Silicones
Siloxanes, Chain Terminators, and Hydrophobic Polymers.
Silicones (chemically known as polysiloxanes) are a group of synthetic organosilicon polymers. Unlike purely inorganic silicates or purely organic plastics, silicones bridge the gap between organic and inorganic chemistry. They feature an inorganic silicon-oxygen backbone clad in an organic (hydrocarbon) skin. Mastering how to control their chain length and structure is a highly-tested topic in competitive exams.
1. Preparation of the Precursors (Müller-Rochow Process)
The starting materials for manufacturing silicones are alkyl or aryl substituted chlorosilanes, having the general formula $R_nSiCl_{4-n}$ (where R is an alkyl or aryl group, and n = 1, 2, or 3).
The Direct Process:
The most common precursor, dialkyldichlorosilane ($R_2SiCl_2$), is prepared by passing an alkyl chloride (e.g., methyl chloride, $CH_3Cl$) over Silicon powder heated to $570\text{K}$ in the presence of Copper (Cu) as a catalyst.
This reaction also produces small amounts of $CH_3SiCl_3$ and $(CH_3)_3SiCl$, which are separated by fractional distillation. These "by-products" play crucial roles in defining the final shape of the polymer.
2. Hydrolysis and Polymerization (Linear Silicones)
Once the chlorosilanes are isolated, they are subjected to hydrolysis followed by condensation polymerization.
Figure 1: Structure of a linear silicone polymer showing the Si-O-Si backbone and outward-facing alkyl groups.
Step 1: Hydrolysis
When dialkyldichlorosilane, $(CH_3)_2SiCl_2$, is treated with water, the chlorine atoms are readily hydrolyzed to form dialkylsilanediol.
Step 2: Condensation Polymerization
The silanediol molecules undergo rapid condensation. Two $-OH$ groups react, eliminating a water molecule and forming a strong Siloxane bond ($Si-O-Si$). Because there are two $-OH$ groups per monomer, they can propagate endlessly in both directions, forming long Linear Silicones.
3. Controlling the Chain: Terminators & Cross-Linkers
The length and nature of the silicone polymer depend entirely on the specific type of chlorosilane used. Examiners love testing the function of $R_3SiCl$ and $RSiCl_3$.
Trialkylchlorosilane ($(CH_3)_3SiCl$) hydrolyzes to form $(CH_3)_3SiOH$.
Notice it only has one $-OH$ group. Therefore, it can only undergo condensation on one side. When it attaches to a growing linear silicone chain, it caps the end. The chain cannot grow any further.
Function: It is used to control the chain length of the polymer. By varying the amount of $R_3SiCl$ added, you determine whether you get silicone oils (short chains) or silicone rubbers (long chains).
Alkyltrichlorosilane ($CH_3SiCl_3$) hydrolyzes to form $CH_3Si(OH)_3$.
It has three $-OH$ groups. This allows it to undergo condensation in three different directions simultaneously. It connects separate linear chains together.
Function: It leads to the formation of complex, highly rigid Cross-Linked (3D) Silicones (Silicone resins).
4. Properties and Industrial Applications
Crucial Chemical Properties:
- Hydrophobic (Water-Repellent): The backbone is surrounded by non-polar organic alkyl groups (like $CH_3$). These hydrocarbon groups face outward, making the surface completely water-repellent.
- High Thermal Stability: The $Si-O$ bond is exceptionally strong ($\approx 368 \text{ kJ/mol}$), much stronger than the $C-C$ bonds found in regular plastics. Silicones can withstand extreme high and low temperatures without decomposing.
- Chemical Inertness: They are highly resistant to oxidation, UV light, and attack by most chemicals.
- Electrical Insulators: Like most covalent network/polymeric structures without mobile electrons, they are excellent electrical insulators.
Major Uses:
- Waterproofing: Used to coat fabrics, tents, and masonry to make them water-repellent.
- Lubricants and Greases: Silicone oils and greases are used in vacuum pumps and high-temperature machinery where standard hydrocarbon oils would burn or freeze.
- Electrical Insulators: Used extensively to coat electrical wiring operating in harsh environments.
- Biocompatibility: Because they are chemically inert and non-toxic, they are widely used in cosmetics, contact lenses, and surgical/medical implants.
Mastery Check: Silicones
15 High-Yield Questions to test your JEE/NEET Preparation
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