Pilot Plant Preparation of Polystyrene of Very Narrow Molecular Weight Distribution Timothy Altares, Jr.’, and E. 1. Clark Pressure Chemical Co., Pittsburgh, Pa. 15201 The availability of polymers of very narrow molecular weight distribution offers the potential of the detailed correlation of polymer properties with molecular weight. A series of polystyrenes, virtually monodisperse, covering the molecular weight range from
600 to 1,800,000, has been prepared in 30-pound batches. The preparative techniques, based orl laboratory work performed at Mellon Institute, are described. These polymers are of considerable utility to calibrate instruments and study molecular weight effects in fundamental and applied polymer research.
THE
bulk flow and dilute solution properties of polymers are dependent on molecular weight and molecular distribution, and methods have been developed to measure these. I t is very difficult to measure properties of a mixture of molecular weights and t o relate to these mixtures a mathematical representation of some average molecular weight. Thus, for years, polymer chemists have had increasing need for polymer samples composed of a single molecular entity-i.e., each molecule in the sample having the same molecular weight and structure. The need has grown as theories relating behavior of polymeric systems to molecular structure have become more quantitative and sophisticated. Unambiguous experimental tests of theory increasingly require very precisely characterized model substances. Standard polymerizations, however, yield a t best fairly heterogeneous distributions of molecular weights. Unfortunately, then, physical and chemical studies have usually had to be made with heterogeneous samples, at least with regard to degree of polymerization, with the result that interpretation of experimental data has sometimes been equivocal, owing to uncertain effects of molecular heterogeneity, in addition to the parameters deliberately being varied. I n the past heterogeneous polymers prepared in a number of ways have been separated into relatively homogeneous samples by fractionation procedures, but these are often difficult, always time-consuming, and do not always produce adequate separation. I n 1940, Flory (1940) published his classic calculations and set forth conditions by which polymers of very narrow distribution ( M , / M , < 1.5) could be prepared. Briefly, he pointed out the requirements of such a hypothetical system to be: 1. Initiation of all polymer chains which are to propagate must be instantaneous, and this initiation rate must
’ Present address, Research Department, Koppers Co., Inc., Monroeville, Pa. 15146. 168
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 2, 1970
be much greater than that of propagation. No new initiating centers must form during propagation. 2 . There must be no chain transfer, interchange, or chain termination during the polymerization. 3. The system must be homogeneous in monomer and initiator, so that each propagating chain would have equal probability of adding monomer a t the same rate; addition must be by monomer (not mixed dimer or trimer). If these conditions were met, the reaction would stop by monomer depletion alone, and would remain active until chemically terminated. Based on these conditions, Flory calculated that the ratio of weight average molecular weight, M u , to the number average, M,, would be
m MU _ -1+ MI? (1 + m)‘ where m equals degree of polymerization, or molecules reacted per polymer molecule (or per initiator). As m increases, the system would be very close to M , / M , = 1.0-that is, the distribution of highest and lowest molecular weight components would be so small in comparison with the major portion of polymer clustered very narrowly about a peak molecular weight that these polymers then could be considered for many applications as molecular entities. I t was further predicted that these polymers would be attained by anionic addition polymerization. I n his mathematical calculations, Flory cited ethylene oxide as the model, although many other monomers could have been considered. Waack et al. (1957) reported the synthesis of polystyrene by anionic techniques, which they called the “living system.” The preparation of polystyrene by this method yielded polymer of fairly narrow molecular weight distribu1.5, but the system was not ideally a tion, M , / M , “Flory system”; thus, distribution was not idealized a t M , / M , = 1.0 (Meyerhoff, 1960; Wenger, 1960).
-
After this, many researchers, too numerous to reference, notably Welsh, Morton, et al., Wenger and Yen, Altares et al., Wyman et al., Szwarc et al., McCormick and Worsfold, and Bywater, etc., used anionic techniques in various solvent media, and various initiators and monomers for the production of monodisperse polymers. Beginning in 1961 Altares (Altares et al., 1964), using techniques developed a t Mellon Institute (Wenger and Yen, 1961) and later the Canadian National Research Council (Bywater and Worsfold, 1962), began to produce 50- to 100-gram laboratory quantities of polystyrene with M , / M , less than or equal to 1.06; this work yielded polymers of molecular weight from 300 up to 7,000,000. System
Theoretical and Laboratory. The system used is basically butyllithium-styrene in benzene solvent (Wenger and Yen, 1961). The idealized simplified scheme is:
BuLi
+ Sty
-+
Busty
-
+
Li
The charges, - and - , are not integers, as represented for simplicity, but partial charges, The reaction is stoichiometric in styrene; therefore, a kinetic molecular weight, M k , can be calculated from the following simple relationship:
grams of styrene I n the ideal case M k = M , moles of initiator
Most of the early work with anionic polymerization, using butyllithium initiation of styrene, yielded polymer of narrow molecular weight distribution, if the molecular weight was high enough. Below molecular weight 30,000, inequities in the initiation system due to association of the initiating ions and equilibrium with growing polymer chains were a problem. Other initiators could be used, but there were problems with these, especially the difunctional initiators. The system butyllithium-styrene was considered the least complex and most workable. Basically, the initiator system, to conform to the Flory conditions of instantaneous propagation, is hampered by association of the initiating ions. Using butyllithium in benzene, the following system actually exists for initiation and propagation:
1. 6BuLi 2. 2BuLi
(BuLi)6
+ 2Sty
+
2(BuSty
Li) 2 (Busty
3. (BuStyLi) + (BuLi)
+ xTHF
+
GBuLi(THF)
6
(termination)
+
BU = butyl Li = lithium Sty = styrene x = No. of monomer units
Mk =
6BuLi
1 (propagation)
+
-+
+
(BuLi)e (initiation)
BuStyLi + Sty + BuStySty Li BuStyLi + x(Sty) Bu(Sty).Sty Li Bu(Sty).Sty - Li + H + Bu(Sty). - + Li ~
Because of step 1, there is not instantaneous initiation. The association of six butyllithium molecules has a finite dissociation time, allowing some polymer chains to be initiated much earlier than others. Since propagation proceeds at the same rate per molecule, once initiation has taken place and no termination or chain transfer occurs, some chains grow longer than others. Additionally, homogeneous propagation is hampered because the growing butylstyryllithium associates in hydrocarbon with an association number of 2, and probably a mixed association number with any unreacted butyllithium. Bywater and Worsfold (1962) have shown that addition of small amounts of tetrahydrofuran breaks down the association of butyllithium, and the result is “instantaneous” initiation.
Li)*
(BuStyLi) (BuLi),
y = association number (unknown) Thus, molecular weights below 30,000 are not monodisperse, and those above 30,000 are statistically smoothed out.
At this point, the obvious solution might be to do the polymerization in pure tetrahydrofuran. The system using 100% tetrahydrofuran is successful in the laboratory, but not desirable for large scale operation. I n our system, the butyllithium-tetrahydrofuran method of initiation in benzene solvent is used. As a laboratory system, briefly, the polymerization is done in an all-glass sealed reactor, under vacuum. All solvents and reactions are rigorously purified. Initiation is instantaneous by addition of tetrahdrofuran a t the proper instant to cold monomer-butyllithium solution and termination is by proton addition, usually by addition of purified methanol. This system yields polystyrene of M , / W , 5 1.1 and in most cases = 1.06. Scale-up to Plant and Problems. The specific system used for the work described here is the butyllithiumtetrahydrofuran-complex-initiated polymerization of styrene described in detail above. The reaction is run to exhaustion or depletion of monomer. Termination is by proton addition. Recovery of product is quantitative. The most difficult portion of this type of polymerization is conformation with point 2 of the Flory conditions for monodispersity-i.e., there must be no chain transfer, interchange, or termination. Such common impurities as oxygen, carbon dioxide, water, peroxides, and hydroperoxides produce deleterious effects, if introduced during polymerization, on the distribution of molecular weights, and possibly on the shape of the resulting polymer by producing branching by coupling, or by premature termination of some of the growing chains (Wyman et al., 1964). Furthermore, since the polystyrene of narrow molecular weight distribution is intended for use as a reference material, it should be free of all foreign matter. An approach similar to that used in the pharmaceutical or food industries was used in choosing processing components. Therefore, a glass-lined, jacketed reactor with anchor stirrer and baffle is the polymerization vessel; all other attendant vessels are stainless steel; all piping is stainless Ind. Eng. Chem. Prod. Res. Develop., Vol. 9 , No. 2, 1970
169
BLANKET
REACTOR
1 7 1 DILUTION
DISTILL.
FINISHED POLYMER
FILTRATION
-
DRYING GRINDING
Figure 1 . Flow diagram of polymer plant
steel, Teflon-lined braided hose, or glass; all gasketing is Teflon or Teflon envelope. Figure 1 is a schematic of the plant. During production of a polymer, an inert atmosphere is necessary as a blanket gas, since admittance of atmospheric oxygen, carbon dioxide, and water would ruin the reaction. As a blanket gas, argon is used, since it is much denser than air and, because of its high molecular weight and density, will diffuse much more slowly than helium or nitrogen, which are commonly used as blanket gases. T o ensure that contaminants are not introduced in the blanketing gas, it is scrubbed through two long tubes containing active solutions of sodium a-methylstyrene anions in THF-benzene before introduction to the plant. Sodium a-methylstyrene anions in THF-benzene are used in the scrubbers, because the solutions can be very concentrated in sodium and monomer and still not be thickened as would be the case for styrene, because a-methylstyrene a t room temperature is above the ceiling temperature required for production of long polymer chains. At room temperature, a system of active w-methylstyrene anions and excess monomer will only grow chains about six units long (Wenger, 1960). The monolayer of moisture, which is adsorbed on all internal surfaces of the plant, could not be removed by heating, as can be done in some cases because of the plant setup. Accordingly, a very strong solution of active butylstyryllithium is pumped through the plant, coming in contact with all surfaces to remove adsorbed water or other terminating contaminants while under argon pressure. With this accomplished, it is just as important to remove the coating of butylstyryllithium cleansing solution from the reactor before polymerization. The butylstyryllithium solution coating the reactor, and the reaction products therefrom with terminators, were removed by distilling benzene throughout the system, filling and flooding the system, and finally dumping the distillate back into the distilling pot. The benzene rinse was distilled from butylstyryllithium anion solution. After three such washes, the system was considered decontaminated. The benzene used for rinse was later the solvent used for polymerization; after three rinses, the required 170
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 2, 1970
amount of benzene was distilled to the polymerizing vessel from anion solution. Monomer (styrene) was painstakingly purified to remove not only the usual contaminants but inhibitor and any polymer dissolved in the monomer. Purification could not be done by distillation from anion solution, as was the case for benzene. Calcium hydride treatment was used to reduce moisture; then the monomer was vacuumdistilled at 60 to 80 mm of Hg, using rectification; purified argon was used as blanket and make-up gas. The distillation column and receiver were repeatedly washed and dumped with distillate until about 50% of the monomer was distilled over. At this point, the center cut of required monomer charge was collected and the receiver was isolated from the still and pressurized to 5 psi with purified argon. When pressurized, the receiver was disconnected and immediately attached to the polymerization unit. The line connecting the polymerization unit and the styrene receiver was flushed, from the unit outward, with argon before the fittings were tightened. A small positive pressure was always maintained on the reaction vessel. The purified monomer was introduced immediately into the reactor, which was previously cooled to about 50°F. The monomer and solvent are cooled prior to addition of initiator in order to retard initiation, and especially propagation, as much as possible before homogeneous mixing takes place. Injection of initiator results in high instantaneous concentrations a t the injection site, but advantage is taken of the fact that when butyllithium is injected alone into cold solution, initiation and propagation are retarded, so that homogeneous mixing is achieved before introduction of tetrahydrofuran. Thus, practically instantaneous initiation results on addition of the tetrahydrofuran. The initiator (BuLi), depending on molecular weight of the polystyrene desired, is usually injected through a rubber-stoppered septum by syringe. Generally, a 10% excess of initiator is added to do an in situ purge; thus, predictability of molecular weight is only &lo%, except in the higher molecular weights, where +20% is expected. Table I compares predicted us. actual molecular weight values. The septum is then closed from the system by a ball valve. The butyllithium obtained from Foote Mineral, Exton, Pa., comes as a hexane solution and is used fresh as received. Tetrahydrofuran complexing agent is purified by simple distillation under argon from sodium-biphenyl anion solution; a 30% forecut is discarded before the required amount of center cut is collected. The required amount is measured and transferred by syringe. Injection is made directly ~~
~
Table I. Comparison of Predictability of Molecular Weights with Measured Values
Predicted
Measured (M,)
i- 7c
550 3.900 10,000 20,000 50,000 100,000 150,000 500,000 1,000,000 2,000,000
524 3,600 10,900 20,200 50,100 97,600 154,000 392,000 773,000 1,610,000
- 4.7 - 7.7 + 9.0 + 1.0 + 0.2 - 1.6 + 2.7 -21.6 -22.1 -18.0
Table 11. Heterogeneity Index of Polystyrene Produced
Weight Au. (M,) Measured by Light Scatter
Number Au. (M,) Measured by Membrane Osmometer or Fractionation
10,000 20,800 50,500 96,200 160,000 394,000 670,000 862,000 1,700,000
10,900 20,200 50,100 97,600 154,000 392,000 640,000 773,000b 1,610,000b
Heterogeneity Index, MdM.
< 1” 1.03 1.01 < 1“ 1.04 1.oo 1.05 1.11 1.06
“Heterogeneity index of less than 1 is not possible. One can get a value less than 1 only because of the narrow molecular weight distribution and because both measurements have experimental error. I n the case of light scatter, j=4% error; osmometry, &5%; thus, if experimental error in one case is plus and in the other is minus, a value less than 1 can be obtained by division. If one includes experimental ermr in the M,/M,, values of 1.06 are more realistic. By fractionation.
into the reactor through the rubber-stoppered septum mentioned above, immediately after initiator injection. Polymerization usually is completed in 2 hours, although the time required is dependent on the molecular weight of the desired polystyrene and temperature of the polymerization. The kinetics are well known (Bywater and Worsfold, 1962). The other requirements of Condition 2 under the Flory conditions are no problem, since the chemistry of this reaction is well known-i.e., no chain transfer or interchange. Condition 3, equal probability of propagating chains adding monomer, is no problem in this system, since in the glass-lined reaction vessel excellent mixing is provided by an anchor agitator driven by a 2-hp motor and baffle, so that batchwise mixing is more efficient than in the laboratory. I n addition to precautions taken to provide homogeneous conditions, one can theorize that since the polymerization is done in solution, concentration gradients of monomer due to viseometric effects after propagation has reached halflife are unlikely, since calculations can be made to adjust the quantities of reactants so that even the highest molecular weight polymer solution made has a final solution viscosity or consistency approximately that of glycerol. The reaction is terminated by injection of degassed methanol through the injection septum by syringe; generally, a 100% excess is used. As methanol is injected, the volume is recorded, so that when the highly colored anion disappears, one can get a check calculation on the molecular weight, even before the polymer is recovered. Polymer is recovered by diluting the solution with benzene to a concentration of about 2.5%. This is pumped
into the bottom of a rapidly stirred tank of boiling distilled water, the benzene being flashed overhead, condensed, and removed. The polymer “popcorn” or “lollipop” is recovered after cooling, chopped in a Cumberland stainless steel grinder, and dried. The polymers of lower molecular weight ( M , 2000 or less) are purified as recovered in solution, by repeated washing with distilled water to remove lithium salts. The polymer solution is concentrated and the polymer finally recovered by freeze-drying. Characterization and Utility
Number average, weight average, molecular weight distribution, volatiles, x-ray examination, nonsolubles, intrinsic viscosity, infrared examination, etc., are usually determined, but these details are not applicable to this presentation. The heterogeneity index of all the polymers produced in the plant described have M , / M , < 1.2 and most have M , / M , = 1.06 (Table 11). The engineering interest in these precisely made polymers is not directly apparent. A very serious handicap to their use as ordinary building materials is their rather high cost at their present rate of production. These polystyrenes sell a t $300 to $500 per pound. Their primary use is as a polymer molecule of a knbwn, and almost single, specific molecular weight and structure. As such, they serve as standards for the calibration of many instruments and techniques used to determine molecular weight. An obvious extension of this for engineering purposes is their use to delineate the effect of molecular weight on polymer properties. Many applied research workers and engineers are correlating such critical functions as flow properties, electrical resistance, thermal stability, and stability to radiation with molecular weight using these materials. This effort is not limited to individual, single polymers. By combining several of these polymers in predetermined fashion, an infinite variety of molecular weight distributions may be synthesized. literature Cited
Altarrs, T., Jr., Wyman, D. P., Allen, V. R., J . Polymer S C ~A-2, . 4533 (1964). Bywater, S., Worsfold, I). J., Can. J . Chem. 40, 1564 (1962). Flory, P. J., J . A m . Chem. SOC. 62,1561 (1940). Meyerhoff, G., 2. Phys. Chem. N F 23,100 (1960). Waack, R., Rembaum, A,, Coombes, J. I).,Szwarc, M., J . Am. Chem. SOC.79, 2026 (1957). Wenger, F., Preprints, Polymer Division, 138th Meeting, ACS, New York, September 1960. Wenger, F., Yen, S. P. S., Makromol. Chem. 43, 1 (1961). Wyman, D. P., Allen, V. R., Altares, T., Jr., J . Polymer S C ~A-2, . 4545 (1964). RECEIVED for review June 20, 1969 ACCEPTED December 23, 1969 AIChE North-Central Regional Symposium, April 25, 1969, Athens, Ohio
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