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Silicon Chemistry Tutorial


Silicone Polymers (Siloxanes)

Silicones are macromolecules comprised of a polymer backbone of alternating silicon and oxygen atoms

(siloxane bonds) with organic side groups, such as methyl, phenyl or vinyl, attached to silicon. The number of repeating units, “x,” can range from zero to several thousand.

Polydimethylsiloxane, the most commonly available silicone:

By adjusting -Si-O- chain lengths, the functionality of the side groups and the crosslinking between molecular chains, silicones can be synthesized into an almost infinite variety of materials, each with unique chemical properties and performance characteristics.

The types of organic groups attached to the silicon atoms can be controlled to create a hybrid chemistry capable of imparting important organic characteristics to the silicone, including compatibility and substantivity.

  • Compatibility – Normally, silicones are not compatible with organic polymers. But if a larger organic group is attached to a silicone, its properties are altered, and it behaves more like an organic polymer.

 

  • Substantivity – Nonreactive silicones lack substantivity. But if an organoreactive group, such as an aminopropyl group, is attached to the silicone, the silicone becomes reactive and will form stable bonds with other organic polymers.

 

By blending the chemistry of silicon with the chemistry of carbon, it is possible to create polymers with unique properties  and superior performance characteristics.

Organoreactive Polysiloxanes.

By adding a number of organoreactive groups to a polysiloxane, it is possible to create a silicon-based organoreactive molecule with the chemical characteristics of a carbon-based reactive molecule and the physical properties of a polysiloxane.

F = Organoreactive group

n = Generally ≥ 3

x = 0 to 1,000

y = 1 to 25

This organoreactive silicon-based molecule can be made to any size. Any number of reactive groups can be designed into it. The organic groups are usually separated from the silicon by at least a three-carbon chain. This chain separates the organic reactivity from the effects of the silicon.

Organoreactive groups can be attached to polysiloxanes in a number of ways, including:

1. Pendant to the polysiloxane backbone, forming “rake” or “comb” structures

2. At the ends of the polysiloxane chain, forming an ABA structure

3. At one end of the polysiloxane chain, forming an AB structure

 

Pendant (“rake”) functionality:

ABA functionality:

AB functionality:

R = Amine, carboxy, hydroxy, epoxy, other

n = Generally ≥ 3

x = 0 to 1,000

y = 1 to 25

In rake structures, organic functionality is distributed along the polysiloxane chain. The resulting silicones are used to chemically bond the polysiloxane polymer to an organic material, or to give unique orientation of the silicone on a metal or glass surface.

AB or ABA organoreactive silicones are preferred when molecular architecture or copolymer synthesis is required because there are only one or two reactive groups per polymer chain, located predictably at the end of the chain. Reactive end groups are more exposed than the pendant reactive groups found in rake structures and are more available to form copolymers with organic intermediates.

AB and ABA functionalities are ideal for tying silicone and organic structures together to form silicone-organic or silicone-organic-silicone block copolymers.

 

Return to the Silicon Chemistry Tutorial home page.

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