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Hydrophobic starch ester synthesis by reaction of
starch with various carboxylic acid imidazolides
BA
BA
The fatty acid imidazolides were prepared by
conversion of one equivalent of the corresponding
fatty acid chloride with two equivalents of
imidazole according to H. A. Staab 7-9. The
conversion of starch with the corresponding fatty
acid imidazolides yields the desired starch
esters with a DS from 1.55-1.90 in good yields
from 79 - 97 (s. tab. 1). Tab. 1 Yield and
degree of substitution (DS) of starch esters
synthesized by reaction of starch with fatty acid
imidazolides.   n.d. not
determined   The by-product imidazole
hydrochloride can be recovered by treatment with
a saturated sodium carbonate solution.
Evaporation of the water, subsequent extraction
of the residue with chloroform and evaporation of
the chloroform leads to a 67 recovery of
imidazole (s. fig 1). A comparison of the
molecular weights (Mn and Mw) of starch esters
which were prepared by the imidazolide method
with starch esters which were synthesized
according to 3 shows that the molecular weights
of starch esters obtained by the imidazolide
method are significantly higher since no acid
hydrolysis occurs in this case (s. tab. 2). Tab.
2 Molecular weights Mn and Mw of starch esters
synthesized by different methods.   W
e gratefully acknowledge the financial support of
this work by the Bundesministerium für
Verbraucherschutz, Ernährung und Landwirtschaft,
Bonn and the Fachagentur Nachwachsende Rohstoffe
e. V., Gülzow.     1 Fachverband der
Stärke-Industrie e. V. Zahlen und Fakten zur
Stärke-Industrie, Bonn 1997 2 A. Vetter, T.
Graf "Markt- und Innovationspotential
pflanzlicher Inhaltsstoffe für die Landwirtschaft
und Industrie", 2. Internationales Symposium
"Werkstoffe aus nachwachsenden Rohstoffen",
Erfurt, Sept. 1999 3 I. A. Gros, R. O. Feuge
"Properties of fatty acid esters of amylose", J.
Am. Oil Chem. Soc. 39, 19 (1962) 4 J. Aburto,
I. Alric, E. Borredon "Preparation of long-chain
esters of starch using fatty acid chlorides in
the absence of an organic solvent", Starch/Stärke
51, 132 (1999) 5 R. Narayan, St. Bloembergen,
A. Lathia, U.S. Patent 5,869,647, 1999 6 Rivard
et al "Effects of Natural Polymer Acetylation on
Anaerobic Bioconversion to methane and carbon
dioxide", Appl. Biochem. Biotech. 34/35, 725-735
(1992) 7 H. A. Staab "Synthesen mit
heterocylischen Amiden (Azoliden)", Angew. Chem.
12, 407-423 (1962) 8 H. A. Staab
"Transacylierungen 1. N-Acyl-Verbindungen
stickstoffhaltiger Heterocyclen", Chem. Ber. 12,
1927-1940 (1927) 9 H. A. Staab, A. Mannschreck
"Synthese von Carbonsäureestern nach der
Imidazolidmethode", Chem. Ber. 95, 1284-1297
(1962)
In 1998 7.7 million tons of starch were produced
in Western Europe from maize, wheat and potatoes.
The use of these starch products is devided
into 4.16 million tons of hydrolysates (53 )
1.83 million tons of native starches (25
) 1.31 million tons of modified starches (17
).  Approximately 3.87 million tons of these 7.3
million tons found application in the food sector
and 3.43 million tons (47 ) in the non-food
sector 1. For comparison 40 of the german
production of starch was used for the non-food
sector in 1996, mainly in paper (308,000 t) and
corrugated cardboards processing (84,000 t) 1,
but only 5,000 t of starch were utilized for the
production of biodegradable plastics 2. Due to
the growing economical interest in bioplastics
caused by the increasing costs for waste disposal
this study will focus on the possibilities of
biodegradable starch esters to find applications
in thermoplastic manufacturing and
processing. Traditonally starch esters were
prepared by direct conversion of starch with the
corresponding carboxylic acid chlorides using
pyridine as catalyst 3. In a recent paper a
solvent free preparation of long-chain esters of
starch using fatty acid chlorides is represented
4. A series of potato starch esters (C8 to C16)
were prepared by gelatinization of native starch
with formic acid, followed by treatment with acyl
chlorides. The degree of substitution diminishes
with increasing fatty acid chain length due to
steric hinderance of the introduced alkyl chain.
However, in this reaction the large excess of
the fatty acid chloride and the formation of HCl,
resulting in a degradation of starch by
hydrolysis, makes this synthesis route
disadvantageous for preparations in a large
scale. Another method of preparing biodegradable,
thermoplastic modified-starch products is
described by Narayan et al. 5. In this
invention different types of starch were
esterified with various carbon acid anhydrids in
anhydrous solvents using a catalytic amount of
4-dimethylamino-pyridine (4-DMAP) as a preferred
catalyst. The starch esters obtained have an
intermediate degree of substitution (DS) from 1.2
to 1.7. Rivard et al showed that under anaerobic
conditions starch esters above a DS 1.7 were
not biodegradable 6. So starch esters with a DS
from 1.2 to 1.7 seem to have the most preferred
balance in mechanical properties, water
resistance and the rate of biodegration 6.
Generally esterification reactions of starch with
carboxylic anhydrides and fatty acid chlorides
are accompanied by degradation of starch, which
causes the use of a neutralizing
reagent.   With regard to the synthesis of
hydrophobic, biodegradable starch esters in a
large scale which are suitable for thermoplastic
processing we developed a synthesis route based
on the conversion of starch with fatty acid
imidazolides. Amylo maize or potato starch has
been reacted with a catalytic amount of a
methanolic KOCH3 solution as catalyst and a
carboxylic acid imidazolide (s. fig.
1).
Fig. 1 Preparation of starch esters by
conversion of starch with fatty
acid imidazolides.