Styrene-Spaced Copolymers Including Anthraquinone and β-O-4 Lignin Model Units: Synthesis, Characterization and Reactivity Under Alkaline Pulping Conditions

A series of random copoly(styrene)s has been synthesized via radical polymerization of functionalized anthraquinone (AQ) and beta-O-4 lignin model monomers. The copolymers were designed to have a different number of styrene spacer groups between the AQ and beta-O-4 lignin side chains aiming at investigating the distance effects on AQ/beta-O-4 electron transfer mechanisms. A detailed molecular characterization, including techniques such as size exclusion chromatography, MALDI-TOF mass spectrometry, and H-1, C-13, P-31 NMR and UV-vis spectroscopies, afforded quantitative information about the composition of the copolymers as well as the average distribution of the AQ and beta-O-4 groups in the macromolecular structures. TGA and DSC thermal analysis have indicated that the copolymers were thermally stable under regular pulping conditions, revealing the inertness of the styrene polymer backbone in the investigation of electron transfer mechanisms. Alkaline pulping experiments showed that close contact between the redox active side chains in the copolymers was fundamental for an efficient degradation of the beta-O-4 lignin model units, highlighting the importance of electron transfer reactions in the lignin degradation mechanisms catalyzed by AQ. In the absence of glucose, AQ units oxidized phenolic beta-O-4 lignin model parts, mainly by electron transfer leading to vanillin as major product. By contrast, in presence of glucose, anthrahydroquinone units (formed by reduction of AQ) reduced the quinone-methide units (issued by dehydration of phenolic beta-O-4 lignin model part) mainly by electron transfer leading to guaiacol as major product. Both processes were distance dependent.