Siloxane, molecular formula: (R2SiO)x. Properties: A polymer composed of a main chain structure formed by Si—O—Si bonds. It is conventionally referred to as organic silicon or polysiloxane, and can be linear, cyclic or cross-linked polymers. 
Introduction
Molecular formula: (R2SiO)x 
Properties: It is a polymer composed of Si—O—Si bonds as the main chain structure. It is conventionally referred to as organic silicon or polysiloxane, and can be linear, cyclic or cross-linked polymers. 
According to the application of the products, they are classified into three major categories: silicone oil, silicone rubber and silicone resin. 
The raw material for synthesizing siloxanes is the derivative of silane SinH2n+2. Linear siloxanes and cyclic siloxanes can be obtained by hydrolyzing R2SiCl2 type organochlorosilanes. Cross-linked siloxanes can be prepared by hydrolyzing and polymerizing R2SiCl2 and RSiCl3 type organochlorosilanes. 
Siloxanes are stable to heat and chemical reagents, do not get wet by water, and have outstanding aging resistance, excellent hydrophobicity and electrical insulation properties. They are used as fire retardants, lubricants and moisture-proofing agents. Silicone resins are used to manufacture fiber-reinforced materials. Silicone rubber is also used as adhesives and sealing materials. 
Classification
There are many types of polysiloxanes. According to their chemical structure and properties, they can be mainly classified into three categories: 
Silicone oil (modified silicone oil) is a low-molecular-weight linear polymer. To achieve certain properties, the molecular structure may also be grafted and modified. It is a colorless or light yellow transparent liquid, hydrophobic and oleophobic. It has a high boiling point and a low freezing point, good heat resistance and chemical stability. It is applied in various industries as a chemical additive; the most common application is as a defoamer in various industries, hydraulic oil, high-grade lubricating oil, filler surface treatment agent and demulsifier, latex coagulant, can also be used as a waterproof/reducing water agent for building materials, textiles, paper and many other substances, lubricant in the plastic industry, release agent, hair moisturizer in personal care, stabilizer for foaming polyurethane, leveling agent in the agricultural ink and coating industry, etc. At the same time, it is also a good electrical insulator. 
② Silicone rubber is a linear polymer with a high molecular weight (in the range of 100,000). After being filled with additives and other agents and undergoing vulcanization, it can be transformed into an elastomer, which is silicone rubber. Silicone rubber possesses characteristics such as electrical insulation, chemical stability, physiological inertness, and the ability to maintain good elasticity over a wide temperature range; it is mainly used for manufacturing rubber products that are used at high or low temperatures and artificial organs required in plastic surgery. 
③ Silicone resin is a linear structure polymer containing reactive groups and capable of further curing. It can be used to produce heat-resistant high-grade electrical insulating varnish (silicone varnish), heat-resistant plastics (silicone plastics), laminated glass cloth and protective coatings, etc. 
Poly siloxane 
Poly siloxane is also known as silicone. The synthesis of low-molecular-weight poly siloxane usually employs a stepwise polymerization reaction. For example, dimethyl dichlorosilane undergoes hydrolysis, dehydration or dehydrogenation reactions to generate polydimethylsiloxane: 
The product is a mixture of cyclized oligomers and linear polymers in approximately equal amounts, and there is a balance relationship between them. Depending on the reaction conditions, the content of cyclized substances can reach 20% to 80%, mainly in the form of tetramers. Usually, after reaching the initial equilibrium, a terminal agent [(CH3)3Si]2O is added to the reaction system to seal the linear polymers, thereby stabilizing the ratio of the product. The polymerization reaction can be carried out under acidic or alkaline conditions. The alkaline conditions are conducive to the formation of polymers with higher molecular weights. The content of cyclized substances can be reduced by vacuum distillation. The mixed product is a liquid and can be directly applied. 
When a trifunctional monomer, methyl trichlorosilane, is added to the above reaction system, a non-linear polysiloxane can be generated. After the product is separated from the aqueous layer, with the action of the catalyst zinc oxalate, the content of the cyclic compound can be reduced and the molecular weight can be further increased. The resulting silicone resin can be cross-linked by adding an alkaline catalyst and heating during its application. 
Silicone elastomers are divided into room-temperature vulcanized silicone rubber (RTV) (with a polymerization degree of 200 to 1500) and heat-cured silicone rubber (with a polymerization degree of 2500 to 11,000). The latter is obtained by the ring-opening polymerization of cyclic monomers. Vulcanization and curing are synonymous terms, but the word "curing" has the additional meaning of further polymerization and cross-linking. One-component room-temperature vulcanized silicone rubber is composed of end-hydroxyl polysiloxane, crosslinking agent (methyl triacetoxy silane), and catalyst (lauroyl dibutyltin ester). Moisture in the air can cause the crosslinking agent to hydrolyze into CH3Si(OH)3 and acetic acid, and the former can complete the curing process with the polysiloxane at room temperature (Equation 2-155). 
The two components of the two-component room-temperature vulcanizing silicone rubber are vinyl-containing polysiloxane and a crosslinking agent (octamethyltetrasiloxane). The crosslinking reaction is as follows: 
Under the catalytic action of chloroplatinic acid or platinum complexes, the addition reaction occurs between Si-H and vinyl groups. Under the catalytic action of tin salts, the reactions between Si-H and Si-OH can also take place, and hydrogen gas is released. 
Polysiloxanes can maintain their physical properties over an extremely wide temperature range (-100°C to 250°C). Its Tg is as low as -127°C, thus possessing outstanding low-temperature flexibility. Secondly, it is resistant to high temperatures and oxidation, and has good stability in chemical and physiological environments. Its weather resistance, water repellency, and dielectric properties are also good. The applications of polysiloxanes are very extensive. The liquid products are used as defoamers, fiber hydrophobic finishing agents, surfactants, lubricants, vacuum greases, and heating oil baths. Resins are used as varnishes, paints, release agents, adhesives, and insulating materials, etc. Elastomers are used as sealing materials, electrical insulating materials, gaskets, and pipes, etc., and can also be used as medical materials, such as artificial heart valves, pacemakers, contact lenses, and inner coatings of plasma bottles, etc. Compared with other organic elastomer materials, silicone elastomers have relatively poor strength, so the addition of fillers for reinforcement is more important.