Structure
Silicones (also known as Polysiloxanes) are organosilicon polymers with a backbone of alternating silicon and oxygen atoms (–Si–O–Si–), while organic groups such as methyl, ethyl, or phenyl are attached to the silicon atoms. This unique structure imparts flexibility and stability.
General Formula: [–SiR2–O–]n, where R = alkyl/aryl group.
Preparation
Silicones are commonly prepared by the controlled hydrolysis and subsequent condensation of alkyl or aryl substituted chlorosilanes:
Direct Synthesis of Chlorosilanes: Silicon (derived from silica sand) reacts with methyl chloride in the presence of a copper catalyst to produce dimethyldichlorosilane (CH3)2SiCl2.
Si + 2CH3Cl → (CH3)2SiCl2
Hydrolysis of Dialkylchlorosilane: The dimethyldichlorosilane is hydrolyzed with water to replace the chlorine atoms with hydroxyl (OH) groups, yielding dimethylsilanol.
(CH3)2SiCl2 + 2H2O → (CH3)2Si(OH)2 + 2HCl
Intermolecular Condensation of Silanols: The silanol molecules undergo a condensation polymerization (releasing water molecules) to form long-chain polymers like polydimethylsiloxane (PDMS).
n (CH3)2Si(OH)2 → [–Si(CH3)2–O–]n + n H2O
- Chain Termination: To control the molecular weight and stop the polymer chain from growing infinitely, a small amount of trimethylchlorosilane, (CH3)3SiCl, is introduced to block active terminal –OH sites.
- Cross-Linking: Introducing monomethyltrichlorosilane, CH3SiCl3, into the starting mixture yields a three-dimensional, cross-linked network known as silicone resins.
Uses
| Application | Purpose |
|---|---|
| Lubricants & Greases | Stable under high temperature and pressure. |
| Waterproofing Agents | Used for fabrics, surfaces, and construction materials. |
| Electrical Insulation | Excellent dielectric properties. |
| Medical Applications | Implants, prosthetics, and surgical tubing due to biocompatibility. |
| Sealants & Adhesives | Used in construction, automotive, and aerospace industries. |
Colour of Silicon
The pure form of silicon is crystalline with a dark grey in colour with a lustre on its surface. Its tetrahedral structure is similar to that of a diamond and shows similar physical and chemical properties.
Frequently Asked Questions
What makes silicones inorganic polymers?
Silicones are considered inorganic polymers because their backbone consists of silicon and oxygen atoms (–Si–O–Si–), unlike organic polymers which have carbon-based backbones.
What kind of catalyst is employed in the production of silicone?
Metal catalysts such as platinum or tin compounds are commonly employed in the production and curing of silicones.
How come silicones are flexible?
The Si–O bond length is longer and the bond angle is wider than C–C bonds, giving the polymer chain high flexibility and elasticity.
What substances are found in silicones?
Silicones contain silicon, oxygen, carbon, and hydrogen atoms. Organic groups (alkyl or aryl) are attached to silicon atoms.
Silicone is what sort of Polymer?
Silicone is a synthetic polymer, specifically an organosilicon polymer, with a repeating –Si–O– backbone.
Is silicone heat-sensitive?
No, silicones are highly heat-resistant and remain stable over a wide temperature range, making them suitable for high-temperature applications.
What are some silicone applications that are useful?
Silicones are used as lubricants, waterproofing agents, electrical insulators, medical implants, sealants, adhesives, and protective coatings.