@
e EfFect of Retarders and Promoters o n .
..
POLYMERIZATION OF ALPHA=METHYLSTYRENE
@
POLYMERIZATIONGlFFlN D. JONES and RALPH E. FRIEDRICH
I
of a-methylstyrene is readily accomplished by alkali metals (3, 72), but without promoters the reaction is characterized by long and variable induction periods together with variable rates and polymer molecular weights. Also, the reaction dies quickly when sodium is removed. Ethers promote the polymerization of styrene and butadiene (7, 9, 7 I > , but they are usually used in high concentrations. In the work reported here, polyglycol ethers in low concentrations were extremely effective promoters.
Physical Research Laboratory, The Dow Chemical Co., Midland, Mich.
Polypropylene glycol ether and polyethylene oxide were found to be good promoters in the polymerization of a-methylstyrene. Induction period and rate of polymerization were increased. While the experiments reported here are specific for a-methylstyrene, similar approaches for other polymerization reactions might pay dividends, especially in using such promoters in reducing the finished polymer’s sensitivity to impurities
Experimental
Undiluted monomer and sodium shot having an average diameter of 1 to 2 mm. were placed in a 3-liter flask agitated with a 5.81 X 1.65 cm. half-moon blade. Oxygen was excluded, and deaerated monomer could be pumped into the vessel. Oxygen in the purge nitrogen was less than 20 p.p.m., but because continuous purge was not used, that in the atmosphere was even less. A bath temperature of 35’ C. was usually used, and in one technique
monomer was added automatically to maintain a constant temperature difference between reaction vessel and bath. However, because of an induction period with each addition, this produced an “on and off” polymerization. Concentration of impurities was determined polarographically @), and their
-
9 4c.
/*
16
g W
g
14
I2
W
>
12
z
>
IO
S
S
8
s
$
6
z
-
z 0
16
0
effect on rate and subsequent polymer viscosity was observed by injecting the impurity into the steadily polymerizing sirup and sampling this. T o determine viscosity of the polymer (10% concentration in toluene), it was separated from monomer by drying in a vacuum oven a t 220’ C. Before separation,
8
4
4
2
2
4
0
0
8
16
24
32
40
48
56
64
72
HOURS
Figure 1. Polymerization rate decreased when ratio ,of sodium to monomer decreased
8
12
16
24
32
40
48
56
64
HOURS
Figure 2. Effect of temperature and added impurities on unpromoted polymerizations. Neither water nor oxygen produced postinhibition retardation VOL. 51, NO. 6
0
JUNE 1959
745
Chain polymerization takes place without promoters, as portrayed above, but reaction is slow and induction period i s variable. With effective promoters, shown at the left as black spheres, induction period i s reduced and rate of polymerization is increased
h
I
however, it was desirable to add an antioxidant ( 4 ) . The propylene glycol ether used as a promoter had a methyl group a t one end, a sec-butyl group at the other, contained 0.3% hydroxyl, and had a molecular weight of 500.
pletion; the ceiling temperature phenomenon was also limiting. The ceiling temperature for the homopolymerization of a-methylstyrene depends on monomer concentration and, therefore, upon conversion, inasmuch as polymer is a diluent. Thus, from the data of McCormick (6), the calculated equilibrium conversion is 12% at 55' C. and 40% at 47' C . This low ceiling temperature acts as a safety feature in stopping runaway polymerization, except that it does not
Results and Discussion
T o facilitate sodium removal, the polymerization was not taken to com-
Table 1.
Rate and Induction Period Were Sensitive to Sodium Monomer Ratio Temp.,
c.
Batch AC B C
CC
D
E
Amount Acetopheof none M ~ n o m e r , ~Content, Liters P.P.M.
9 9 10 21 21 21
1.0 1.3 2.5 1.0 2.0 1.0
17 17 17 17
1.0 1.0 2.0 2.0
Induction Polymer Period, Rate, Hr. %/Hr.
37 33 37 135 135 135 25 25 25 25
12 12 17 4 8 4 1.9 3.7
7.5 8.5
1.1 0.40 0.30 0.84 0.52 0.74 3.0 2.7 2.2 2.5
ConVisc0sity.b version, Centi% poises 15.0 44.7 14.5 18.2 12.0 17.6
17.0 25.0 17.6 19.3 16.8
47.3 50.8
17.0 16.0 16.2 34.0 49.2 63.6 60.9
At a constant amount of sodium, 160 cc. 10 centipoises corresponds t o M.W. 165,000, 30 centipoises t o 340,000, and 100 centipoises t o 630,000 (6). c Figure 1.
Table II.
Polymer Viscosity Was Greatly Influenced b y Some of the Impurities Injected into Unpromoted Polymerizations of a-Methylstyrene
Inhibition, Rate, Conversion, Polymer Impurity Added T ~ ~ ~Hr.. , %/Hr. % Viscosity Batch" Type P.p.m. C. Initial After Before After Before After Before After Ab 45.0 1.5 14.0 29.4 Bbgc H20 5 11.5 7.6 6.8 0.42 0.51 7.3 14.0 61.5 68.5 Cb 0 2 0.22 10.0 20.1 1.0 0.26 0.29 8.0 13.8 30.9 30.8 Dd HnO 20 23.0 6.5 10.0 0.4 1.7 2.4 7.0 65.3e 154.0 Af CHzO 0.26 20.0 2.4 1.4 1.4 2.4 6.0 20.8 70.6" 67.7 Bf (CH*O), 25 20.0 2.5 2.3 1.5 1.3 8.0 13.9 65.6" 156.0 Cd$' CsH5COCHs 50 22.0 6.2 10.3 0.8 0.45 5.2 11.8 104.0 35.8
...
f
...
... ... ...
...
...
a Initial acetophenone content, 35 t o 38 p.p.m.; 4% sodium. Figure 2 . 6.4% sodium. Initial acetophenone content, 9 p.p.m. a From control experiment immediately preceding. Figure 3.
746
INDUSTRIAL AND ENGINEERING CHEMISTRY
apply in dimerization. The effect of reversibility upon the molecular weight of the polymer is to increase the probability of termination a t low molecular weight by reaction with impurities.
Fn = 4' + r3
where r l = rate of propagation 72 = rate of depolymerization 73 = rate of termination The dependence of polymerization rate and induction period upon the ratio of the amount of sodium to the amount of monomer was studied in batch polymerization experiments (Table I). The conversion curves of the first three examples are given in Figure 1. The viscosity of the polymer appears to be controlled by impurities such as acetophenone rather than by conversion or by the sodium to monomer ratio (Table 11). For example, samples were withdrawn a t 12.5 and 1770 conversion in a particular polymerization experiment, and the polymer viscosity found to be 63.7 and 63.2 centipoises, respectively. The polymerization rate increases with increase in temperature (curve A , Figure 2). The dependence of induction period upon temperature is even more pronounced (1.5 hours a t 45' C. and 20.1 hours a t 10' C.). Generally, the induction periods ranged from several hours at 35' to 40' C., to days a t room temperature, and weeks below 0" c. Water produces a sharp inhibition period, and sometimes an increase in polymer viscosity results. The length of the inhibition period is not, however, directly proportional to the amount of water added. While water deactivates the surface of the sodium temporarily, it causes flaking of sodium and an eventual increase in active surface area.
Stirring speeds in excess of 600 r.p.m. also caused excessive flaking. Stirring speed was unimportant between 250 and 600 r.p.m. Lower speeds were insufficient to keep the sodium suspended. The sensitivity to oxygen as an inhibitor is shown in curve C, Figure 2. Like water, oxygen produces no postinhibition retardation. Acetophenone is a retarder (Figure 3, curve C ) and depresses the molecular weight of the polymer. Formaldehyde is an inhibitor without noticeable postinduction effect at the low concentration tested. Paraformaldehyde seems to give both inhibition and a postinduction increase in molecular weight. Possibly some monomeric formaldehyde is generated from the paraformaldehyde, and the remainder is to be classed with the polyethers as a promoter. The adventitious presence of paraformaldehyde might explain the occasional runaway copolymerization reported in the sodiumcatalyzed copolymerization of styrene and butadiene (7). Prior to the use of promoters in this work, it was found desirable to eliminate carbonyl compounds from the monomer. By refluxing over molten sodium at 100 mm. of mercury and subsequent distillation, it was possible to prepare purified monomer containing less than 1 p.p.m. of carbonyl compounds, according to polarographic analysis. Promoters were then discovered which made this degree of purity unnecessary. An ether of poly(propy1ene glycol) was especially active ; however, poly(ethy1ene oxide) was also a promoter. Not only did the promoter reduce the induction period and increase the rate of polymerization, but it greatly increased the polymer viscosity when used a t a low concentration (Table 111). The promoter reduced the sensitivity to impurities. For example, in a polymerization in the presence of the ether of poly(propy1ene glycol), the injection of llG p.p.m. of oxygen gave only a 70minute inhibition period. Prior to the use of the polymeric promoter, experiments (Table IV) with dioxane as a promoter showed an interesting carry-over of effect from one experiment to the next as though the surface of the sodium remained activated for a time, despite rinsing with monomer several times between experiments. In
Table 111.
Poly(propy1ene Glycol Ether) Promoter Increased Polymer Viscosity When Used at Low Concentration
Monomer Batch Ai
Polyglycol,
A2
267 535 535
A8
AP
P.P.M.
Induction Period, Hr.
Temp.,
5.2 0.5 1.7 1.5
23 35 21 25
0
Rate, %/Hr.
c.
O
1.6
fast 10.8 14.4
Viscosity, Centipoises
Conversion,
68.2 2180.0 440.0 140.0
17.1
%
... 7.0 8.2
Acetophenone (100 p.p.m.) was added.
some instances, polymerization continued and caused plugging of lines after removal of the sirup from the sodium. Under these circumstances, the polymer chains could be considered to be "living" (70). The promoter is believed to favor homogeneous propagation. A high temperature coefficient of polymer molecular weight was observed despite the presence of promoter (Table V). Color development is characteristic of
the polymerization of styrene derivatives with sodium. During polymerization the color was usually straw yellow, but the polyglycol intensified the color and a t higher conceritrations produced the deep red color of the a-methylstyrene-sodium complex in ether solution. Most of the color was bleached instantly upon the addition of water after removal of the sodium shot; however, yellowness sometimes developed in the polymer as a
25
Figure 3. Effect of monomer oxidation products on unpromoted polymerizations. Acetophenone was a retarder and depressed molecular weight
2o
A
t
9
2ooc.
B
w
> 2
0 0
IO
5
0
5
0
10
15
25
20 HOURS
30
35
14
Figure 4. Viscosity was lower in continuous polymerization than in batch polymerization or one with intermittent feed. Feed rates were adjusted to control temperature at 35.5" C. Product was withdrawn in 500-ml. increments
1 2 0
1 1 1
,
0
100
0 x--x
50
1 150
,
,
I
250
300
350
I
200
0 400
MINUTES
VOL. 51, NO. 6
JUNE 1959
747
Table IV.
Monomer Batch
Dioxane Promoter Showed a Carry-Over Effect" Induction Polymer Dioxane, Period, Rate, Viscosity, % Hr. %/Hr. Centipoises
Alb
0
Ai
0.28 0 2.8 0 0
Aa A4 Aa
BiC
BP
a
B3
5.6 0
B4
0
20' C.
*
38.0 11.7 10.0 3.0 14.0 20.0 4.5 4.0 5.3
61 p.p.m. acetophenone.
Acetophenone
A1
18 218 121 121 19 19 19
c1
cz c3 a
and had a viscosity of 141 centipoises. I t is evident that cumene is an inert diluent and that a high boiling impurity was mildly regulating the polymer viscosity. Furthermore, this impurity was probably not a,@-dimethylstyrene because the propylene used was essentially free of butenes, and no impurity of mass 132 was detected, nor did 6-methylstyrene produce an appreciable modifier effect when added. Indene, however, is a potent retarder and may be the impurity concerned. Indene (100 p.p.m.) used with 250 p.p.m. of poly(propylcne glycol ether) decreased the polymer viscosity from 39.2 to 34.2 centipoises at 6y0 conversion and also decreased the rate from 39.0 to 21.670 per hour. A modifier was sought which would control polymer viscosity without being a retarder. Either the modifier was a retarder, such as indene, or it was insufficiently acidic to have any effect as was triphenylmethane. Vinylcarbazole and anthracene acted as sluggish comonomers, reducing both viscosity and rate. With continuous monomer feed, an inhibitor would be expected to act as a modifier and retarder, and it was generally observed that the viscosity was lower in a continuous polymerization (Figure 4) than in a batch polymerization or one with intermittent feed. The mechanism of chain termination, in the absence of sufficient aldehyde or impurities having active hydrogen, is
1.6 1.6 5.9 56.0 8.3 8.3 8.3
26 26 210 210 81 81 81
Oxygen
Yellowness," %
29 58
3.5 3.5 14.0 14.0 13.0 33.0 42.0
Difference in transmittance a t 620 and 420 mp of a 0.1-inch specimen.
748
INDUSTRIAL AND ENGINEERING CHEMISTRY
c.
a
O.lycindene
Impurity, P.P.M. tert-Butylcatechol Water
Temp.,
Rate %/Hr.
Polymer Viscosity,
Centipoises
46.0
9.6
11.2
36.0
2.02
52.9
31.2
1.7
79.0
I
Yellowness Was Intensified b y Oxygen Addition
Batch Az B1 B2
51.4 84.4 145.0 60.7 183.0 103 0 28.8 6804.0 2670.0
28 p.p.m. acetophenone.
result of heat treatment during drying and molding. As shown in Table VI, this permanent yellowness was intensified by the addition of oxygen during polymerization and not by acetophenone or tezt-butylcatechol. Autoxidation products of a-methylstyrene other than acetophenone-for example, hydratropic aldehyde (2)-seem to produce more yellowness. There appears to exist in the monomer an additional unidentified high boiling impurity which affects polymer viscosity. This was of significance prior to the use of the promoter when the viscosity of the polymer was often too low. For example, a sample of monomer was fractionated into lower boiling (80Y0) and higher boiling (ZOy0) portions. These were then purified to remove acetophenone. Polymerizations were carried out in sealed tubes, and a polymer viscosity of 36.5 centipoises was obtained with the lower boiling fraction and 8.45 and 6.85 centipoises with samples of the higher boiling fraction. I n another experiment, propylene of 99 mole Yopurity was used to prepare a-methylstyrene which was then polymerized at 9' C. after sodium treatment as above. This monomer was prepared in two grades of purity. The lower boiling sample contained 4.770 cumene and 0.2y0 indene and had a viscosity of 175 centipoises. The higher boiling samples contained 0.2yc cumene and
Table VI.
0.86 1.35 0.67 12.5 1.05 0.89 21.6 0.86 0.2
Table V. Higher Temperatures Increased Rate with Poly(propy1ene Glycol Ether) and Decreased Viscosity"
267 p.p.m. promoter: 4% aodium.
considered to be hydride transfer. The presence of unsaturation in the polymer has been reported (4). I t could arise, alternatively, by a nondegradative metalation of monomer, but this reaction would be expected to be accentuated by the promoter. The indication of a n extreme maximum in the relationship of polymer molecular weight to increasing promoter concentration suggests that the promoter stabilizes the growing chain toward elimination of sodium hydride in addition to increasing propagation rate. At higher promoter concentration, cleavage of the ether becomes a controlling termination step depending on temperature and choice of ether. Acknowledgment
The contribution of Frank M. Bolton, who suggested the use of polyglycols as promoter, is acknowledged.
Literature Cited (1) Ebert, G., Fries, F. A. (to I. G. Farbenindustrie, A. G.), Ger. Patent 520,104 (Jan. 26, 1929). (2) Hock, H . , Siebert, S.,Chem. Ber. 87, 546 (1954). (3) Jones, G. D. (to Dow Chemical Co.), U. S. Patent 2,621,171 (Dec. 9, 1952). (4) Jones, G. D., Friedrich, R. E., Werkema, T. E., Zimmerman, R. L., IND. ENG.CHEV.48, 2123 (1956). (5) McCormick, H. W., Dow Chemical Co.,
Midland, Mich., unpublished work.
(6) McCormick, H. W., J. Polymer Scz. 2 5 , 4 8 8 (1957). (7) Marvel, C. S., Bailey, W. J., Inskeep, G. E., J.Polymer Sci. 1, 275 (1946). (8) Pasternak, R., Helv. Chim. Acta 31, 753 (1948). ( 9 ) Scott, N. E. (to E. I. du Pont de Nemours & Go.), U. S.Patent 2,181,771 (Nov. 28, 1939). (10) Fzwarc, M., J. Am. Chem. SOC.78, 2636 (1956). (11) Walker, J . F. (to E. I. du Pont de Nemours & Co.), U. S. Patent 2,327,082 (Aug. 17, 1943). (12) Werkema, T. E. (to Dow Chemical Co.), Ibid., 2,658,058 (Nov. 3, 1953).
RECEIVED for review June 5, 1958 ACCEPTED March 2, 1959 Division of Polymer Chemistry, 135th Meeting, ACS, Boston, Mass., April 1959.
