1. Introduction

Silicones are a class of inorganic-organic polymers containing a backbone of alternating silicon and oxygen atoms (–Si–O–Si–) with organic side groups (usually methyl, –CH₃). General formula: (R₂SiO)_n, where R = alkyl or aryl group. Silicones are thermally stable, chemically inert, and water-repellent. They are also called polysiloxanes.

2. Structure

- Backbone: –Si–O–Si–O– (similar to –C–C– in organic polymers)
- Side groups (R) attached to silicon: methyl (–CH₃), phenyl (–C₆H₅), vinyl (–CH=CH₂)
- Repeating unit: –[R₂Si–O]_n–

Example: Polydimethylsiloxane (PDMS)
–[Si(CH₃)₂–O]_n–

3. Methods of Preparation

3.1 Hydrolysis of Chlorosilanes

Common route: Hydrolysis of dimethyldichlorosilane

3.2 Condensation of Silanols

Silanols (Si–OH) condense to form Si–O–Si linkages, eliminating water:

4. Properties

• Thermal: Stable over wide temperature range (–50 to 200°C)
• Mechanical: Flexible, elastic, rubber-like consistency
• Chemical: Resistant to oxidation, ozone, water, and UV
• Electrical: Excellent insulator
• Physical: Low surface tension, water-repellent

5. Types of Silicones

• Fluid Silicones: Low viscosity, used as lubricants, hydraulic fluids
• Elastomeric Silicones: Rubber-like, used in seals, gaskets, and medical implants
• Resin Silicones: Hard, glass-like, used in coatings and adhesives
• Gels: Semi-solid, used in cosmetics and cushioning

6. Applications

• Medical: Implants, catheters, prosthetics
• Industrial: Seals, gaskets, lubricants, adhesives
• Electrical: Insulators, coatings, encapsulation of components
• Consumer products: Cookware, hair care, cosmetics
• Construction: Sealants, paints, water-repellent coatings