Ring-opening polymerization is an additive process but tends to give condensation-like polymers but follows the stoichiometry of addition polymerization.  For example, polyethylene glycol is formed by opening ethylene oxide rings:
:HOCHCHOH  +  n CHO   →   HO(CHCHO)</sub>H
Nylon 6 (developed to thwart the patent on nylon 6,6) is produced by addition polymerization, but chemically resembles typical polyamides.