Addition (chain-growth) polymerization: Monomers must have C=C double bonds. The pi bond breaks and monomers add one-by-one to the growing chain. NO by-product formed. Molecular weight of polymer = n x (monomer MW). Examples: PE, PVC, PS, PTFE, PAN, PMMA.

Condensation (step-growth) polymerization: Monomers must have TWO functional groups (bifunctional). React with loss of small molecule (H2O, HCl, NH3). Any two monomers can react at any time (not just chain ends). Examples: nylon-6,6, Terylene, Bakelite, melamine resin.

Key distinguishing test: Does the polymer's repeating unit have the same molecular formula as the monomer? YES → addition. Does it lose atoms (e.g., fewer H, O than expected)? YES → condensation.

Special case: Nylon-6 is formed by ring-opening polymerization of caprolactam — technically neither classical addition nor condensation, but classified as condensation because the product has amide linkages.