ANTIOXIDANT PROPERTIES OF PHENOLIC LIGNIN MODEL COMPOUNDS

Antioxidant activities of phenolic lignin model compounds were determi ned by measurements of inhibition rate constants (k(inh)) during inhib ited peroxidation of styrene in chlorobenzene initiated by azobisisobu tyrylnitrile with known rates of initiation (R-i). The number of perox yl 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), i soeugenol (3), coniferyl alcohol (4), coniferyl aldehyde (5), and 4-al lyl-2,6-dimethoxyphenol (6) were all more active antioxidants than the commercial inhibitor 2,6-di-tert-butyl-4-methylphenol (BHT). Two dime r models, bis(2-hydroxy-3-methoxyl-5-allylphenyl)methane (7) and ydrox y-3,3'-dimethoxy-5,5'-dimethoxymethylbiphenyl (8) and a synthetic tetr amer, '-methoxy-5'-methylbenzyl)-3-methoxyphenyl]methane (9) were more active antioxidants. The overall relative activity was tetramer > dim ers > 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 radica l peroxidation.