Cyclic polysiloxanes

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

Hydrolysis of a reactive silane derivative under the proper conditions first produces a silanol, but except in the case of a few that are exceptionally stable, this product is not isolated, since the acid or alkaline reagents that are normally present cause most silanols to condense rapidly to form siloxanes. The product generally isolated when a dialkyl-or diaryl-substituted silane is hydrolyzed is, of course, a mixture of cyclic polysiloxanes, (R2SiO)n, from which high polymers (often elastomeric) are obtained by catalytic rearrangement and polymerization reactions. 

Silanediols R2Si(OH)2. In the early studies the only diols isolated were those in which R is an aromatic radical, but even these compounds condense to linear and cyclic polysiloxanes. 

Cyclic polysiloxanes. Particular interest attaches to the configurations of these molecules, for the related carbon compounds paraldehyde and metaldehyde have puckered rings. The true silicon analogue of paraldehyde, tri-methylcyclo-trisiloxane, has been prepared but its structure is not known. 

New, silicon-based, cross-coupling reactions with the participation of silatranes, cyclic polysiloxanes, and Si-derivatives of heterocycles 02ACR835. 

Cyclic polysiloxanes also absorb in this region, with a characteristic cyclotrisiloxane band at 9.8 /z281. Cyclic tetramers and pentamers have a single band at about 9.2// which gradually widens and splits with larger rings284. The positions of these bands are essentially independent of the other silicon substituents. The shift of the siloxane band to... 

The RTV silicones, in general, exhibit considerably greater outgassing of both condensable and noncondensable materials than the HV silicones. Their condensable-time curves were usually linear and only one condensable product was being evolved. The Infrared spectra of the condensable products from all of the silicones evaluated indicated these materials to be predominately low molecular weight alkyl linear and/or cyclic polysiloxanes (Table II). Silicone A gave a nonlinear condensable-time curve and evolved two types of condensable products an alkyl linear and/or cyclic polysiloxane, and an aromatic ester. 

The HV silicones evolved less condensable and noncondensable compounds than their RTV counterparts. Their condensable-time curves were linear and essentially only one type of condensable compound was evolved. The condensable products from the HV silicones were also identified as alkyl linear and/or cyclic polysiloxanes. ... 

Silicones Polymers. The outgassing products from silicone polymers initially condense on the glazing as liquid alkyl linear and/or cyclic polysiloxanes. These liquid condensates have been observed to form on solar collectors after several days of testing in the Arizona desert. The liquid polysiloxanes do not appreciably affect the relative light transmittance. However, as the thin film is exposed to the harsh environmental conditions encountered during three months of desert testing, it is slowly converted into a white colloidal silica powder through the follov/ing type of overall reaction ( ). 

This polymer was also characterised by SEC. Its bimodal molar mass distribution pointed to a significant contribution from the cyclic polysiloxane fraction. The dialysis of this polymer in water permitted the separation of cyclics into the lower molar mass fraction, while the linear polymer constituted the higher molar mass fraction. The gel chromatograms of both these fractions were also compared. The two-step dialysis performed for the precursor and biocidal polymer led to a narrow polydispersity of the former, MW / Mn = 1.21 [95]. 

Clarson SJ, Semiyen JA (1986) Cyclic polysiloxanes. 1. Preparation and characterization of poly(phenylmethyl siloxane). Polymer 27 1633-1636... 

There have been many comparisons of cyclic polysiloxanes with the corresponding linear chains of the same molecular weight. Table 3.3 provides... 

Table 3.3. COMPARISONS BETWEEN CYCLIC POLYSILOXANES AND LINEAR POLYSILOXANES OF THE SAME MOLECULAR WEIGHT ...

Semiyen, J. A., Cyclic Polysiloxanes. Makromol. Chem., Macromol. Symp. 1985, 6,155-163. 

Clarson, S. J. Semiyen, J. A., Cyclic Polysiloxanes 1. Preparation and Characterization of Poly(phenylmethylsiloxane). Polymer 1986, 27,1633-1636. Clarson, S. J. Dodgson, K. Semiyen, J. A., Cyclic Polysiloxanes 2. Neutron Scattering from Poly(phenylmethylsiloxane). Polymer 1986,28,189-192. 

MALDI-tof-ms has been used to study the preparation of linear polysilox-anes with hydride end-capped and cyclic polysiloxanes from the ring-opening anionic polymerization reaction with hexamethylcyclotrisiloxane and octamethylcy-clotrisiloxane (99). The PDMS reaction was followed by MALDI and the incorporation of the hydride end cap and the creation of the cyclics was studied as a function of time. Further studies (100) employed MALDI and sims it was found that MALDI provides more reproducible relative intensities of the n-mers than does SIMS. 

Conditions that form low molecular weight linear or cyclic polysiloxanes are the following ... 

The product mixture from a typical MCS reaction is subjected to several distillation and isolation steps. The product mixture can be roughly divided into monomers and residue. The monomers are separated from the residue stream by distillation the residue contains siloxanes and disilanes. Some monomers can be recovered by various redistribution reactions of the residue mixture (15). The individual monomers are separated by distillation where the separation of Di from Tri is difficult. With Di as an example, equation 3 shows the hydrolysis and condensation to form linear and cyclic polysiloxanes. Another useful material is hexamethyldisiloxane (MM) which forms from hydrolysis/condensation of Me3SiCl (mono), equation 4. 

In Table 16.3, the most frequently mentioned (based on ca. 400 scientific sources) pathological symptoms caused by the presence of linear and cyclic polysiloxanes in vivo are listed. It has been shown that silicone-tissue interactions are the cause of both minor health problems—such as irritation, headache, skin reactions, chronic fatigue syndrome—and very serious diseases, such as rheumatoid arthritis, systemic... 

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