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Thermally induced homolytic scissions of interunitary bonds in a softwood lignin solution: A spin‐trapping study
Author(s) -
Seino T.,
Yoshioka A.,
Takai M.,
Tabata M.
Publication year - 2004
Publication title -
journal of applied polymer science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.575
H-Index - 166
eISSN - 1097-4628
pISSN - 0021-8995
DOI - 10.1002/app.20698
Subject(s) - radical , chemistry , spin trapping , homolysis , electron paramagnetic resonance , lignin , nucleophile , steric effects , photochemistry , softwood , medicinal chemistry , adduct , organic chemistry , polymer chemistry , materials science , physics , nuclear magnetic resonance , composite material , catalysis
Unstable chemical species, that is, radicals generated by the thermal treatment of a dimethyl sulfoxide (DMSO) solution of the lignin of a softwood, Yezo spruce ( Picea jezoensis Carr.), were studied in detail with an electron spin resonance (ESR) method combined with a spin‐trapping technique. An unstable secondary carbon radical (∼CH ·) in the solution was trapped as a stable nitroxide spin adduct [ R (NO ·)CH∼ ( R = tert ‐butyl benzene)] when the DMSO solution was heat‐treated in the presence of a spin‐trapping reagent [2,4,6‐tri‐ tert ‐butylnitrosobenzene (BNB)] at about 40°C. This meant that alkyl phenyl ether bonds (∼CHO‐phenyl), known as interunitary lignin bonds, were homolytically scissioned by the thermal treatment in the lignin solution. A detailed analysis of the ESR spectrum revealed that three kinds of radicals—primary (∼CH 2 ·), secondary (∼CH ·), and tertiary (∼C ·) carbon radicals—were trapped as stable spin adducts at about 60°C, although the phenoxy radical (PhO ·) was not trapped by the BNB spin trap as the counter radical of the secondary carbon radical. This suggested that a fairly large steric hindrance existed between the so‐called guaiacoxy radical with a methoxy group in the ortho position and the BNB molecule bearing two butyl groups as bulky moieties in the ortho positions. However, the phenoxy radicals in the lignin solution were stable up to about 60°C. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2136–2141, 2004