Polymers: Addition and Condensation Polymerisation

Introduction

Polymers are large molecules composed of repeating structural units called monomers. They are ubiquitous in everyday life, found in plastics, textiles, rubbers, and many other materials. Polymerisation is the process of joining monomers together to form a polymer.

There are two main types of polymerisation: addition polymerisation and condensation polymerisation. These processes differ in the mechanism of monomer joining and the types of monomers involved.

Addition Polymerisation

Mechanism

Addition polymerisation involves the direct joining of monomers without the loss of any atoms. Typically, monomers with double or triple bonds (e.g., alkenes) participate in this process.

The mechanism usually involves a radical initiator, which generates free radicals. These radicals then attack the monomers, adding to the double bond and creating a new radical. This process repeats, leading to a long polymer chain.

Types of Addition Polymerisation

• Radical Polymerisation: Initiated by free radicals. Common monomers include ethene, propene, vinyl chloride.
• Cationic Polymerisation: Initiated by strong acids or Lewis acids.
• Anionic Polymerisation: Initiated by strong bases.

Examples

Polymer	Monomer	Typical Uses
Polyethylene (PE)	Ethene (C2H4)	Plastic bags, films, bottles
Polypropylene (PP)	Propene (C3H6)	Containers, fibres, car parts
Polyvinyl Chloride (PVC)	Vinyl Chloride (C2H3Cl)	Pipes, flooring, window frames
Polytetrafluoroethylene (PTFE) - Teflon	Tetrafluoroethylene (C2F4)	Non-stick coatings, seals, electrical insulation

Factors Affecting Addition Polymerisation

Several factors can influence the rate and properties of addition polymers:

• Temperature: Higher temperatures generally increase the rate of reaction.
• Pressure: Higher pressure can increase the concentration of monomers, leading to a faster reaction.
• Initiator Concentration: Higher initiator concentration increases the rate of initiation.
• Inhibitors: Substances that slow down the reaction by reacting with free radicals.

Condensation Polymerisation

Mechanism

Condensation polymerisation involves the joining of monomers with the loss of a small molecule, such as water (H₂O), methanol (CH₃OH), or hydrochloric acid (HCl).

The reaction proceeds step-by-step, with monomers reacting in pairs to form dimers, then trimers, and so on, until a long polymer chain is formed.

Types of Condensation Polymerisation

• Polyesterification: Involves the reaction of a diol (alcohol with two hydroxyl groups) and a dicarboxylic acid. Water is eliminated.
• Polyamidation: Involves the reaction of a diamine (amine with two amine groups) and a dicarboxylic acid. Water is eliminated.
• Polyurethane Formation: Involves the reaction of a diisocyanate and a polyol.

Examples

Polymer	Monomer(s)	Typical Uses
Polyester (e.g., PET)	Ethylene Glycol + Terephthalic Acid	Plastic bottles, fibres (e.g., polyester clothing)
Polyamide (e.g., Nylon)	Diamino-hexanedicarboxylic Acid	Tires, ropes, textiles
Polyurethane	Diisocyanate + Polyol	Foams, coatings, adhesives
Protein (e.g., Collagen, Silk)	Amino Acids	Structural components of living organisms, textiles

Factors Affecting Condensation Polymerisation

The rate and properties of condensation polymers are affected by:

• Temperature: Higher temperatures generally increase the rate of reaction.
• Catalyst: Catalysts are often used to speed up the reaction.
• Monomer Ratio: The ratio of monomers can affect the properties of the polymer.
• Water Removal: Removing the water produced during the reaction helps to drive the equilibrium towards polymer formation.

Comparison of Addition and Condensation Polymerisation

Feature	Addition Polymerisation	Condensation Polymerisation
Monomer Type	Typically unsaturated (alkenes)	Typically bifunctional (e.g., diols, diamines)
Mechanism	Chain reaction (radical, cationic, anionic)	Step-by-step reaction with loss of a small molecule
Byproducts	None	Small molecules (e.g., water, methanol)
Molecular Weight	High molecular weight polymers are formed quickly	Molecular weight can be lower, and requires careful control

Conclusion

Addition and condensation polymerisation are fundamental processes in polymer chemistry. Understanding the mechanisms, types, and factors affecting these processes is crucial for designing and producing polymers with desired properties for a wide range of applications.