Polymethylhydrosiloxane

Description

Methylpolysilicones (also called dimethylpolysiloxanes) are high-molecular weight organic compounds of silicon which are similar in chemical structure to inorganic silicates. They are members of a large family in which silicon and oxygen atoms form a siloxy skeleton to which various alkyl and aryl groups are attached in regular, repetitive fashion. The methylpolysilicones may be characterized by the following formula: Si(CH3)3O[Si(CH3)2O] nSi(CH3)3
where "n" can be varied from 0 to 10,000 or higher.
The commercial methylpolysilicones are usually linear polymers, although cyclic forms may also be utilized.
Methylpolysilicones can be prepared in viscosity grades ranging from one to several hundred thousand centistokes (cs) depending on the degree of polymerization. Those employed in food uses generally have "n" values of 200-400, with molecular weights usually between 14,000 and 21,000. In this molecular weight range, methylpolysilicones are clear, colorless, viscous liquids, which may contain up to 5% silicon dioxide. With higher degrees of polymerization, resins and rubbers are produced. The methylpolysilicones are insoluble in water and the lower alcohols, but soluble in most aliphatic and aromatic hydrocarbon solvents. The polymers are nonionic, inert compounds strongly resistant to chemical and thermal attack. They are hydrophobic, greasy, and markedly antiadhesive.
silica gel as well as emulsifying agents and preservatives. Foodgrade methylpolysilicones must meet the following specifications:
Refractive index: 1.400 to 1.404.
Specific gravity: 0.964 to 0.973.
Viscosity: 300 to 600 cs.
Loss on heating: Not more than 18%.
Limit of impurities: Arsenic, not more than 3 parts per million (ppm); heavy metals, not more than 10 ppm (expressed as lead).
The antifoaming and hydrophobic properties of dimethylpolysilicones are also utilized medically in patients suffering from excessive gas production or retention such as flatulence, gastric bloating, or postoperative gaseous distention. They have also been used to reduce gas shadows in radiography of the bowel and to improve visualization in gastroscopy. The usual therapeutic adult dose is 150-400 mg daily. Silicones are also employed to prevent dermal irritation in bedridden patients. Medical grade preparations (simethicone) must contain 93-99% dimethylpolysilicone, not more than 4.5% silicon dioxide, and have a viscosity not less than 300 cs.
Because of their chemical and thermal resistance together with their antiadhesive and hydrophobic properties, methylpolysilicones have found various uses in the food industry. They are used to coat containers and molds for bakery products, candies, and confections; to impregnate packaging and wrapping paper which contact foods; and to reduce foaming in various fermentation and canning processes. Methylpolysilicones at low concentrations (0.03-0.05 ppm) reduce thermal and oxidative deterioration of frying fat by forming a protective film at the oil-air interface. At higher levels (1-10 ppm), silicones added to shortening and other frying fats may eliminate smoking difficulties by raising the smoke point 20-30°F. Silicones have also been incorporated into cellulosic sausage casings to facilitate the casing removal from the enclosed meat mass.

Chemical properties

clear colorless viscous liquid

Physical properties

colorless free flowing liquid; average molecular weight 1500-2200 g mol^?1 (supplier dependent); effective mass per hydride of 60 g mol^?1; d = 1.006.

The Uses of Polymethylhydrosiloxane

Polymethylhydrosiloxane (PMHS) is an easily handled, inexpensive, non-toxic, and mild reducing agent. Although relatively inert towards organic functionality, PMHS can transfer its hydride to a variety of metal catalysts (including Sn, Ti, Zn, Cu, and Pd) which can then participate in a wide range of reductions. Alternatively, when made hypercoordinate by the action of fluoride or other nucleophiles, PMHS can act directly as a reducing agent. Polymethylhydrosiloxane is widely used as reducing agent often used in conjunction with metal catalysts or nucleophilic activators.

The Uses of Polymethylhydrosiloxane

methicone is a type of silicone used primarily in the formulation of free-flowing cosmetic powders. Methicone can also be found in cosmetic preparations as a skin surface sealant to reduce transepidermal water loss.

Preparation

hydrolysis of methyldichlorosilane followed by heating (60–150 °C) the resultant mixture of cyclic silanes in the presence of hexamethyldisiloxane generates the linear polysiloxane.