METHOD FOR PREPARING POLYIMIDE AND POLYIMIDE PREPARED
THEREBY
BACKGROUND OF THE INVENTION
One known process for the preparation of polyimides is solution polymerization. This process includes a first step, in which a diamine compound and a dianhydride compound react in solution to form a partially polymerized polyimide, and a second step, in which the polymerization is completed and solvent is removed. The second step typically requires a residence time on the order of hours, and it also requires substantial capital investment in the form of gear pumps required to transport the molten polymer, and evaporation equipment required to remove the solvent and any water formed as a product of the polymerization and imidization reactions. This process allows carefully controlled proportions of the diamine and dianhydride reactant, but its long residence times, high temperatures, and exposure of the molten polyimide to atmospheric air often result in the degradation of the polyimide and its physical properties.
Another known process for the preparation of polyimides is melt polymerization. Melt polymerization is conducted in a single reactive extrusion step in which the diamine and dianhydride reactants are fed to an extruder, where they react to form a polyimide. This process is substantially faster and less capital-intensive than solution polymerization. It is also useful for the preparation of high molecular weight polyimides, as well as polyimides having high glass transition temperatures. However, it does not allow for precise control of reactant stoichiometry. As a result the polyimide produced is not consistent in quality. A further disadvantage of conventional melt polymerization techniques is that the reaction mixture passes through a so-called "cement stage" as the polyamic acid intermediate is formed. During this phase of the reaction, the mixture of reactants becomes very viscous and thus difficult to process. Because of these disadvantages, melt polymerization is not widely practiced commercially.
It would therefore be advantageous to synthesize polyimide by a method that provides the precise control of reactant stoichiometry and manageable viscosities offered by the

solution polymerization, as offered by melt polymerization

well as the substantial time and equipment cost savings

BRIEF DESCRIPTION OF THE INVENTION
The above-described and otter drawbacks are alleviated by a method of preparing a polyimide, comprising: introducing to an extruder an oligomer mixture comprising a solvent and an oligomer foi nied by the reaction of a dianhydride compound and a diamine compound; removing solvent from the oligomer mixture via a vent in the extruder; and melt kneading he oligomer to form a polyimide.
Other embodiments, includii g a polyetherimide prepared by the method, are described in detail below.
BRIEF DESCRIPTION OF fHE DRAWINGS

Referring now to the drawing FIGURES:

gs wherein like elements are numbered alike in several

FIG. 1 is a simplified dia; downstream vent is employe 1
jram of a system for practicing the method; a single

FIG. 2 is a simplified dia upstream vent and five dow the extruder are condensed;

p-am of another system for practicing the method; an nstream vents are employed; volatile components exiting

FIG. 3 is a simplified diagram upstream vents and the slightly sub-atmospheric pressure maintained at sub-atmosphei ic pressure: side feeders; melt temperature, used to adjust the process agent may, optionally, be i facilitate solvent removal.
of another system for practicing the method; three st downstream vent are maintained at atmospheric or and the four remaining downstream vents are ; two of the upstream vents are associated with :, die pressure, and extruder torque are monitored and asing monomer inputs to the oligomer tank; a stripping ihjected into the downstream portion of the extruder to

DETAILED DESCRIPTIONS

OF THE INVENTION
One embodiment is a method of preparing a polyimide, comprising: introducing to an extruder an oligomer mixture comprising a solvent and an oligomer formed by the reaction of a dianhydride compound and a diamine compound; removing solvent from the oligomer mixture via a vent in the extruder; and melt kneading the oligomer to form a polyimide. The present inventors have discovered that the present method provides an advantageous and heretofore unobtainable combination of precise control of reactant stoichiometry and substantial time and equipment cost savings.
The method comprises introducing to an extruder an oligomer mixture comprising a solvent and an oligomer formed by the reaction of a dianhydride compound and a diamine compound. The dianhydride compound may have the structure

(Figure Removed)

wherein V is a tetravalent linker selected from (a) substituted or unsubstituted, saturated, unsaturated or aromatic monocyclic and polycyclic groups having about 5 to about 50 carbon atoms, (b) substituted or unsubstituted, linear or branched, saturated or unsaturated alkyl groups having 1 to about 30 carbon atoms, and (c) combinations thereof, wherein the substitutions are ethers, epoxides, amides, esters, or combinations thereof. Preferred dianhydride compounds include those having the structure
(Figure Removed)


wherein the divalent T moi rings of the respective aryl i

:ty bridges the 3,3', 3,4', 4,3', or 4,4' positions of the aryl nide moieties; T is -O- or a group of the formula -O-Z-O-

; Z is a divalent radical selected from the following formulae
(Figure Removed)
wherein y is an integer of 1 to about 5, and q is 0 or 1. In one embodiment, the dianhydride compound comprises bisphenol A dianhydride (BPADA), which consists of one or more isomers having the structure
(Figure Removed)
In another embodiment, the dianhydride compound comprises 4,4'-oxy-diphthalic anhydride (OOP A), which consists of one or more isomers having the structure
(Figure Removed)
In another embodiment, the dianhydride compound comprises BPADA and ODPA.
The diamine compound may have the structure
H2N R NH2
where Q is a covalent bond or a member selected from the formulae
wherein R is a divalent organic radical selected from (a) aromatic hydrocarbon radicals having 6 to about 20 carbon atoms and halogenated derivatives thereof, (b) alkylene radicals having 2 to about 20 carbon atoms, (c) cycloalkylene radicals having 3 to about 20 carbon atoms, and (d) divalent radicals of the general formula
(Figure Removed)

(Figure Removed)
bis(4-aminophenyl)ether, hexamethylenediamine, 1,4-cyclohexanediamine, diaminodiphenylsulfones svch as 4,4'-diaminodiphenylsulfone, and the like, and mixtures thereof. In one embodiment, the diamine compound comprises meta-phenylenediamine (m-PD). 1 n another embodiment, the diamine compound comprises diaminodiphenylsulfone (DI >S), which consists of one or more isomers having the structure
where y' is an integer from] example, m-phenylenediair


•NH,
1 to about 5. Specific diamine compound include, for ine, p-phenylenediamine, bis(4-aminophenyl)methane,



In another embodiment, the d

amine compound comprises m-PD and DDS.

take place in the presence o reacting with free amine end)
The reaction of the dianhydrjde compound and diamine compound may, optionally,
a so-called chain stopper. Chain stoppers capable of groups on the oligomer or the polyimide include, for
example, phthalic anhydride. Chain stoppers capable of reacting with free anhydride end groups on the oligomei or the polyimide include, for example, aniline and substituted anilines.
Other specific dianhydride compounds, diamine compounds, and chain stoppers suitable for use in the present invention include those described in, for example, U.S. Patent Nos. 3,847,867 to Heafh et al., 3,850,885 to Takekoshi et al., 3,852,242 and
3,855,178 to White, 3,983,093
to Williams et al., and 5,189,137 to Howson et al.
The reaction of the diamine solvent. Suitable solvents
dompound and dianhydride compound takes place in a nclude halogenated aromatic solvents, halogenated
aliphatic solvents, non-halogenated aromatic solvents, non-halogenated aliphatic solvents, and mixtures thereof. Halogenated aromatic solvents are illustrated by ortho-dichlorobenzene (ODCB), chlorobenzene, and the like, and mixtures thereof. Non-halogenated aromatic solvents are illustrated by toluene, xylene, anisole, veratrole, trimethoxybenzenes, and the like, and mixtures thereof. Halogenated aliphatic solvents are illustrated by methylene chloride, chloroform, 1,2-dichloroethane, and the like,. and mixtures thereof. Non-halogenated aliphatic solvents are illustrated by ethanol, acetone, ethyl acetate, and the like, and mixtures thereof. In one embodiment, the solvent comprises a halogenated aromatic solvent. In one embodiment, the solvent comprises ortho-dichlorobenzene.
The reaction of the diamine compound and dianhydride compound may, optionally, be conducted in the presence of an imidization catalyst, which catalyzes the conversion of amic acid functionality to cyclized imide functionality. Suitable imidization catalysts are known in the art; they include salts of organophosphorus acids, particularly phosphinates such as sodium phenyl phosphinate and heterocyclic amines such as 4-diaminopyridine. Sodium phenyl phosphinate is presently preferred.
Reaction of the dianhydride compound with the diamine compound in the solvent generates an oligomer mixture. In one embodiment, the oligomer mixture is an oligomer solution, which is herein defined as comprising less than 0.1 weight percent of solids. In one embodiment, the oligomer comprises amic acid repeating units having the structure
O O
HO C C N R—5-
\v/ H *
-I-N C C OH
? H
O O
wherein V is a tetravalent linker selected from (a) substituted or unsubstituted, saturated, unsaturated or aromatic monocyclic and polycyclic groups having about 5 to about 50 carbon atoms, (b) substituted or unsubstituted, linear or branched, saturated

or unsarurated alkyl groups thereof, wherein the substitij thereof. In the structure divalent organic radical selej 6 to about 20 carbon atoms chain alkylene radicals hav radicals having about 3 to general formula
laving 1 to about 30 carbon atoms, and (c) combinations ions are ethers, epoxides, amides, esters, or combinations mmediately above, R is a substituted or unsubstituted ted from (a) aromatic hydrocarbon radicals having about halogenated derivatives thereof, (b) straight or branched ,g about 2 to about 20 carbon atoms; (c) cycloalkylene bout 20 carbon atoms, and (d) divalent radicals of the



(Figure Removed)



wherein Q is a divalent m halogenated derivatives then of polymerization (i.e., cha oligomer may comprise im repeat units described abovi one imidized repeating unit
ety selected from -O-, -S-, -C(O)-, -SOz-, CyH2y-, and of, wherein y is an integer from 1 to 5. Because the rate
growth) may be similar to the rate of imidization, the dized repeat units. Thus, in addition to the amic acid
the oligomer may, optionally, further comprise at least aving a structure selected from


(Figure Removed
)An oligomer is herein de: repeating units. The oligomer three repeating units, mon preferably at least five repeal
In one embodiment, the olig<
,[ied as comprising a number average of at least two may preferably comprise a number average of at least preferably at least four repeating units, even more ng units.
r comprises repeating units having the structure
(Figure Removed)
Such units may be derived, for example, from oligomerization of BPADA and m-PD in a suitable solvent. In another embodiment, the oligomer comprises repeating units having the structure
(Figure Removed)
Such units may be derived, for example, from oligomerization of OOP A and DDS in a suitable solvent. In another embodiment, the oligomer is formed from co-oligomerization of at least three monomers selected from bisphenol A dianhydride, 4,4'-oxy-diphthalic anhydride, meta-phenylenediamine, and diaminodiphenylsulfone.
The oligomer may preferably have a weight average molecular weight of about 5,000 to about 40,000 atomic mass units (AMU). Within this range, a weight average molecular weight of at least 10,000 AMU is more preferred. Also within this range, a weight average molecular weight up to 30,000 AMU is more preferred.
The oligomer mixture may comprise about 5 to about 90 weight percent of the oligomer. Within this range, the oligomer content is preferably of at least about 10 weight percent, more preferably at least about 20 weight percent, still more preferably at least about 30 weight percent. Also within this range, the oligomer weight content is preferably up to about 80 weight percent, more preferably up to about 70 weight percent, still more preferably up to about 60 weight percent.
The oligomer mixture ma> solvent. Within this rang* weight percent, more at least about 40 weight { preferably up to about 90 percent, still more preferabl;
comprise about 10 to about 95 weight percent of the the solvent content is preferably of at least about 20 preferably at least about 30 weight percent, still more preferably ercent. Also within this range, the solvent content is weight percent, more preferably up to about 80 weight up to about 70 weight percent.
The method comprises me embodiment, the polyimide
(Figure Removed)
kneading the oligomer to form a polyimide. In one omprises repeating units having the structure
wherein V is a tetravalen saturated, unsaturated or about 50 carbon atoms, (b) or unsaturated alkyl groups thereof, wherein the thereof. In the structure divalent organic radical 6 to about 20 carbon atoms chain alkylene radicals radicals having about 3 to general formula
havi ng
linker selected from (a) substituted or unsubstituted, arojnatic monocyclic and polycyclic groups having about 5 to ubstiruted or unsubstituted, linear or branched, saturated laving 1 to about 30 carbon atoms, and (c) combinations substitu ions are ethers, epoxides, amides, esters, or combinations nmediately above, R is a substituted or unsubstituted selected from (a) aromatic hydrocarbon radicals having about r halogenated derivatives thereof, (b) straight or branched about 2 to about 20 carbon atoms; (c) cycloalkylene ibout 20 carbon atoms, and (d) divalent radicals of the
(Figure Removed)
wherein Q is a divalent m halogenated derivatives the
iety selected from -O-, -S-, -C(O)-, -SO2-, CyH2y-, and eof, wherein y is an integer from 1 to 5. In a preferred
embodiment, the polyimide is a polyetherimide. In this embodiment, the ether oxygen may be derived from the dianhydride compound, the diamine compound, .or both. In one embodiment, in which the oligomer is prepared from BPADA and m-PD, the polyimide is a polyetherimide comprising repeating units having the structure

(Figure Removed)

In another embodiment, in which the oligomer is prepared from ODPA and DOS, the polyimide is a polyetherimide comprising repeating units having the structure
(Figure Removed)
There is no particular molecular weight limit on the polyimide, except that it has a higher molecular weight than the oligomer. In one embodiment, the polyimide comprises a number average of repeating units at least two, preferably at least three, more preferably at least 4, still more preferably at least five repeating units greater than the number average of repeating units in the oligomer.
In one embodiment, the polyimide comprises a number average of repeating units at least 1.2, preferably at least 1.3, more preferably at least 1.4, still more preferably at least 1.5 times greater than the number average of repeating units in the oligomer. Similarly, in one embodiment, the ratio of the polyimide weight average molecular weight to the oligomer weight average molecular weight is about 1.2 to about 10. Within this range, the ratio is at least about 1.5, more preferably at least about 1.8. Also within this range, the ratio is up to about 8, more preferably up to about 6, still more preferably up to about 4.
In one embodiment, the polyimide has a weight average molecular weight greater than 30,000 to about 80,000 AMU. Within this range, the polyimide weight average molecular weight is preferably at least about 35,000 AMU, more preferably at least about 40,000 AMU. Also within this range, the polyimide weight average molecular

weight is preferably up to AMU. One advantage of th polydispersity than polyimid dianhydride compound and
aut 70,000 AMU, more preferably up to about 60,000 j method is that it produces a polyimide having a lower ;s produced by reactive extrusion methods in which the he diamine compound are fed directly to the extruder.

Thus, in one embodiment, the polyimide has a polydispersity index less than about 4, preferably less than about 3, more preferably less than about 2.5. The polydispersity index is the ratio of the weight average molecular weight to the number average molecular weight. Another advantage of the method is that it can produce polyimides of very high molecular weight, which could not be obtained using, for instance, a solution polymerization method alone. The molecular weight characteristics of the oligomer and the polyimide may be determined by methods known in the art such as, for example, gel permeation chromatography using appropriate standards.
Another advantage of the method is that it produces a polyimide extrudate with low residual solvent. For example polyimide produced by the method may comprise less than 1,000, preferably less than 500, more preferably less than 250, still more preferably less than 100, even more preferably less than 50 parts per million by weight of residual solvent. Solvent removal may, optionally, be facilitated by addition of a stripping agent to the extruder. The stripping agent will typically have an atmospheric boiling point below the operating barrel temperature(s) of the extruder. Suitable stripping agents include, for example, nitrogen, water, carbon dioxide including supercritical carbon dioxide, and air. Because some unwanted decomposition pathways for the oligomer and the polyimide are oxidative, it is preferred that the stripping gas is free of oxygen. A presently preferred stripping agent is nitrogen.
Another advantage of the method is that it produces a polyimide with low concentrations of residual anhydride and amine end groups. For example, the polyimide may comprise less than about 1 mole percent, preferably less than 0.5 mole percent, more preferably less tha|n 0.25 mole percent, even more preferably less than
0.1 mole percent, of unreacted anhydride end groups, based on moles of polyimide. A procedure for determining anhydride end groups is described in the working examples, below. Similarly, the polyimide may comprise less than about 1 mole percent, preferably less than 0.5 mole percent, more preferably less than 0.25 mole percent, even more preferably less than 0.1 mole percent, still more preferably less than 0.01 mole percent, of unreacted amine end groups, based on moles of polyimide. A procedure for determining amine end groups is described in the working examples, below.
In a preferred embodiment, the oligomer mixture is superheated when it is introduced to the extruder. The oligomer mixture is superheated when it has a temperature greater than the boiling point of the solvent at atmospheric pressure. Typically, the temperature of the superheated oligomer mixture will be about 2°C to about 200°C higher than the boiling point of the solvent at atmospheric pressure. Within this range, a temperature of less than or equal to about 150°C above the boiling point of the solvent is preferred, with a temperature less than or equal to about 100°C above the boiling point of the solvent being more preferred. Also preferred within this range, a temperature of greater than or equal to about 10°C above the boiling point of the solvent is preferred, with a temperature greater than or equal to-about 50°C above the boiling point of the solvent being more preferred. In instances where there are multiple solvents present, the oligomer mixture is superheated with respect to at least one of the solvent components. Where the polymer-solvent mixture contains significant amounts of both high and low boiling solvents, it is sometimes advantageous to superheat the oligomer mixture with respect to all solvents present (i.e., above the boiling point at atmospheric pressure of the highest boiling solvent). Superheating of the oligomer mixture may be achieved by heating the mixture under pressure. Superheating the oligomer mixture facilitates solvent removal because a substantial portion of the solvent evaporates as the mixture enters the extruder.
A condensable gas is considered superheated when it has a temperature above its boiling point at its current pressure. A solvent is therefore super heated when it has a positive degree of superheat, where the degree of superheat is defined by the expression (P|V-P,), which represents the difference between the equilibrium pressure
of the solvent in the vapor phase (P|V) and the total pressure in the space of the extruder where the devolatilization process takes place (Pt). As described above, one embodiment of superheating the oligomer mixture occurs when the oligomer mixture has a temperature greater than the atmospheric boiling point of at least one of its constituent solvents. Another embodiment of superheating the oligomer mixture occurs when the flash separation of the solvent from the oligomer mixture is accomplished by applying vacuum to the heated mixture so the surrounding pressure is lower than the vapor pressure of the solvent in the mixture. This method is also described herein as superhea ing as the degree of superheat (Piv-Pt) is a positive value. In other words, an oligomer mixture that is kept at a temperature below the boiling point of the solvent at atmospheric pressure can be in a superheated state as long as the surrounding pressure is lower than the vapor pressure of the solvent at the temperature of the oligomer mixture.
the extruder via a feed inlet mixtures comprising less than
The temperature of the oligonier mixture may be adjusted by conventional means such as, for example, a heat exchanger. The oligomer mixture is preferably introduced to
in fluid communication with the extruder. Oligomer about 30 percent by weight solvent may be too viscous
to be pumped through a heat exchanger, one of the preferred methods for heating the oligomer mixture. In such instances it is possible to heat the oligomer mixture by other means, for example, hea ing the oligomer mixture in an extruder, or a helicone mixer, or the like. The oligomer mixture may be heated by means of a first extruder. The heated oligomer mixture emerging from the first extruder may be transferred through a pressure control valve into a second devolatilizing extruder equipped according to the method with at least one vent operated at subatmospheric pressure, optionally one or more vents operated at about atmospheric pressure, and at least one side feeder equipped with at least one vent being operated at atmospheric pressure. In one embodiment, the die face df the first extruder may serve as the pressure control valve, which regulates the flpw of heated oligomer mixture into the second devolatilizing extruder. In this embodiment the heated oligomer mixture is introduced directly from the die face of the first extruder into the feed zone of the second
devolatilizing extruder. The first extruder may be any single-screw extruder or twin-screw extruder capable of heating the oligomer mixture.
When the oligomer mixture is heated above the atmospheric boiling point of the solvent (i.e., when it is pressurized), the system may comprise a pressure control valve downstream of the heat exchanger, if used, or downstream of the feed tank. The pressure control valve preferably has a cracking pressure higher than atmospheric pressure. The cracking pressure of the pressure control valve may be set electronically or manually and is typically maintained at a pressure in the range of about 1 pound per square inch (psi) (0.07 kgf/cm2) to about 350 psi (25 kgf/cm2) above atmospheric pressure. Within this range, the cracking pressure may preferably be at least about 5 psi (0.35 kgf/cm2), more preferably at least about 10 psi (0.7 kgf/cm2) above atmospheric pressure. Also within this range, the cracking pressure may preferably be up to less than about 100 psi (7.0 kgf/cm ), more preferably up to about 50 psi (3.5 kgf/cm2) above atmospheric pressure. The back pressure generated by the pressure control valve is typically controlled by increasing or decreasing the cross sectional area of the valve opening. Typically, the degree to which the valve is open is expressed as percent (%) open, meaning the cross sectional area of valve opening actually being used relative to the cross sectional area of the valve when fully opened. The pressure control valve prevents evaporation of the solvent as it is heated above its boiling point. Typically, the pressure control valve is attached (plumbed) directly to an extruder and serves as the feed inlet of the extruder. Suitable pressure control valves include, for example, those sold as RESEARCH® Control Valve, manufactured by BadgerMeter, Inc.
In one embodiment the oligomer mixture is introduced through multiple pressure control valves located on the extruder and the side feeder. The system may comprise two side feeders and two pressure control valves, the first of the pressure control valves communicating directly with the feed zone of the extruder (i.e., attached directly to the extruder), and the second of the pressure control valves being attached to one of the side feeders, the second of the pressure control valves communicating with the extruder via the side feeder. Alternatively, it is possible to have a system in which there is no pressure control valve in direct communication with the extruder,
having instead multiple si pressure control valve.
The oligomer is converte preferably conducted at a range, the melt temperature about 320°C. Also within t more preferably up to abt selecting and maintaining a ban-el temperatures of the transition temperature of th<
In general, as the feed rat increase in the screw spec material being fed to the residence time of whatevei solvent mixture. Thus, the is useful to characterize this Typically the extruder is op into the extruder in kilogr revolutions per minute (rp about 0.5, depending on th rate to screw speed where i at 400 kilograms per hour maximum and minimum among other factors, the s extruder the higher the max
; feeders each of which is equipped with at least one
to polyimide by melt kneading. Melt kneading is jmperature of about 280 to about 400°C. Within this s preferably at least about 300°C, more preferably at least is range, the temperature is preferably up to about 380°C, at 360°C. The temperature is typically controlled by separate temperature for each barrel of the extruder. The xtruder are usually set at about 120°C above the glass polyimide product.
of the oligomer mixture is increased, a corresponding must be made in order to accommodate the additional xtruder. Moreover, the screw speed contributes to the material is being fed to the extruder, here an oligomer-crew speed and feed rate are typically interdependent. It relationship between feed rate and screw speed as a ratio.
erated such that the ratio of oligomer mixture introduced
ams per hour (kg/hr) to the screw speed expressed in i) is about 0.005 to about 50, preferably about 0.01 to diameter of the extruder. For example, the ratio of feed e oligomer mixture is being introduced into the extruder into an extruder being operated at 400 rpm is 1. The fbed rates and extruder screw speeds are determined by,
ze of the extruder, the general rule being the larger the
mum and minimum feed rates.

The extruder may compris long as it is configured tc solvent as well as the down product. Exemplary exrruc screw co-rotating extruder
any number of ban-els, type of screw elements, etc., as provide sufficient volume for flash evaporation of the tream devolatilization of remaining solvent and water by-rs include a twin-screw counter-rotating extruder, a twin-a single-screw extruder, or a single-screw reciprocating
extruder. A preferred extruder is the co-rotating, intermeshing (i.e., self wiping) twin-screw extruder.
A system for carrying out the method may, optionally, further comprise one or more condensing systems to collect the solvent removed via the vent(s). The vents may be connected to a solvent removal and recovery manifold comprising solvent vapor removal lines, a condenser, and a liquid solvent receiving vessel. Any solvent collection system known in the art may be used to recover solvent via the vents.
Solvent may, optionally, be removed from the oligomer mixture before it is introduced to the extruder. This solvent removal may be effected by conventional means in, for example, the reaction vessel used to prepare the oligomer mixture. When an oligomer mixture feed tank separate from the reaction vessel is employed, solvent removal may be effected from the feed tank. Alternatively, solvent removal may be effected in an evaporator or distillation apparatus separate from the reaction vessel or feed tank.
One of the advantages of the invention is that it allows for precise control of reaction stoichiometry. This stoichiometry is primarily controlled via the additions of dianhydride compound, diamine compound, and any other reactants at the beginning of oligomer synthesis. However, the stoichiometry of the resulting oligomer mixture may be analyzed, and any unexpected stoichiometric imbalance may be corrected via addition of dianhydride compound, diamine compound, or other reactants prior to introduction of the oligomer mixture to the extruder. Such a correction may be effected, for example, in the oligomer reaction vessel, or in a separate oligomer mixture feed tank.
When the oligomer mixture is an oligomer solution, it may, optionally, be filtered before introduction to the extruder. Such a filtration may occur before and/or after heating above the solvent boiling point. A preferred solution filtration system is one that is in direct communication with the extruder via a pressure control valve attached directly to the extruder. A highly preferred solution filtration system is an in-line metal filter. Alternatively, the extruder may optionally comprise a melt filtration system for filtering the polymer melt in the extruder. Filtration may also be conducted
on the polyimide melt, melt include, for example Corporation. Additional equipment may be found i: 10/648,604, both filed Augu
Suitable filtering devices for oligomer solution and polyimide the 13 micron sintered metal filters sold by PALL d sscriptions of melt and solution filtration methods and :n co-pending U.S. Application Serial Nos. 10/648,647 and it 26, 2003.

The method may, optionaly. polyimide and another polyr i< extruder, downstream fron polycarbonate, a poly(arylei polyolefin, a polysiloxane, polymer, or the like, or a mi their preparation are knowi solid. Alternatively, the comprising the additional polymer may be added in a: the total of the polyimide
r, be used to form polymer blends comprising the ter. Thus, the method may further comprise adding to the
oligomer mixture introduction, a polymer such as a e ether), a polyester, a polysulfone, a polyetherketone, a a poly(alkenyl aromatic) compound, a liquid crystalline
ture thereof. These additional polymers and methods for
in the art. The additional polymer may be added as a additional polymer may be added as a polymer mixture polymer and a solvent. When present, the additional
amount of about 1 to about 95 weight percent, based on
:an I
the additional polymer.

The method may, optionally example, the method introduction, a filler or an fillers, non-conductive fill mixtures thereof. When about 50 weight percent about 1 to about 20 wei colorants, pigments, dyes, foaming agents, mold present, the additive may b percent, preferably about 0. of the composition.
based
rele ise
, be used to add other components to the polyimide. For may comprise adding to the extruder, downstream from solution jdditive. Suitable fillers include, for example, conductive ;, particulate fillers, fibrous fillers, and the like, and present, the filler may be used in an amount of about 0.1 to on the total weight of the composition, preferably from ,ht percent. Suitable additives include, for example, Hraviolet light stabilizers, antioxidants, heat stabilizers, agents, and the like, and mixture thereof. Where $ used in an amount of about 0.0001 to about 10 weight )00l to about 1 weight percent, based on the total weight

In another embodiment, the at least one additive prior t<
oligomer mixture may comprise at least one filler and/or its introduction into the extruder. It has been found that
the pre-dispersal of filler into the oligomer mixture allows for the efficient and uniform distribution of the filler in the resulting isolated polymer product matrix. The lower viscosity of the oligomer mixture allows for efficient mixing of the filler and polyimide with a minimized usage of energy as compared to compounding the filler and polyimide in an extruder or similar device. A further advantage of adding the' filler to the oligomer mixture rather than compounding it in an extruder is to minimize the heat history of the polyimide.
In some situations, it may be desirable to add more components than can be conveniently added to a single extruder, or to reduce the residual solvent level below that achievable with a single extruder. In such situations, the method may, optionally, further comprise introducing the polyimide into a second extruder. Thus, the extruder into which the oligomer mixture is first introduced may be coupled to a second extruder, the second extruder optionally being equipped with one or more subatmospheric or atmospheric vents for the removal of residual solvent. The second extruder may be closely coupled to the initial extruder thereby avoiding any intermediate isolation and re-melting steps. The use of a second extruder in this manner is especially beneficial during operation at high throughput rates where the residence time of the polyimide in the initial extruder is insufficient to achieve the desired low level of residual solvent. The second extruder may be any extruder such as a twin-screw counter-rotating extruder, a twin-screw co-rotating extruder, a single-screw extruder, or a single-screw reciprocating extruder. Where the second extruder comprises a plurality of vents, some vents may be operated at atmospheric pressure while others are operated at subatmospheric pressure.
One embodiment is a method of preparing a polyimide, comprising: introducing to an extruder an oligomer .mixture comprising an oligomer and a solvent, wherein the oligomer mixture is superheated; removing solvent from the oligomer mixture via a vent upstream from oligomer mixture introduction and a vent downstream from oligomer mixture introduction; and melt kneading the oligomer to form a polyetherimide. Solvent removal may, optionally, be effected by removing solvent via at least two vents downstream from solution introduction. The vent upstream from oligomer mixture introduction is preferably maintained at a pressure in the range of

about 10 to about 760 mil! pressure may preferably be range, the upstream vent p mercury. The vent down maintained at a pressure of range, the downstream ven of mercury, more preferabl; range, the downstream ven mercury, more preferably u may, optionally, be located feeder provides for added entrained by the escaping percent, preferably from al oligomer mixture is removi any solvent remaining is
icters of mercury. Within this range, the upstream vent t least about 50 millimeters of mercury. Also within this ssure may preferably be up to about 750 millimeters of ream from oligomer mixture introduction is preferably jout 10 to about 500 millimeters of mercury. Within this pressure may preferably be at least about 25 millimeters at least about 50 millimeters of mercury. Also within this jressure may preferably be up to about 300 millimeters of to about 200 millimeters of mercury. The upstream vent n a side feeder. Location of the upstream vent on a side olume and serves to trap and return polymer particles olvent vapors. Generally, from about 50 to about 99 ut 90 to about 99 percent of the solvent present in the through the upstream vent(s) and a substantial portion of through the downstream vent(s).
One embodiment is a meth to an extruder an oligomer oligomer is formed by phenylenediamine, wherein the oligomer mixture is sup a vent upstream from olig from oligomer mixture int from oligomer mixture ir mixture introduction is mai millimeters of mercury, th maintained at a pressure o vent still further down stre pressure of about 10 to a oligomer to form a polye weight of residual solvent.
of preparing a polyetherimide, comprising: introducing ixture comprising an oligomer and a solvent, wherein the reaction of bisphenol A dianhydride and meta-ic solvent comprises ortho-dichlorobenzene, and wherein rheated; removing solvent from the oligomer mixture via ner mixture introduction, at least one vent downstream iduction, and at least one vent still further downstream reduction; wherein the vent upstream from oligomer tained at a pressure in the range of about 10 to about 760 vent downstream from oligomer mixture introduction is about 250 to about 500 millimeters of mercury, and the m from oligomer mixture introduction is maintained at a ut 100 millimeters of mercury; and melt kneading the erimide comprising less than 500 parts per million by
The method may include oligomer synthesis. Thus, one embodiment is a method of preparing a polyimide, comprising: reacting a dianhydride compound and a diamine compound in the presence of a solvent to form an oligomer mixture comprising an oligomer and a solvent; introducing the oligomer mixture to an extruder; removing solvent from the oligomer mixture via a vent in the extruder; and melt kneading the oligomer to form a polyimide. Oligomer synthesis may, optionally, comprise removing water formed as a product of oligomerization and/or imidization.
The invention further includes the polyimides prepared by any of the above methods. Polyimides isolated according to the methods described herein may be transformed into useful articles directly, or they may be blended with one or more additional polymers or polymer additives and subjected to injection molding, compression molding, extrusion methods, solution casting methods, and like techniques to provide useful articles. Injection molding is frequently the preferred method of forming the useful articles.
The figures provide non-limiting, illustrative examples of systems suitable for performing the method. FIG. 1 is a simplified diagram of a polyimide preparation system 10. Reactor 20 is charged via inputs including dianhydride compound addition 510, diamine compound addition 520, and solvent addition 530. Reaction forms an oligomer mixture 540, which flows to oligomer mixture holding tank 50. A gear pump 30 transfers the oligomer mixture to the extruder 80. The extruder has a single vent 100, located downstream of oligomer mixture addition. Volatiles are pumped to a vacuum 560 source via a condenser 110. The extruder employs mixing and kneading elements in a reaction zone 120 to convert the oligomer to polyimide, which exits the extruder at die 130 as an extrudate 600.
FIG. 2 is a simplified diagram of another polyimide preparation system 10. Reactor 20 is charged via inputs including dianhydride compound addition 510, diamine compound addition 520, and solvent addition 530. Other components, such as a monoanhydride chain-stopper, may, optionally, be added. Reaction forms an oligomer mixture 540, which is preferably an oligomer solution. The oligomer mixture flows to oligomer mixture holding tank 50. A gear pump 30 transfers the
oligomer mixture to the elements in four reaction zo the extruder, conveying one upstream and five dowi four vents (counting upstrea and maintained at a low source, such as a vacuum condenser 110. The fifth an 1 mm Hg) by a high vacuum volatiles are removed by a c<
extruder 80. The extruder employs mixing and kneading 120, which are in between vents. In other sections of elements are used. The extruder pictured has six vents 100, stream of the oligomer mixture addition point. The first n to downstream) are combined at vacuum manifold 3 60 pressure (e.g., 25-100 mm Hg) by a moderate vacuum 580 pump. Any condensable volatiles are removed by a sixth vents are maintained at a low pressure (e.g., 25-50 590 source, such as a vacuum pump. Any condensable Id trap 170. The extrudate 600 is formed at the die 130.

thin
condens able maintained
FIG. 3 is a simplified diagn 20 is charged via inputs i compound addition 520, s Reaction forms an oligomei The oligomer mixture is mixture holding tank 50. The stoichiometry may, compound addition 510 and transfers the oligomer mix mixture to a temperature oligomer mixture is a unit 70. The oligomer extruder employs mixing sections of the extruder, con vents 100, three upstream point. The first four vents pressure only slightly less 570 source, and any and sixth vents are moderate vacuum 580 sou condenser 110. The sevent
m of another polyimide preparation system 10. Reactor eluding dianhydride compound addition 510, diamine jlvent addition 530, and chain stopper addition 535. mixture 540, which is preferably an oligomer solution, nped via gear pump 30 and flow meter 40 to oligomer ere, evaporated solvent 550 may, optionally, be removed. r, be adjusted via inputs including dianhydride diamine compound addition 520. Another gear pump 30 ure through a heat exchanger 60 to heat the oligomer the atmospheric boiling point of the solvent. If the , it may, optionally, be filtered by a solution filtration enters the extruder 80 via the inlet valve 90. The kneading elements in the reaction zones 120. In other eying elements are used. The extruder pictured has eight md five downstream of the oligomer mixture addition (counting upstream to downstream) are maintained at a atmospheric (e.g., 500-760 mm Hg) by a weak vacuum volatiles are removed by a condenser 110. The fifth at a moderate pressure (e.g., 250-500 mm Hg) by a ce, and any condensable volatiles are removed by a and eighth vents are maintained at a low pressure (e.g.,
25-100 mm Hg) by a high vacuum 590 source, and any condensable volatiles are removed by a condenser 110. For high vacuum maintained via a vacuum pump, it may be preferred that the condenser is a cold trap. The extrudate 600 is formed at the die 130. Melt temperature and pressure may, optionally, be monitored at the die. Low levels of residual solvent in the extrudate 600 are facilitated by stripping agent injection 610, which is effected by a stripping agent source 140 and a pump 150.
All ranges disclosed herein are inclusive of the endpoints, and the endpoints are combinable with each other.
The invention is further illustrated by the following non-limiting examples. PREPARATIVE EXAMPLE 1
This example describes the preparation of an oligomer mixture. The reactive mixture consisted of 14,000 grams of bisphenol A dianhydride (BPADA), 2,940.98 grams of meta-phenylenediamine (mPD), 177.1 grams of phthalic anhydride (PA), and 30,109 grams of ortho-dichlorobenzene (o-DCB). A 50-gallon glass reactor was charged with o-DCB, BPADA, and PA, and the mixture stirred at room temperature overnight. The mixture was then heated to 160°C under nitrogen, and molten mPD was added in increments to the reactive mixture over a period of about 24 minutes. As water started to form in the reactor, the temperature of the reactive mixture decreased from about 157.TC at the beginning of the mPD addition to about 146.8°C at the end. Samples of the solution mixture taken from the reactor at different times showed that the molecular weight of the oligomer (Mw/Mn/PDI) was 18966/8767/2.163 after 30 minutes of reaction, and 25013/11197/2.234 after about 45 minutes from the time when the mPD was completely added in the reactor. This solution containing oligomers having a weight average molecular weight of about 25,000 AMU was dropped into a non-stirred heated tank and'used as feed to the reactive extruder for polymerization and devolatilization.
EXAMPLES 1-5
The oligomer solution frond Preparative Example 1, containing about 35 weight
percent of oligomer in o-DC kept at about 165-170°C. Tl
J was continuously fed to an extruder from a heated tank e extruder set-up was similar to that described for Figure



2, above. The extruder w is a 25 millimeter diameter, co-rotating, intermeshing
ype, and it included 14 barrels (total extruder length to
extruder of the twin-screw diameter ratio, L/D, of 56), The oligomer solution was DCB was removed from the 1 (back vent), and 5, 7, 9,1 operated at a relatively h
»nd six vents for the elimination of volatile components, idded to the extruder at barrel number 4. The solvent o-process through vacuum vents located at barrels number , and 13 (forward vents). All of the extruder vents were gh level of vacuum corresponding to a pressure of



approximately 25 millimete s of mercury. Vent 2, at barrel 5, did not have a vacuum
gauge. The screws of the e: times for reaction, and
trader were designed to provide relatively long residence ntense surface area renewal for efficient flash/trace

devolatilization balance. Tl e extruder was run at about 5 kilograms/hour of polymer, 250 rpm screw speed, 80-'0% of maximum torque (maximum torque is typically

about 100 newton-meters)
and about 385°C melt temperature. There were no
processing problems (stran 1 dropping, vent plugging, etc.) with the polymerization
and devolatilization of olig up of molecular weight o
mer solution through the reactive extruder. Some build-the polymer in the solution inside the feed tank was
observed during the appn iximately two hours it took for the experiment to be
completed, from a weight
average molecular weight of about 25,000 AMU at the
beginning of the experime: it to about 39,000 AMU at the end. No stabilizers were added to the oligomer solut on fed to the extruder.
The polyimide produced b this method showed a weight average molecular weight of
about 52,500 to 54,500, \rith a polydispersity between about 2.5 and 2.6. These molecular weight properties are similar to those of the polyetherimide sold commercially by General Electric Company as ULTEM® 1000. The polyimide also exhibited rheological prop( rties indistinguishable from those of ULTEM® 1000. Five extrudates were taken at A arious times corresponding to modest variations in barrel temperatures during the tw o hours of extrusion. Barrel temperatures were monitored
in eight zones; a ninth temperature measurement was conducted at the die face. See Table 1.
Levels of residual ortho-dichlorobenzene (o-DCB) were determined by gas chromatography using a Hewlett Packard 6890 gas chromatograph as follows. A precisely weighed sample of about 0.25 grams of polyimide pellets was placed in a 20 milliliter vial. To the vial was added five milliliters of a solution of 4 milliliters bromobenzene in 4 liters HPLC-grade dichloromethane. The vial was capped and shaken until all solids had dissolved. A portion of the resulting solution was transferred to a gas chromatography sample vial and capped. The sample was analyzed by gas chromatography using flame ionization detection. Ortho-dichlorobenzene was quantified by comparison of the sample o-DCB peak area to peak areas for previously run o-DCB standard solutions. Each of the five extrudates had an o-DCB concentration less than the 10 parts per million by weight limit of detection for the test.
The extrudates had solution yellowness index (YI) values between 13 and 14 measured according to ASTM E313, molded Yl values between 80 and 84 measured according to ASTM D6290, and they were 0.3-0.4 mole percent amine rich relative to the target stoichiometry of equimolar dianhydride and diamine. The glass transition temperature (Tg) of the polyimide was 219.5°C.
Results are given in Table 1. They show that the polyimides had high molecular weights, low polydispersities, and acceptable yellowness indices.
Table 1

(Table Removed) PREPARATIVE EXAMPLE 2
The reactive mixture consisted of 14,050 grams of BPADA, 2,941 grams of mPD, 177.1 grams of PA, and 30.1 kilograms of o-DCB. The BPADA amount was 50 grams greater than that in Preparative Example 1 to compensate for the amine-rich product of that example. A 50-gallon glass reactor was charged with o-DCB, BPADA and PA, and the mixture stirred at room temperature overnight. The mixture was then heated to 160°C under nitrogen, and molten mPD was added in increments .to the reactive mixture over a period of about 36 minutes. As water started to form in the reactor, the temperature of the reactive mixture decreased from about 155.2°C at the beginning of the mPD addition to about 152.5°C at the end. Samples of the solution mixture taken from the reactor at different times showed that the molecular weight of the oligomer (Mw/Mn/PDI) was 25873/11060/2.339 after 15 minutes of reaction, and 28544/12225/2.335 after about 45 minutes, both taken from the time when the mPD was completely added in the reactor. This solution containing oligomers having a weight average molecular weight of about 28,500 was dropped into a non-stirred, heated tank and used as feed to the reactive extruder for polymerization and devolatilization. Since the sample taken after 15 minutes in the reactor was shown to

have aim stoichioraetry (0.12 correction was made in the minutes (without stirring) befij
EXAMPLES 6-13
mole% of di anhydride and 0.127 mole% of amine), no reacting solution as this was left to react for another 30 ire dropping it into the feed tank of the extruder.

A solution from Preparative oligomer in o-DCB, was con1 about 140-150°C. The ej
Example 2, containing about 35 percent by weight of linuously fed to an extruder from a heated tank kept at (trader was a 25 millimeter diameter co-rotating,

intermeshing extruder of the rjvin-screw type, and it included 14 barrels (L/D=56), and

six vents for the eliminatiori
of volatile components. The oligomer solution was
added to the extruder at barrejl number 4. The solvent o-DCB was removed from the process through vacuum vents located at barrels number 1 (back vent), and 5, 7, 9, 11

and 13 (forward vents). All

pf the extruder vents were operated at a relatively high



level of vacuum (corresponding to an absolute pressure of about 25-50 millimeters of
mercury). Vents 2 (at barre] screws of the extruder were reaction, and intense surface]5) and 6 (at barrel 13) lacked pressure gauges. The
designed to provide relatively long residence time for
area renewal for efficient flash/trace devolatilization

balance. The extruder was ru^i at about 5 kilograms/hour of polymer, 250 rpm screw

speed, 80-90% of maximum

:orque, and about 385°C melt temperature. There were

no processing problems (strand dropping, vent plugging, etc.) with the polymerization and devolatilization of the olij ;omer solution through the reactive extruder.

Some build-up of molecular observed during the approxj completed, from a weight a beginning of the experiment to the feed tank was turned reaction in the holding tank, the extruder.
weight in the oligomer solution inside the feed tank was imately 2.5 hours it took for the experiment to be rerage molecular weight of about 28,500 AMU at the about 47,000 AMU at the end. The heater connected off during the experiment in an effort to prevent further stabilizers were added to the oligomer solution fed to

Product polymer composition > were analyzed for the concentration of residual amine and anhydride endgroups by ] TIR on films prepared from the product polymers on a Carver press. The Fourier transform infrared (FTIR) absorption spectrum was

measured and the intensities of the IR absorption bands for amine and anhydride functional groups were compared with the intensity of an absorption band chosen from the sample spectrum to serve as the reference absorption band. The resultant values were then compared to a set of calibration values obtained from a series of films prepared from standards, of the product polymer containing known amounts of amine and anhydride functional groups and analyzed using an identical FTIR method to produce a quantitative measure of amine and anhydride endgroup concentration in the product polymer composition. End group concentrations are expressed in mole percent relative to moles of polyimide.
Extrusion conditions and analytical results for eight extrudates are summarized in Table 2. The polyimide produced by this method showed a weight average molecular weight of about 52,200 to 54,900, with a polydispersity of about 2.6 to 2.8. These molecular weight properties are similar to those of the polyetherimide sold commercially by General Electric Company as ULTEM® 1000. The polyimide also exhibited Theological properties indistinguishable from those of ULTEM® 1000. The eight extrudates analyzed had residual o-DCB levels less than or equal to6 ppm, and solution YI values of about 14 to 17, and they were slightly (0.02-O.OS mole percent) dianhydride rich.
Table 2

(Table Removed)
COMPARATIVE EXAMPLE 1
A lab-scale experiment was run using starting materials similar to those used in Preparative Example 1, but rather than stopping at an oligomer solution, the reaction was run to completion of polymerization and imidization. The reactive mixture consisted of 75 grams of BPADA, 15.76 grams of mPD, 0.95 grams of PA, and 99.76 milliliters of o-DCB. A 250 ml glass reactor was charged with o-DCB, BPADA,
mPD and FA, and the mixtu e the bulk of the water of i After that, the at the boiling'point and the collected from the 8 and 11 hours. The results show that react a mixture of BPADA, obtain a polyetherimide ha illustrated by the working obtained using the process o: ortho-dichlorobenzene for intermediate molecular weigl t designed for reaction and polymer to the final level in. The residence time of tl e less than about 2 minutes, produce a polyetherimide o solvent left was therefore sul
: temperature of the heating
; reactor at i eaction Molecular w sight analysis of the relativ ;ly long:
The heated to 165 C for approximately one hour whereby irajidization was removed by co-distillation, with o-DCB. oil bath was raised to allow reflux of o-DCB reaction mixture was stirred for 11 hours. Samples were times of 30,50, and 75 minutes, and 2,3,4, 5,6, lese samples is summarized in Table 3. reaction times (up to 11 hours) were needed to ijiPD and PA in a solution with ortho-dichlorobenzene to 'ing an ULTEM® 1000-like molecular weight. As ;xamples above, a polymer of similar properties was this invention by pre-reacting BPADA, mPD and PA in 45 minutes in a batch reactor to obtain oligomers of , and then processing this mixture on a reactive extruder devolatilization to build up the molecular weight of the separate the polymer from the solvent it was dissolved oligomer mixture in the devolatilization extruder was total time required for the process of this invention to high molecular weight having only traces of residual slant i ally shorter than that for the conventional method. While not wishing to be bcjund believe that the better effr polyetherimides of high batch reactor but no extrude extruder to generate interfa rotation of the screws inside for the chemical reaction to reaction so the equilibrium' relatively large quantities o combination of heat, vacuun that the polydispersities of t|he method and the reactor-
by any particular hypothesis, the present inventors iency of the reactor-extruder combination to produce mcjlecular weight, compared with the method that uses a may be explained in terms of the ability of the reactive ial surface area for reaction and devolatilization. The the extruder contribute to bringing the reactants together ([ccur, eliminating the water produced by the condensation can be moved forward, and ultimately eliminating the solvent contained in the reactive solution through the and surface area renewal. It is also worth mentioning polyetherimide polymers produced by the fully batch extnjder combination process of this invention are very similar
suggesting that the residence time provided by the process of this invention to prepare polyetherimides may be adequate to make polyetherimides of the same molecular architecture than the current commercial process. In this respect, it can be said that the process of this invention combines the advantages of the traditional solution polymerization and melt polymerization processes by pre-reacting the monomers in a batch step to ensure a better control of the stoichiometry of the reactive system, and then, once oligomers of the right polydispersity have been formed, the extruder provides the right environment to finish off the polymerization reaction while separating the polymer from the solvent in one single processing step at relatively short residence times.
Table 3
(Table Removed) PREPARATIVE EXAMPLE

Using a procedure similar to weight percent solution of o igo: BPADA, 2954.91 grams of InPD DCB. The oligomers had a A phosphorus stabilizer, obti ined the oligomer solution at 2,200
that described for the preparative examples above, a 35 was prepared by the reaction of 14,075 grams of ; and 197.45 grams of PA in 30.1 kilograms of o-veight average molecular weight of about 32,000 AMU. from Ciba Geigy as IRGAFOS® 168, was added to parts by weight per million weight oligomer.

EXAMPLES 14-19

'WO
solu :ion
point
The oligomer solution fron extruder from a heated feed rotating intermeshing extrud die plate. The extruder had side feeder attached at barrel of the oligomer feed, which barrels 1, 2, and 4 were mai 735 millimeters of mercury), lower pressures (see values ir shell-and-tube condensers. ./ measured barrel temperatures introducing the oligomer a 30 weight percent solution run continuously through thi heated above the boiling oligomer solution tank and
Preparative Example 3 was continuously fed to an tank. The extruder was a 25 millimeter diameter co-r with ten barrels (length/diameter=40) and a two-hole
vents upstream (one at barrel 1 and one located on a 2) and four vents downstream (at barrels 4, 5, 7, and 9) was at the downstream edge of barrel 2. The vents at tained at slightly less than atmospheric pressure (about The vents at barrels 5, 7, and 9 were maintained at much Table 4). Volatiles from all vents were condensed with 11 extruder barrels were set at a temperature of 350°C; are given in Table 4 as a function of sample. Prior to
, the extruder was purged with about 39 kilograms of f ULTEM® 1010 polyetherimide in o-DCB, which was extruder for over an hour. The oligomer solution was
of o-DCB via a heat exchanger in line between the
oligomer feed valve on the extruder. A 13-micron
sintered metal filter, obtained from PALL, was installed in the oligomer solution feed line upstream of the extruder to eliminate particulate contaminants. The extrusion isolation process ran well for the entire three-hour experiment without any operator intervention or vent maintenance. Six extrudates were collected and analyzed. Each looked clear with a glossy surface, like a commercial ULTEM® polyetherimide. Concentrations of anhydride end groups and amine end groups were determined as described above. Extrusion conditions and results are summarized in Table 4. Comparisons of weight average molecular weight of oligomer contained in the feed solutions and the corresponding extrudates indicate increases by factors of about 1.25 to about 1.55. The results also show that extruded samples contained acceptably low levels of residual solvent, ranging irom 273 to 458 ppm, even when the extruder had a relatively low length to diameter ratio of 40. Residual o-DCB concentrations were all below 500 ppm. Surprisingly, solution yellowness index values were similar to those of commercial ULTEM resins prepared by a much for time- and capital-intensive process. Relatively high concentrations of residual amine and anhydride end groups in the final polyimide suggest that the reaction was not carried out to completion in the extruder, possibly due to the shorter residence time in the ten-barrel extruder compared to the fourteen-barrel extruder. This is also reflected in the weight average molecular weights of these extrudates, where were lower than the expected theoretical molecular weights of about 52,000 to 54,000. Nevertheless, molded plaques prepared from the resins of these examples exhibited good properties under Dynatup (falling dart) conditions (52.5 foot-pound = 71.2 joules of energy, 100% ductility at 100°C, compared to a commercial control (54.4 foot-pound = 73.8 joules of energy, 100% ductile).
Table 4

(Table Removed)
PREPARATIVE EXAMPLE 4
Using a procedure similar to weight percent solution BPADA, 3,546.8 grams of mP
at described for the preparative examples above, a 35 of oligbmer was prepared by the reaction of 16,890 grams of >, and 236.9 grams of PA in 36.5 kilograms of o-DCB.

EXAMPLES 20-29

The oligomer solution from extruder from an insulated, diameter co-rotating intermes 56) and a two-hole die plate, barrels 1 and 4) and four v oligomer feed, which was at t 7, and 9 were maintained at the house vacuum system, pressure of about 25 millime vents 1,4,7, and 9 were con vents 11 and 13 were condens under the feed and all vents, The oligomer solution was oligomer solution for Examp solution for Examples 23-29 barrels were set at a temperati Table 4 as a function of extruder was purged with UL' dispensed from an insulated analyzed. The results, in T weight average molecular wei show that the final molecul insensitive to the molecular v/ levels were extremely low. commercial resins. The ve polymerization reaction has residence time of the fourte
and
Preparative Example 4 was continuously fed to an uilheated feed tank. The extruder was a 25 millimeter ng twin-screw extruder with fourteen barrels (L/D = Fhe extruder had two vents upstream (just upstream of its downstream (at barrels 7, 9, 11, and 13) of the e upstream edge of barrel 5. The vents at barrels 1, 4, pressure of about 25-50 millimeters of mercury using ic vents at barrels 11 and 13 were maintained at a rs of mercury using a vacuum pump. Volatiles from ensed with a shell-and-tube condenser. Volatiles from d in a cold trap. The extruder had conveying elements kneading blocks in the reaction zones between vents, ed using a positive displacement gear pump. The s 20-22 contained.no added stabilizer; the oligomer ntained 2,200 ppm of ER.GAFOS® 168. All extruder e of 350°C; measured barrel temperatures are given in Prior to introducing the oligomer solution, the iM® 1000 polyetherimide. The oligomer solution was unheated tank. Ten extrudates were collected and le 5, show that the extrusion process increased the it by factors of about 1.1 to about 1.9. The results also weight distribution of the polyimide is remarkably ght distribution of the feed solution. Residual o-DCB The extruded resin appeared slightly darker than low levels of amine end groups suggest that the pproached completion, possibly due to the longer n-barrel extruder compared to that of the ten-barrel
extruder used in Examples 14-19. The resin was formulated to be slightly anhydride-rich for increased thermal stability.
Table 5 (Table Removed)
PREPARATIVE EXAMPLE
Using a procedure similar to weight percent solution of ol BPADA, 2,588 grams of 4,4' diphthalic anhydride (ODPA) reaction was carried out by required about one hour at reactor temperature was reaction temperature was abo like a thin paste. There stoichiometric analysis i overnight and further molecu the o-DCB was removed. Th and a sampling after 40
that described for the preparative examples above, a 30 gomer was prepared by the reaction of 3,648 grams of diaminodiphenylsulfone (DDS), 982 grams of 4,4'-oxy-and 108 grams of PA in about 32 liters of o-DCB. The fjrst dissolving the DDS and BPADA in o-DCB, which 00°C. The ODPA and PA were then added, and the gradually raised. The water product evolved when the 1145-150°C. At that point, the reaction mixture looked was a slow molecular weight build. Preliminary indicated an amine excess. The reaction was cooled to 160°C ar weight build took place. The next morning, most of anhydride correction was made by addition of BPADA, minutes showed a slight anhydride excess. A slight amine
correction was made by addition of DDS with the intention of leaving a slight excess of anhydride.
EXAMPLES 30-33
The oligomer solution from Preparative Example 5 was continuously fed to an extruder from the titanium reactor in which it was prepared. The extruder was a 25 millimeter diameter co-rotating intermeshing twin-screw extruder with fourteen barrels (L/D = 56), a two-hole die plate, and two vents upstream of oligomer addition and four vents downstream of oligomer addition. Conveying screw elements were used under the feed and all vents. Screw kneading blocks were used in the reaction zones between vents. Oligomer was fed just upstream of barrel 5. Vents at barrels 1, 4, 7, and 9 were connected to house vacuum (ca. 50-75 mm Hg) with a shell-and-tube condenser, and vents at barrels 11 and 13 were connected to a vacuum pump (ca. 25 mm Hg absolute pressure) with a cold trap. All barrels were set to a temperature of 350-375°C. The oligomer solution was fed by a gear pump from the reactor, which was maintained at 180°C. Extrusion conditions and results are summarized in Table 6. Based on a pre-extrusion weight average molecular weight of about 19,800 AMU, the weight average molecular weight increased by a factor of about 1.4 on extrusion. These examples demonstrate that the process of the invention can be used to prepare polyimides with relatively high glass transition temperatures. The glass transition temperatures of the polyimides prepared in these examples were about 255-257°C for the four samples measured. These values may be compared to values of about 217°C for polyetherimides prepared from BPADA and mPD. Residual solvent levels were extremely low, and the color of the resin was similar to that of other commercially-available polyimides. The residual concentration of amine end groups in the polyimide was relatively low, suggesting that the polymerization was carried out to near-completion in the extruder. The polyimide was anhydride-rich, as it was purposefully formulated to be. The polyimide exhibited low polydispersity.
Table 6
(Table Removed)
*TLTM = too low to measure EXAMPLES 34-37
This experiment used an oligomer solution similar to that described in Preparative Example 5 . It was continuously fed to an extruder from the titanium reactor in which it was prepared. The extruder was a 25 millimeter diameter co-rotating intermeshing twin-screw extruder with fourteen barrels (L/D = 56), a two-hole die plate, and two vents upstream of oligomer addition and four vents downstream of oligomer addition. Conveying screw elements were used under the feed and all vents. Screw kneading blocks were used in the reaction zones between vents. Oligomer was fed just upstream of barrel 5. Vents at barrels 1, 4, and 7 were connected to house vacuum (ca. 50 mm Hg of absolute pressure) with a shell-and-tube condenser. The vent at barrel 9 was connected to its own vacuum pump (ca. 25 mm Hg of absolute pressure) and cold trap, and the vents at barrels 11 and 13 shared a vacuum pump (ca. 25 mm Hg of absolute pressure) and cold trap. All barrels were set to a temperature of 375°C. The oligomer solution was fed by a gear pump from the reactor, which was maintained at 180°C. Extrusion conditions and results are summarized in Table 7 (extrusion conditions were not recorded for Example 37). The results show that polyimides of high molecular weight, low polydispersity, and low residual solvent can be prepared using the method of the invention. The low concentration of amine end groups in the polyimide suggests that the polymerization reaction in the extruder was nearly complete.
Table 7(Table Removed)
While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
All cited patents, patent applications, and other references are incorporated herein by reference in their entirety.

CLAIMS:
1. A method of preparinj; a polyimide, comprising:

introducing to an extruder oligomer formed by the reaction compound;
0) an oligomer mixture comprising a solvent and an of a dianhydride compound and a diamine

removing solvent from the oli joiner mixture via a vent (100) in the extruder (80); and melt kneading the oligomer to form a polyimide.
2. The method of claim 4, wherein the oligomer comprises a number average of at least three repeating units.

3. The method of claim repeating units at least two the oligomer.
4. The method of claim per million by weight of resid
5. The method of claim is introduced to the extruder
, wherein the polyimide comprises a number average of ts greater than the number average of repeating units in
, wherein the polyimide comprises less than 1,000 parts al solvent.
1, wherein the oligomer mixture is superheated when it 0).

6. The method of introduction rate, in kilogran s minute, of about 0.005 to abo
clai n
1, characterized by a ratio of oligomer mixture per hour, to extruder screw speed, in rotations per t50.

7. The method of claim and/or a diamine compound t mixture to the extruder (80).
8.
The method of claim
1, further comprising adding a dianhydride compound the oligomer mixture prior to introducing the oligomer
, further comprising melt filtering the polyimide.
9. The method of claim 1, further comprising filtering the oligomer mixture prior
to introducing it to the extruder (80).
10. The method of claim 1,
wherein the dianhydride compound comprises bisphenol A dianhydride, the diamine compound comprises meta-phenylenediamine, and the solvent comprises ortho-dichlorobenzene; and wherein the oligomer mixture is superheated;
wherein removing solvent from the oligomer mixture comprises removing solvent via a vent (100) upstream from oligomer mixture introduction, at least one vent (100) downstream from oligomer mixture introduction, and at least one vent (100) still further downstream from oligomer mixture introduction; wherein the vent (100) upstream from oligomer mixture introduction is maintained at a pressure in the range of about 10 to about 760 millimeters of mercury, the vent (100) downstream from oligomer mixture introduction is maintained at a pressure of about 250 to about 500 millimeters of mercury, and the vent (100) still further down stream from oligomer mixture introduction is maintained at a pressure of about 10 to about 100 millimeters of mercury; and
wherein the polyimide comprises less than 1000 parts per million by weight of residual solvent.