Process for the hydrolysis of poly(iminoimidazolidinediones) to polyparabanic acids

Processes for the efficient sulfuric acid hydrolysis of poly(iminoimidazolidinediones) (PIPA) to the corresponding polyparabanic acids (PPA) have been developed. Process conditions can be manipulated to favor the formation of large crystalline particles of ammonium sulfate, a byproduct of the reaction, so that they can be readily separated from the reaction solution.

BACKGROUND OF THE INVENTION 
It is pointed out in U.S. Pat. No. 3,661,859 that poly(parabanic acids) 
(PPA) are made by hydrolyzing poly(iminoimidazolidinediones) which are 
formed by the reaction of hydrogen cyanide with diisocyanates or by the 
reaction of dicyanoformamides with diisocyanates or by the polymerization 
of cyanoformamidyl isocyanates as fully set forth therein. 
The hydrolysis reaction can be carried out by contacting a solution of a 
poly(iminoimidazolidinedione) with an aqueous solution of a Bronsted acid 
such as hydrochloric, hydrobromic, sulfuric, nitric, and the like, or with 
anhydrous hydrogen chloride or hydrogen bromide such that when the polymer 
is mixed with water, hydrolysis of the imino groups will occur to convert 
the polymer to the desired poly(parabanic acid) which is characterized by 
1,3-imidazolidine-2,4,5-trione-1,3-diyl rings. 
The solution of polyiminoimidazolidinedione may be the polymerization 
solution in which it was prepared or it may be one which was prepared by 
dissolving the isolated polymer in a preferable solvent. 
When an aqueous solution of acid is added to a solution of the heterocyclic 
polymer characterized by imino-1,3-imidazolidinedione rings, the extent of 
hydrolysis may be controlled by the quantity of acid used. Complete 
hydrolysis requires a mole of hydrogen ion per mole of imino group to be 
hydrolyzed. The heterocyclic polymers before and after hydrolysis are both 
insoluble in water; therefore, the quantity of water used to dilute the 
acid must be limited so that precipitation of polymer does not occur. 
To prevent precipitation of the polymer by localized high concentrations of 
water during the addition of the aqueous acid to the polymer solution, the 
aqueous acid may be first diluted with the same solvent in which the 
polymer is dissolved and this mixture then added to the polymer solution. 
Hydrolysis occurs rapidly in the presence of an excess amount of acid and 
may be complete within a few minutes at room temperature. The general 
temperature conditions for hydrolysis are between 10.degree. and 
120.degree. C., preferably between 30.degree. to 60.degree. C. The 
hydrolysis conditions may be maintained for a few minutes or for several 
hours. (See U.S. Pat. No. 3,661,859.) 
Thus, poly(iminoimidazolidinediones) are readily converted to 
poly(parabanic acids) by acid hydrolysis. 
##STR1## 
SUMMARY OF THE INVENTION 
Processes for hydrolyzing poly(iminoimidazolidinediones) to poly(parabanic 
acids) using stoichiometric quantities of sulfuric acid and conditions 
which promote the formation of large crystals of ammonium sulfate, a 
by-product of the hydrolysis reaction, are described. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Sulfuric acid is the preferred acid to use because ammonium sulfate is less 
soluble and thus more easily separated from the polymer solution than the 
ammonium salts of other acids such as hydrochloric, hydrobromic and nitric 
in dimethylformamide, dimethylacetamide, dimethylsulfoxide, and N-methyl 
pyrrolidone and other dipolar aprotic solvents. 
The reaction is: 
##STR2## 
Stoichiometrically, only 0.5 mole of sulfuric acid and 1 mole of water per 
mole of imino group in the repeating unit of the polymer chain is required 
for complete hydrolysis. However, at ambient and up to 60.degree. C. 
reaction temperatures during hydrolysis, it was necessary to use at least 
one mole of sulfuric acid per mole of imino group to hydrolyze all of the 
imino groups present. 
It was observed that when clarified (by centrifugation) reaction solutions 
of PPA-M which contained dissolved ammonium bisulfate were gently heated, 
ammonium sulfate separated from solution. As the temperature was increased 
to 100.degree. C., the rate of separation increased. The salt remained 
insoluble after the solutions had cooled to room temperature. It was also 
observed that the quantity of insoluble ammonium sulfate which separated 
from polyparabanic acid reaction solutions at room temperature increased 
slowly over a period of several weeks. 
This observation was extended to a clear saturated solution of ammonium 
bisulfate in DMF. It was found that ammonium sulfate formed and separated 
from the solution at a rate which increased rapidly as the temperature was 
increased. In addition, the pH of the solution decreased from about 1.2 to 
1.05 (measured after cooling to 25.degree. C.). Thus, it became obvious 
that the complete hydrolysis of PIPA to PPA using only 0.5 mole of 
sulfuric acid per mole of imino group should be possible at elevated 
temperatures. It is probably due in large part to the increased rate at 
this higher temperature of the following reaction which converts the 
bisulfate salt to ammonium sulfate and the strongly acidic sulfuric acid: 
##STR3## 
Apparently, ammonium bisulfate is not acidic enough in this environment to 
hydrolyze the imino groups. 
Conditions have been found wherein complete hydrolysis occurs when only 0.5 
mole of sulfuric acid per mole of imino group is used in the reaction. By 
using a solution which contains at least 0.50 mole, preferably 0.5 to 0.8, 
most preferably 0.502-0.506 mole of sulfuric acid and 1-20 moles, 
preferably 4-10 moles of water per mole of imino group, complete 
hydrolysis of poly(iminoimidazolidinedione) to poly(parabanic acid) occurs 
if the polymer solution is heated at 80.degree.-110.degree. C., preferably 
85.degree.-90.degree. C., for 5-120 minutes, preferably 15-20 minutes. At 
this temperature, the initially formed ammonium bisulfate is converted to 
sulfuric acid and the less soluble ammonium sulfate. The sulfuric acid is 
then available for further reaction with another imino group. This 
continues until the sulfuric acid is depleted or all imino groups are 
hydrolyzed. 
Since most of the ammonium sulfate precipitates from solution, it can be 
removed by filtration or centrifugation. In addition to other factors, 
such as solution viscosity and filter porosity, the rate of clarification 
by centrifugation and filtration is dependent on the size of the ammonium 
sulfate crystals. 
It has been found that the size of the ammonium sulfate crystals is 
dependent upon the quantity of water used to hydrolyze the 
poly(iminoimidazoldienedione) and upon the procedure used to add the 
sulfuric acid and water to the polymer in organic solvent solution. The 
polymer solution which is to be mixed with the acid and water normally 
contains 5 to 15 grams of polymer per 100 ml. of solution, although 
solutions containing less than 5 grams or more than 15 grams of polymer 
per 100 ml. may be used. 
The dramatic effect of the quantity of water on the average crystal size of 
the ammonium sulfate formed during the hydrolysis reaction is revealed by 
the results described in Examples 10-15. The crystals were very small when 
the concentration of water was low. 
Several procedures for adding the acid and water to a 
poly(iminoimidazolidinedione) solution have been used to hydrolyze the 
polymer to poly(parabanic acid). Insofar as the desired polymer reaction 
is concerned, all of the procedures give equally satisfactory results. 
However, the average sizes of the ammonium sulfate crystals formed during 
the reactions are not the same by all of the procedures. The procedures 
are identical with regard to the relative quantities of sulfuric acid and 
water used, the temperature (80.degree.-120.degree. C., preferably 
85.degree.-95.degree. C.) and time (10-60 minutes, preferably 15-20 
minutes) of reaction. The procedures differ from one another by the 
technique used to add the sulfuric acid and water to the polymer solution. 
The following procedures for adding the acid and water to solutions of 
poly(iminoimidazolidinediones) in dimethylformamide, N-methylpyrrolidone, 
N,N-dimethylacetamide, dimethylsulfoxide, or other suitable solvents may 
be used. 
(A) An acid solution containing sulfuric acid, water and the solvent in 
which the polymer is dissolved is added to the 
poly(iminomidazolidinedione) solution. The temperature of the polymer 
solution may be 20.degree.-100.degree. C., preferably 
50.degree.-80.degree. C. Although the temperature of the acid solution may 
be as high as 120.degree. C., it is preferable that it be no higher than 
10.degree. C. if relatively large, easily filtered crystals of ammonium 
sulfate are to be formed. (See Examples 16-22.) 
(B) The required quantity of 90-100%, preferably 95-97%, sulfuric acid is 
added to the poly(iminoimidazolidinedione) solution; then a solution of 
the water in the polymer solvent is added to the solution (Examples 23 and 
24). Initially the temperature of the polymer solution has a temperature 
of 20.degree.-100.degree. C., preferably 40.degree.-80.degree. C., while 
the temperature of the acid is conveniently 15.degree.-30.degree. C., and 
that of the aqueous dimethylformamide solution is 5.degree.-100.degree. 
C., preferably 15.degree.-50.degree. C. 
(C) An aqueous solution of sulfuric acid is added to the 
poly(iminoimidazolidinedione) solution (Example 25). Since the acid 
solution is not diluted with the polymer solvent, it is necessary to 
control the rate of addition and to provide adequate agitation of the 
polymer solution so that the aqueous solution does not precipitate 
polymer. The temperature of the acid solution may be 
10.degree.-100.degree. C., but preferably is 15.degree.-50.degree. C. 
(D) The calculated quantity of 96% sulfuric acid is added to the 
poly(iminoimidazolidinedione) solution; then the water required to 
complete the hydrolysis is added at a rate and with adequate agitation of 
the polymer solution so that precipitation of the polymer does not occur 
(Example 26). The initial temperature of the polymer solution may be 
20.degree.-100.degree. C., preferably 50.degree.-80.degree. C. The 
temperature of the acid and water are conveniently ambient, usually 
15.degree.-30.degree. C. 
The average sizes of ammonium sulfate crystals formed in the hydrolysis 
reaction are acceptably large if procedures B, C, and D are used; 
procedure A may be used satisfactorily if the temperature of the acid 
solution is 10.degree. C. or lower. Procedure D is preferred because it 
does not require additional solvent, it is not necessary to prepare an 
acid solution and cool it, and it utilizes the heats of solutions of the 
acid and water in each other and in the polymer solvent to increase the 
temperature of the polymer solution toward the desired range. Experiments 
in which each of these procedures have been used are described in the 
Examples of this disclosure.

EXAMPLE 1 
A solution of poly(iminoimidazolidinedione) was prepared by the reaction of 
125.0 gms. (0.50 mole) of diphenylmethane diisocyanate with 13.9 gms. 
(0.51 mole) of hydrogen cyanide in 1160 ml. of dimethylformamide. The 
reaction was catalyzed by 0.08 gm. of sodium cyanide in 20 ml. of 
dimethylformamide. After 30 minutes, the polymerization was complete. A 
solution of 54.1 gms. (0.53 mole) of 96% sulfuric acid, 36.0 gms. (2 
moles) of water, and 36 ml. dimethylformamide was then added to the 
solution. An exothermic reaction ensued and the temperature increased from 
36.degree. C. up to 45.degree. C. After 30 minutes the reaction solution 
was filtered and poured into water to precipitate the poly(parabanic acid) 
which had an inherent viscosity of 0.92. 
In this example, the equivalent of 1.06 moles of sulfuric acid per mole of 
imino group (which is equivalent to the moles of diphenylmethane 
diisocyanate used in the polymerization) was used for the hydrolysis. The 
infrared spectrum of the product showed that it was the desired 
poly(parabanic acid); there was no evidence of bands at 3.06.mu. (NH) and 
5.98.mu. (C=N) which characterize the iminoimidazolidinedione ring in the 
poly(iminoimidazolidinedione) which was hydrolyzed. 
EXAMPLE 2 
This example is identical to Example 1, except that the equivalent of only 
0.56 moles of sulfuric acid per mole of imino group was used for the 
hydrolysis. The infrared spectrum of the polymer product indicated that 
some of the imino groups had not been hydrolyzed. In addition to the 
absorption peaks which were characteristic of the poly(parabanic acid), 
there were absorption peaks at 3.04 and 5.98.mu. which are characteristic 
of the N--H and C=N groups which are in the poly(iminoimidazolidinedione). 
These results show that under these reaction conditions (maximum 
temperature of 45.degree. C.), poly(iminoimidazolidinedione) was not 
completely hydrolyzed to the corresponding poly(parabanic acid) by the use 
of only 0.56 mole of sulfuric acid per mole of imino group. 
EXAMPLE 3 
In this example, exactly one mole of sulfuric acid per mole of imino group 
in the poly(iminoimidazolidinedione) solution was used for the hydrolysis. 
This illustrates the necessity of the use of an excess of sulfuric acid in 
the hydrolysis reaction when the reaction temperature is no higher than 
about 50.degree. C. 
The polymerization in Example 1 was repeated using the same quantities of 
starting materials. Then, only 51.0 gms. (0.50 mole) of 96% sulfuric acid 
in 36.0 gms. (2 moles) of water and 36 ml. of dimethylformamide was added 
to the poly(iminoimidazolidinedione) solution to hydrolyze it to the 
corresponding poly(parabanic acid). The infrared spectrum of the product 
showed that some of the imino groups had not been hydrolyzed. The polymer 
was mostly the poly(parabanic acid) but contained some 
iminoimidazolidinedione rings as evidenced by the presence of weak 
absorption bands at 3.06.mu. (N--H) and 5.98.mu. (C.dbd.N). 
EXAMPLE 4 
This example demonstrates that 0.502 moles of sulfuric acid will completely 
hydrolyze one mole of repeating unit in a poly(iminoimidazolidinedione) 
when the solution is heated at 85.degree. C. for at least 15 minutes. 
To a solution of 193 gms. (0.772 mole) of diphenylmethane diisocyanate and 
21.3 gms. (0.79 mole) of hydrogen cyanide in 1800 ml. of 
dimethylformamide, was added a solution of 0.075 gm. of sodium cyanide in 
9.5 ml. of dimethylformamide. The exothermic reaction raised the 
temperature from 29.degree. C. up to 47.degree. C. After 30 minutes an 
acid solution composed of 39.6 gms. (0.388 mole) of 96% sulfuric acid, 
13.9 gms. (0.77 mole) of water and 100 ml. of dimethylformamide was added 
slowly to the viscous solution of poly(iminoimidazolidinedione). The 
temperature was increased to 85.degree. C. and held there for 15 minutes. 
The polymer was then precipitated by pouring the solution into water. The 
poly(parabanic acid) had an inherent viscosity of 1.30. The infrared 
spectrum of a thin film of the product was identical to that of a thin 
film of an authentic specimen of the poly(parabanic acid) expected. The 
complete absence of absorption bands at 3.04 and 5.98.mu. which are 
characteristic of the N--H group and the exocyclic C.dbd.N, respectively, 
and are present in the polymer before hydrolysis provided proof that 
hydrolysis had been complete. 
EXAMPLES 5-8 
Poly(iminoimidazolidinediones) were hydrolyzed to the corresponding 
poly(parabanic acids) using 0.502-0.507 moles of sulfuric acid per mole of 
imino group in the polymer solutions. The quantity of water was varied 
from one (theoretically all that is required) to four moles per mole of 
imino group. The results (see table below) show that only one mole of 
water per mole of imino group was necessary, particularly if the polymer 
was precipitated in water. Each solution was heated 15 minutes at 
85.degree.-90.degree. C. 
______________________________________ 
Moles of Imino 
Moles Moles Hydrolysis.sup.(a) 
Ex. Groups to Hydrolyze 
of H.sub.2 SO.sub.4 
of H.sub.2 O 
Complete 
______________________________________ 
5 1 0.503 1 Yes 
6 1 0.506 2 Yes 
7 1 0.502 3 Yes 
8 1 0.507 4 Yes 
______________________________________ 
.sup.(a) Hydrolysis was considered complete if no absorption peaks at 
3.04.mu. (.dbd.NH) and 5.98.mu. (exocyclic C.dbd.N) were observed; they 
are characteristic of iminoimidazolidinedione rings. 
EXAMPLE 9 
A solution of a poly(iminoimidazolidinedione) was prepared from 670 g. 
(2.66 moles) of 4,4'-diphenyl ether diisocyanate and 73.9 g. (2.73 moles) 
of hydrogen cyanide in 6.2 l. of dimethylformamide using 0.3 g. of sodium 
cyanide as catalyst. The polymer was hydrolyzed by mixing it with 137.5 g. 
(1.35 moles) of 96% sulfuric acid (0.506 mole per mole of imino group) and 
191.5 g. (10.6 moles) of water (4 moles per mole of imino group). The 
solution was stirred and heated at 85.degree.-90.degree. C. for 15 
minutes. The polymer was precipitated by pouring the reaction solution 
into water with vigorous agitation. The polymer had an inherent viscosity 
of 0.72. The infrared spectrum of the polymer indicated that it was the 
expected poly(parabanic acid) and that hydrolysis of the precursor polymer 
had been complete. 
EXAMPLES 10-15 
The results of these examples reveal the effect of water concentration in 
the hydrolysis reaction solution on the crystalline particle sizes of 
ammonium sulfate which formed and precipitated from solution. 
The poly(iminoimidazolidinedione) which was hydrolyzed in each of these 
examples was prepared as follows. To a solution of 663 g. (2.65 moles) of 
diphenylmethane diisocyanate, 2.2 g. (0.018 mole) of phenyl isocyanate and 
73.5 g. (2.72 moles) of hydrogen cyanide in 6140 ml. of dimethylformamide 
was added a solution which contained 0.26 g. of sodium cyanide in 35 ml. 
of dimethylformamide. The exothermic reaction raised the temperature from 
26.degree. C. up to 48.degree. C. After 30 minutes the 
poly(iminoimidazolidinedione) which had formed was hydrolyzed by adding 
0.506 moles of sulfuric acid per mole of imino group and water to the 
reaction solution; the quantity of water used in each experiment is 
recorded in the table below. Th hydrolysis solutions were heated 15 
minutes at 80.degree.-100.degree. C. 
The ammonium sulfate which precipitated was separated from each solution by 
filtration. Poly(parabanic acid) was precipitated by mixing each filtrate 
with distilled water. The inherent viscosities of the polymers varied from 
0.80-0.95. 
The ammonium sulfate crystals from each experiment were examined under a 
microscope. The sizes of the crystals were determined by comparing 
photomicrographs of the crystals with that of a calibrated scale at the 
same magnification. The results show that significantly larger crystals 
are formed in those reactions wherein 10 moles of water per mole of imino 
group was used to hydrolyze the poly(iminoimidazolidinedione) than in 
those reactions wherein only 1.5 moles of water was used. In the former, 
the water concentration in the final polymer solution was only about 0.3%, 
while it was about 5.5% in the latter polymer solutions. 
______________________________________ 
Ammonium Sulfate Crystal 
Moles of Sizes (Microns) 
Ex. No. H.sub.2 O Width Length 
______________________________________ 
10 1.5 3-8 10-20 
11 1.5 5-8 10-30 
12 1.5 5-8 10-30 
13 10 5-10 30-70 
14 10 5-10 30-70 
15 10 5-10 30-70 
______________________________________ 
EXAMPLES 16-22 
In these examples, solutions of poly(iminoimidazolidinediones) which had 
been made from diphenylmethane diisocyanate and hydrogen cyanide were 
mixed with an acid solution which contained 0.51 moles of sulfuric acid, 
10 moles of water, and 180 ml. of dimethylformamide for every mole of 
imino group in the polymer solution. The moles of imino group were assumed 
to be equal to the moles of diphenylmethane diisocyanate used to make the 
polymer. The temperature of each acid solution when it was added to the 
polymer solution is recorded in the table below. Before adding the acid 
solution, the temperature of each polymer solution was 
40.degree.-45.degree. C. After the acid solution had been added, each 
reaction solution was heated 15 minutes at 85.degree.-90.degree. C. 
Ammonium sulfate crystals were collected from each solution and they were 
examined and measured by the same technique used in Examples 10-15. The 
results (see table below) indicate that relatively large crystals of 
ammonium sulfate formed when the temperature of the acid solution was less 
than 10.degree. C. at addition. 
______________________________________ 
Acid Solution 
Ex. No. Temperature, .degree. C. 
Salt Size (Microns) 
______________________________________ 
16 15 10-40 
17 14 10-40 
18 24 10-30 
19 14 10-40 
20 24 5-20 
21 7 40-80 
22 7 40-80 
______________________________________ 
EXAMPLE 23 
A solution which contained 250 g. of dimethylformamide and 31.0 g. (0.112 
moles of imino groups) of a poly(iminoimidazolidinedione) which had been 
prepared from diphenylmethane diisocyanate and hydrogen cyanide was used 
in this example. To the stirred solution was added 6.0 g. (0.058 mole) of 
96% sulfuric acid. The temperature increased from 30.degree. C. up to 
39.degree. C. After 15 minutes a solution of 20.2 g. (1.12 moles) of water 
in 25 ml. of dimethylformamide was added to the solution with rapid 
stirring. It was heated 15 minutes at 85.degree. C. The polymer was 
precipitated from solution and shown to be identical to an authentic 
specimen of the expected poly(parabanic acid). The crystals of ammonium 
sulfate were found to be 30-50 microns long and 8-10 microns wide. 
EXAMPLE 24 
A poly(iminoimidazolidinedione) was prepared by reacting 16.1 g. (0.596 
mole) of hydrogen cyanide with 144.5 g. (0.578 mole) of diphenylmethane 
diisocyanate in 1340 ml. of dimethylformamide in the presence of 0.05 g. 
of sodium cyanide. The polymer was hydrolyzed by adding 30.1 g. (0.295 
mole) of 96% sulfuric acid to the stirred reaction solution. The 
temperature of the polymer solution was 52.degree. C. After one minute a 
solution of 104 g. (5.78 moles) of water in 104 ml. of dimethylformamide 
was added. After heating the solution for 15 minutes at 85.degree. C., it 
was filtered to remove the ammonium sulfate which had formed. The polymer 
which was precipitated from solution was the expected poly(parabanic 
acid), and the infrared spectrum of the product indicated that hydrolysis 
of the precursor polymer was complete. 
The crystals of ammonium sulfate were relatively large, measuring 40-100 
microns long and 8-10 microns wide. 
EXAMPLE 25 
To a solution of 270 ml. of dimethylformamide and 36 g. (0.13 mole of imino 
groups) of a poly(iminoimidazolidinedione) which had been prepared from 
diphenylmethane diisocyanate and hydrogen cyanide was added a hot 
(82.degree. C.) solution of 6.8 g. (0.067 mole) of 96% sulfuric acid in 
23.4 ml. of water. Stirring was vigorous. The solution was heated 15 
minutes at 85.degree.-93.degree. C. The salts were separated by 
filtration, and the polymer was precipitated by pouring the clear filtrate 
into water. The polymer was the expected poly(parabanic acid). 
The ammonium sulfate salt crystals were 8-10 microns wide and 20-40 microns 
long. 
EXAMPLE 26 
A solution of a poly(iminoimidazolidinedione) was prepared by the reaction 
of 610 g. (2.44 mole) of diphenylmethane diisocyanate with 67.5 g. (2.5 
mole) of hydrogen cyanide in 5600 ml. of dimethylformamide in the presence 
of 0.24 g. of sodium cyanide. It was hydrolyzed by adding 127 g. (1.24 
moles) of 96% sulfuric acid to the stirred reaction solution (44.degree. 
C.). As soon as the acid has been added, 439 g. (24.4 mole) of water was 
added and the solution was heated at 85.degree.-90.degree. C. for 15 
minutes. The ammonium sulfate which was formed and precipitated from 
solution was separated by filtration. Polymer which was precipitated from 
the filtrate was identical to an authentic specimen of the expected 
poly(parabanic acid). 
The crystals of ammonium sulfate were 8-10 microns wide and 40-80 microns 
long.