AbstractCopolymerization of cationic monomer and nonionic macromonomer in water‐ether mixture solution is useful for preparing the functional polymer. Copolymer of diallyldimethylammonium chloride (DMDAAC) and a nonionic block polyether macromonomer (PEP‐65) has been found that has very well reverse demulsification performance, but the polymerization phenomenon cannot be described by any reported polymerization method. In this paper, the copolymerization behavior including polymer mechanism, polymerization kinetics, and reactivity rate of DMDAAC and PEP‐65 in the mixture solution of water and propylene glycol methyl ether (PGME) was studied for explaining the special copolymerization phenomenon and its flotation performance for treating oily wastewater was also investigated. The results showed that DMDAAC‐PEP‐65 copolymerization in water‐PGME mixture solution has four stages and has both characteristics of emulsion polymerization and dispersion polymerization. In addition, the copolymer also has excellent flotation performance.