Siloxane polymers degradation

Clarson, S.J., Depolymerisation, degradation and thermal properties of siloxane polymers. In Clarson, S.J. and Semiyen, J.A. (Eds.), Siloxane Polymers, Polymer Science and Technology Series. PTR Prentice Hall, Englewood Cliffs, NJ, 1993, pp. 216-244. 

Clarson, S. J. Depolymerization, Degradation and Thermal Properties of Siloxane Polymers. In Siloxane Polymers, Clarson, S. J., Semiyen, J. A., Eds. Prentice Hall, 1993 pp 21 244. 

Progress in the study of the degradation and stabilisation of siloxane polymers Carbon functional silicones... 

Various reactions, both of polymerization and of polymer degradation, can produce cyclic polymer molecules. A well-known process is the ring-chain equilibration reaction, which may be used to produce cyclic siloxanes and o er cyclic polymers. The linear chain reacts intramolecularly and yields a cyclic and a linear chain. In the initial stages, the molar fraction of cyclics increases at the expense of the linear chains. After some time, equilibrium conditions are achieved and the molar fraction of cyclics remains constant. In some cases, all the sites in the macromolecular backbone are equivalent and no peculiar bond exists which is preferentially attacked. This case is referred to as thermodynamically controlled cyclization. 

Grassie, N. and K.R Francey, Thermal degradation of polysiloxanes. Part 3. Poly(dunethyPmethylphenyl siloxanes), Polym. Degr. Stab., 2 53 (1980). 

While siloxanes are also amenable to ADMET polymerization, they display trends similar to silanes, in that the steric bulk near the reacting olefin prevents productive metathesis [107]. Specifically, l,l,3,3-tetramethyl-l,3-divinyl-disiloxane proved unreactive, while l,3-diallyl-l,l,3,3-tetramethyldisiloxane almost exclusively cyclized. Still, longer di- and tri-siloxane monomers have been shown to polymerize via ADMET. Ring-chain equilibria are well known for polysiloxanes, and some of these polymers degrade over time via intrachain siloxane exchange reactions to form cyclic oligomers [108]. 

Tang, Z., Li, Y., Zhang, Y.J., Jiang, P., 2012. OUgomeric siloxane containing triphenylphos-phonium phosphate as a novel flame retardant for polycarbonate. Polymer Degradation and StabiUty 97, 638—644. 

Silicone elastomers are well known for their superior resistance to elevated (and reduced) temperatures, with little or no loss of properties. Again, the siloxane polymer structure is responsible for properties not seen in carbon-based polymers one reason is the inherent flexibility of the siloxane molecule, and another is its greater resistance to oxidative degradation. When these basic properties are enhanced with appropriate heat-resistant fillers and additives (often iron oxide), a high temperature elastomeric adhesive can be made that will withstand heating in air for up to a year at 400 F without significant property losses. Resistance to even higher temperatures can be achieved for shorter times, as shown in Table 2. 

Today, the thermal degradation chemistry of comparatively simple siloxane polymer systems has been well studied and... 

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. 

The thermostability of siloxane-silazane copolymers of both random and block structure is found to be much higher (i.e. 100-200°C) with respect to polysiloxanes. This effect is brought about by introducing only a few silazane entities into the polymer chain. The reasons for the effect are not clear and the mechanism of thermal degradation of polysilazoxanes will require further experimental studies. 

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