Synthesis of a lignin polymer model consisting of only phenolic beta-O-4 linkages and testing its reactivity under alkaline conditions

A trimeric beta-O-4 lignin model bearing a styrene unit was polymerized in the presence of azobisisobutyronitrile as an initiator of free radical polymerization. The polymer analysis achieved by size exclusion chromatography (SEC), H-1, C-13, P-31 NMR, matrix-assisted laser desorption ionization combined with time-of-flight mass spectrometry (MALDI-ToF MS), differential scanning calorimetry, and thermogravimetry indicated its well-defined structure with good thermal stability at temperatures usually applied for alkaline pulping. SEC analysis proved a high degree of polymerization despite the size of the phenolic beta-O-4 units in the polymer structure. MALDI-ToF spectrometry indicated that the polymer is composed of phenolic beta-O-4 elements only. The chemical reactivity under alkaline conditions of the polymer was studied by both HPLC and P-31 NMR. These two methods revealed two main concurrent processes, which include an oxidation of the phenol part followed by C-alpha-C-beta bond cleavage, and formation of a quinone methide followed by its beta-O-4 bond cleavage.