A trimeric β-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), 1H, 13C, 31P 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 β-O-4 units in the polymer structure. MALDI-ToF spectrometry indicated that the polymer is composed of phenolic β-O-4 elements only. The chemical reactivity under alkaline conditions of the polymer was studied by both HPLC and 31P NMR. These two methods revealed two main concurrent processes, which include an oxidation of the phenol part followed by Cα-Cβ bond cleavage, and a formation of a quinone methide followed by its β-O-4 bond cleavage.