Chain-growth polymerization

A type of polymerization where monomers add to a growing chain one at a time

Chain-growth polymerization is a method of polymerization in which the growth of a polymer chain proceeds by successive addition of monomer molecules to an active site on the growing polymer chain. This process is distinct from step-growth polymerization, where polymer chains grow by the stepwise reaction between functional groups of monomers.

Mechanism

Chain-growth polymerization involves three main steps: initiation, propagation, and termination.

Initiation

The initiation step involves the generation of an active center, which can be a free radical, cation, or anion, depending on the type of chain-growth polymerization. This active center is typically created by the action of an initiator, which can be a chemical compound or a physical process such as heat or light. For example, in free radical polymerization, an initiator like benzoyl peroxide decomposes to form free radicals that start the polymerization process.

Propagation

During the propagation step, monomer molecules add to the active center, extending the polymer chain. This step is characterized by rapid addition of monomers, leading to high molecular weight polymers. The rate of propagation is influenced by factors such as the concentration of monomers and the nature of the active center.

Termination

Termination occurs when the active center is deactivated, stopping the growth of the polymer chain. This can happen through various mechanisms, such as combination, where two active chains join together, or disproportionation, where a hydrogen atom is transferred from one chain to another, resulting in the formation of two inactive chains.

Types of Chain-Growth Polymerization

Free Radical Polymerization

Free radical polymerization is the most common type of chain-growth polymerization. It involves the use of free radicals as the active centers. This method is widely used for the production of polymers such as polyethylene, polystyrene, and polyvinyl chloride.

Cationic Polymerization

In cationic polymerization, the active center is a positively charged ion. This type of polymerization is suitable for monomers with electron-donating groups, such as isobutylene and styrene.

Anionic Polymerization

Anionic polymerization involves negatively charged active centers. It is used for monomers with electron-withdrawing groups, such as acrylonitrile and methyl methacrylate. This method allows for precise control over the molecular weight and structure of the polymer.

Applications

Chain-growth polymerization is used in the production of a wide range of commercial polymers. These polymers have applications in various industries, including packaging, construction, automotive, and electronics. For example, polyethylene is used in packaging materials, while polystyrene is used in insulation and disposable cutlery.

Advantages and Disadvantages

Advantages

• High molecular weight polymers can be produced rapidly.
• The process can be controlled to produce polymers with specific properties.

Disadvantages

• The reaction can be difficult to control, leading to broad molecular weight distributions.
• Some chain-growth polymerizations require stringent conditions to prevent premature termination.

Related pages

• Polymer chemistry
• Step-growth polymerization
• Free radical polymerization
• Cationic polymerization
• Anionic polymerization