Interfacial polymerization is a type of step growth polymerization of two reactants occurring on the interface between two liquid phases, each containing one of the reactants. American polymer scientists Paul Morgan and Stephanie Kwolek used the term “interfacial polycondensation” when they studied this method in 1959. The polymer formed on the interface needs to precipitate from the liquid phases, which makes sense for the term polycondensation. The most common polymer made by this method is polyamide, where diamine and diacid chloride react to form polyamide and hydrogen chloride. This reaction is done at room temperature by placing an aqueous phase containing diamine on top of an organic phase containing the acid chloride. The polymer precipitates and can be withdrawn continuously. The common classroom experiment “Nylon rope trick” involves the Schotten-Baumann reaction in this manner. For mass production in commercial processes, two phases are stirred to generate more interface for the reaction.
Both reactants have very low solubility in the other liquid phase. This ensures a controlled introduction of aqueous reactant into an excess of reactant in the organic phase. The reaction rate is very high, so that reactants diffuse to the interface and combine almost stoichiometrically to form high molecular weight polymer. The high reaction rate is required otherwise diamine will have time to penetrate through the polymer to start new polymer chains in the organic phase.
Interfacial polymerization occurs at room temperature. Polymers that are unstable at high temperatures can go through this pathway. High molecular weight polymer is made without matching the stoichiometric ratio.
Large amounts of solvents need to be handled and recycled, which can be costly in energy and money. The chemical nature of this method limits the product that can be made.