(C2H4)n — Polyethylene
Polyethylene is a versatile polymer made by polymerizing ethene molecules. It is widely used in packaging, plastic bags, bottles, and containers due to its durability, flexibility, and chemical resistance.
Interactive 3D Molecular Structure — (C2H4)n
Properties
| Chemical Formula | (C2H4)n |
|---|---|
| Molecular Mass | Varies with chain length (typically 28n g/mol) |
| Physical State | Solid |
| Melting Point | 110°C to 135°C (depending on type) |
| Boiling Point | Not applicable (decomposes before boiling) |
| Density | 0.91 to 0.97 g/cm³ |
| Odor | Odorless |
| Color | Translucent to white |
| Taste | Tasteless |
| Polarity | Non-polar |
| Type of Bond | C–C single bonds (covalent) |
| Solubility | Insoluble in water; soluble in hot toluene and xylene |
| Common Forms | Low-Density Polyethylene (LDPE), High-Density Polyethylene (HDPE), Linear Low-Density Polyethylene (LLDPE) |
Introduction to Polyethylene
Polyethylene (PE) is one of the most common and widely used thermoplastic polymers in the world. It is produced through the polymerization of ethene (ethylene), a simple alkene with the formula \(C_2H_4\). The polymerization process links many ethene monomers together to form long chains of \(-CH_2-CH_2-\) repeating units, represented as \((C_2H_4)_n\). Polyethylene is known for its toughness, flexibility, and resistance to moisture and chemicals. It plays an essential role in everyday life—from grocery bags and plastic films to medical devices and insulation materials.
Types of Polyethylene
There are several types of polyethylene depending on the density, branching, and polymerization process. The most common types are:
- Low-Density Polyethylene (LDPE): It has a highly branched structure, making it soft, flexible, and suitable for films and packaging materials.
- High-Density Polyethylene (HDPE): It is more crystalline and less branched, providing strength and rigidity. It is used in bottles, pipes, and containers.
- Linear Low-Density Polyethylene (LLDPE): Combines properties of LDPE and HDPE, used in stretch films and flexible tubing.
- Ultra-High-Molecular-Weight Polyethylene (UHMWPE): Extremely strong and durable, used in bulletproof vests and artificial joints.
Structure and Polymerization Process
The polymerization of ethene to form polyethylene occurs via addition polymerization. The process can be represented by the equation:
\( n\,CH_2=CH_2 \xrightarrow[]{catalyst, pressure, temperature} [-CH_2-CH_2-]_n \)
This reaction is typically catalyzed using Ziegler-Natta catalysts or metallocene catalysts. Depending on the catalyst and reaction conditions, polyethylene with different molecular weights and branching can be obtained. The molecular structure largely determines its physical properties such as flexibility, crystallinity, and melting point.
Physical and Chemical Properties
Polyethylene is a non-polar hydrocarbon polymer that exhibits high chemical resistance. It does not react with acids, bases, or salts and is unaffected by most solvents at room temperature. The material is lightweight, electrically insulating, and can withstand low temperatures without cracking.
Some key characteristics include:
- Soft and flexible (especially LDPE).
- Strong and rigid (especially HDPE).
- Water-resistant and excellent barrier properties.
- Low coefficient of friction and good wear resistance.
Uses and Applications of Polyethylene
Due to its versatility and low cost, polyethylene is used across numerous industries. Common applications include:
- Packaging: Plastic bags, shrink wraps, cling films, and containers.
- Household goods: Bottles, buckets, and toys.
- Construction: Pipes, fittings, and insulation materials.
- Medical devices: Prosthetics, catheters, and drug delivery systems.
- Automotive: Fuel tanks, bumpers, and wiring insulation.
Polyethylene’s durability and chemical resistance make it ideal for long-term applications and recyclability initiatives.
Environmental Impact and Recycling
Although polyethylene is highly useful, it poses significant environmental challenges due to its non-biodegradability. It accumulates in landfills and oceans, contributing to plastic pollution. However, advances in recycling technologies and biodegradable polyethylene alternatives are mitigating these effects. Recycling codes #2 (HDPE) and #4 (LDPE) are used to identify and sort polyethylene for reprocessing. Recycled PE is repurposed for manufacturing pipes, pallets, and outdoor furniture, reducing environmental impact.
Key Reactions of Polyethylene
Polymerization Reaction
\( n\,CH_2=CH_2 \rightarrow [-CH_2-CH_2-]_n \)
This is an addition polymerization reaction, where multiple ethene monomers join without the loss of any small molecule. The reaction is initiated by free radicals, cations, or coordination catalysts like titanium chloride and aluminum alkyls in the Ziegler-Natta process.
Thermal Decomposition
When heated excessively in the absence of oxygen, polyethylene undergoes thermal cracking or pyrolysis, breaking down into smaller hydrocarbons like alkanes and alkenes:
\( [-CH_2-CH_2-]_n \xrightarrow[]{\Delta} \text{Alkanes + Alkenes} \)
This reaction forms the basis of chemical recycling, where waste polyethylene is converted back into useful petrochemical feedstocks.