Addition polymerization: no byproducts, just chain growth
Addition polymerization starts with unsaturated monomers (typically alkenes like ethylene, propylene, or vinyl chloride) and builds chains by breaking the double bond and adding monomers to the growing chain. The process is initiated by a free radical, carbocation, or carbanion, which attacks the pi bond and creates a new reactive site at the end of the chain. Monomers keep adding until the chain is terminated by loss of the active site. No byproducts are released; all the atoms in the monomers end up in the polymer. Polyethylene, polypropylene, and PVC are addition polymers.
Condensation polymerization: linking with water loss
Condensation polymerization joins monomers by forming a covalent bond between two functional groups (such as carboxylic acids and amines, or alcohols and carboxylic acids) while releasing a small molecule, typically water. The reaction is reversible in principle; polymer chains can hydrolyze back to monomers. Polyesters are made by condensing alcohols with carboxylic acids; polyamides (nylons) are made by condensing amines with carboxylic acids. Condensation polymers tend to be stronger and more heat-resistant than addition polymers because the backbone often contains polar groups (C=O, N-H) that promote intermolecular attraction.
Controlling chain length and properties
The degree of polymerization (number of monomer units in the chain) is controlled by reaction time, temperature, and catalyst concentration in addition polymerization. In condensation polymerization, it is also affected by the stoichiometric balance of the two functional groups; excess of one group limits the final chain length. Longer chains produce stronger, tougher polymers. Cross-linking (connecting different polymer chains) creates thermosets like epoxy resins, which are rigid and heat-resistant. Understanding polymerization is essential for producing materials with desired properties for applications from plastic bags to engineering plastics.