Interplay between two radical species in the formation of periodic patterns during a polymerization reaction.

Periodic patterns are ubiquitous in nature and spontaneously form on molecular to cosmic scales by the interplay between reaction and diffusion. Understanding how these patterns form is important to understand the construction rules of nature and apply them in the synthesis of functional artificial materials. This work clarifies how radical (R˙) species affect pattern formation in periodic precipitated and depleted zones during a polymerization process in an agarose gel. When a monomer (Mon) solution was poured on top of the gel doped with an initiator (In) in a test tube, periodic and continuous precipitation occurred near and far away from the solution/gel interface, respectively. In contrast, a system without In exhibited only a continuous band of precipitates beyond a depleted zone without precipitates at a certain distance from the interface. In the depleted region, an inhibitor (Q) added to the solution limited the polymerization triggered by R˙ formed thermally from Mon. With the addition of enough In to overcome the quenching effect of Q, periodic bands appeared near the solution/gel interface. These results suggest the involvement of two independent polymerization processes: (i) polymerization triggered by R˙ formed from In, which is the dominant process up to 100 h and yields periodic structures near the interface. After 100 h, the dominant process is the polymerization triggered by R˙ generated thermally from Mon, which yields a continuous precipitation zone. These two R˙ species compete and generate periodic bands near the interface (<100 h) and a continuous band far away from the interface (>100 h).