Polysiloxanes (Silicones)

Silicone polymers or polydialkylsiloxanes are an important class of inorganic polymers that find many industrial uses. The primary reason for their great popularity is their outstanding temperature and oxidative stability, excellent low temperature flexibility, and high resistance to weathering and many chemicals. These polymers are also capable of wetting most surfaces due to their low surface tension.

The name silicone usually refers to organosilicon polymers with the general structure -[Si(R2)-O]- where R = -CH3 is called poly(dimethyl siloxane) which is often abbreviated as PDMS:

PEI

The methyl groups along the chain can be substituted by many other groups such as ethyl, phenyl, or vinyl, which allows for tailoring the chemical, mechanical and thermophysical properties for a wide variety of applications. 

Unlike most other polymers, silicones possess an inorganic backbone of -(Si-O)- repeat units. The Si-O bonds are strongly polarized and without side groups, should lead to strong intermolecular interactions. However, the nonpolar methyl groups shield the polar backbone. For this reason, silicone polymers have a very low critical surface tension despite a very polar backbone. In fact, PDMS has one of the lowest critical surface tension of all polymers which is comparable to that of Teflon.

Due to the low rotation barriers, most siloxanes are very flexible. For example, the rotation energy around a CH2-CH2 bond in polyethylene is about 12.1 kJ/mol but only 3.8 kJ/mol around a Me2Si-O bond, corresponding to a nearly free rotation. Furthermore, chain-to-chain interaction is rather week due to the low cohesive energy, and the distance between adjacent chains is noticeably larger in silicones than in alkanes which also contributes to the greater flexibility of PDMS. Due to great flexibility of the chain backbone, the activation energy of viscous flow is rather low and the viscosity is less dependent on temperature compared to hydrocarbon polymers.

Polydimethysiloxanes have a low surface tension in the range of 20 to 25 mN/m and consequently can wet most surfaces. With the methyl groups located on the outside, silicones produce very hydrophobic films.  Due to the large free volume, most gases have a high solubility and high diffusion coefficient in silicones. That is, silicones have a high permeability to oxygen, nitrogen and water vapor, even if in this case liquid water is not capable of wetting the silicone surface! As expected, silicone compressibility is also high.

Many other groups like phenyl, vinyl, alkyl, or trifluoropropyl can substitute the methyl groups along the chain. The simultaneous presence of other organic groups attached to the inorganic backbone leads to many unique properties and allows their use in a broad range of fields. One general drawback of the presence of other organic groups along the chain backbone is the reduction of the polymer's thermal stability. But with these substitutions, many other properties can be (greatly) improved. For example, a small percentage of phenyl groups along the chain reduces the tendency to crystallization and allows the polymer to remain flexible even at very low temperatures. The phenyl groups also increase the refractive index. Trifluoropropyl groups along the chain change the solubility parameter of the polymer from 15.3 MPa1/2 to 19.4 MPa1/2, which reduces the swelling of silicone elastomers in alkane and aromatic solvents. Silicone copolymers can also be prepared with excellent surfactant properties, with the silicone as the hydrophobic part.

 

Synthesis & Crosslinking of Silicones

Commerical (linear) silcones are prepared by hydrolysis of dichlorosilanes3 in the presence of an excess of water. This reaction is exothermic and yields in a first step disilanols, Me2Si(OH)2, which then form linear and cyclic oligomers (mainly cyclic trimers and tetramers)5 by inter- and intramolecular condensation catalyzed by the released hydrochloric acid, HCl:

Siloxane

The oligomers are separated from the acidic aqueous layer, washed with water, and then neutralized and dried. The linear and cyclic oligomers obtained by hydrolysis of the dialkyldichlorosilanes are typically of relative low molecular weight are often further condensed (linear form) or polymerized (cyclic form) to yield polymers of appropriate chain length. In some cases, the reaction mixture includes trichlorosilanes to increase the molecular weight and to achieve crosslinking.
The average molecular weight and the ratio of cyclic and linear oligomers in the silicone resin mixture depend on the type of substituents along the chain and on the reaction conditions such as monomer concentration, temperature, pH, and type of solvent. For example, large and bulky side groups shift the equilibrium to the linear form and basic conditions favor higher molecular weight oligomers.

Chlorosilanes can be converted to many other silane monomers and silicone oligomers. For example, reactive alkoxy and vinyl end groups can be formed by the reaction of chlorosilanes with alcohols and vinylmethylsiloxanes:

Siloxane

These oligomers and monomers can be further modified to produce a large number of products such as room temperature and moisture curing silicone rubbers that find uses in silicone adhesives, coatings, sealants, and caulks.
The silicone chemistry is remarkable versatile because silanol groups readily react with many other functional groups such as acetoxy, oxime, amine, or alkoxy:

The crosslinking of siloxane resins can be catalyzed by many metal-organics or acid/base catalysts, whereas one-part acetoxy resins are typically cured with tin catalysts and vinyl terminated resins with organic peroxides or platinum catalysts.

Notes & References

  1. Walter Noll, Chemistry and Technology of Silicone, Academic Press, New York 1968

  2. J.E. Mark, D.W. Schaefer, & G. Lin, The Polysiloxanes, Oxford University Press, New Yory 2015

  3. Methyl chlorosilanes can be obtained by the reaction of silicon with chloromethane at elevated temperatures (ususally 200-450°C) and in the presence of a copper catalyst in a fluidized-bed reactor4.The reaction yields a mixture of different methylchlorosilanes with dimethyl-dichlorosilane as the main component.

  4. US 4,500,724, Method for Making Alkylhalosilanes; WO2016091551A1, Method for the direct synthesis of methyl chlorosilanes in fluidized-bed reactors

  5. The cyclic siloxanes can be removed by distillation and converted to polyalkylsiloxanes by acid- or base-catalyzed ring-opening polymerization.

  6. C.J. Brinker and G.W. Scherer, Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing, Academic Press, San Diego 1990

  • Summary

    Polysiloxanes

    are the most important class of inorganic polymers. Unlike most other polymers, silicones possess a fully inorganic backbone of -(Si-O)- repeat units.

  • The name silicone usually refers to linear polymers with a silicon-oxygen backbone (-Si-O-) and with alkyl side groups.

  • The most important polyalkylsiloxane possesses only methyl side groups, called polydimethyl siloxane and often abbreviated as PDMS.

  • The Si-O bond energy is greater than that of a C-C bond whereas the energy barrier for rotation is singnificantly smaller. This has far-reaching effects on the thermal and mechanical properties of silicones.

  • Due to the strong Si-O and Si-C bonds, silcone polymers have a very high heat and oxidative stability and outstanding chemical resistance.

  • With the alkyl groups pointing to the outside, silicones produce hydrophobic films. Like Teflon, they have a very low critical surface tension despite a very polar backbone and are capable of wetting most surfaces.

  • Due to the low rotation barrier and weak chain-to-chain interaction, most siloxanes are rather soft and flexible and thus have excellent low temperature flexibility.

  • Commerical silcones are often prepared by hydrolysis of chlorosilanes in the presence of  water. This reaction is exothermic and yields in a first step silanols,  which are condensed to linear and cyclic oligomers by inter- and intramolecular condensation reactions.

  • The properties of polysiloxanes depend on the type and number of side groups, the molecular weight and crosslink density.

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