An alternating copolymer has regular alternating A and B units, and is often described by the formula: -A-B-A-B-A-B-A-B-A-B-, or -(-A-B-)-. The molar ratio of each monomer in the polymer is normally close to one, which happens when the reactivity ratios r and r are close to zero, as can be seen from the Mayo–Lewis equation. For example, in the free-radical copolymerization of styrene maleic anhydride copolymer, r = 0.097 and r = 0.001, so that most chains ending in styrene add a maleic anhydride unit, and almost all chains ending in maleic anhydride add a styrene unit. This leads to a predominantly alternating structure.
A step-growth copolymer -(-A-A-B-B-)- formed by the condensation of two bifunctional monomers A–A and B–B is in principle a perfectly alternating copolymer of these two monomers, but is usually considered as a homopolymer of the dimeric repeat unit A-A-B-B. An example is nylon 66 with repeat unit -OC-( CH)-CO-NH-(CH)-NH-, formed from a dicarboxylic acid monomer and a diamine monomer.