Cyclic dimethylsiloxanes

The x-meric cyclic dimethylsiloxane and diphenylsiloxane are denoted as (DJ and (D") respectively. Linear species terminated with trimethylsilyl units, having (x + 2) silicon atoms are denoted by (MD,M). 

Cyclic compounds, unnatural, 24 59 Cyclic cooligomerization, 16 238 Cyclic corrosion tests, 16 218 Cyclic diacyl peroxides, 18 473, 477 Cyclic dimethylsiloxanes, 22 575 Cyclic diperoxides, 18 461 Cyclic diperoxyketals, 18 457 Cyclic disulfides, antimicrobial properties of, 23 713... 

Humans emit a variety of VOCs such as pentane and isoprene (e.g., Gelmont et al., 1981 Mendis et al., 1994 Phillips et al., 1994 Jones et al., 1995 Foster et al., 1996). In addition, emissions from personal care products have been observed. Decamethylcyclopen-tasiloxane (D5), a cyclic dimethylsiloxane with five Si-O units in the ring, and the smaller D4 analog, octameth-ylcyclotetrasiloxane, are used in such products as underarm deodorant and antiperspirants at concentrations up to 40-60% by weight (Shields and Weschler, 1992 Shields et al., 1996). Increased concentrations of D5 have been measured in offices and are correlated to human activity, as expected if personal care products were the major source (Shields and Weschler, 1992). In some cases, increased concentrations attributable to emissions from silicone-based caulking materials were also observed (Shields et al., 1996). 

DeBolt and Mark [71] reported that a 30-membered cyclic dimethylsiloxane was not threaded by linear polysiloxane via Method 2, but 38-membered rings were threaded to the extent of 5.0% (neat, molar ratio cyclic/linear=43) in a sil-oxane rotaxane. 

The hydrolytic condensation of dimethyldichlorosilane is carried out in apparatus 4 with a jacket, an agitator, and backflow condenser 6. First, the apparatus is loaded with a necessary amount of water then the agitator is switched on and the reactor receives dimethyldichlorosilane at such speed that the hydrolytic condensation temperature does not exceed 40 °C. The reactive mixture from the hydrolyser enters separator 11, where hydrochloric acid and the products of hydrolytic condensation (a mixture of cyclic dimethylsiloxanes) are continuously separated. The hydrochloric acid from the lower part of the separator enters a special collector (not shown in the diagram), and the mixture of cyclic dimethylsiloxanes from the top of the separator is sent into collector 10 and from there into apparatus 7, where it is neutralised with soda. 

After neutralising, cyclic dimethylsiloxanes are sent into collector 8 and from there through weight batch box 9 into distillation tank 17. The distillation of the light fractions, which are low-molecular cyclic dimethylsiloxanes, is conducted at a temperature below 230°C in vapours and at 260-280 °C in the tank at a residual pressure of 5.2—9.3 GPa. The distilled product enters batch box 16 and then is subjected to catalytic rearrangement. 

If the viscosity of the rearrangement product does not meet the necessary requirements, it should be supplemented with a corresponding component (cyclic dimethylsiloxanes or hexamethyldisiloxane) to achieve a necessary viscosity. 

During the hydrolytic condensation diorganosiloxanes have a great tendency to form cycles. For example, dimethyldichlorosilane is hydrolysed with water (in the absence of solvents) by the mechanism of polycondensation, forming a mixture of linear and cyclic dimethylsiloxanes. 

The actual polymerization process involves a ring-opening reaction of dimethyl-substituted cyclic siloxanes. The preparation of the cyclic materials starts with the production of pure silicon via the reduction of quartz with coke in an electric arc furnace. The silicon metal then reacts with methyl chloride to give a mixture of silicones, from which dimethyldichlorosilane is removed by distillation.65 Subsequent hydrolysis gives the cyclic dimethylsiloxane. 

The first advantage is the ease with which the cyclic and linear polymers can be separated and analysed. Much use is made of the techniques of gas-liquid chromatography (g.l.c.) for volatile molecules, and gel permeation chromatography (g.p.c.) for the less volatile molecules. As examples of the power of tlttse metlmds, it is noted that cyclic dimethylsiloxanes [(CH3)2SiO]x, with up to a hundred skeletal bonds,... 

The hydrolysis of, for example, dimethyl dichlorosilane to cyclic dimethylsiloxanes and linear diols can be regarded as following a similar pattern, if OHs+Cl" is taken as the analogue of NH4+C1". Increase of acid concentration, or the use of a mutual solvent, increases the proportion of cyclic polymers their proportion is decreased by the use of insufficient, alkaline or scarce water in the foi-m of hydrated salts (see pp. 80 ff. in ref. 65). 

The evidence is strengthened by the preparation from the cyclic trimer or tetramer of the related linear compounds (PNCUin PCI-, (Section II, C), which have quite different properties. The stable existence of such large rings has in any case seemed less unreasonable since the isolation of the separate members of the cyclic dimethylsiloxane series (Me2SiO) up to the nonamer, which has a 18-membered ring (see p. 80 in ref. 65). 

Dimethyldichlorosilane ist then hydrolyzed to yield intermediate silanol functional species which are normally unstable under the conditions of hydrolysis so condense to form an oligomer mixture consisting of cyclic dimethylsiloxanes (3a) and hydroxylterminated linear dimethylsiloxanes (3b). Hydrogen chloride is liberated in this reaction. 

Another Py-GC/MS experiment was performed on polyacrylic /nfer-net-polysiloxane, a copolymer used as impact properties modifier. This is a polymer of butyl acrylate with low levels of allyl, methyl, and 3-(dimethoxymethylsilyl)propyl methacrylates interpenetrated with cyclic dimethylsiloxane. The copolymer has CAS 143106-82-5. The pyrolysis was done at 600 C in He similar to other experiments previously discussed. The pyrogram is shown in Figure 6.7.14 and peak identification is given in Table 6.7.10. 

Table 6.7.10. Compounds identified in the a copolymer of butyl acrylate with low levels ofallyl, methyl, and 3-(dimethoxymethylsilyl)propyl methacrylates interpenetrated with cyclic dimethylsiloxane as shown in Figure 6.7.14.

The complete hydrolysis of dimethyldichlorosilane leads to an oligomeric mixture, consisting of cyclic dimethylsiloxanes ... 

Anionic Ring Opening Polymerization of Cyclic Dimethylsiloxanes (A) Promoted Organolithium/Trimer Reactions... 

Cyclic dimethylsiloxane tetramer. See Octamethyicyciotetrasiloxane Cyclic methylethylene carbonate. See Propylene carbonate Cyclic neopentanetetrayl bis (2,4-di-t-butylpheny ) ester phosphorous acid. See Bis (2,4-di-t-butylphenyi) pentaerythritoi diphosphite Cyclic neopentanetetrayl bis (octadecyl phosphite). See Distearyi penta-erythrityi diphosphite... 

CAS 556-67-2 EINECS/ELINCS 209-136-7 Synonyms Cyclic dimethylsiloxane tetramer Cyclotetrasiloxane, octamethyl- Octamethyltetracyclosiloxane Classification Silicone oligomer cyclic siloxane Empiricai CsH240[Pg.1228]

Cyclic dimethyl polysiloxane Cyclic dimethyl polysiloxane with n= 3-6 Cyclic dimethylsiloxane. See Cyclomethicone Cyclic dimethylsiloxane pentamer. See Decamethylcyclopentasiloxane Cyclic dimethylsiloxane tetramer. See Octamethylcyclotetrasiloxane Cyclic ethylene carbonate. See Ethylene carbonate... 

Decamethrin. See Deltamethrin Decamethylcyclopentasiloxane CAS 541-02-6 EINECS/ELINCS 208-764-9 Synonyms Cyclic dimethylsiloxane pentamer Cyclic pentamer-D5 Decamethylpentacyclosiloxane Dimethylsiloxane pentamer... 

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