Antioxidant properties of phenolic lignin model compounds

Antioxidant activities of phenolic lignin model compounds were determined by measurements of inhibition rate constants (k(inh)) during inhibited peroxidation of styrene in chlorobenzene initiated by azobisisobutyrylnitrile with known rates of initiation (R-i). The number of peroxyl radicals trapped by each antioxidant, the stoichiometric factors (n), were determined by comparison with pentamethyl-hydroxychroman, PMHC, n = 2. Monomeric lignin models, 4-propylguaiacol (1), eugenol (2), isoeugenol (3), coniferyl alcohol (4), coniferyl aldehyde (5), and 4-allyl-2,6-dimethoxyphenol (6) were all more active antioxidants than the commercial inhibitor 2,6-di-tert-butyl-4-methylphenol (BHT). Two dimer models, bis(2-hydroxy-3-methoxyl-5-allylphenyl)methane (7) and 2,2'-dihydroxy-3,3'-dimethoxy-5,5'-dimethoxymethylbiphenyl (8) and a synthetic tetramer, bis[2-hydroxy-5-(2'-hydroxy-3'-methoxy-5'-methylbenzyl)-3-methoxyphenyl]methane (9) were more active antioxidants. The overall relative activity was tetramer > dimers > monomers > BHT. The stoichiometric factors of 1 to 6 ranged from 1.6 to 1.7 compared to PMHC. The n factors for 7, 8, and 9, showed an additive effect per phenolic hydroxyl. Phenolic groups in lignin may protect lignin-containing pulps and paper against damaging free radical peroxidation.