Handbook of Size Exclusion Chromatography and Related ...

higher when using pyridine than when using DMSO, whereas no difference was
observed for microcrystalline cellulose or filter paper samples. The higher DP for
the softwood kraft pulp samples was ascribed to incomplete derivatization in
pyridine leading to aggregation of the cellulose part of the sample. Using LiCl/
DMAc as solvent and only catalytic amounts of pyridine, the reaction proceeds
homogeneously and the CTCs are formed within three hours for various samples
(125). The chromatographic condition used was, however, not adequate for high M
samples, giving a nonquantitative response and about half of the expected Mw as
obtained from off-line LS measurements.
Precipitation and removal of byproducts (N,N-diphenylurea, methyl
phenylcarbamate, and the phenylisocyanate trimer) are important for determination
of the elemental composition, that is, determination of the DS. Precipitation
has been carried out in neat EtOH (123,127,128) or neat MeOH (121,125,129).
The conditions chosen for precipitation are a trade-off between complete removal
of byproducts and complete recovery of the CTC (120). To circumvent the
incomplete precipitation of the CTC in neat solvents, mixtures of water and MeOH
(30:70 or 50:50) have been used with or without addition of salt (117,119,120).
Coprecipitated trimer can be removed by extraction with toluene (120). Precipitation
of CTC from the reaction medium has also been achieved using a mixture of
MeOH, water, and acetic acid (122). In those cases where purification is not
needed, a complete recovery of the derivative can be ensured by evaporation of the
solvent (118,130–132).
Efforts to catalyze the carbanilation reaction have been made by adding
different kinds of amines to the reaction mixtures consisting either of pyridine,
DMSO, or DMF as solvents (124,133). 1,4-Diazobicyclo(2.2.2)octane (DABCO)
and 4-N,N-dimethylaminopyridine accelerated the dissolution of cellulose during
the reaction and DABCO in pyridine made it possible to carbanilate samples,
which were otherwise unreactive in pyridine. However, several disadvantages
were reported such as severe tailing of the elution curves due to incomplete
carbanilation, loss of phenyl isocyanate by formation of phenyl isocyanate depolymerization,
and retardation of the carbanilation reactions by some amines (133).
Presence of pyridine or its derivatives in carbanilation reactions of Avicel or cotton
linter samples in DMSO was found to cause severe depolymerization of the
cellulose (124).
A method for carbanilation and direct SEC of lignin-containing hardwood
kraft pulps and softwood kraft pulps using LiCl/DMAc has recently been reported
(134). The samples were successfully carbanilated using phenyl, ethyl, or propyl
isocyanate according to the procedure described by McCormick and Lichatowich
(135), but without addition of catalyst. For studies of the pulp lignin and its
interference with cellulose and hemicellulose the preferred reactants are ethyl
isocyanate or propyl isocyanate, since the UVabsorbance of the phenyl carbanilate
interferes with the UV absorbance of lignin.
© 2004 by Marcel Dekker, Inc.