Catechol polymerization in the presence of silver surface

Polymerization of catechol (o-diphenol) was studied by UV-visible absorptio n and surface-enhanced Raman spectroscopies. Catechol was found to polymeri ze in solution, both in ethanol and in water, leading to a browning of the solution. This process may take place in nature for a-diphenols similar to catechol, either by enzymatic or by abiotic mechanisms implying inorganic c atalysts, leading to the formation of humic substances in soils. Catechol i s an excellent molecular model for studying oxidative polymerization proces ses since it is the simplest molecule containing the very reactive o-diphen ol group, like other polyphenols existing in nature. The results found in t his study reveal that the oxidative condensation of catechol is remarkably enhanced in the presence of the silver colloids employed to enhance the Ram an signal. The possible polymerization mechanism that catechol may follow c an be (i) formation of C-C and ether C-O-C bonds between aromatic rings, wh ich is the predominant mechanism at neutral pH, (ii) the creation of benzof uran and benzodioxane moieties, also probable, although this mechanism seem s to be less important than in the case of caffeic acid, and (iii) a ring o pening mechanism yielding aliphatic chains with carboxylic acids, mainly in duced at alkaline pH in the absence of light. The influence of pH, light ir radiation and storage time on the catechol condensation was investigated in relation to the above mechanisms. (C) 2001 Elsevier Science B.V. All right s reserved.