Note: Descriptions are shown in the official language in which they were submitted.
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,, PRODUCTION OF POLYSUCCINIMIDE IN AN ORGANIC MEDIUM
This invention relates to the production of
polysuccinimide by thermal polymerization of L-
aspartic acid in an organic medium. More
particularly, this invention relates to the thermal
polymerization of L-aspartic acid in a high boiling
alkyl alcohol or alkane.
BACKGROUND OF 'I'HF: INVENTTnN
Polyaspartic acid has been produced by
thermal polymerization of L-aspartic acid which
involves heating the acid to a temperature in the
range of from about 200°C to about 400°C. Water is
driven off as the acid polymerizes to form
polysuccinimide. The imide is easily converted to
polyaspartic acid by basic hydrolysis. Early
interest in such processes related to theories for
formation of prebiotic polypeptides. For the purpose
of testing such theories laboratory experiments used
powdered L-aspartic acid, usually packed in the bottom
of a flask which was then heated below the melting
point of the acid. Such reactions were slow and took
place over many hours. One such example is reported
by Kokufuta et al. in Bulletin of the Chemical Society
of Japan Vol. 51 (5) 1555-1556 (1978) "Temperature
Effect on the Molecular Weight and the Optical Purity
of Anhydropolyaspartic Acid Prepared by Thermal
Polycondensation." The structure of
anhydropolyaspartic acid has been thoroughly
investigated such as by J. Kovacs et al. in J.O.C.S.
Vol. 26 1084-1091 (1961).
In recent years many utilities have
been suggested for anhydropolyamino acid. Such
polyamides have been suggested as potential drug
carriers by Neuse et al. in Die Angewandte
Makronmolekulare Chemie 192 35-50 (1991) "Water-
soluble polyamides as potential drug carriers." They
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have also been tested as scale inhibitors with respect
to natural sea water and calcium sulfate in particular
by Sarig et al. as reported by the National Council on
Research and Development (NRCD 8-76, Seawater
Y
Desalination 150-157 (1977). Polyaspartic acid has
been well known for its ability to disperse solid
particles in detergent formulations, having been
mentioned as a dispersant in numerous patents, a few
of which are U.S. Patents 4,363,797; 4,333,844;
4,407,722 and 4,428,749. Also, as described in U.S.
Patent 4,971,724 to Kalota et al., it has been
discovered that compositions comprising polyamino
acids such as aspartic acid, when ionized at alkaline
pH, effectively inhibit corrosion of ferrous metals in
the presence of aqueous medium. Various derivatives
of polyamino acids have also been made wherein
attributes have been supplied by groups attached to
reactive sites on the molecule. One such example is
disclosed in U.S. Patent 3,846,380 to Fujimoto et al.
Because of the various impending
potential utilities of anhydropolyamino acids,
interest in processes for preparing such compounds in
large volume, particularly polyaspartic acid, has
increased. This interest has resulted in several
recent patents being issued which are directed to
fluid bed systems; in particular, U.S. Patent
5,219,986 to Cassata. Other such patents are U.S.
5,057,597 and 5,221,733 to Koskan and Koskan et al.
respectively. More recently, patents have issued
covering a process for preparing the polysuccinimide
by means of tray driers such as a rotary tray drier in
U.S. 5,319,145 to Palk et al. and an indirectly heated
tray drier in U.S. 5,315,010 to Koskan et al. When
phosphoric acid is employed in these processes the
aspartic acid undergoes polymerization to form the
polysuccinimide, forming a course powder containing
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lumps of up to about two and one-half centimeters in
diameter. Prior to subsequent processing the course
powder must be treated to remove the lumps.
Furthermore, in the use of an acidic catalyst such as
phosphoric the powder undergoes a tacky phase which
makes the powder difficult to handle in these dryers.
It has been often noted in the literature
that the color of the polysuccinimide is related in
rough manner to the time/temperature relationship in
its production. For example, the color of the product
from processes employing relatively longer reaction
time under elevated temperature produces more darkly
colored polysuccinimide than is produced with shorter
reaction time and lower temperatures. A typical
teaching of this phenomenon is found in a publication
entitled "Temperature Effect on the Molecular Weight
and the Optical Purity of Anhydropolyaspartic acid
Prepared by Thermal Polycondensation" by Kokufuta et
al., Bulletin of the Chemical Society of Japan, Vol.
51, pp. 1555-1556, 1978.
Many efforts have been made to lower the
time/temperature relationship by employing the above
noted dryers. However, the polysuccinimide produced
by such processes possesses an undesirable color. The
color of the polysuccinimide is transferred to the
water soluble salt upon hydrolysis of the initial
product.
It has been reported in the literature that
the use of acidic catalysts such as phosphoric acid
reduces the color of the resulting polysuccinimide.
It is believed that phosphoric acid increases the
reaction rate and therefore reduces the amount of time
required at high temperature for completion of the
reaction. While color of the polysuccinimide produced
in a solution of phosphoric acid is improved, the use
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of large amounts of acid is inconvenient. Another attempt
to employ liquid media for the L-aspartic polymerization
process is found in U.S. 5,371,179 wherein the use of
poly(alkylene glycols) is employed.
In one attempt to overcome the color problem it has
been found that the polyaspartate formed by thermal
polymerization followed by alkaline hydrolysis is treated
with bleach in water solution. This treatment is reported
to decolorize the solution of polyaspartate as noted in U.S.
Patent 5,292,864 to Wood et al. However, in many instances
the color of the polysuccinimide becomes a disadvantage as
this initial product is employed without first converting
it to the water soluble polyaspartate salt as was done in
the above noted patent to Wood et al. For example, in U.S.
Patent 5,266,237 to Freeman et al and Australian Patent
646728 published 1994-03-03, the polysuccinimide is added
directly to other ingredients to form a detergent
composition. The color of the polysuccinimide, particularly
in detergent applications, is desirably white. Thus the
method of decolorizing the water solution of the salt is not
useful in preparing desirable compositions of the above-
noted patent to Freeman et al which incorporate the
succinimide. Accordingly, there is needed a convenient
process for the production of polysuccinimide which has
acceptable color for detergent applications without the need
for decolorization. More convenient liquid media are needed
for large scale production of the polysuccinimide which has
very little color.
BRIEF DESCRIPTION OF THE INVENTION
In accordance with one embodiment of the present
invention there is provided a process for preparing
polysuccinimide by the thermal condensation of L-aspartic
acid which comprises providing a high boiling organic
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reaction medium selected from the group consisting of high
boiling straight or branched chain alkyl alcohols having
from 7 to 14 carbon atoms and mixtures thereof and straight
or branched chain alkanes having from 10 to 20 carbon atoms
and mixtures thereof, heating the amino acid in the medium
for a sufficient time to polymerize the amino acid.
In accordance with another embodiment of the
present invention there is provided a process for preparing
polysuccinimide by the thermal condensation of L-aspartic
acid which comprises providing a high boiling organic
reaction medium selected from the group consisting of high
boiling straight or branched chain alkyl alcohols having
from 7 to 14 carbon atoms and mixtures thereof and straight
or branched chain alkanes having from 10 to 20 carbon atoms
and mixtures thereof and an acid catalyst for the
condensation reaction, heating the acid in the medium in the
presence of the catalyst for a sufficient time to polymerize
said amino acid.
In accordance with a further embodiment of the
present invention there is provided a process for the
production of polysuccinimide by the thermal condensation of
L-aspartic acid in the presence of phosphoric acid catalyst
in an organic medium comprising dodecane at a temperature in
the range of from about 140°C to about 200°C for a period of
from about 1 to about 2 hours.
There has been discovered a process for the
polymerization of L-aspartic acid wherein the L-aspartic
acid is dispersed in a liquid medium
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comprising an organic medium selected from the group
consisting of high boiling alkyl alcohols and alkanes
or mixtures thereof. The term ~~high boiling~~ as used
in this application and claims means a liquid having a
boiling point at atmospheric pressure above the
reaction temperature for the thermal condensation of
L-aspartic acid. In operation, the L-aspartic acid
is slurried in the organic medium. The optional
catalyst such as phosphoric acid can be added after
the slurry is formed, preferred, or dispersed evenly
throughout the monomer prior to being added to the
organic liquid. The slurry is heated within the range
of from about 140°C .to about 260°C with agitation
sufficient to prevent the solids from congealing. As
the reaction proceeds, water, which is insoluble in
the organic medium of this invention, is removed from
the mixture by distillation. Reaction times range
from about 1 to about 5 hours. The reaction product
is a solid which is easily removed from the reaction
medium by conventional means such as filtration, etc.
The liquid medium can be recycled for preparation of
additional amounts of polysuccinimide.
In most instances the polysuccinimide
prepared in accordance with this invention is
hydrolyzed in basic medium to provide polyaspartic
acid which has many utilities as noted above. The
hydrolysis is typically carried out in situ, without
removing the polysuccinimide from the organic medium
by adding an aqueous 5-20o solution of sodium
hydroxide at a temperature of <100°C for a period
sufficient to hydrolyze the polymer thereby producing
the sodium salt of the acid. The hydrolyzed product
will remain in the aqueous phase and is easily
separated from the immiscible organic medium by
decantation or any suitable means for liquid-liquid
separation.
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. DETArT,FD DESCRIp'T'T~N OF THE INVENTTnt~
The organic medium of this invention is
selected from the group consisting of high boiling
alkyl alcohols and alkanes. The high boiling alcohols
are those having from 7 to 14 carbon atoms and can be
straight or branched chain. High boiling alkanes are
straight or branched chain alkanes having from 10 to
20 carbon atoms.
Typical examples of high boiling alkyl
alcohols are heptanol, octanol, decanol, isodecanol,
dodecanol and tridecanol and tetradecanol and mixtures
thereof. Because of its boiling range and ease of
separation, isodecanol is preferred.
Typical examples of high boiling alkanes are
decane, dodecane, tridecane, tetradecane, pentadecane,
hexadecane, octadecane and eicosane. A commercially
available mixture of high boiling alkanes is sold
under the trade name Therminol D12° by Monsanto
Company, St. Louis MO. Therminol D12 is a mixture of
synthetic hydrocarbons having a liquid range of -48°C
to 260°C. The straight chain alkanes, particularly
dodecane, are preferred.
The high boiling alkanes are preferred in
the process of this invention because of their
relative inactivity with respect to the acid catalyst,
phosphoric acid. In practice, it has been found that
the high boiling alcohols require purification to
remove color from the medium more frequently than is
the case with high boiling alkanes. Also, losses of
the alcohol in the range of about twenty percent due
to reactivity with the acid catalyst are expenses
which are not encountered in the use of high boiling
alkanes. High boiling alkanes are relatively non-
reactive with respect to phosphoric acid and the other
reactants in the process of this invention.
Furthermore, it has been found that the high boiling
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alkanes can be reused repeatedly without noticeable
. degradation or need of a special purification step.
One of the most important aspects of the
process of this invention is the use of a uniform
mixture of the starting acid in the reaction medium,
particularly if a catalyst is employed. It has been
found desirable to provide agitation in the reaction
medium, particularly during the early phase of the -
reaction. The starting acid has a tendency to become
tacky during the polymerization reaction causing the
particulate material to congeal into large particles
which impede the progress of the reaction. This
condition can be avoided by employing adequate
agitation, particularly during the early phase of the
reaction. In a preferred embodiment, the reaction
medium is subjected to agitation during the reaction
to maintain the solids in the reaction medium
dispersed in the liquid medium as much as possible.
When a catalyst is employed, such as the
well known phosphoric acid, it is important that the
starting acid and catalyst be thoroughly and uniformly
mixed. This can be achieved by combining the acid
catalyst with the starting acid before being added to
the organic reaction medium of this invention.
Preferably, the acid catalyst is added to the organic
medium directly and then thoroughly mixed so as to
achieve uniform dispersal of the catalyst.
One result of inadequate mixing of the
reaction medium is an increase in the color of the
final product. As noted above, one of the desired
results of any process for producing polysuccinimide
is to provide a product having a color as close to
white as possible. If the catalyst is not uniformly
mixed with the starting acid or if a uniform
suspension in the organic medium is not maintained,
some of the material will be polymerized and will
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remain in the medium, at elevated temperature, longer
than desired while the remainder of the starting acid
is being polymerized. However, a uniform mixture of
catalyst with the starting acid and a dispersed
uniform slurry in the organic reaction medium,
typically achieved by means of agitation, provides the
optimum process conditions.
The most widely used catalyst is phosphoric
acid, employed at a concentration in the range of from -
about 0.1 percent to about 40 percent, by weight of
starting acid. In one embodiment, 85o phosphoric acid
is first blended thoroughly with the starting acid.
While any means which achieves such mixing may be
employed, the typical means is a centrifugal mixer
such as that commercially available under the trade
name Turbulizer. The dry particulate acid, such as
aspartic acid, is fed to the mixer while being
contacted with an aqueous solution of the acid
catalyst. The starting acid, with catalyst, is then
introduced into a reactor containing the organic
reaction medium of this invention directly from the
mixer. In a more preferred embodiment, the acid
catalyst, typically phosphoric acid is combined with
the organic medium after the addition of starting
acid. Thorough mixing of the acid in the medium
assures the uniform catalytic action on the starting
acid thereby allowing uniform conversion of the
starting acid to the polymeric product. This results
in holding the polymer at reaction temperature the
shortest possible time thereby preventing degradation
and consequent increase in the color of the polymer.
The condensation reaction of L-aspartic acid
to form polysuccinimide takes place in the range of ,,
from about 140°C to about 200°C, and optimally in the
range of from 160°C to about 180°C. Such temperature
range provides for adequately rapid polymerization so
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that the reaction time is reasonably short, i.e, the
optimum balance between completion of the reaction and
limited exposure of the reaction product to high
temperature. As noted above a short reaction time at
elevated temperature favors the production of light
colored material while holding the polymer at reaction
temperature for an extended period of time increases
the,amount of color in the product and darkens it.
Reaction time in the range of from about 1 to 2 hours,
preferably 1.5 hours, is usually sufficient to provide
maximum conversion of the starting acid while not
allowing excessive color to develop in the product.
However, as also observed, the molecular weight of the
polymer is increased with increase in reaction time in
the temperature range noted above.
Because color is an important property of
polysuccinimide, reflectance measurements by standard
means provide an important guide to optimum reaction
conditions. It has been found that the color (as
measured by reflectance) of the polymer product of the
process of this invention is superior to the color of
the polymer product produced by the comparable thermal
processes conducted in various dryer means typical of
the prior art, i.e., tray dryers, ovens and rotary
dryers. Reflectance measurement of a typical starting
acid, L-aspartic acid, indicates a reflectance of
white light of about 96%. Polymer produced by
typical, comparable prior art processes indicates a
reflectance of white light in the range of from about
40% to about 55%. Surprisingly, polysuccinimide
product produced in accordance with the process of the
present invention indicates a reflectance of white
A
light in the range of from about 80o to above 90% as
will be shown by the experimental data provided below.
35- As noted above, it is advantageous to employ
an acid catalyst in the process of this invention and
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the most typical of such acids is phosphoric acid.
While uniformity of admixture with the starting acid
provides the most optimum condition in this regard, it
has been found that in some instances wherein there is
5- observed only a partial completion of the reaction
after raising the slurry in the reactor to reaction
temperature (about 140°C), there has been non-uniform
admixture of the catalyst thereby allowing rapid
condensation reaction of only a portion of the
starting acid. It has been found that, to avoid such
conditions, the catalyst is added in two portions.
The first portion of acid catalyst is added before
raising the reaction mass to the reaction temperature.
Then, a second addition of acid catalyst to the
reaction slurry after achieving reaction temperature
provides an increase in the reaction rate thereby
completing the reaction more quickly and without
unduly long exposure of the polysuccinimide in the
reactor to elevated temperature. For example, in the
use of phosphoric acid as a catalyst, an additional
amount of catalyst up to that equal to the initial
amount charged to the reactor will quickly increase
the reaction rate. Such increase in reaction rate
decreases the time required for completion of the
condensation reaction and shortens the time of
exposure of the reaction product to the elevated
temperature required for the reaction. Improved color
and more uniform particle size of the resulting
product are observed in processes wherein the
phosphoric acid catalyst is added in at least two
steps. -
To further illustrate the process of the
present invention there is described below non-
limiting preferred embodiments. In the following
examples percentage values are percent by weight
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unless otherwise specified. All reactions were
r conducted under a nitrogen atmosphere.
RXAMPT~R 1
A series of runs were conducted employing a
L-aspartic acid with phosphoric acid catalyst.
Approximately 80 g of L-aspartic acid was employed in
. each run in approximately 400 g of isodecanol. The
catalyst was added to the system directly to the
reaction medium. The reaction vessel was a round
bottom flask containing isodecanol as the organic
medium and was fitted with a heating mantle and a
condenser. The reaction time was measured from the
time the reaction medium reaches approximately 140°C
until termination by removal from the heat source.
Molecular weight is an average of two measurements as
determined by the GPC method.
TABLE 1
Run % Temp. ReactionMolecular
2 0 No. Catalyst Range ~C Time Weight
1 149.9 - 160 58 min. 9296
2 21 159 -164.5 85 min. 9817
3 " 140.5 -163.8 210 min.
4 " 137.8 - 180 150 min.11,993
2 5 5 42.5 135.2 -180 230 min.11,850
RXAMPT,R 2
The reaction medium employed in Runs 1-5 of
Example 1 above was distilled to recover purified
30 isodecanol for reuse. A portion, 276.2 g was
distilled at a temperature in the range of from
209.6°C to 218.3°C over a 1 hour period. A residue of
10.9 g was left in the pot while 204.8 g of isodecanol
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and 51.6 g of water were recovered. This represented
91.1 percent recovery of the organic medium for reuse.
y
EXAMPLE 3
In this example the L-aspartic acid was
added to the reactor step-wise to increase the amount
in the reaction medium. Into a reactor were added
400.53 g of isodecanol, 40 g of 85o phosphoric acid
with stirring. The mixture was heated to 160°C at
which time 80 g of L-aspartic acid was added. With
continued heating for 40 minutes at 160°C, a second
portion, 20 g of L-aspartic, was added. Thirty
minutes later an additional 20 g of L-aspartic acid
was added to the reaction medium while maintaining a
reaction temperature of 160°C. The reaction continued
for an additional 3 hrs. 15 minutes for a total
reaction time of about 4.5 hrs. The reaction provided
a yield of 960 of theoretical polysuccinimide having a
molecular weight of 10,694 and exhibiting a highly
acceptable light color.
EXAMPLE 4
Into a reactor was charged 403.5 g of
isodecanol and 20 g of 85o phosphoric acid. This
charge was heated to 95°C and a charge of 120 g of L-
aspartic acid was made to the reactor. The
temperature was raised to 140°C in 30 minutes and then
raised to 200°C gradually over a period of about 3 hrs
then held at that temperature for an additional hour.
Large crystalline product was removed from the
reaction medium by filtration to yield 94 g of
polysuccinimide having a molecular weight of 13,264.
The product represented a 100% conversion of the
starting acid to polymer.
EXAMPLE 5
Into a reactor equipped with heating mantle
and a standard agitator~configuration was placed 370 g
of dodecane and 150 g of L-aspartic acid. The mixture
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was agitated to provide a uniform mixture and then
12.2 g of 85% phosphoric acid was added. The reaction
temperature was held at 180°C and was 100a complete
after 90 - 100 minutes. To the reactor was added 343
g of a 15o aqueous solution of sodium hydroxide. The
mixture was held at 50-70°C for 30 minutes with
agitation to hydrolyze the polymer to polyaspartic
acid. The polymer remained in the aqueous phase and
was recovered by decanting the organic medium thereby
leaving an aqueous solution of polyasparatic acid.
EXAMPLE 6
The polysuccinimide produced in Examples 1-4
above was subjected to light reflectance measurements
by means of a Hunter Labscan spectrophotometer. A
white porcelain plate was measured to establish a
reference reading. The parameters for the
reflectance measured at different wave lengths for
these readings are as follows:
Black=0 White=100 R2
green=-8 red=+8 (a)
blue=-28 yellow=+28 (b)
The results of the reflectance measurements are
present in Table 2 below.
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TABLE 2
Sample Rd (a) (b)
Standard 85.31 0.98 -0.19
L-aspartic
acid 96.15 .50 2.39
Run 1 91.57 -1.53 7.08
Run 2 86.0 -0.88 10.98
Run 3 94.42 -1.96 6.49
Run 4 88.92 -0.91 9.51
Run 5 88.47 .016 6.83
Ex. 3 91.2 -1.89 10.5
Ex. 4 77.09 -1.891 9.68
Ex. 5 86.25 -2.10 11.16
Prior
art* 52.48 4.56 21.06
Prior
art** 43.09 8.56 25.23
* Product of a process as described U.S. Patent
in
5,319,145 but phosphoric acid catalyst.
with about 20o
** Product of a process as described U.S. Patent
in
5,319,145 with no catalyst.
EXAMPLE 7
Into a reactor equ ipped with stirrer there
a
was charged 379.8 ecane and 0.3 g of L-
g of hexad 15
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aspartic acid. The acid and alkane were mixed well
and then 26.5 g of 85o phosphoric acid was added
slowly in two separate charges over 50 min. The
reaction slurry was heated to 180°C with agitation and
the slurry held for an additional 1 hour for a total
reaction time of 110 min. The product was composed of
large particles which filtered easily to provide 149.5
g of polysuccinimide exhibiting very light color. The
molecular weight of the product was 11,350. The
reaction medium remained nearly colorless and was
easily recovered and was found to have a neutral pH of -
7. The hexadecane was easily recovered and reused
without need for purification.
EXAMPLE 8
The process of Example 5 was repeated except
that the phosphoric acid catalyst was added in two
stages_ Initially, 4.2 g of phosphoric acid and 60 g
of L-aspartic were added to the dodecane. The slurry
was heated to 150°C until the solids agglomerated
slightly. Then, and additional 8.6 g of 85%
phosphoric acid was added and the temperature of the
slurry raised to 190°C for an additional 30 minutes
for a total reaction time of about 90 min. A pale
yellow polymer was recovered from the reaction mixture
by filtration having a molecular weight of 17,640.
The yield of polymer was 41 g.
EXAMPLE 9
To an Ace reactor was added 374.2 g of
dodecane which was heated to 107.3 C. A pre-mix of
108_4 g of L-aspartic acid containing 7 g of 850
phosphoric acid which was ground to a fine powder was
then added. In this reaction the reaction medium was
subjected to agitation which maintained the solids in
the reactor in a substantially dispersed condition
throughout the organic reaction medium. The slurry
was heated gradually to 180°C over a period of 50
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minutes. After a reaction time of 35 minutes a second
addition of 7 g of 85% phosphoric acid was made which
appeared to increase the reaction rate by a rapid
increase in the release of water from the reaction
mixture. The reaction was discontinued after a total
reaction time above 140°C of 1 hr. 25 minutes. The
reaction mixture was held at 180°C for a period of 45
minutes. Analysis of the resulting polysuccinimide
indicated nearly 100 percent conversion and a
molecular weight of 12,993. The dried product color
was slightly off white.
There has been described above a convenient,
economical method for producing polysuccinimide having
an acceptable color for use directly into such
utilities as detergent and other uses where light
color is required. The high boiling organic medium of
this invention has been shown to be reusable, in many
instances without purification. When purification is
required it is conveniently accomplished by simple
distillation thereby providing an efficient means to
provide light colored polysuccinimide not heretofore
conveniently obtainable in large quantities in an
industrial process.
Although the invention has been
described in terms of specific embodiments which are
set forth in considerable detail, it should be
understood that this description is by way of
illustration only and that the invention is not
necessarily limited thereto since alternative
embodiments and operating techniques will become
apparent to those skilled in the art in view of the
disclosure. Accordingly, modifications are
contemplated which can be made without departing from ,.
the spirit of the described invention.
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EXAMPLE 10
Into a reactor equipped with a stirrer,
heating mantle and means to provide a nitrogen purge
_ there was charged 453.3 g of hexadecane. After
raising the temperature of the hexadecane to about
185°C, 199.6 g of L-aspartic acid was added. The acid
and alkane were mixed well and heated over a period of
about 30 minutes to a temperature of 200°C at which
time water began to be released rapidly from the
mixture. The reaction slurry was held at about 200°C
for about 1 hr. and 40 minutes at which time water
release had reduced markedly. A sample indicated that
conversion of the starting acid to polymer was about
42o at that time. The temperature was then raised to
220°C and held at that temperature for an additional 1
hr. and 20 minutes which provided 1000 conversion of
the starting acid to polymer. The entire reaction was
run with constant agitation. The product, 143 g of
polysuccinimide, was composed of fine particles
exhibiting a pink color. The reaction medium remained
nearly colorless and about 419 g of the alkane was
easily recovered by filtration.
