Catechol polymerization in the presence of silver surface

Polymerization of catechol (o-diphenol) was studied by UV-visible absorption and surface-enhanced Raman spectroscopies. Catechol was found to polymerize in solution, both in ethanol and in water, leading to a browning of the solution. This process may take place in nature for o-diphenols similar to catechol, either by enzymatic or by abiotic mechanisms implying inorganic catalysts, leading to the formation of humic substances in soils. Catechol is an excellent molecular model for studying oxidative polymerization processes since it is the simplest molecule containing the very reactive o-diphenol group, like other polyphenols existing in nature. The results found in this study reveal that the oxidative condensation of catechol is remarkably enhanced in the presence of the silver colloids employed to enhance the Raman signal. The possible polymerization mechanism that catechol may follow can be (i) formation of C-C and ether C-O-C bonds between aromatic rings, which is the predominant mechanism at neutral pH, (ii) the creation of benzofuran and benzodioxane moieties, also probable, although this mechanism seems to be less important than in the case of caffeic acid, and (iii) a ring opening mechanism yielding aliphatic chains with carboxylic acids, mainly induced at alkaline pH in the absence of light. The influence of pH, light irradiation and storage time on the catechol condensation was investigated in relation to the above mechanisms. (C) 2001 Elsevier Science B.V.