Addition and Condensation Polymerization - SS3 Chemistry Lesson Note
Addition polymerization and condensation polymerization are two major methods by which monomers are joined together to form polymers. These polymerization processes have distinct mechanisms, resulting in different types of polymers with varied properties. Understanding the differences between addition and condensation polymerization is crucial in the design and synthesis of specific polymers for various applications. Let's explore addition and condensation polymerization in detail:
Addition Polymerization:
1. Mechanism: Addition polymerization involves the polymerization of unsaturated monomers that contain double or triple bonds. During the reaction, the pi (π) bonds in the monomers break, and new σ bonds are formed between the carbon atoms, leading to the creation of a polymer chain. There is no elimination of small molecules during addition polymerization.
2. Initiation: The reaction is initiated by a radical initiator, a strong acid, or a metal catalyst. The initiator generates reactive free radicals, which start the polymerization process by attacking the unsaturated double bond in the monomer.
3. Propagation: Once the polymerization is initiated, the active radical repeatedly attacks other monomer molecules and incorporates them into the growing polymer chain. This process continues until all available monomers are consumed or until the reaction is terminated.
4. Termination: Additional polymerization can be terminated in several ways, including combination termination (two radicals combine to form a non-radical species) or disproportionation termination (one radical transfers a hydrogen atom to another radical).
5. Examples: Common examples of addition polymers include polyethene (from ethylene monomer) and polypropylene (from propylene monomer).
6. Characteristics: Additional polymers are usually formed under mild reaction conditions and result in high molecular weight polymers. The polymerization process is rapid and straightforward, and the resulting polymers are often thermoplastic, meaning they can be melted and reshaped without chemical degradation.
Condensation Polymerization:
1. Mechanism: Condensation polymerization involves the reaction between bifunctional or multifunctional monomers, where two functional groups (e.g., hydroxyl and carboxyl groups) combine, leading to the elimination of small molecules (such as water, alcohol, or ammonia) as byproducts.
2. Initiation: Unlike addition polymerization, condensation polymerization does not require a separate initiator. The reaction can be initiated by heating, acid catalysis, or base catalysis.
3. Step-Growth Polymerization: Condensation polymerization is also known as step-growth polymerization because the polymer chains grow one monomer unit at a time. Monomers with different functional groups combine in a stepwise manner to form the polymer.
4. Examples: Common examples of condensation polymers include nylon (formed from the reaction of diamine and diacid chloride monomers), polyester (formed from the reaction of diol and diacid monomers), and polyurethane (formed from the reaction of diisocyanate and diol monomers).
5. Characteristics: Condensation polymers often have lower molecular weights compared to addition polymers. They can form crosslinked networks during polymerization, resulting in thermosetting plastics with excellent heat resistance and mechanical properties. Unlike thermoplastics, thermosetting plastics cannot be melted and reshaped once they have been set.
In summary, addition polymerization involves the polymerization of unsaturated monomers without the elimination of small molecules, resulting in thermoplastic polymers. Condensation polymerization, on the other hand, involves the reaction between bifunctional or multifunctional monomers with the elimination of small molecules as byproducts, leading to thermosetting polymers. The choice between addition and condensation polymerization depends on the desired properties of the final polymer and the specific application requirements.