Process for the preparation of polyetherimides

An improved solution polymerization process for the preparation of polyetherimides comprises prereacting an aromatic bis(ether anhydride) with an aromatic diamine, followed by the addition of phthalic anhydride end capping agent and further reaction to form an end capped prepolymer. The prepolymer is converted to polyetherimide. The improved process advantageously greatly reduces the formation of PAMI, an undesired byproduct.

BACKGROUND OF THE INVENTION 
The present invention relates to processes for the preparation of 
polyetherimides and, more particularly, to processes which reduce the 
formation of the undesirable byproduct phthalic anhydride 
m-phenylenediamine imide (hereinafter "PAMI") during the preparation of 
polyetherimides. 
Because of their light weight, durability and strength, engineering 
thermoplastics are widely used. One class of engineering themoplastics are 
the polyetherimides to which the present invention is directed. 
Polyetherimides are sold by the General Electric Company under the 
trademark Ultem.RTM.. Such products of the General Electric Company 
include Ultem.RTM.1000 and ULTEM.RTM.1010 which are polyetherimide resins 
derived from bisphenol A dianhydride and m-phenylenediamine with phthalic 
anhydride used as an end-capping and chain-stopping agent. Polyetherimide 
resins are well known in the art and are of considerable commercial value 
for use in molding compositions because of their excellent physical, 
chemical and thermal properties. The high glass transition and heat 
deflection temperatures exhibited by these polymers permit their use in 
high performance applications. The Ultem.RTM.1000 and 1010 products 
mentioned above, for example, find applications in the automotive, 
aerospace and electrical industries. 
A number of processes for making polyetherimides have been disclosed. 
Generally, these polymers are prepared by reacting an organic diamine with 
an aromatic bis(ether dicarbonyl), i.e., an aromatic bis(etheranhydride) 
or an aromatic bis(ether dicarboxylic acid). Two processes which have been 
of particular interest are the so-called melt polymerization and solution 
polymerization processes. The basic melt polymerization process was 
described by T. Takekoshi and J. Kochanowski in U.S. Pat. No. 3,803,805. 
This process involves combining an aromatic bis(ether anhydride) and an 
organic diamine and heating the mixture under an inert atmosphere to form 
a homogeneous melt. Water formed during the polymerization reaction is 
removed at a temperature of up to 350.degree. C. In a preferred embodiment 
of the process, the final stage of the reaction is conducted under reduced 
pressure to facilitate removal of water. The basic polyetherimide 
polymerization technique has been improved by employing catalysts to 
enhance yields or reaction rates (for example, see Takekoshi, et al. U.S. 
Pat. No. 3,833,544 and F. Williams III, et al., U.S. Pat. No. 3,998,840, 
and Takekoshi, U.S. Pat. No. 4,324,882). In addition, the melt 
polymerization method has been adapted to the continuous mode by 
conducting the reaction in extrusion apparatus (for example, see 
Takekoshi, et al. U.S. Pat. No. 4,011,198 and Banucci, et al. U.S. Pat. 
No. 4,073,773). 
Solution polymerization is generally conducted by reacting an aromatic 
bis(ether anhydride) and an organic diamine in an inert solvent at 
temperatures up to about 200.degree. C. With this procedure, water of 
reaction is typically removed by azeotropic distillation. The resulting 
polymer is generally recovered by mixing the reaction solution with a 
precipitant, such as methanol. 
The reaction solvents employed for solution polymerization reactions are 
selected for their solvent properties and their compatibility with the 
reactants and products. High-boiling nonpolar organic solvents are 
preferred. (E.g., see Takekoshi, et al., U.S. Pat. No. 3,991,004). 
Dipolar, aprotic solvents and phenolic solvents can also be used, 
particularly when an aromatic bis(ether dicarboxylic acid) is used as the 
starting material (e.g., see Takekoshi, et al., U.S. Pat. No. 3,905,942). 
D. Heath and J. Wirth (U.S. Pat. No. 3,847,867) disclose a method for 
preparing polyetherimides which involves stirring a solution of an 
aromatic bis(ether anhydride) and an organic diamine in a dipolar, aprotic 
solvent under ambient conditions to produce a polyamide acid and casting 
the polyamide acid solution on a substrate to facilitate the removal of 
the organic solvent. The cast polyamide acid film can then be heated at 
temperatures of 150.degree. C. or higher. After the initial heating, the 
cast film can then be heated to temperatures of from 200.degree. C. to 
300.degree. C. to convert the polyamide acid to the polyetherimide. 
A process for making polyetherimides which is particularly preferred from 
the commercial standpoint is disclosed in U.S. Pat. No. 4,417,044 to 
Parekh. This disclosure is incorporated by reference herein Parekh 
discloses the reaction of an aromatic bis(ether anhydride) with an organic 
diamine and a "chain stopping agent" in an inert solvent mixture to form a 
prepolymer. The prepolymer generally contains a substantial amount of 
polyetherimide, but also typically contains partially reacted oligomers 
and polyamide acid intermediate compounds. The prepolymer is subsequently 
subjected to a second process step wherein the mixture is formed into a 
thin film under solvent-volatilizing conditions to effect substantially 
complete solvent and water removal. Further heating of the reaction 
product, preferably in a second thin film evaporator, substantially 
completes the polymerization to the desired polyetherimide. Polyetherimide 
removed from the second thin film evaporator can be continuously extruded, 
air cooled and pelletized to form a resin product suitable for injection 
molding and other applications such as sheet production. 
The Parekh solution polymerization process has proven to be a highly 
efficient process for the production of polyetherimides. Unfortunately, 
the production of sheet materials via thin film extrusion of 
polyetherimide resins produced via the Parekh process described above has 
been accompanied by the frequent formation (termed "plateout" in the 
industry) of an intractable coating upon the nip rollers of the 
sheet-forming equipment. The coating appears as a powder and can interfere 
with the quality of the sheet materials, for example by causing visible 
imperfections in the sheets. 
Accordingly it is an object of the present invention to provide an improved 
solution polymerization process for the production of polyethrimides which 
results in lower formation of plateout materials. It is another object of 
the invention to modify the Parekh solution polymerization process to 
reduce the formation of plateout materials while retaining the many 
advantages of that process. 
SUMMARY OF THE INVENTION 
The plateout material which builds up on the nip rollers during production 
of polyetherimide sheet materials was analyzed and found to consist of a 
low molecular weight byproduct formed from the reaction of two moles of 
phthalic anhydride chain stopper and one mole of m-phenylenediamine. The 
byproduct is of formula 
##STR1## 
and referred to herein as phthalic anhydride m-phenylenediamine imide or 
"PAMI." The PAMI plateout material appears as a white powder which 
sublimes at temperatures above about 300.degree. C. The analysis of 
polyetherimide resins produced according to the Parekh process described 
above revealed that PAMI content within the resins can vary from batch to 
batch within a typical range of from about 400 to over 800 parts per 
million. 
It has now been discovered that the formation of PAMI can be greatly 
minimized via the present invention in which a process of forming a 
polyetherimide comprises: 
(a) partially reacting an aromatic bis(ether anhydride) with an organic 
diamine under polyetherimide-forming conditions to form a prepolymer; 
(b) reacting the prepolymer with a phthalic anhydride end capping agent 
under conditions to form an end-capped prepolymer; and 
(c) heating the end-capped prepolymer under conditions to convert 
substantially all of said prepolymer to polyetherimide.

DETAILED DESCRIPTION OF THE INVENTION 
The first step of the process of the present invention involves reacting an 
aromatic bis(ether anhydride) of the formula 
##STR2## 
with at least one organic diamine having the formula 
EQU H.sub.2 N--R--NH.sub.2 (II) 
in an inert solvent under polyetherimide-forming conditions, wherein the 
group 
##STR3## 
is selected from: 
##STR4## 
R' being hydrogen, lower alkyl or lower alkoxy, Z is a member selected 
from the group consisting of (A) divalent organic radicals of the formula: 
##STR5## 
and (B) divalent organic radicals of the general formula 
##STR6## 
where X is a member selected from the group consisting of divalent 
radicals of the formulas 
##STR7## 
where y is an integer from 1 to about 5; and R is a divalent organic 
radical selected from the group consisting of (a) aromatic hydrocarbon 
radicals having from 6 to about 20 carbon atoms and halogenated 
derivatives thereof, (b) alkylene radicals having from 2 to about 20 
carbon atoms and cycloalkylene radicals having from 3 to about 20 carbon 
atoms, (c) from C.sub.2 to about C.sub.8 alkylene terminated 
polydiorganosiloxane, and (d) divalent radicals of the general formula 
##STR8## 
where Q is a member selected from the group consisting of: 
##STR9## 
and x is an integer from 1 to about 5. 
Bis(ether anhydride)s of formula I include for example, 
1,3-bis(2,3-dicarboxyphenoxy)benzene dianhydride; 
1,4-bis(2,3-dicarboxyphenoxy)benzene dianhydride; 
1,3-bis(3,4-dicarboxyphenoxy)benzene dianhydride; and 
1,4-bis(3,4-dicarboxyphenoxy)benzene dianhydride. 
A preferred class of aromatic bis(ether anhydride)s included by formula I 
includes compounds of formulas III, IV and V, which follow: 
##STR10## 
and mixtures thereof, where Y is selected from the group consisting of 
--O--, --S--, 
##STR11## 
Aromatic bis(ether anhydride)s of formula III include, for example: 
2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride, 
4,4'-bis(3,4-dicarboxyphenoxy)diphenyl ether dianhydride; 
4,4'-bis(3,4-dicarboxyphenoxy)diphenyl sulfide dianhydride; 
4,4'-bis(3,4-dicarboxyphenoxy)benzophenone dianhydride; 
4,4'-bis(3,4-dicarboxyphenoxy)diphenyl sulfone dianhydride; and mixtures 
thereof. 
Aromatic bis(ether anhydride)s of formula IV include, for example: 
2,2-bis[4-(2,3-dicarboxyphenoxy)phenyl]propane dianhydride; 
4,4'-bis(2,3-dicarboxyphenoxy)diphenyl ether dianhydride; 
4,4'-bis(2,3-dicarboxyphenoxy)diphenyl sulfide dianhydride; 
4,4'-bis(2,3-dicarboxyphenoxy)benzophenone dianhydride; 
4,4'-bis(2,3-dicarboxyphenoxy)diphenyl sulfone dianhydride; and mixtures 
thereof. 
The aromatic bis(ether anhydride) of formula V may be, for example, 
4-(2,3-dicarboxyphenoxy)-4'-(3,4-dicarboxyphenoxy)-diphenyl-2,2-propane 
dianhydride. 
Some of the aromatic bis(ether anhydride)s of formula (I) are shown in U.S. 
Pat. No. 3,972,902 (Darrell Heath and Joseph Wirth). As described therein, 
the bis(ether anhydride)s can be prepared by the hydrolysis, followed by 
dehydration, of the reaction product of a nitrosubstituted phenyldinitrile 
with a metal salt of a dihydric phenol compound in the presence of a 
dipolar, aprotic solvent. 
Additional aromatic bis(ether anhydride)s also included by Formula (I) are 
shown by Koton, M. M.; Florinski, F. S.; Bessonov, M. I.; Rudakov, A. P. 
(Institute of Heteroorganic Compounds, Academy of Sciences, U.S.S.R.), 
U.S.S.R. No. 257,010, Nov. 11, 1969, Appl. May 3, 1967, and by M. M. 
Koton, F. S. Florinski, Zh. Org. Khin, 4(5), 774 (1968). 
Other dianhydrides may also be used in combination with the dianhydrides of 
Formula I to form copolymers. Examples of such dianhydrides include 
pyromellitic dianhydride, sulfur dianhydride, benzophenone dianhydride and 
the like. 
The organic diamines of Formula (II) include, for example: 
m-phenylenediamine, 
p-phenylenediamine, 
4-4'-diaminodiphenylpropane, 
4-4'-diaminodiphenylmethane (commonly named 4,4'-methylenedianiline). 
4,4'-diaminodiphenyl sulfide, 
4,4'-diaminodiphenyl sulfone, 
4,4'-diaminodiphenyl ether (commonly named 4,4'-oxydianiline), 
1,5-diaminonaphthalene, 
3,3'-dimethylbenzidine, 
3,3'-dimethoxybenzidine; 
2,4-bis(.beta.-amino-t-butyl)toluene, 
bis(p-.beta.-amino-t-butylphenyl)ether, 
bis(p-.beta.-methyl-o-aminopentyl)benzene, 
1,3-diamino-4-isopropylbenzene, 
1,2-bis(3-aminopropoxy)ethane, benzidine, 
m-xylylenediamine, 
p-xylylenediamine, 
2,4-diaminotoluene 
2,6-diaminotoluene, 
bis(4-aminocyclohexyl)methane; 
3-methylheptamethylenediamine, 
4,4-dimethylheptamethylenediamine; 
2,11-dodecanediamine, 
2,2-dimethylpropylenediamine, 
octamethylenediamine, 
3-methoxyhexamethylenediamine; 
2,5-dimethylhexamethylenediamine, 
2,5-dimethylheptamethylenediamine, 
3-methylheptamethylenediamine, 
5-methylnonamethylenediamine, 
1,4-cyclohexanediamine, 
1,12-octadecanediamine, 
bis(3-aminopropyl)sulfide 
N-methyl-bis(3-aminopropyl)amine, hexamethylenediamine, 
heptamethylenediamine, nonamethylenediamine, decamethylenediamine, 
bis(3-aminopropyl)tetramethyldisiloxane, 
bis(4-aminobutyl)tetramethyldisiloxane, and mixtures of such diamines. 
The polyetherimide-forming conditions employed in the first process step 
are similar to those disclosed in the Parekh patent and generally include 
a reaction temperature of from about 40.degree. C. to about 200.degree. 
C., preferably from about 80.degree. C. to about 180.degree. C. The 
solvent can be an inert nonpolar organic solvent or an inert polar solvent 
that does not deleteriously affect the reaction. Relatively high-boiling 
solvents are preferred, and examples of such solvents are chlorobenzene, 
dichlorobenzenes, trichlorobenzenes, diphenylether, diphenylsulfide, 
acetophenone, chlorinated biphenyl, chlorinated diphenylethers, 
methylcyclohexane, and the like. o-Dichlorobenzene is preferred. 
Polar reaction solvents that can be used include phenolic solvents, such as 
phenols, cresols, ethylphenols, isopropylphenols, t-butylphenols, 
xylenols, chlorophenols, dichlorophenols, phenylphenols, and the like. In 
addition, dipolar, aprotic solvents can be employed as reaction solvents. 
Such solvents are generally non-acid, oxygen-containing, 
nitrogen-containing organic solvents and include, for example, 
N,N-dimethylacetamide, N-methylpyrrolidone, N,N-dimethylformamide, 
dimethylsulfoxide, hexamethylphosphoramide, and the like. Mixtures of 
solvents can also be employed. 
The order of addition of reactants is not critical, provided that the end 
capping agent is added subsequent to the partial reaction of the anhydride 
with the diamine in accordance with the invention. It is preferred to 
effect the reaction of the dianhydride and the organic diamine under 
reflux and in an inert atmosphere, such as nitrogen or helium. Sufficient 
solvent is generally utilized to provide a solids content in the range 
between 1% and 90%, preferably in the range between about 15% and about 
60%. 
It has been found that substantially equal molar amounts of the organic 
amine end groups and the aromatic bis(ether anhydride) end groups provide 
optimum results. Stoichiometry should be closely controlled, the preferred 
range being from about 0.95 to about 1.05 mole of organic diamine end 
groups per mole of anhydride end groups. 
Various catalysts can be employed in catalytic amounts. Such catalysts 
include inorganic salts, such as alkali metal carbonates, sodium chlorate 
or ferric sulfate, and oxygenated phosphorous compounds of various alkali 
metals, such as sodium phenyl phosphonate. 
Reaction time for the first "partial reaction" process step can vary from 
about 0.5 to about 5 hours, more typically about 0.75 to 1.5 hours, 
depending upon such factors as the temperature employed, degree of 
agitation, nature of reactants, solvent, and the like. The progress of 
this first partial reaction step is monitored by monitoring the opacity of 
the reaction mixture. Upon becoming homogeneous, the reaction mixture 
turns from opaque to clear. 
A phthalic anhydride end capping agent is added to the reaction mixture 
following the partial reaction of the bis(ether anhydride) with the 
diamine. The reaction proceeds, optionally under reflux, following the 
addition of the end-capping agent. Temperature and other reaction 
conditions remain as before the addition of the end-capping agent. 
Following the addition of the end-capping agent, the reaction can be 
conveniently monitored by measuring the melt viscosity (melt flow) of the 
polymer that is produced. Generally, higher melt viscosities indicate 
greater degrees of polymerization. 
During the course of the reaction, water of reaction is removed. The amount 
of water generated, as a percentage of theoretical, can also be used to 
monitor the course of the reaction. Water ca be conveniently removed o a 
continuous basis by azeotropic distillation, employing a low-boiling 
azeotropic solvent. 
The prereaction process sep followed by the end-capping process step 
produces an end-capped prepolymer-in-solvent mixture. As used herein, the 
term prepolymer means a material which generally contains a substantial 
amount of polyetherimide, but also typically contains partially reacted 
oligomers and polyacid amide intermediate compounds. 
The prepolymer-solvent mixture from the first two reaction steps is 
subjected to a further process step, wherein the mixture is heated to 
temperatures generally higher than in the previous steps in order to 
complete the conversion to polyetherimide and to drive off remaining 
solvent. This step can advantageously be conducted in a continuous manner 
using conventional thin-film evaporation equipment whereby the 
prepolymer/solvent mixture is formed into a thin film under conditions to 
effect substantially complete solvent and water removal. Such equipment 
can take a variety of forms, and the process of the present invention is 
not limited to any particular form of equipment. Typical thin-film 
evaporation equipment consists of a heated, large-diameter, cylindrical or 
tapered tube in which is rotated a series of wipers, either maintaining a 
fixed close clearance from the wall or riding on a film or liquid on the 
wall. The continuous forming and reforming of the film permits 
concentration of viscous materials. Reduced pressure may be employed to 
accelerate solvent removal, and an evaporation temperature of from about 
200.degree. C. to about 450.degree. C. preferably from about 250.degree. 
C. to about 350.degree. C. is employed. Lower temperatures result in very 
viscous mixtures, which are difficult to process and can damage equipment, 
whereas higher temperature can cause decomposition of the produce. 
Thin-film evaporation permits efficient solvent recovery, which is 
advantageous from both economical and ecological standpoints. 
The elevated temperatures employed in this subsequent heating process step 
result in further polymerization of the prepolymer. The degree of 
polymerization is dependent on a number of factors, including throughput 
rate, temperature, pressure and surface renewal rate. 
In a particularly preferred embodiment, the `heating` process step is 
accomplished in two phases. The first phase encompasses the formation of a 
thin film as described above and at temperatures generally ranging from 
about 150.degree. to about 190.degree. C. The product of this process step 
is generally a prepolymer having a substantially reduced solvent content. 
In a second phase, the prepolymer from the first phase is heated to a 
temperature above the glass transition temperature of the polyetherimide 
polymer product and less than about 450.degree. C. to form a 
polyetherimide. Preferred temperatures for this step range from about 
250.degree. C. to about 350.degree. C. Substantially complete 
polymerization and solvent and water removal occur i this second phase. 
From a processing standpoint, there might not be a clear separation 
between the first and second phases of the `heating` process step. For 
example, the prepolymer may be retained in a thin-film evaporator beyond 
the point at which a substantial portion of the solvent has been removed, 
thus effecting substantially complete polymerization. 
In a preferred embodiment of the process, the prepolymer is continuously 
transferred from the outlet of a first thin-film evaporator to the inlet 
of a second thin-film evaporator maintained at melt polymerization 
temperatures. The second phase is conducted in the second thin-film 
evaporator and advantageously employs reduced pressure to facilitate 
removal of remaining traces of solvent and water. From the second 
thin-film evaporator the polyetherimide can be continuously extruded, air 
cooled, and pelletized t form a resin product suitable for injection 
molding and other applications. Such extrusion can be effected, for 
example, by means of a pump which pumps the heated polymer from the second 
thin-film evaporator through a suitable die. 
In an alternative embodiment, the first and second phases of the `heating` 
process step are conducted in a combined thin-film evaporator-screw 
extrusion apparatus. In this embodiment, the first phase (solvent removal) 
occurs in the thin-film evaporator and the second phase (final conversion) 
occurs in the screw extruder. Of course, other combinations of equipment 
can also be employed (e.g., a plurality of thin-film evaporators in 
parallel or series, followed by extruders), and the process of the 
invention is not limited to any particular apparatus. The second phase 
only need be conducted until substantially complete polymerization and 
solvent removal, has been achieved. Generally, the processing times are 
relatively short (depending on the equipment used), e.g., less than about 
15 minutes for the first phase and less than about 5-6 minutes for the 
second phase. 
The present process overcomes a disadvantage of the Parekh process, namely 
the formation of PAMI and its plateout during film and sheet production, 
while retaining its many advantages. The lengthy reaction times and 
incomplete reactions associated with other solution polymerizations are 
avoided by the solvent removal and high-temperature processing. On the 
other hand, by conducting a prepolymerization reaction in solution, the 
problems commonly associated with melt polymerization techniques are 
avoided. The losses of volatile reactants are minimized, and the so-called 
"cement stage" does not occur. 
Practice of the present invention is further illustrated by the following 
examples which should not be viewed as limiting the scope of the 
invention. 
COMATIVE EXAMPLE 
The formation of the undesirable byproduct PAMI during the preparation of 
polyetherimide via the Parekh process of U.S. Pat. No. 4,417,044 was 
illustrated as follows. A mixture of bisphenol A dianhydride (BPA-DA), 
phthalic anhydride (PA) and m-phenylene diamine (mPD) in o-dichlorobenzene 
was prepared by first dissolving 5.0225 g (9.650 millimoles) of BPA-DA and 
0.1037 g (0.70 millimoles) PA in o-dichlorobenzene at approx. 90.degree. 
C. and thereafter adding 1.0814 g (10.00 millimoles) mPD. The solution was 
slowly warmed to about 180.degree. C. and refluxed under N.sub.2 for 4 
hours at this temperature to produce a prepolymer containing 
polyetherimide groups as well as acid amide groups. A 0.10 g sample was 
then removed to a small test tube and heated at approx. 350.degree. C. for 
15 minutes under N.sub.2 to substantially finish the conversion to 
polyetherimide. The resulting polymer was dissolved in dichloromethane and 
its molecular weight was determined, by gel permeation chromatography 
(GPC) against a polystyrene standard, to be 24,000 M.sub.N (typical of the 
desired polyetherimides. The reaction solution was then analyzed for PAMI 
level via GPC using Polymer Science PL GEL columns (one micron mixed bed, 
one 5 micron 500 .ANG. bed and one 5 micron 50 .ANG. bed, the latter to 
separate PAMI from other low molecular weight monomers). PAMI level was 
determined against a known sample. The PAMI level was determined to be 730 
ppm .+-.10 %. 
EXAMPLE 1 
A procedure similar to that described in the above Comparative Example was 
followed except that, according to the inventive process, a mixture of 
BPA-DA and mPD in o-dichlorobenzene was prereacted (refluxed) for 1 hour 
at approximately 150.degree. C. prior to the addition of PA end capping 
agent. The mixture was refluxed for 3 additional hours following PA 
addition, heated to approx. 350.degree. C. to finish the conversion to 
polyetherimide and then analyzed, all as above. GPC revealed a molecular 
weight of 24,700 M.sub.N, demonstrating that the process according to the 
present invention produced polyetherimide of equivalent molecular weight 
as compared to the Comparative Example process. Further GPC analysis 
confirmed that PAMI content had been reduced to 90 ppm. Thus, the addition 
of PA following the prereaction of the BPA-DA and mPD is seen to greatly 
reduce the formation of PAMI. 
EXAMPLES 2-11 
Another series of reactions of BPA-DA, mPD and PA in o-dichlorobenzene, 
following the experimental procedures detailed in the Comparative Example 
and followed in Example 1 above, was conducted. In each of the following 
Examples the amounts of BPA-DA and mPD remained constant as set forth in 
the Comparative Example. The timing and amount of the PA addition was 
varied, however, to determine their affect on final PAMI content. For 
example, in some of the following reactions a portion of the PA was added 
along with the initial mixture of BPA-DA and mPD and the remainder of the 
PA was added after the reaction had progressed for the indicated period of 
time. 
EXAMPLE 2 
BPA-DA, mPD and 0.7 mmoles PA were reacted as set forth in the Comparative 
Example as a control. The PAMI concentration in the final polyetherimide 
product was 730 ppm. 
EXAMPLE 3 
The solution of BPA-DA and mPD was refluxed at approx. 190.degree. C. (i.e. 
prereacted) for one hour. 0.7 mmoles of PA was added to the prereaction 
product and the solution was refluxed for 3 additional hours. The PAMI 
concentration in the final product was 100 ppm. 
EXAMPLE 4 
The procedure of Example 3 was repeated and resulted in 90 ppm of PAMI in 
the final product. 
EXAMPLE 5 
A solution of BPA-DA, mPD and 0.3 mmoles of PA was refluxed for one hour, 
at which point an additional 0.4 mmoles of PA was added. The mixture was 
refluxed an additional 3 hours. PAMI concentration was 90 ppm following 
the first hour of reflux but rose to 430 ppm following the addition of the 
remainder portion of PA, further reflux and heating to convert the 
prepolymer to polyetherimide. Thus it is seen that pre-reaction in the 
presence of even a small portion of the PA end capping agent results in 
comparatively high PAMI formation. 
EXAMPLE 6 
An initial solution of BPA-D,, mPD and 0.3 mmoles PA was prepared. After 20 
minutes of warming the temperature of the solution had reached 140.degree. 
C. and at that time the PAMI content was 100 ppm. The remaining 0.4 mmoles 
of PA was added. The solution was brought to reflux temperature and held 
udder reflux for 4 hours. The PAMI content in the final product was 500 
ppm. 
EXAMPLE 7 
An initial solution of BPA-DA, mPD and 0.3 mmole PA was refluxed for 1 
hour. The mixture was then cooled to approx. 170.degree. C. and extracted 
with 100 ml of water to remove any unreacted mPD. 0.4 mmoles PA was added 
to the reaction solution which was then refluxed an additional 3 hours. 
The final PAMI content was 410 ppm. 
EXAMPLE 8 
An initial solution of BPA-DA, mPD and 0.15 mmoles of PA was refluxed for 1 
hour. The remaining 0.55 mmoles of PA was added, followed by 3 hours of 
further reflux. The final product contained 310 ppm of PAMI. 
EXAMPLE 9 
A solution of BPA-DA and mPD was refluxed for one hour. Then 0.7 mmoles PA 
and an additional 0.35 mmoles mPD was added and the resulting solution was 
refluxed an additional 3 hours. The final concentration of PAMI rose to 
4200 ppm. due to the additional mPD. 
EXAMPLE 10 
A solution of BPA-DA, mPD and 0.3 mmoles of PA was refluxed for 1 hour. At 
that point the PAMI concentration was 100 ppm. 20 ml of water was added to 
the solution and was subsequently extracted. The remaining 0.4 mmoles of 
PA was added and the solution was refluxed for an additional 3 hours. The 
final PAMI concentration was 400 ppm. 
EXAMPLE 11 
In this Example the usefulness of phthalic acid as a PAMI-reducing end 
capping agent was evaluated. 5.0225 g (9.65 mmoles) of BPA-DA was 
dissolved in 14 ml o-dichlorobenzene in a 50 ml boiling flask. 0.1162 g 
(0.7 mmoles) phthalic acid was added but did not appear to go into 
solution. After 5 minutes 1.0814 g (10 mmoles) of mPD was added and the 
mixture was heated to approx. 180.degree. C. for 4 hours. Subsequent 
heating at 350.degree. C. as in the previous examples afforded a final 
product containing 500 ppm PAMI.