Polymerization Reaction — the NEET Chemistry reaction: mechanism, reagents, conditions, structures and exam traps.
Polymerization Reaction Polymerization is a chemical reaction process in which monomer molecules react together in a chemical reaction to form three-dimensional networks or polymer chains. This process can be broadly classified into addition polymerization, where monomers add to one another in such a way that the resulting polymer contains all atoms of the monomer unit, and condensation polymerization, where monomers react to form a polymer with the simultaneous elimination of small molecules such as water, methanol, or HCl. Typically, the reaction mixture transitions from a low-viscosity liquid to a highly viscous liquid or a solid. For some reactions, heat is evolved (exothermic for many addition polymerizations). No significant color changes are usually observed unless chromophoric monomers or initiators are used. Odors may be present from monomers or byproducts. Addition polymerization is exothermic (ΔH < 0) because σ bonds formed are stronger than the π bonds broken. Condensation polymerization is also exothermic but less so, and is entropy-driven at higher temperatures due to release of small molecules (H₂O, HCl). Chain-growth polymerization (e.g., Free radical): 1. Initiation: Initiator (e.g., peroxide) forms radicals, which react with a monomer to start the chain. 2. Propagation: The radical polymer chain rapidly adds more monomer units. 3. Termination: Two growing chains combine or disproportionate to end the polymerization. Step-growth polymerization (e.g., Condensation): 1. Monomers with at least two functional groups react to form dimers, trimers, and longer oligomers, typically with the elimination of a small molecule. 2. These oligomers then react with each other or with monomers, gradually increasing chain length. The monomer concentration decreases rapidly in the early stages, but high molecular weight polymers are only formed in the later stages. Confusing addition vs. condensation polymerization: remember addition polymers retain all monomer atoms, condensation polymers lose a small molecule. Incorrectly identifying the monomer unit from a given polymer structure (especially for condensation polymers where a small molecule is lost). Failing to balance equations by including 'n' for monomers and repeating units, and 'nH2O' or other byproducts for condensation. Assuming all polymerizations are radical-initiated; remembering other types like cationic, anionic, and coordination. Not recognizing common commercial polymers and their monomers (e.g., PVC from vinyl chloride, PET from terephthalic acid and ethylene glycol).