Processes for the anionic polymerization of lactam accelerated by the presence of nitrothiophene and certain derivatives thereof. The processes are especially applicable to the preparation of poly-2-pyrrolidone which can be molded into filaments, films, and shaped articles in general.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to improved polymerization processes for the 
polymerization of lactams which are conducted in the presence of 
nitrothiophene and/or certain derivatives thereof which accelerate the 
polymerization. In a further aspect, this invention relates to accelerator 
compositions comprising such nitrothiophene compounds. 
2. The Prior Art 
Polylactams, such as poly-2-pyrrolidone (nylon-4) and polycaprolactam 
(nylon-6), are produced by the anionic (alkaline-catalyzed) polymerization 
of the lactam. The catalyst usually used comprises the reaction product of 
a lactam with an alkali metal, or quaternary ammonium hydroxide, or a 
source of alkali or alkaline earth metal, such as the hydroxide or 
alkoxide. The reaction product is generally recognized to be a lactamate, 
e.g., a salt, such as potassium pyrrolidonate, the product of the reaction 
between potassium and 2-pyrrolidone, having the formula: 
##STR1## 
The salt consists of a cationic species such as K.sup.+, Na.sup.+, 
Ca.sup.++, N(CH.sub.3).sub.4.sup.+, etc., depending on the source of the 
catalyst, and an anionic species which can be a pyrrolidonate ion, a 
caprolactamate ion, etc. depending on the choice of lactam. Polymerization 
initiators and/or activators can be present during the polymerization 
reaction. 
Various polymerization process have been suggested by the prior art, one of 
which is disclosed in U.S. Pat. No. 3,721,652, in which carbon dioxide is 
used as an activator for the polymerization of pyrrolidone. This patent 
also states that although it is preferable to use carbon dioxide as the 
sole activator, that other activators could also be used in combination 
with carbon dioxide. 
As with most commercial processes, it would be desirable to increase the 
polymerization rate, thus reducing the size of the process equipment 
capacity and processing time required. One of the problems with 
polymerization accelerators is that they frequently cause an inferior, 
very low-molecular-weight polymer to be produced. Accordingly, it has now 
been discovered that by the use of certain nitrothiophene compounds, the 
rate of polymerization can be substantially increased, without 
substantially affecting polymer quality. 
BRIEF SUMMARY OF THE INVENTION 
In summary, the process of the present invention comprises polymerizing a 
mixture of lactam, anionic catalyst an activator (e.g. carbon dioxide) and 
a catalytically effective amount of 2-nitrothiophene, alkyl derivatives 
thereof, or mixtures thereof. 
In summary, another process of the present invention comprises polymerizing 
a mixture of lactam, anionic catalyst, a polymerization activator, e.g., 
carbon dioxide, a catalytically effective amount of 2-nitrothiophene, 
and/or alkyl derivatives thereof, and an amount of a tetra(lower alkyl) 
ammonium halide effective to increase the molecular weight of the polymer 
product without substantially reducing the rate of polymerization. 
FURTHER DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS 
In the present process a mixture comprising the desired lactam monomer, 
anion catalyst, an activator, e.g., carbon dioxide, and a catalytically 
effective amount of certain 2-nitrothiophene compounds or mixtures thereof 
is polymerized. Although broadly described as a catalytically effective 
amount, the 2-nitrothiophene compound more specifically acts as an 
accelerator increasing the polymerization rate. 
The 2-nitrothiophene compounds which can be used in the present process are 
nitrothiophenes and alkyl-2-nitrothiophenes and can be conveniently 
represented by the following generic formula 
##STR2## 
wherein R is hydrogen or lower alkyl having 1 through 4 carbon atoms, and 
can be attached at the 3, 4 or 5-position of the thiophene ring. 
Suitable 2-nitrothiophene compounds which can be used include, for example, 
2-nitrothiophene; 3-methyl-2-nitrothiophene; 4-methyl-2-nitrothiophene; 
5-ethyl-2-nitrothiophene; 3-n-propyl-2-nitrothiophene; 
4-isopropyl-2-nitrothiophene; 5-n-butyl-2-nitrothiophene; 
3-t-butyl-2-nitrothiophene, and the like. Also, mixtures of different 
2-nitrothiophene compounds can be used. Generally, it is preferred to use 
2-nitrothiophene, since it affords very good results and is relatively 
inexpensive as compared with the other 2-nitrothiophene compounds. 
The 2-nitrothiophene compounds are generally known compounds and can be 
obtained from commercial sources or prepared by known procedures or 
obvious modifications thereof (e.g., substitution of appropriate 
substrates and solvents). 
Typically about from 0.005 to 1%, preferably about from 0.01 to 0.5% by 
weight, based on the total lactam, of the 2-nitrothiophene compound is 
used. Lower amounts of the 2-nitrothiophene compound are generally 
ineffective to produce a significant increase in the polymerization rate, 
and higher amounts produce very low molecular weight polymers which have 
inferior properties. Best results are typically obtained using about from 
0.01 to 0.2 weight percent, based on total lactam. 
Polymerization Conditions 
The polymerization process of this invention is applicable to the 
polymerization of lactams under anionic polymerization conditions, i.e. 
alkaline catalyzed polymerization. Also, as is well recognized by the art, 
the polymerization should preferably be conducted under substantially 
anhydrous conditions. The process is generally applicable to the 
polymerization of lactams having from 5 through 13 atoms in their rings 
and is especially applicable to the polymerization of 5-7 membered-ring 
lactams, such as, for example, the polymerization of epsilon-caprolactam 
to polycaprolactam (nylon-6) and the polymerization of 2-pyrrolidone to 
poly-2-pyrrolidone (nylon-4). The process is especially preferred for the 
polymerization of 2-pyrrolidone, since it affords good yields of high 
molecular weight poly-2-pyrrolidone in relatively short polymerization 
times. The polymer can be made into film and shaped articles in general by 
molding or extrusion. The polymer can also be melt-spun, wet-spun or 
dry-spun into filaments having substantial orientation along the 
filamentary axes, high tensile strength, and other properties desirable 
for textile fibers. This is especially important since the primary 
commercial use for polymers such as nylon-4 and nylon-6 is as synthetic 
fibers. 
The anionic catalyst used in the present process is typically and 
conveniently a lactamate salt. This catalyst can be conveniently prepared 
by the reaction of a lactam with a compound having an alkaline reaction, 
such as for example, alkali metal hydroxides, alkaline earth metal 
hydroxides, alkali metal alkoxides, alkali metals, etc. Preferably, an 
alkali metal, or a source of alkali metal or alkaline earth metal is used. 
Most preferably, an alkali metal hydroxide such as potassium hydroxide or 
sodium hydroxide is used, and potassium hydroxide is most preferred. Also, 
a different lactamate can be used as the catalyst than the lactam which is 
polymerized. Obviously, however, for process convenience it is preferred 
to use the same lactam for both. 
The polymerization mixture used in the present process contains a catalytic 
effective amount of the anionic catalyst (e.g., the lactamate salt), 
usually about from 0.5-30 mol percent, preferably about 1-20 mol percent, 
based on total lactam. In accordance with the present invention, the 
polymerization mixture is polymerized in the presence of a catalytically 
effective amount, typically about from 0.005 to 0.3 weight percent, 
preferably about from 0.01 to 0.1 weight percent, based on monomer, of the 
nitrothiophene compound. Best results are obtained by activating the 
polymerization mixture by the addition of carbon dioxide and/or sulfur 
dioxide. The polymerization mixture can also contain a suitable inert 
organic solvent, but typically it is preferred to merely use excess 
lactam. 
In a preferred embodiment of this invention, illustrating, for convenience, 
the polymerization of pyrrolidone, an alkali metal hydroxide is added to 
excess 2-pyrrolidone in an amount about from 0.5-30 mol percent, 
preferably about from 1-20 mol percent and most preferably about 10 mol 
percent based on total pyrrolidone. "Total pyrrolidone" refers to the 
2-pyrrolidonate catalyst, 2-pyrrolidone provided as solvent to the 
catalyst, 2-pyrrolidone catalyst having formed an adduct or complex with 
an activator or initiator, and any additional monomer charged to the 
reaction. The alkali metal hydroxide reacts with 2-pyrrolidone to form a 
solution of alkali metal pyrrolidonate and water in 2-pyrrolidone. This 
solution is dehydrated until it contains less than about 0.1-0.2 weight 
percent water. Then carbon dioxide, and/or sulfur dioxide, preferably 
carbon dioxide, is added in an amount corresponding to about 5-80, 
preferably about from 10-50 mol percent of the alkali metal 
2-pyrrolidonate present in solution, e.g., 10-50 mol percent, based on 
cationic species such as sodium or potassium. The dioxide is believed to 
function as an initiator or activator of polymerization. Where sulfur 
dioxide is used, it is desirable to operate in the lower sulfur dioxide 
ranges, as the use of large amounts of sulfur dioxide leads to the 
production of an off-color yellow product. The nitrothiophene compound 
accelerator is then added in a catalytically effective amount. Most 
preferably, the accelerator is added to the KOH-lactam solution after 
dehydration. In addition to carbon dioxide and/or sulfur dioxide, 
additional initiators and/or activators can also be present in small 
amounts, such as, for example, a small amount of acetic anhydride, or 
N-acyl lactamate, e.g., amounts of about 0.05-2.0 weight percent. 
The addition of a tetra(lower alkyl) ammonium halide, preferably a 
chloride, can be used to particular advantage, since this activator has 
been found to increase the molecular weight of the polymer product, in the 
present process, with only a modest reduction in polymerization rate. Thus 
the use of this activator affords added flexibility to the process. 
Suitable tetra(lower alkyl ammonium halides which can be used include, for 
example, tetramethyl ammonium chloride, tetraethyl ammonium chloride, 
(dimethyl-dipropyl) ammonium chloride, and the corresponding bromides and 
the like and mixtures thereof. Typically about from 0.05 to 3%, preferably 
about from 0.2 to 2.5% by weight, based on total lactam, of these 
compounds is used. 
Polymerization conditions for the readily polymerizable lactams are well 
known and can be used for the present process, with the exception that 
shorter polymerization times can be used. For example, using 
2-pyrrolidone, the polymerization can be conducted in the present process 
at temperatures in the range at about from 15.degree. C. to 100.degree. 
C., preferably about from 25.degree. C. to 70.degree. C., and most 
preferably about from 40.degree. C. to 60.degree. C., under a pressure 
ranging from subatmospheric to superatmospheric, for a period about from 2 
to 20 hours, preferably about from 3 to 10 hours. Rest results in terms 
of optimizing process reaction time efficiency are obtained by conducting 
the polymerization at temperatures in the range of about from 45.degree. 
to 55.degree. C. for about from 4 to 8 hours. Higher polymerization times 
can be used but afford no significant advantage and hence, merely increase 
process time. 
Also, as is well recognized by the art, in order to produce high-quality 
polylactam capable of being formed into fibers, filaments and yarn of 
commercial textile quality, high-purity lactam should be used for the 
polymerization. Thus, where it is desired to use the polymer product for 
this purpose, care should be taken to use high purity lactam. Any suitable 
purification procedure can be used to purify the lactam and such 
procedures are well known to the art. 
The process of the present invention can be generally applied to the 
production of polymers of lactams such as, for example, polymers of 
2-pyrrolidone or caprolactam, and also to the production of copolymers of 
lactams, such as, for example, copolymers of caprolactam or 2-pyrrolidone 
with each other or different lactams. Consequently, in general, and unless 
otherwise indicated in the above description where the terms "lactam" or 
"monomer" have been used, it should be appreciated that the teachings are 
applicable both to homopolymerization and also to the copolymerization of 
different lactams which copolymerize under the stated conditions of 
alkaline polymerization catalysis. 
The polymerizations of the present invention can be carried out with 
various amounts of lactam, catalysts, inert organic solvents (solvents for 
the initial polymerization mixture but not for the desired polymer), 
initiators and other activators-the amount of each being properly 
coordinated to produce the most effective polymerization. The 
polymerization can be conducted as a bulk polymerization, a solution 
polymerization or a dispersion polymerization, and can be conducted as a 
batch process or a continuous process or a semi-continuous process. 
The lactam starting materials are known compounds and can be obtained from 
commercial sources or prepared by known procedures or obvious 
modifications thereof (e.g., substitution of appropriate substrates and 
solvents). 
Where typical reaction condition ranges have been given, it should be 
appreciated that reaction conditions both above and below these ranges can 
also be used though typically with poorer results or economies. 
Definitions 
As used herein, the following terms have the following meanings unless 
expressly stated to the contrary. 
The term "alkyl" refers to alkyl groups having from 1 through 10 carbon 
atoms and includes both straight-chain and branched-chain alkyl groups. 
The term "lower alkyl" refers to such alkyl groups having 1 through 4 
carbon atoms such as, for example, methyl, ethyl, isopropyl, and butyl. 
The term "catalytically effective" in the context of the accelerators of 
the present invention refers to an amount of accelerator which is 
sufficient to significantly increase the polymerization rate with respect 
to a finite period time as compared with the unaccelerated polymerization. 
The term "2-nitrothiophene compound" refers to compounds having the Formula 
I given herein on page 4, and mixtures of such compounds. 
The term "total lactam" refers to the total amount of lactam in the 
polymerization mixture, including, for example, that present as substrate, 
solvent, and lactamate catalyst, etc. 
The term "lactam" refers to lactams having 5 through 13 ring atoms, such 
as, for example, 2-pyrrolidone, epsilon-caprolactam, enantholactam, 
capryllactam, laurolactam, and the like. 
The term "pyrrolidone" refers to 2-pyrrolidone. 
As used herein, the term "shaped articles" is a generic term referring 
broadly to one or more useful products which are generally referred to as 
plastic or in that context as synthetic (e.g. synthetic fibers). The term 
thus includes, for example, filaments, films, sheets, containers, 
moldings, equipment cases and parts, etc. 
A further understanding of the invention can be had from the following 
non-limiting examples.

EXAMPLE 1 
A flask equipped with stirrer and a reduced-pressure distillation head was 
charged with 500 g (5.9 mols) of 2-pyrrolidone and 39.5 g of potassium 
hydroxide pellets (0.6 mol, 85.3% KOH). The resulting mixture was heated 
to about 80.degree. C. under 1-2 mm pressure in about 10 minutes to remove 
water. Then the pressure was reduced further to about 0.5 mm, and the 
reaction mixture heated to about 113.degree. C. to take 5-10 cc of 
pyrrolidone overhead. The mixture was cooled to 30.degree. C., and carbon 
dioxide was added to give a potassium pyrrolidonate:carbon dioxide mol 
ratio of 1:0.3. (1a) A portion of the solution (50.0 g) was poured into an 
empty polyethylene bottle. (1b) Another portion (49.5 g) was poured into a 
polyethylene bottle containing 0.50 g of a 10% 2-nitrothiophene solution 
in pyrrolidone. Both bottles were well shaken and then held at 50.degree. 
C. for 5 hours. At the end of this time the polymer was removed from the 
bottle, crushed and extracted with water. After drying, the polymer was 
weighed to determine conversion, and a viscosity measurement was made to 
determine molecular weight. The results are given in Run 1 and Run 4 of 
Table I. 
The procedure described above was repeated, but modified as indicated in 
Table I, hereinbelow, and in some instances using tetramethyl ammonium 
chloride (Runs 5 and 6) or tetraethylammonium chloride (Run 7) in addition 
to carbon dioxide. The results of these tests are summarized in Table I 
hereinbelow. 
TABLE l 
__________________________________________________________________________ 
Polymerization*.sup.1 of 2-Pyrrolidone 
Polymeriza- 
CO.sub.2 Accel., 
tion time, 
Conver- 
Molecular 
Run No. 
Mol%*.sup.1 
Accelerator 
wt.%*.sup.2 
Hrs. at 50.degree. C. 
sion,%*.sup.3 
Weight*.sup.4 
__________________________________________________________________________ 
1 3 None -- 5 8 320,000 
2 3 2-Nitrothiophene 
0.01 
5 27 340,000 
3 3 2-Nitrothiophene 
0.05 
5 53 255,000 
4 3 2-Nitrothiophene 
0.10 
5 62 175,000 
Runs using 2 weight percent TMAC*.sup.5 in addition to CO.sub.2 
5 3 None -- 5 13 &gt;400,000 
6 3 2-Nitrothiophene 
0.06 
5 59 320,000 
Run using 2 weight percent TEAC*.sup.6 in addition to CO.sub.2 
7 3 2-Nitrothiophene 
0.06 
5 58 265,000 
__________________________________________________________________________ 
*.sup.1 Potassium 2pyrrolidonate in polymerization mixture is 10 mol 
percent based on total 2pyrrolidone; CO.sub.2, if used, is 3 mol percent, 
based on total 2pyrrolidone. 
*.sup.2 Weight percent of accelerator based on total mixture. 
*.sup.3 Percent conversion is calculated as 100 .times. (weight of 
polymer)/weight of total monomer, and total monomer has been defined 
heretofore. 
*.sup.4 All molecular weights are reporated as weight average molecular 
weight as determined from Gardner viscosities (of a solution of 1.00 g of 
polymer in 20 ml of 88.4 wt. % aqueous formic acid), using a Gardner 
viscositymolecular weight relationship developed from specific viscositie 
(of 0.1 g of polymer/100 cc of mcresol solution at 25.degree. C.). 
*.sup.5 TMAC is tetramethyl ammonium chloride; weight percent is based on 
total 2pyrrolidone. 
*.sup.6 TEAC is tetraethyl ammonium chloride; weight percent is based on 
total 2pyrrolidone. 
As can be seen from the above Table, the inclusion of 0.1% 2-nitrothiophene 
produced almost eight-fold acceleration of the polymerization in terms of 
conversion at five hours. Also, although the molecular weight of the 
product is lower, as compared with the unaccelerated polymerization, the 
molecular weight is still substantially above 100,000 and can be 
considered a very good quality polymer. Also as can be generally seen from 
Table I, the addition of 2% TMAC or TEAC increased molecular weight 
without significantly reducing conversion. 
EXAMPLE 2 
The polymerization is also conducted following the procedure of Example 1, 
except that 2-nitrothiophene is respectively replaced with 0.50 g of a 10% 
solution in pyrrolidone of 3-methyl-2-nitrothiophene, 
4-methyl-2-nitrothiophene, 5-methyl-2-nitrothiophene; 
3-ethyl-2-nitrothiophene; 4-n-propyl-2-nitrothiophene, and 
5-t-butyl-2-nitrothiophene, respectively. 
Obviously many modifications and variations of the invention, described 
hereinabove and below in the claims, can be made without departing from 
the essence and scope thereof.