Continuous bulk polymerization of 2-pyrrolidone

A process for forming polymers of 2-pyrrolidone in pellet form via bulk polymerization is disclosed. Polymerization accelerators are empolyed to markedly shorten the polymerization time thus making possible a compact continuous polymerization apparatus which produces pellets of high polymer content requiring no further polymerization. The pellets are formed by forcing the polymerizing mixture through small tubes to form rods of polymer which are then cut into short lengths.

BACKGROUND OF THE INVENTION 
This invention relates to the polymerization of 2-pyrrolidone and more 
particularly to the formation of pellets of this polymer by means of a 
continuous bulk polymerization process. 
The polymerization of 2-pyrrolidone using an alkali metal salt of 
2-pyrrolidone as a catalyst combined with a co-catalyst or "activator" 
under anhydrous conditions is well known in the art. Relative prior art is 
recited in our co-pending application Ser. No. 039,773. Most of the prior 
art disclosed methods of polymerization which are known as "bulk 
polymerization" resulting in the formation of an exceptionally hard and 
tough polymer cake which is very difficult to comminute. A method commonly 
employed is to grind the polymer cake in a Wiley mill or similar larger 
grinder. This is not only very expensive but the cake is so hard that it 
breaks particles of steel from the blades of the grinder thus introducing 
metal particles into the ground polymer which interfere with subsequent 
operations such as melt spinning through the tiny holes of a spinnerette 
to form fibers. 
Some attempts to avoid this grinding step by forming pellets directly via 
bulk polymerization have been disclosed in the prior art. Thus U.S. Pat. 
No. 3,681,293 discloses a method of extruding the partially polymerized 
mass while still soft, cutting the extruded soft rod into pellets and then 
further polymerizing the pellets. U.S. Pat. No. 3,804,813 discloses a 
similar method in which the partially polymerized mass is cut into small 
pieces which are then further polymerized. Such methods in actual practice 
are virtually inoperable because it is difficult or impossible to carry 
out the various steps of the operation in the complete absence of moisture 
with the result that the further polymerization of the pellets does not 
occur since contact with even small amounts of water vapor stops the 
polymerization reaction. It is therefore essential that the bulk 
polymerization be carried out in a completely sealed apparatus until a 
conversion of 50 percent or more is reached before exposure to the 
atmosphere. Pellets having this polymer content are suitable for any 
subsequent use. The unpolymerized monomer may be recovered readily in the 
washing step which is necessary to remove the alkali. To facilitate 
washing it is desirable that the polymerization be stopped at about 50 to 
60 percent rather than allow it to procede to maximum conversion. 
Attempts have been made by us in the past to carry out the polymerization 
of 2-pyrrolidone continuously in small tubing completely sealed from the 
atmosphere using metal tubing coated with a low-friction material such as 
polytetrafluoroethylene ("Teflon") or by using Teflon or polyethylene 
tubing. The intention was to keep the polymerizing mass continuously 
moving through the tubing until a solid rod was formed which would emerge 
from the end. The difficulty encountered was that the residence time in 
the tubing was of necessity so long to reach even 40 percent conversion 
that the method became impractical. Not only were very long lengths of 
tubing required but the resistance to flow in such long lengths made the 
system inoperable. To make matters even worse, the polymerizing mass 
characteristically passes through a sticky plastic phase in the earlier 
stages of the polymerization which contributes greatly to the resistance 
to flow. 
It is an object of this invention to provide a method of increasing the 
polymerization rate to such a degree that only relatively short lengths of 
tubing are required to provide the necessary residence time. 
It is a further object of the invention to provide a method of minimizing 
the duration of the sticky plastic phase which normally occurs during the 
polymerization so that it is no longer a problem. 
Other objects will be apparent in the detailed disclosure which follows. 
SUMMARY OF THE INVENTION 
We have discovered that by using the polymerization accelerators disclosed 
in our co-pending application Ser. No. 039,773 that the residence time is 
shortened to the point where relatively short lengths of tubing may be 
used in the continuous rod polymerization method. Also the use of the 
proper accelerators results in minimizing the duration of the plastic 
sticky phase resulting instead in the rapid formation of a gel with 
relatively low adhesion. 
By following the polymerization procedure disclosed in Example 8 of the 
above referred to application, the total time required for a conversion of 
about 60 percent is only about 60 minutes compared to 24 hours or more for 
only about 40 percent conversion by older methods. This cuts the length of 
tubing required by at least 1/24 thus making this continuous 
polymerization method practical.

DETAILED DESCRIPTON OF A PREFERRED EMBODIMENT 
Referring to the drawing which is a flow diagram of the process, the 
invention is practiced by first forming the alkali metal salt of 
2-pyrrolidone, preferably the potassium salt, by by adding an aqueous 
solution of the alkali metal hydroxide to 2-pyrrolidone (line 1) and 
distilling off the water under vacuum until the solution is anhydrous 
(line 2). This is best insured by distilling over up to about 10 percent 
of the 2-pyrrolidone whereby all the water is removed. It is preferred to 
form a solution of about 8 to 10 percent of the alkali metal pyrrolidonate 
(conveniently referred to as the "K-salt" solution). 
Next an anhydrous solution containing preferably about 2 to 3 mol percent 
of the quaternary ammonium salt ("Q-salt") is prepared by adding the 
hygroscopic compound, preferably tetra n-butyl ammonium bisulfate or 
methyl tri n-butyl ammonium sulfate, to 2-pyrrolidone (line 3) and 
distilling over, under vacuum, about 10 percent of the pyrrolidone (line 
4). In a plant operation both these steps, i.e. the preparation of the 
K-salt solution and the Q-salt solution may be advantageously carried out 
in wiped film evaporators as a continuous process. 
The polymerization initiator, if it is SO.sub.2, may be added to the Q-salt 
solution (line 5) in which it is soluble (this solution containing 
SO.sub.2 is stable for several days and hence may be stored in a holding 
tank if desired). The K-salt and Q-salt solutions are then pumped 
continuously (lines 6 and 7) through a static mixer into a manifold (line 
8). The two solutions are pumped at essentially the same rate although the 
speed of either pump may be varied slightly for purposes of optimization. 
The volume of the manifold should be small in order to insure that the 
dwell time of the mixture is short thus avoiding the build up of polymer. 
If the initiator to be used is CO.sub.2 it cannot be added to the Q-salt 
solution since it is not soluble in it but must be bled into the K-salt 
flow line (line 10). Alternatively the SO.sub.2 may be added at this point 
instead of being dissolved in the Q-salt solution. 
The final mixture in the manifold then consists of an anhydrous 
2-pyrrolidone solution containing about 4 to 5 mol percent of K-salt and 
about 1 to 1.5 mol percent of Q-salt together with the polymerization 
initiator which is preferably present in about 0.005 mol of SO.sub.2 per 
mol of 2-pyrrolidone or about 0.03 mol of CO.sub.2 per mol of 
2-pyrrolidone. 
From the manifold the pressure forces the polymerization mixture into the 
tubing lines (9) attached to the manifold where polymerization takes 
place. The tubing lines may be stainless steel coated with a low friction 
material such as Teflon or a silicone or may be simply Teflon or 
polyethylene tubing. An inside diameter of about 1/8 inch is preferred 
since this size rod when cut into pellets is ideal both for the washing 
step and for feeding to an extruder after drying. The tubing lines are 
surrounded with a jacket (11) so that they may be maintained at an optimum 
polymerization temperature by any means such as blowing heated air into 
the chamber. If the initiator used is SO.sub.2 we prefer maintaining a 
temperature of about 40.degree. C. but if CO.sub.2 is used we prefer the 
temperature to be 50.degree. C. The length of tubing and the number of 
tubing lines are adjusted to match the flow rate and the required 
polymerization time. 
The solid rods (12) which emerge are then fed to a standard pellet cutter 
to form pellets. The pellets must then be washed to remove the alkali, 
unpolymerized monomer and the Q-salt. Both the monomer and the Q-salt may 
be removed and reused by methods disclosed in our copending application 
Ser. No. 039,773. 
The following examples are intended to further illustrate the invention and 
are not intended to limit its scope: 
EXAMPLE 1. 
1140 grams (3.4 mols) of tetra n-butyl ammonium bisulfate was added to 7600 
grams of purified 2-pyrrolidone and this solution was passed through a 
small laboratory wiped-film evaporator at a pressure of 10 mm mercury and 
a jacket temperature of 132.degree. C. taking overhead about 10 percent of 
the 2-pyrrolidone along with the water to insure the formation of an 
anhydrous solution of the Q-salt which was collected at the bottom of the 
evaporator and contained about 80.5 mols of 2-pyrrolidone. 51.5 grams 
(0.8045 mol) of SO.sub.2 was dissolved in this solution which was stored 
until the following solution was prepared in the same wiped-film 
evaporator. 
1050 grams of a 50 percent aqueous solution of potassium hydroxide was 
added to 7600 grams of purified 2-pyrrolidone and this mixture passed 
through the wiped-film evaporator under reduced pressure to form an 
anhydrous solution of the K-salt. In addition to the water about 10 
percent of the 2-pyrrolidone was also distilled overhead to insure the 
formation of an anhydrous solution containing about 9.4 mols of K-salt and 
80.5 mols of 2-pyrrolidone. 
Each solution was fed to a high pressure metering pump which delivered the 
solution to a static mixer. The pumps used were Milton-Roy Model No. 
MR1-33-32SM of the packed plunger type capable of developing pressures up 
to 7500 psi and metering from about 5 ml per minute to 50 ml per minute. 
All parts contacted by the solutions were made of stainless steel. 
Each pump was adjusted to deliver 7.5 ml per minute of the solution to the 
manifold thus giving a flow rate of 15 ml per minute for the combined 
solutions. The manifold was a 1 inch stainless steel pipe capped at both 
ends with 4 inches between the caps. All tubing used for liquid flow to 
the manifold was heavy guage (0.049 in.) 1/4 in. O.D. stainless steel. On 
the outlet side of the manifold there were attached 4 Teflon tubes, 1/8 
inch inside diameter and 10 feet long. These tubes were not coiled but 
straight in order to minimize resistance to flow and were enclosed by 
means of a box constructed of Styrofoam (foamed polystyrene). Air heated 
to 40.degree. C. was circulated through the box to maintain the optimum 
polymerization temperature. 
The polymerized rods which emerged at the end of approximately one hour 
polymerization time were collected and cut into pellets by feeding them 
into a Wiley mill. After washing the pellets with water and drying the 
conversion was found to be 52 percent and the viscosity determined via the 
Gardner Bubble Tube method was 5.5 Stokes (tube "T") when measured as a 5% 
solution in 85% formic acid. 
EXAMPLE 2. 
The method of Example 1 was followed except that an equivalent amount of 
methyl tri n-butyl ammonium bisulfate was substituted for the tetra 
n-butyl ammonium bisulfate with essentially the same results. 
EXAMPLE 3. 
The method of Example 1 was followed except that an equivalent amount of 
tetra n-butyl ammonium sulfate was substituted for the tetra n-butyl 
ammonium bisulfate. After washing the conversion in the pellets was found 
to be 40 percent and the viscosity was 63 Stokes. 
EXAMPLE 4. 
The method of Example 1 was followed except that an equivalent amount of 
methyl tri n-butyl ammonium sulfate was used in place of the tetra n-butyl 
ammonium bisulfate. The results were the same as in Example 3. 
EXAMPLE 5. 
The apparatus described in Example 1 was modified to utilize only one high 
pressure metering pump. The anhydrous solutions of the K-salt and the 
Q-salt were mixed in a holding tank under dry nitrogen to protect them 
from atmospheric moisture and this mixture fed into one pump of the same 
type described in Example 1. The stainless steel tubing connected to the 
output of the pump contained a tee ahead of the static mixer through which 
was bled either SO.sub.2 or CO.sub.2 to initiate the polymerization. The 
feed rate from the single pump was adjusted to 15 ml per minute. Although 
no fine control by varying the feed rate of either the K-salt solution or 
the Q-salt solution is possible with this system, the results were 
essentially the same. Care was taken to adjust the concentrations of the 
K-salt and Q-salt solutions in the original mixture of optimum conditions 
and when this was done the polymerization rate (conversion) and the 
viscosity of the product was the same as that obtained in Example 1.