Revealing the formation mechanism of insoluble polydopamine by using a simplified model system

Dopamine (DA) and its polymer (PDA) have attracted broad interest in recent years. Due to the insolubility of PDA, however, it has been difficult to determine the exact structure and the polymerization mechanism. PDA is usually prepared under alkaline conditions (pH 8.5). In this case, the polymerization process may follow an equilibrium pathway to form indole-like repeating units, which lead to cross-linked structures. However, PDA can be prepared even under acidic conditions (pH 4.0) in the presence of an oxidant, ammonium persulfate (APS), which cannot be interpreted by previous mechanisms. Therefore, there should exist several pathways in parallel on the formation of PDA. Herein, a derivative of DA, 2-(4-methoxy-3-methylphenyl) ethylamine (MOE) that has fewer active sites, is used as a simplified model system to study the polymerization mechanism of DA. Experimental results from UV-Vis spectroscopy, single-molecule force spectroscopy and other measurements indicate that free radical polymerization of MOE occurs under both acidic and alkaline conditions in the presence of a polymerization initiator, APS. According to the mathematical principle of Set Theory and the fact that MOE has fewer active sites than DA, we speculate that free radical polymerization as a possible pathway should exist in parallel along with previously proposed pathways during the formation of PDA. These parallel pathways may be the main reason for the structural complexity of PDA.