Polydimethylsiloxanes depolymerization

The depolymerization of polydimethylsiloxane to dimethylsilanediol (10) on exposure to dry soil was reported in 1979 by Buch and Ingebridtson79. The presence of moisture markedly decreased the rate of depolymerization. 

The mechanism of this depolymerization is unlikely to be biological but rather is thought to involve catalysis by clay minerals within the soil. At high water levels, access of the hydrophobic polydimethylsiloxane to the active sites on the clay may be limited, thus slowing the depolymerization. The dimethylsilanediol (10) is the major product species... 

Kucera et al. combined anionic and cationic polydimethylsiloxane. With the ratio of active centres 1 1, a perfectly stable polymer was produced which did not depolymerize even under conditions where a trace of acid or base would lead to a rapid decomposition of all polymer chains [105]. This was the first combination of macroions described in the literature. 

Protection of the linear polydimethylsiloxane chain with the bulky ladder polyphenyl-silsesquioxane (PPhSO) block copolymer molecule inhibits its cyclic depolymerization and retains the organic framework to a large extent. 

Many industrial applications for polydimethylsiloxanes (PDMS) make use of their stability towards heat and radiation. However, under extreme conditions these macromolecules degrade and/or depolymerise and there is an ever present need for more thermally stable polymers and a better understanding of the degradation and depolymerization processes. The siloxane bond is the only bond involved in the depolymerization process whilst in the degradation process all bonds are involved. Indeed Hyde148 and... 

In a general article about fluorosilicone elastomers [41], Kim analyzed the properties of classical fluorosilicones - [(R)(RF)SiO] - that are an excellent resistance to solvents, a good thermal and oxidative stability, an outstanding flexibility at low temperature. He concluded that fluorosilicones are superior to fluorocarbon elastomers, but they were not very good at high temperatures (above 450 C). Conventional polydimethylsiloxanes, and classical fluorosilicones, present the drawback to give reversion or depolymerization at high temperature, which deteriorates the physical properties. 

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