Note: Descriptions are shown in the official language in which they were submitted.
A PROCESS FOR DISTILLING
A 2-ISOCYANATOALKYL ESTER
OF AN ~,~-ETHYLENICALLY
UNSATURATED CARBOXYLIC ACID
This invention relates to the use of ga~eous
nitrogen dioxide or nitric oxide to inhibit the vinyl
: polymerization of a 2-isocyanatoalkyl ester of an
: N, ~-ethylenically unsaturated carboxylic acid.
: :
Certain conventional polymerization inhibitors
have been used to inhibit during distillation the vinyl
polymerization of a 2-isocyanatoalkyl ester of an
ethylenically unsaturated carboxylic acid. For
example, Europeani Patent Office Application Nv. 78190156.5,
: I0 Publication No. 144,:published January 10, 1979, discloses
: the use of phenothiazine to inhibit the polymerization
of 2-lsocyanatoethyl methacrylate during distillation.
While conventional polymerization inhibitors
:are generally effective in inhibiting polymerization of
2:-isocyanatoalkyl esters during storage, they have not
proven as effective during the distillation of the iso-
; : cyanatoal]~yl ester. In particular, these isocyanatoalkylesters are susceptible to the formation of a hard,
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brittle, highly cross~linked polymer, which is referred
to as popcorn polymer in the prior art because of its
physical appearance. This popcorn polymer is especially
deleterious, because once formed it tends to initiate
further polymerization.
Relatively volatile inhibitors, such as p-meth-
oxyphenol, have been employed during the distillation of
2-isocyanatoalkyl methacrylate to inhibit polymer forma-
tion in the gas phase and in the distillate. However,
these volatile inhibitors have been observed to be rela-
tively ineffective as polymerization inhibitors in this
application.
Nitrogen oxides are known in the art to inhibit
the polymerization of certain unsaturated compounds
U.S. Patent 3,964,978 discloses the distillation of vinyl
aromatic compounds in the presence of nitrogen dioxide to
inhibit polymerization. U.S. Patent 3,964,979 teaches
; the use of nitric oxide to inhibit the polymerization of
vinyl aromatic compounds during distillation. British
Patent 1,265,419 teaches that acrylic acid can be distilled
in the presence of nitric oxide in the gas phase and
phenothiazine in the li~uid phase to minimize polymeriza-
tion. Eowever, the presence o~ nitrogen dioxide is
taught to promote the polymerization of acrylic acid and
to discolor the distillate.
Nitric oxide is disclosed by J. F. Villa and
H. B. Powellj Syn. React. Inorg. Metal-Org. Chem., 6, pp.
59-63 (1976), to catalyze the trimerization of certain
aliphatic isocyanates. This disclosure suggests that
nitric oxide would not be suitable as a polymerization
inhibitor for a 2-isocyanatoalkyl ester.
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This invention is a process for distilling a
2-isocyanatoalkyl ester of an ~ ethylenically unsatu-
rated carboxylic acid from a liquid mixture, character~
ized by distilling the isocyanatoalkyl ester in the
presence of an amount of gaseous nitrogen oxide effective
to inhibit the vinyl polymeriz.ation of the isocyanato-
alkyl ester. An effective amount of the nitrogen oxide
is an amount which reduces the polymerization of the
isocyanatoalkyl ester compared to the polymerization
which occurs at the same conditions in the absence of
the nitrogen o~ide.
The 2-isocyanatoalkyl esters of ~ ethyleni-
cally unsaturated carboxylic acids form a known class
of compounds, which can be represented by the formula I
O ~R~
, ~
R'-C-O- C-~N=C=O
R
n
wherein each R is indep~ndently hydrogen, alkyl, alkenyl,
alkoxy, alkaryl, aral~yl or aryli R' is alkenyl; and n
is 2 or 3. of course, R can represent a wide ~ariety
of moieties, such as, methyl, ethyl, cyclohexyl, isopro-
penyl, vinyl, etho~y, tolyl, phenylethyl or phenyl.
Preferably, R is hydrogen and n is 2. Preferably, R'
is vinyl or isopropenyl, more preferably isopropenyl.
;~ Hereafter, the compound of formula I will be referred
to as an isocyanatoalkyl ester for the sake of brevity.
The isocyanatoalkyl ester can be present in
~; 30 any mixture from which it can be separated by distilla-
tion, so long as said mixture is substantially inert in
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the subject reaction. In order to conserve the nitrogen
oxides added, diluents which may react with the nitrogen
oxides are advantageously avoided. In one preferred
embodiment, the isocyanatoalkyl ester to be distilled
is prepared by reacting a 2-alkenyl-2-oxazoline or
2-alkenyl-2-oxazine in a water-immiscible solvent with
phosgene in the presence of an aqueous hydrogen chloride
acceptor, as is described in British Patent 1,252,099.
On completion of the phosgenation reaction, the organic
phase containing the 2-isocyanatoalkyl ester is conven-
iently separated and optionally dried with a conventional
drying agent, such as CaCl2 or zeolite. The 2-isocyanato-
alkyl ester of the unsaturated acid is then separated
by distillation in the presence of a nitrogen oxide, as
is disclosed hereinafter.
Nitrogen dioxide and nitric oxide, which are
used as vinyl polymerization inhibitors herein, are
both well-known compounds. For the sake of brevity
these two polymerization inhibitors are hereafter
referred to as nitrogen oxides. Either one of these
nitrogen oxides, a mixture of such oxides or a diluent
gas containing at least one of these oxides of nitrogen
can be employed to inhibit polymerization. Mitric
oxide is preferred as a polymerization inhi~itor ~ecause
it adds fewer colored impurities to the typically
colorless distilled isocyanatoalkyl ester than the
other nitrogen oxides. Surprisingly, no isocyanate
trimer is observed in the isocyanatoalkyl ester follow-
ing treatment with nitric oxide. Any gaseous diluent
used with the nitrogen oxides should be inert in the
instant reaction. Preferably, the diluent gas is sub-
stantially free of oxygen and water, more preferably
the diluent gas, if present, is nitrogen.
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The nitrogen oxide can operably be introduced
in the region immediately above the liquid mass to be
distilled to inhibit polymerization in the gas phase
and in the liquid which condenses overhead. However,
care must be taken in this embodiment to insure that
the nitrogen oxide permeates the space above the medium
or polymerization can occur. Of course, where a nitro-
gen oxide is not introduced to the liquid medium, a
polymerization inhibitor should al50 be employed in the
liquid. However, some of the oxides of nitrogen inter~
act with other polymerization inhibitors to produce
colored impurities. For example, p-methoxyphenol
reacts with nitric oxide to produce a yellow-colored
impurity, which will codistill with the isocyanatoalkyl
lS ester, Less volatile polymerization inhibitors, such
~ as phenothiazine, may also form colored impurities, but
- ~ these colored impurities typically will not distill
with the isocyanatoalkyl ester. It is preferred,
therefore, to sparge the nitrogèn oxide through the
li~uid mixture during distillation and to use a polym-
erization inhibitor in the mixture which is signifi-
cantly less volatile than the isocyanatoalkyl ester.
~: :
If the distillation is conducted in a continu-
,~
~ ous process, the nitrogen oxide can conveniently be
~: :
added to the incoming mixture containing the isocyanato-
alkyl ester. The gaseous nitrogen oxides are virtually
insoluble in the isocyanatoalkyl ester and these gases
can be readily recovered and recycled. Generally, the
inhibitor is gradually depleted during operation, but
wlth recycle the intermittent replacement of only small
amounts of the oxides o~ nitrogen is generally necessary.
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The concentration of nitrogen oxides necessary
to inhiblt polymerization varies over a wide range
dependent upon numerous factors, including the identity
of the nitrogen oxide and the isocyanatoalkyl ester,
the distlllation temperature, the distillation pressure,
residence time and the amount of reflux. Even the
purity of the isocyanatoalkyl ester af~ects the forma-
tion of polymers, as a crude starting material appears
in general less susceptible to the formation of popcorn
polymer during distillation than a comparatively pure
isocyanatoalkyl ester. Generally, a concentration of
nitric oxide in the gas phase immediately above the
liquid medium of at least 0.01, preferably at least
0.02, more preferably at least 0.1 percent by weight,
is efective to inhibit polymerization. At or near th~
minimum effective concentration care must be taken to
see that the concentration is maintained uniformly
above the liquid medium or else polymerization can
occur. One convenient method distributing -the nitrogen
oxide uniformly in a large volume is to introduce the
nitrogen o~ide to the liquid in a diluent gas. The
upper limit on the concentration of nitrogen oxide is
; ~ determined primarily by economic considerations. The
~; concentration of nitrogen oxides is preferably less
than 20 percent, more preferably less than 3 percent by
weight of the gases above the liquid distilled. At
concentrations of nitrogen oxide greater than 2 percent
by weight the distillate may be visibly colored.
:`:
The manner in which the distillation is car-
ried out is not critical. The distillation is advanta-
; geously conducted at reduced pressure, so as to avoid
distillation at temperatures which are deleterious to
~` the isocyanatoalkyl ester. The distillation vessel
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should be closed, because some isocyanatoalkyl esters
are very toxic, as are the oxides of nitrogen. Impuri-
ties which are lower boiling than the isocyanatoalkyl
ester are conveniently distilled before the 1socyanato-
alkyl ester. Azeotropic mixtures can be created tomodify the order of distillation, if desired.
The temperature of the liquid during distil-
lation is preferably from 65C to 110C, more preferably
from 80~C to 95C. Higher temperatures than those in
the preferred range, though op~rable, can result in
poor separation and substantial decomposition or polym-
erization of the isocyanatoalkyl ester at long residence
times. Lower temperatures are not generally operable
to distill the isocyanatoalkyl ester, even when dist11-
lation is attempted under vacuum. Preferably, thepressure over the liquid to be distilled is from 5 to
15 millimeters of mercury (0.6-2.0 kPa~ during
distillation.
The foIlowing examples are presented to
illustrate the process of this invention, but are~ot
to be taken as limiting its scope. All parts and
percentages are by weight unless otherwise indicated.
.~
Example 1
To a round-bottom flask equipped with a
; 25 6 inch, straight tube distillation column topped with a
distillation head, a means for stirring and a means for
measuring temperature, was charged 121 grams of crude
2-i~ocyanatoethyl methacrylate (IEM) containing 82.2
percent IEM, 0.6 percent methylene chloride, 1080 parts
~ 30 per million (ppm) phenothiazine, less than 1 percent of
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an epoxy resin, and a remaining amount of relatively
non-volatile impurities resul-ting from the preparation
of IEM. The epoxy resin reacts during distillation
with the hydrolyzable chloride present to eliminate
this impurity from the distilla-te.
The flask was heated to 95C under a pressure
of lO mm of mercury (1.3 kPa), while the liquid was
sparged with 20 cubic centimeters per minute of a
nitrogen gas containing 0.8 percent nitric oxide. The
IEM was refluxed for 1.25 hours and then distilled for
2 hours. Assuming a constant rate of distillation, the
concentration of nitric oxide in the gas phase was 376
ppm.
,
The distillate recovered was analyzed by con-
ventional gas and liquid chromatographic techniques and
was found to be essentially pure monomeric IEM. The 85
grams of distillate recovered represented a yield of 70
pe~cent. The undistilled tars were clear fluids. No
popcorn polymer was observed.
Comparative_Experiment
In -the manner described in Example 1, 50
` grams of crude IEM was refluxed at 95C under a pressure
of 10 mm ~f mercury (1.3 kPa), except that no nitric
~ oxide was introduced. After 15 minutes of heating,
; 25 popcorn polymer was observed in the column and the
flask. After an additional hour, the entire contents
~; of the flask polymerized.
Exam~le 2
In a manner otherwise identical to Example 1,
pure nitric oxide was sparged through the crude IEM,
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while the IEM was first refluxed at 95C for 1 hour and
then distilled over a period of 2 hours. No popcorn
polymer was observed in either the distillate or the
undistilled material.
Example 3
In a manner otherwise similar to Example 2,
204.4 grams of crude IEM, containing 86.6 percent IEM
and 700 ppm pheno-thiazine, were refluxed at 90~C for 1
hour and then distilled. During reflux and distilla-
tion, a nitrogen gas stream containing 0.8 percentnitric oxide was introduced to the distillation vessel
immediately above the liquid medium. Two and one-half
hours after the reflux was initiated, all of the material
remaining in the flask was popcorn polymer. The distil-
late contained 112.5 grams of essentially pure IEM,representing a yield of 55 percent.
:
Example 4
In a manner similar to Example 1, 151 grams
of crude IEM, containing 81 percent IEM, 1000 ppm
phenothiazine and small amounts of methyIene chloride
~` and an epoxy resin, were refluxed at 92C under a
pressure of 9 millimeters of mercury, while a gaseous
mixture con-taining 0.16 percent nitxogen dioxide in
nitrogen was sparged through the liquid. The gas was
passed through the liquid at a rate of 168.8 cubic
centimeters per minu-te. A~ter 2 hours, distillation of
the gas-sparged liquid was initiated. Over a period of
3 hours, 85.6 grams of essentially pure IEM was recovered
in th distillate, representing a yield of 70 percent.
The undistilled residue remained fluid and no popcorn
polymer was produced in the system.
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