society Washington, DC, 2000 pp 81—97. 

Cypryk, M. Chojnowski, J. Kurjata, J. Tertiary Trisilyloxonium Ion and Silylenium Cation in Cationic Ring-opening Polymerization of Cyclic Siloxanes. In Organosilicon Chemistry VI From Molecules to Materials Auner, N., Weis, J., Eds. Wiley-VCH Weinheim, 2005 pp 85-92. 

Uses. As a catalyst in the polymerization of cyclic siloxanes for electrolytes in batteries... 

The commercial importance of polysiloxanes (silicone polymers) was discussed in Sec. 2-12f. The higher-MW polysiloxanes are synthesized by anionic or cationic polymerizations of cyclic siloxanes [Bostick, 1969 Kendrick et al., 1989 Noll, 1968 Saam, 1989 Wright, 1984]. The most commonly encountered polymerizations are those of the cyclic trimers and tetra-... 

The anionic polymerization of cyclic siloxanes can be initiated by alkali metal hydroxides, alkyls, and alkoxides, silanolates such as potassium trimethylsilanoate, (CH3)3SiOK, and other bases. Both initiation... 

Ring-opening polymerization of cyclic siloxanes with cationic initiators allows the possibility of introducing stable end groups by the use of suitable chain transfer agents. Thus, polysiloxanes with trimethylsilyl end groups are formed when the cationic polymerization of octamethylcyclotetrasiloxane is carried out in the presence of hexamethyldisiloxane as transfer agent ... 

Cesium hydroxide is used as electrolyte in alkahne storage batteries. Other apphcations of this compound involve catalytic use in polymerization of cyclic siloxane and treatment of hazardous wastes. 

In polymerizations of cyclic siloxanes, because of the higher coordination number of silicon the active centre can be formed by the decomposition of the transition complex with pentavalent silicon [208]... 

In our studies of the polymerization of cyclic siloxane monomers we observed anomalies in the kinetics of the process and in the viscosity changes of the medium. The growing polydimethylsiloxane chain ends were found to associate... 

The system in cationic ring-opening polymerization, which most closely approaches this ideal situation, is the polymerization of cyclic siloxanes [86-88]. The chain, composed of —O—Si— units is highly flexible and provides a reaction sites for cyclization. It is also well suited for experimental studies, because concentrations of cyclic oligomers in equilibrium, due to their volatility, can be determined with accuracy by gas-liquid chromatography. The slope of the experimentally derived plot is indeed very close to the theoretically predicted value of -2.5. 

Radiation-Induced Polymerization. In 1956 it was discovered that D can be polymerized in the solid state by y-irradiation (145). Since that time a number of papers have reported radiation-induced polymerization of D and D in the solid state (146,147). The first successfiil polymerization of cyclic siloxanes in the liquid state (148) and later work (149) showed that the polymerization of cyclic siloxanes induced by y-irradiation has a cationic nature. The polymerization is initiated by a cleavage of Si—C bond and formation of silylenium cation. 

Tertiary Trisilyloxonium Ion and Silylenium Cation in Cationic Ring-Opening Polymerization of Cyclic Siloxanes... 

The mechanism of polymerization of cyclic siloxanes is consistent with the observed features of polymerization of D2. The kinetic differences between the polymerizations of D3, D4 and D2 can be explained by differences in basicity and ring strain of these monomers. 

Through steric hindrance and conjugative effects, these ionic phosphonium salts are very stable to hydrolysis. This, coupled with the lipophilic nature of the cation, results in a very soft, loosely bound ion pair, making materials of this type suitable for use as catalysts in anionic polymerization [8 - 13]. Phosphazene bases have been found to be suitable catalysts for the anionic polymerization of cyclic siloxanes, with very fast polymerization rates observed. In many cases, both thermodynamic and kinetic equilibrium can be achieved in minutes, several orders of magnitude faster than that seen with traditional catalysts used in cyclosiloxane polymerization. Exploiting catalysts of this type on an industrial scale for siloxane polymerization processes has been prevented because of the cost and availability of the pho hazene bases. This p r describes a facile route to materials of this type and their applicability to siloxane synthesis [14]. 

Scheme 18 Formation and ring-opening polymerization of cyclic siloxane.

The discussion following will deal mainly with the important equilibrations and polymerizations of cyclic siloxanes. For further information concerning these methods and for excellent discussions of other types of siloxane polymerizations, the reader is referred to Voronkov (O, Noll O), and Meals (10). In addition, Noll (7, Eaborn (11), and Arkles and Peterson ( 1 2) offer reviews of the general chemistry of silicon compounds. 

Polymers of this type are produced by either acid- or base-catalyzed polymerizations of cyclic siloxanes where trisubstituted siloxanes are used to end-cap the polymer chains and control the viscosity (VII). 

Which themselves initiate polymerization of cyclic siloxanes. 

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