Q.What is the fundamental difference between addition polymerization and condensation polymerization? Give one example of a polymer formed by each method.

Addition (chain-growth) polymerization joins together unsaturated monomers (each containing a reactive $\text{C=C}$ double bond) one at a time onto a single continuously growing chain, typically via a free-radical initiation/propagation/termination mechanism, with NO atom lost in the process -- the polymer's formula is an exact multiple of the monomer's, e.g. $n\,\text{CH}_2{=}\text{CH}_2 \rightarrow -(\text{CH}_2\text{CH}_2)_n-$ (polythene). Condensation (step-growth) polymerization instead joins together bifunctional (or polyfunctional) monomers -- each carrying two reactive end groups -- at many independent points across the reaction mixture, eliminating a small molecule, most often water, at every linking step, e.g. nylon-6,6 forms from hexamethylenediamine and adipic acid with loss of water at each amide linkage.

Compare whether the monomer needs a reactive double bond (addition) or two reactive end-groups (condensation), and whether a small molecule is eliminated during linking.

Assuming both mechanisms always use the same type of monomer; forgetting that condensation polymers lose mass (as the eliminated small molecule) relative to the sum of their monomers, while addition polymers do not.