---
title: "Short note: Silicones"
subject: "Engineering Chemistry"
chapter: 4
key: "semester_2/engineering_chemistry:silicones-note"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/silicones-note
site: BCT Engineering
---

# Short note: Silicones

Engineering Chemistry · Chapter 4: Engineering Polymers

- Asked in: 2070 Bhadra, 3 marks; 2068 Magh, 2.5 marks (asked 2 times)
- Video: [SILICONES ELASTOMERS (RUBBERS) PROPERTIES SYNTHESIS AND USES](https://www.youtube.com/watch?v=hyKMJ9Ut4u8) — UMA BIO-Chemistry
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/silicones-note
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-4

## Question

Write a short note on silicones.

## Short answer

Silicones are polymers with a –Si–O– backbone and methyl side groups, made by hydrolyzing (CH3)2SiCl2 and condensing the silanediol. They are heat-stable (–50 to 250 °C), water-repellent, and electrically insulating. Uses include lubricants, sealants, flexible rubber, and heat-resistant coatings.

## Answer

**Silicones** are inorganic–organic polymers with a backbone of alternating silicon and oxygen atoms (–Si–O–Si–) and organic groups (usually methyl) attached to silicon, general formula [R₂SiO]ₙ.

**Preparation**
1. Dimethyldichlorosilane is made by passing CH₃Cl over silicon at 300 °C with copper catalyst: Si + 2CH₃Cl → (CH₃)₂SiCl₂
2. Hydrolysis: (CH₃)₂SiCl₂ + 2H₂O → (CH₃)₂Si(OH)₂ + 2HCl
3. Condensation (loss of water) of the silanediol gives the linear polymer: n (CH₃)₂Si(OH)₂ → [–Si(CH₃)₂–O–]ₙ + n H₂O

Adding a little (CH₃)₃SiCl stops the chain (chain length control); adding CH₃SiCl₃ gives cross-links.

#### Types
| Type | Structure | Uses |
|---|---|---|
| Silicone oils/fluids | Short linear chains | Lubricants, hydraulic fluids, polishes, anti-foaming agents, cosmetics |
| Silicone greases | Oil + filler (silica) | High and low temperature lubricant, vacuum grease |
| Silicone rubbers | Long chains lightly cross-linked | Gaskets, seals, tubing, medical implants, cable insulation |
| Silicone resins | Highly cross-linked network | Heat-resistant paints, varnishes, electrical insulation, water repellents |

#### Properties and uses
Silicones are stable to heat (–50 to 250 °C), water repellent, chemically inert, good electrical insulators, and physiologically harmless.

- Lubricants and oils stable over a wide temperature range.
- Water-repellent coating for cloth, glass and building surfaces.
- Electrical insulation (high-temperature) and sealants for electronic devices.
- Rubber for gaskets, tubes, medical implants, keypads and bakeware.
- Cosmetics, polishes, anti-foam and mould-release agents.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is the backbone of a silicone polymer?
   A. Alternating Si and O atoms with organic groups on Si
   B. C-C chain
   C. Alternating P and N atoms
   D. S-S chain
   Answer: A. Silicones are [R2SiO]n with Si-O-Si backbone and usually methyl groups on silicon. (Key point: Backbone)

2. What is the sequence of silicone preparation?
   A. Hydrolysis of SiCl4, then oxidation
   B. CH3Cl over Si/Cu gives (CH3)2SiCl2, hydrolysis gives silanediol, condensation gives polymer
   C. Polymerising dimethylamine
   D. Heating sand with carbon
   Answer: B. Si + 2CH3Cl gives dimethyldichlorosilane, which hydrolyses to a diol that condenses with loss of water. (Key point: Preparation)

3. What does adding a little (CH3)3SiCl do?
   A. Makes cross-links
   B. Speeds up hydrolysis
   C. Stops the chain, controlling chain length
   D. Makes the silicone conducting
   Answer: C. It has only one reactive Cl so it caps the chain; CH3SiCl3 gives cross-links instead. (Key point: Chain control)

4. Which silicone type has a highly cross-linked network?
   A. Silicone oils
   B. Silicone greases
   C. Silicone rubbers
   D. Silicone resins
   Answer: D. Resins are highly cross-linked; rubbers are lightly cross-linked, oils are short linear chains. (Key point: Types)