T h e product was weighed and analyzed. Product identification was by mass spectrometry in terms of peak heights representing the different mass levels of the components. Decaborane conversions were obtained by combining the amount of decaborane isolated with that found in the product. Results and Discussion
The results of the alkylations in which aluminum chloride, zinc chloride, titanium chloride, antimony chloride, and mixed halides were used as catalysts are presented in Table I. Table I1 contains the results of the alkylations in which the sesquihalides and triethylaluminum were used as catalysts. Included in Table 11, for comparison, are the results of an aluminum chloride experiment. Reactions using aluminum chloride were conducted to provide a standard by which the results obtained with other catalysts might be compared. The weaker Friedel-Crafts catalysts were of insufficient activity to catalyze this reaction. The mixed catalyst (SbC13AlRr3) also failed to catalyze the reaction. The synthesis of the sesquihalides, utilizing proper precautions, was not difficult. T h e sesquiiodides and bromides were prepared because these aryl and alkyl halides were the most readily available and the preparation did not require pressure equipment. The mixing of the sesquihalides with pentane in a dry box was considered the most effective means of handling these pyrophoric compounds. This mixture could be briefly exposed to air in comparative safety when the transfer to the reactor was made. T h e aryl sesquiiodide, although more difficult to prepare, had the advantage of being nonpyrophoric in air for short periods of time. T h e initial alkylation using 2.5 grams of methyl aluminum sesquiiodide did not occur, probably because of the presence of small amounts of impurities in the decaborane. T h e alkylation proceeded briskly a t 70' to 75' C. when 5- and 10-gram quantities of these catalysts were used. This represents a lower mole per cent of catalyst than necessary for aluminum chloride. When decaborane conversions, using these sesquihalide catalysts, were increased to the 50% range, polymethylation was also increased, lowering the boron content of the product below 50%. KO evidence of the presence of ethyldecaboranes was found when the ethylaluminum sesquihalides were used.
The catalyst activity of phenylaluminum sesquiiodide was considerably less than that of the alkyl sesquihalides. Only an 11 to 13% conversion was realized as compared to the 34 to 38% conversion of alkyl compounds. The use of triethylaluminum as a catalyst was interesting in view of the fact that no reaction occurred. An active compound such as this would be expected to enhance alkylation. The sesquihalides compare in general with aluminum chloride as follows: Slightly higher decaborane conversions ( 5 to 10% over aluminum chloride) can be obtained in the presence of the alkylaluminum sesquihalides without materially affecting product distribution. The alkylation using these sesquihalides proceeded more rapidly and at lower temperatures than the corresponding aluminum chloride reaction. The sesquihalides are, in general, more expensive and more difficult to handle than aluminum chloride. Conclusions
Zinc chloride, antimony chloride, titanium chloride, mixed halides, and triethylaluminum were unsuitable catalysts in the methylation of decaborane under the conditions investigated. The use of alkylaluminum sesquihalides as catalysts does not sufficiently increase decaborane conversions to warrant their replacement of aluminum chloride. Acknowledgment
T h e authors thank John Norman, who furnished all the spectrometry data. literature Cited
(1) Grasse, A., Mavitz, J., J. Org. Chem. 5 , 106-21 (1940). (2) Ohenland, C., Newherry, J., Olin Mathieson Chemical
Corp., New Haven, Conn., unpublished paper on methylation of decahorane. ( 3 ) Olin Mathieson Chemical Corp., Niagara Falls, N. Y.,
unpublished work. (4) \Villiams, R. L., Dunstan, I., Blay, N. J., J . Chem. Soc. 1960, pp. 5006-12. RECEIVED for review March 10, 1965 ACCEPTEDJune 1, 1965
STEREOREGULAR POLYMERIZATION OF BUTADIENE WITH ALKYLALUMINUM CHLORIDES AND COBALT OCTOATE M O R R I S G l P P l N
Central Research Laboratories, The Firestone Tire CY Rubber Co., Akron 17, Ohio
earlier publication (70) data were presented which established the unique character of one of the known catalysts for the polymerization of butadiene to a polymer of up to 9870 cis-l,4 content. This catalyst, consisting of diethylaluminum chloride and cobalt chloride, required a modification of a minor proporrion of the diethylaluminum chloride with either water or oxygen in order to activate it. The data reported and discussed at that time included, mainly, the
I
N AN
160
l&EC P R O D U C T R E S E A R C H A N D D E V E L O P M E N T
effect of water and oxygen on the heterogeneous EtZAlC1CoCl? system and the solvent-soluble EtpAlCI-CoClp.py catalyst. T h e mole ratio variations in the catalyst components, the effects of polymerization temperature and solvent, and some experimental work on polymerization rates were also discussed. The technical importance of configurational uniformity in high polymers is an accepted fact. Among the synthetic polydienes known, only the cobalt-catalyzed polybutadiene
The water or oxygen requirement for the activation of the diethylaluminum chloride cocatalyst with cobalt to produce a 98% cis-lr4-polybutadiene was reported previously. The activation i s considered to be the formation of two alkylaluminum compounds differing in degree of Lewis acidity. Several reactions presumed to lead to the higher acid were carried out on a minor proportion of the diethylalurninum chloride. Water i s neither necessary nor desirable in the ethylaluminum sesquichloride-cobalt octoate catalyst system. Polar solvents d o not appreciably alter the cis-lr4 catalysis with diethylaluminurn chloride-water-cobalt octoate, but do decrease the rate or extent of polymerization with increase in the dielectric constant of the solvent. A mechanism of cobalt catalysis of polybutadiene i s proposed.
has shown the highest degree of cis-l,4 stereoregularity. This Ziegler-type catalyst system involving cobalt as the transition metal component has received much attention. The consensus in published literature favors, as the cocatalyst, a n alkylaluminum halide such as diethylaluminum chloride (4, 75-27) or the alkylaluminum sesquichloride types (3, 5-7, 7 7 , 73, 23-26). There are very few instances where a trialkylaluminum and a cobalt compound have been cited. The advantages in the use of a soluble cobalt compound have been recognized. Benzene-soluble cobalt compounds have been obtained by formation of cobalt chloride complexes with pyridine (27)> with the nitroso group (79)> with aluminum chloride (2. 3>6 . 73, 23. 25: 26): Lvith butanol (7)>with 6diketones, such as the acetylacetonate (76, 77) and the alkyl esters of phosphoric and phosphorus acids ( 4 ) . Currently, the cobalt salts of the fatty acids have found wide use in this catalyst system, the benzene-soluble cobalt octoate being the most commonly used. T h e treatment of diethylaluminum chloride with water or oxygen is believed to result in its partial dealkylation and substitution of an electronegative group for an ethyl group, a condition requisite to its function as cocatalyst ivith cobalt. This dealkylation reaction was also carried out through several different routes involving certain classes of organic compounds with Lvhich diethylaluminum chloride will presumably so react. Although essential in the case of diethylaluminum chloride, this was not found to be true with ethylaluminum sesquichloride. I t appears, therefore, that \\!hatever the structure of the catalyst and its mechanism of catalysis of an elastomeric polymer, there must be present a mixture of alkylaluminum chlorides, or the equivalent, in which the over-all ratio of alkyl to aluminum is less than 2 and more than 1. Experimental
T h e follo\ving reagents were used in carrying out the polymerizations : Butadiene, special purity, 99.5 mole 70pure Benzene, thiophene-free, ACS reagent grade Ethylaluminum chlorides Cobalt octoate. 12% cobalt in benzene solution The benzene used as solvent was purified in an apparatus containing no joints other than the necessary stopcocks. It was distilled, the first 10 to 20y0 by volume discarded, and the subsequent distillate passed through a silica gel column and then transferred into a holding flask. The benzene was moved through the apparatus by application of nitrogen pressure tapped from a manifold. The entire apparatus was maintained at a slight excess pressure at all times. In the preparation of a polymerization run. the required amount of benzene was drawn into a clean oven-dried bottle from the holding flask. Thirty to 40 grams of calcium hydride were added and butadiene was distilled into the bottle in a closed system after a similar forerun had been discarded. The bottle was capped after insertion of a dry glass-wool plug into the neck of the bottle and was allowed to stand 24 hours. After the 24-hour contact, the solution was transferred under dry nitrogen to another oven-dried bottle by means of a double
needle valve, desiccant being retained on the glass-wool filter. The jvater content of liquid hydrocarbon has been reported to be reduced from 200 p.p.m. to 1 p.p.m. in 30 minutes of contact timr with calcium hydride a t room temperature (74). T h e method of drying the benzene solution of butadiene, as described above, should therefore result in a water residue not to exceed 1 p,p,m. Since the reaction betiveen calcium hydride and water is known to be irreversible and quantitative, it is highly improbable that any amount of water, measurable by any means, Lvould remain in the solution just prior to adding the catalyst. Aluminum alkyls were used a t approximately 1M concentrations in dried, purified benzene. T h e cobalt octoate was diluted to 0.02.21 solutions Lvith the same solvent. All the experimental \vork \vas carried out \vith 50 grams of butadiene dissolved in a total of 450 grams of benzene. The usual order of catalyst addition to the monomer-solvent was alkylaluminum solution, water (Xvhich was added as a saturated solution in benzene), follo\ved by the cobalt octoate. All polymerizations \\.ere carried out a t 5" C. for 19 hours, except wherc stated otherwise. Polymers Ivere isolated by coagulation in 2-propanol containing some phenyl-2-naphthylamine follo\ved by leaching in fresh alcohol-antioxidant for 24 hours. The polymers were blot-dried and covered with a pentane solution of antioxidant. The swelled polymers were finally dried in a vacuum oven a t 50' C. Infrared measurements \\.ere made on polymer samples purified by solution in toluene and reprecipitation with alcohol. The film technique described in the patent literature (20) was used to determine the per cent cis-1.4, trans-1,4, and vinyl configurations. Other methods for determining the microstructure of polybutadiene have been studied and used in this laboratory. These methods indicate lower cis-1 :4 values, but for purposes of comparison with published infrared microstructure data, the patent literature method was used throughout this \vork. Water Substitutes
The theory of the function of the water component of the Et?AlCl-H?O-cobalt octoate catalyst has been discussed (70). T h e most effective form of this catalyst is the one in \vhich the ethylaluminum chloride seems to exist in nvo states of relative Le\vis acidiLy-for example. EtPXlCl and Et.41C1,. T h e dealkylation of a portion of the Et?.AlCl with H20 to give EtAl(0H)Cl E t H provides the Et.AlX2 (X = electronegative group) necessary to form the co-species with the remaining E,t,.AlCl for the activation of the Ivhole catalyst. The experimental data given in Table I indicate a n optimum in polymer conversion and microstructure at 5 to 1 5 mole % of Lvater based on the diethylaluminum chloride. At 100 mole 76 of Ivater. practically no polymerization took place. This experimental optimum seems to parallel the findings reported by Adema. Bartelink. and Smidt ( 7 ) to the effect that polymer yield is a t a maximum \\.hen 10 mole yc of ivater or oxygen is present in thc Et2.41CI-TiCl, catalyst for the polymerization of olefins. These authors observed that the concentration of unpaired electrons. as measured by electron spin resonance studies. is a t a maximum \\.hen 10 mole :5 of ivater or oxygen based on the Et2.%lC1is present, and that complete elimination
+
VOL. 4
NO. 3
SEPTEMBER
1965
161
of unpaired electrons occurs a t a mole ratio of H20!Et2A1C1 = 1. They suggested that because the curves for the polymerization rate of ethylene and for the concentration of unpaired electrons us. time are identical in shape, there is a direct relationship between the concentration of unpaired electrons and the polymerization rate of ethylene. A chemical reaction which results in the partial substitution of certain electronegative groups (limited in these studies to -OH and -C1) for an ethyl group in EtQAIC1 should provide the required cospecies of higher Lewis acidity necessary to promote the catalysis of butadiene polymerization with cobalt. The following compounds, which were assumed to be capable of entering into this type of reaction, were tested in place of water. The polymerization results are given in Table 11. Organic Hydroperoxide. The possible reaction Etp41C1 ROOH + EtAl(0H)Cl f ROR could furnish EtAlX?. T o test the activation by peroxides, 2.0 to 8.0 mmoles of cumene hydroperoxide (CHP) were used with a ratio of 20j0.04 mmoles of Et2AlCl/cobalt octoate per 100 grams of monomer (phgm). The control catalyst containing 2.0 mmoles of H 2 0 produced the typically high conversion polybutadiene of 98% cis-1,4 structure and 5.9 dilute solution viscosity (DSV) in deciliters per gram. When both H20 and C H P were omitted in the catalyst, no polymerization took place. However, the substitution of 2.0 to 6.0 mmoles of C H P for the water successfully produced high conversion polymers of 97 to %yocis-1>4 and 3.4 to 6.2 DSV. Halogen. EtAIX, results from the reaction Et,AlCl Br2-P EtAlBrCl EtBr. Elemental bromine should therefore activate the catalyst. This was actually found to be the case. From 4.0 to 8.0 mmoles of bromine (phgm as monoatomic) activated the catalyst to produce polybutadiene of 957, cis1,4 structure. Conversion was high, but considerable gel was formed a t the higher bromine levels. Elemental iodine used in the same manner did not react to form the active compound EtAlXZ required for polymerization. Alcohol. Another route to Et.41X2 could be according to the reaction Et2AlCl ROH + EtAl(0H)Cl REt, in which the alcoholic hydroxyl is of sufficient lability to react. O u r earlier experience has shown that the substitution of an equivalent amount of ethanol in place of water failed to activate the catalyst. But the lability conferred upon a n alcoholic hydroxyl through its attachment to a tertiary carbon atom proved to be effective in activating the catalyst, presumably according to the reaction stated above. A solution of tert-butyl alcohol in benzene (the alcohol was dried over activated alumina in a warm oven for 24 hours, then distilled under nitrogen) was used to provide 4.0 to 10.0 mmoles phgm of the alcohol component of the catalyst. High polymer conversions were obtained in every case, and the cis-l,4 contents of these polymers ranged from 92 to 94%. Organic Halides. ~-CHLOROETHANOL. This contains a highly reactive chlorine atom. It could therefore be possible to produce controlled dealkylation of the Et2AlCl such as is believed to occur Lvith Xvater, CHP, Brl, and tert-BuOH to give EtAlX,. Ft'hether the -OH also participates to any extent in the dealkylation reaction is not known, but the desired effect \vas obtained with 2.0 to 10.0 mmoles phgm of the 2-chloroethanol. At each level, high conversion polymers \vere obtained with cis-1,4 contents of 93 to 967,. ALLYLCHLORIDE.This possesses a chlorine atom Lvhose reactivity is enhanced by the carbon-carbon double bond adjacent to the carbon to ivhich it is attached. The reaction EtyAICl CHFCH--CH2-C1 could lead to the desired
+
+
+
+
162
Mole
70
H?O Based on EtZAlC1
0 0.5 1 .o 2.5 5.0 10.0 25.0 50.0 100.0 110.0 a Based analyzed.
l&EC PRODUCT RESEARCH AND DEVELOPMEN1
70
P.P..M.a
CIS-
H20 Cow. 1,4 0 0 1.8 9.9 92.4 3.6 52.5 94.5 9.0 92.7 96.5 97.5 18.0 93.8 92.8 97.8 36.0 85.3 97.7 90.0 60.2 97.8 180 7.3 94.4 360 396 2.6h on total benzene-butadiene.
Table II.
Per Cent trans-
1,2 DSV
1,4
1.8 1.5 1.7 1.2 0.9 0.8 1.0 4.2
5.8 4.0 1.8 1.3 1.2 1.5 1.2 1.4
0.9 1.5 3.2 4.9 6.2 7.6 8.4 4.9
%
Gel 0 0 0 0 0
0 5.0 28.0
Powder which could no6 be
Substitutes for Water
Et*AlCl/Cobalt octoate = 20/0.04 mmoles phgm H,O Substitutea H20 CHP
yo
Conu. 100
Mmoles
2.0
0 91.6 99.7 91.4 17.0 0
0 2.0 4.0 6.0 8.0
+
+
Variation of Water Component
Table 1.
Et*AlCl/Cobalt octoate = Z0/0.04 mmoles phgm
Br
0
tBA
0 4.0 6.0 8.0 10 0
CE
0
2.0 4.0 6.0 8.0 10.0 AC 0 2.0 4.0 8.0 a C H P , Cumene Chloroethanol. A C ,
Table 111.
Per Cent trans1,3 1,4 1,2
cis-
70
98
1
I
DSV 5.9
97 98 98 98
2 1 1 1
2 1 1 1
3.4 4.9 6.2 6.1
0 0 0 0
95 95 95
3 3 3
2 2 2
3.2 1.8 1.1
0 43.5 65.7
0 2.1 94 3 3 100 4 4 1.6 99.6 93 4 4 1.4 94.6 92 96.9 94 4 3 2.0 0 96 2 3 2.5 98.7 1.9 3 4 1OQ 93 3 4 1.9 98.6 94 3 3 2.8 100 95 3 3 2.2 9 9 . 3 94 0 1 1 3.7 66.0 98 3.5 98 1 1 85.2 1 1 2.5 7 1 . 6 98 hydrofieroxide. t B A , tert-Butyl alcohol. Allyl chloride.
Gel 0
0 0 0 0
0.5 0 0 0 0 0 0 0 C E , 2-
Relative Effectiveness of the Isomeric Chlorobutanes as Substitutes for Water
Et?AlCl/Cobalt octoate = 20/0.04 mmoles phgm C'
GI Isomer H20 n-
Iso-
tert-
,c
Per Cent trans1,4 1,4 7,2 98 1 1
DSV 5.7
Gel
c /o
as-
Mmoles 2.0 0 2.0 4.0 0 2.0 4.0
Conv. 95.5 0 0 0
88.4 85.5
95 95
2 3
3 3
1.8 1.9
0 0
0 2.0 4.0
0 88.4 81.8
98 98
2 1
1 1
3.5 3.0
0 0.5
0
0
AlEtCI, and either 1-pentene or other possible products. From 2.0 to 8.0 mrnoles phgm of allyl chloride resulted in high yield of polymer containing 98% cis-1,4 structure. The controlled dealkylation of I S O M E RCHLOROBUTANES. IC a portion of the Et2AlCl by means of reactive electronegative groups, as illustrated in the above examples, is further demonstrated by a comparison of the effectiveness of the isomeric chlorobutanes. T h e reactivity of the chlorine atom in these compounds is a t a minimum in n-chlorobutane, and increases with the iso-, secondary-, and tertiary chloro isomers, in the order given. In these experimental runs, 2.0 to 4.0 mmoles phgm of the chlorobutanes were substituted for water. The results shown in Table I11 indicate that the chlorine atom attached to a primary carbon in a straight chain is completely unreactive in the dealkylation of EtQAIC1, whereas the chlorine atom bound to a tertiary carbon presumably reacted with Et2AlC1 to result in a n active catalyst capable of producing high yields of 98y0 cis-1,4 polymer. The increase in cis-1,4 content from 95 to 97 and 98YC is also proportional to the relative degree of reactivity of the chlorine atom in this series of isomers. Those cases Ivhere polymer yield us. concentration of activating agent did not fol1oLv a direct relationship but instead passed through a peak, as Lvith CHP and AC, may be attributed to the presence of side reactions or the effect of accumulated reaction products on chain termination. .ALU>lINU>$ METAL. Pure commercial Et2AlCl used in this laboratory alxvays required the addition of water. However, on one or tw.0 occasions a sample \vas used which gave variable degrees of conversion without the addition of water. These EtZAlC1 samples \\.ere sometimes cloudy, and on standing: a fine gray precipitate settled out. The resemblance of this solid material to powdered aluminum metal suggested a n investigation of the effrct of elemental aluminum on EtsAlCl in relation to catalyst activation. The results of several experimental runs in ivhich aluminum metal \\-asused to convert the inert Et,AlCl-cobalt catalyst into its active form are reported in Table 11.. A normal sample of EtrAlC1 which cannot form an active catalyst ivith cobalt octoate in the absence of water !vi11 d o so after its reaction lvith elemental aluminum. A reaction period of 24 hours bet\veen the aluminum metal and the Et,AlCl resulted in considerable improvement in catalyst efficiency compared xvith the in situ addition of the three components to the benzene-butadiene charge. A mechanism for the activation of the catalyst with aluminum metal must take into account the formation of EtZ41Cl2 from Et2.41Cl. to conform with the proposed theory of the requirement for a Leivis acidity "potential difference" in alkylaluminum halide. ,4 suggested route to EtAlCl, from EtzAlCl and A1 is the following:
Et-
AI,
/''
+ A(
4-
Et
In this reversible reaction tivo molecules of Et2AlCl become coordinated through the A1 atom into a n intermediate containing the elements of EtAlC12, with which the EtzAIC1 present in excess can combine to form the cocatalyst species. In summarizing these results, the formation of a n active diethylaluminum chloride cocatalyst requires the conversion of part of it to the higher Lewis acid derivative. This is accomplished by reaction with water, oxygen, organic materials having a reactive electronegative group capable of substitution for one ethyl group, or aluminum metal itself. By careful control over the amount of these activating agents added to the diethylaluminum chloride, polymerization runs can be made in which the rate of polymerization, yield, microstructure, and polymer DSV are remarkably reproducible from run to run. The suitability of each sample of EtsA41C1received in this laboratory, lvith respect to reproducibility of results, is determined by its performance in controlled experiments which include a test run without added water. The usual absence of any polymerization, except in the rare instances of an EtzAIC1 sample which Ivould form a polymer-producing catalyst with cobalt without the addition of water. signified its chemical purity within tolerable allowances. Effect of Water on Ethylaluminum Sesquichloride Cocatalyst
The polymerization results obtained with the Et2.41C1-Hz0cobalt octoate catalyst in which the H,O/Et,AICl mole ratios were varied, as shown in Table I , indicate that maximum conversion to polymer occurred ivhen from about 5 to 15 of water based on the Et2AlCl was included in the mole catalyst, and that no polymerization took place in the absence of water. A t these cocatalyst compositions the cis-l,4 addition was high. and showed a somexvhat lesser dependence on the water content of the catalyst than did the polymer conversions. However. the DSV of these polymers varied directly with the amount of water in the catalyst. I n contrast to the gradual improvement in catalyst activity with respect to final polymer conversion and cis-1,4 content as the mole ratio of water to Et2.41Cl was increased u p to the optimum levels indicated, the effect of water on the ethylaluminum sesquichloride catalyst component was such that
~~
Table IV.
Activation of EtrAlCl with Aluminum
Et,XlCl/Cobalt octoate Source of EtgZ4IC1 A" Bb Bb
AI, G.
0 0
.+fmoles Hr 0
0 0
r' Coni,. 92.1
=
20/0.04 mmoles phgm Pi'r Cent cis-
7.1 96
trans1.J
7 2
D VS
2
2
3.4
%
Gel 0
0
0 2.0 100 98 1 1 5.7 0 0.04 0 53.3 91 3 6 0.8 0 Dd 0.04 0 89.8 95 2 3 2.0 0 Sample of Et2AIC1 found not to require ivaicr f o r artii'ation. Snrnple of Et24ICI in use 0 1 er pitended period and found to require added ztsater f o r actication. Sample of B . A1 added in situ, Sample of B in contort w i t h gran:rlnr aIurninum f o r 2.1 hours brfore U S E .
CC
VOL. 4
NO. 3
SEPTEMBER
1965
163
100
I
0
L20p,, 0 DIETHYLALUMINUMCHLORIDE
40
ETHYLALUMINUMSESQUlCHlORlDE
,
2
6
4
8
,
,
,
,
10
12
14
16
18
Rpm. H20 Added Figure 1 . Effect on final polymer conversion of water added to diethylaluminum chloride and ethylaluminum sesquichloride
2
6 8 10 12 14 Rpm. H20 In Catalyst Deterioration of poly4
Figure 2. merization rate resulting from addition of water to ethylaluminum sesquichloride
maximum catalyst activity took place when water was omitted altogether. These contrasting results were obtained with solutions of butadiene in benzene which were prepared and dried under identical conditions. I n Figure 1 the final polymer conversions obtained with the ethylaluminum sesquichloride cocatalyst to which variable amounts of water, expressed in parts per million, were added are contrasted with the conversions obtained with the diethylaluminum chloride cocatalyst containing the same range of water. The ethylaluminum sesquichloride type cocatalyst gave maximum conversion beginning with zero water in the catalyst. At most a very slight decline in final conversion a t the higher water concentrations may be interpreted from the curve. The diethylaluminum chloride type cocatalyst, on the other hand, is inert toward polymerization in the absence of water, but becomes as efficient in producing high conversion of butadiene to polymer as the sesquichloride type only when a t least 10 p.p.m. of water is added. From the data given in Table V, the per cent conversion us. time of polymerization was plotted a t each level of water contained in the catalyst, including zero added water. From these curves, the time in minutes when 507, conversion was reached ( t l i 2 ) was obtained and plotted against parts per million of water in the catalyst, as shown in Figure 2. The curve 164
l & E C PRODUCT RESEARCH A N D DEVELOPMENT
demonstrates the adverse influence of water on the rate of polymerization of butadiene catalyzed with ethylaluminum sesquichloride and cobalt octoate. The initial solution of butadiene in benzene (containing no added water), which gave no polymerization with diethylaluminum chloride and cobalt octoate, gave a polymerization half time of 31 minutes under the stated experimental conditions. Thereafter, beginning with the addition of 1.0 p.p.m. of water, the half-time period increased a t a rate which was proportional to the amount of water added. The performance of the ethylaluminum sesquichloride cocatalyst was found to differ from published data (6) which indicate that optimum polymerization rates required 2 to 5 p.p.m. of water. The polymer DSV at full conversion showed the expected increase with higher levels of water in the catalyst. The minimum DSV a t all conversions was found in the series of polymers where no water was added. While higher than normally desired in most practical applications, the DSV is nevertheless the lowest for this catalyst where water is excluded. Within each water level, however, the DSV increased with conversion to a peak a t around 50 to 60% conversion, then decreased somewhat. This was observed to be true with the EtzAlCl-HnO-cobalt catalyst which was discussed in the where it was suggested that the decrease earlierpublication (70), in DSV after 50 to 60% conversion might be due to cyclization of the polymer chains. Within the limits of the water range tested, the microstructure of the polymer is not affected to any appreciable extent, if a t all, by the presence or absence of water in the catalyst. The precision of the infrared film analysis technique used in the determination of the cir-1,4 content is shown by the following values obtained in seven replicates of the total conversion polymer made with the water-free catalyst: 97.7, 97.8, 98.1, 97.9, 97.6, 97.5? 97.5. The average is 97.7 and the range is 0.6 unit. The cis-1,4 values from Table V were plotted against the corresponding per cent conversion at each level of water, including zero water, in the catalyst. A comparison of the cis-1,4 values found at different levels of water in the catalyst, both a t 60% conversion and at maximum conversion, is shown in Table VI. According to these data, it would be difficult to say that there was any change in the ris-124content of the 60% conversion polymers in view of the 0.6 cis-1,4 variation in measurement observed in the experiment on replicate determinations. Nor can i t be stated with confidence that there is any change in cis-l,4 content among the maximum conversion polymers made with the water containing catalysts compared with the polymer obtained with the water-free catalyst.
Polar Solvents
Throughout all the work done with the cobalt catalyst, polymerizations were carried out in benzene solvent, which is ideally suited to the rapid and efficient catalysis of butadiene to high yields of polymer of 98% cir-1,4 structure and 3.5 to 6.0 DSV. Also, consistency of polymerization results is another outstanding feature of benzene as solvent. Studies were also made of catalysis in hexane solvent and in mixtures of benzene and hexane, the latter proving to be one method for controlling the polymer molecular weight without affecting the microstructure (70). It was a matter of interest to observe the effect of solvent polarity upon the course and outcome of a butadiene polymerization with Et2A1C1-HaO-cobalt octoate. Several aromatic solvents of dielectric constants ( E ) varying from 2.3 for
Table V.
Polymerization Time, Min. 10
7
0 2 4 17 0 49 . - . 9" ~
50 60 90 19 hr.
77.2b 87.5" 87.Y 94.0 97.4
20 30 40 50 60 90 19 hr.
98.4 98.5 98.2 98.0 98.1 97,8 97.7
10 20 30 40 50 60 90 19 hr.
3.4 4.6 5.3 4.8 4.6 4.6 4.5 4.4
0 11 2 71 7 49.4 71.9 81.3 93.8' 95.6
98.8 98.5 98.4 98.3 98.4 98.2 97.9
Addition of Water to Ethylaluminum Sesquichloride Cocatalyst
(Et3A12C13)l,2/Cobaltoctoate = 20/0.04 mmoles phgm P.P.M. H20 2 3 4 5 6
yo Conversion
8
70
14
55.4b 78.7 86.7 92.7 96.9
0 6 7 20 . 5 40.8 53.6 66.8 86.7 96.7
4 9 1~. 5 .2 32.9 45.2 65.7 89.5j 95.6
2 5 11 9 ii.2b 33.4d 41 . O 86.6 95.1
0 2 1 10 8 20.5 32.9' 43.00 83.7j 94.5
0 0 4 1 9.2 13.4" 20,Oh 53.9 95.7
0 0 0 5 4.2 8.1 13.50 29.2i 92.8
0 0 0 1.7 5.0 7.7/ 13.6i 94.6
98.7 98.4 98.2 98.0 98.2 98.1 97.7
98.9 98.9 98.7 98.7 98.8 98.6 98.2
98.7 98.8 98.8 98.6 98.6 98.2 98.2
98.7 99.0 98 8 98.8 98.9 98.5 98.1
98.7 98.8 98.7 98.8 98.5 98.5 98.3
98.7 99 . 0 98.9 98.8 98.8 98.5
98.1 98.2 98.8 98.4 98.9 97.9
98.1 98.7 98.5 97.9 97.9
4.3 5.1 5.4 7.1 7.0 6.3 5.4
4.6 5.8 6.2 6.2 7.4 5.9
4.8 5.8 6.7 7.7 6.2
0 16.4 1 2 8-
~~
0
0
.
Dilute Solution Viscosity 4.5 5.5 5.6 5.6 5.4 4.9 4.7
4.4 4.8 5.1 4.5 4.8 4.5 4.5
Corrections,polymerization time in minuies.
a
4.3 5.4 6.0 6.0 5.9 5.3 4.9
4.4 5.7 6.2 6.2 6.3 6.0 5.4
5.2 6.0 6.7 6.8 6.1 5.2
19.5% gel.
31, 39, 49, 52,e 51, f 62,0 61, 59, 92,1 91.
benzene to 10.2 for rn-dichlorobenzene were tested with the Et2AlCl-HZO-cobalt catalyst. These solvents were. generally, the mono- and dichlorinated derivatives of benzene and toluene. According to the data in Table VII, the rate or extent of polymerization was reduced with the increase in magnitude of the dielectric constant of the solvent, but there was little effect on the polymer microstructure. These two phases of the polymerization process, the amount of polymerization and the configuration of the polymer units, would seem, therefore, to be distinct and separate. T h e Ziegler-Natta catalyst structure usually regarded as most probable is a type of complex formed by the transition metal and the alkylaluminum, upon which monomer adsorption and rearrangement represent the unit growth of the polymer chain. If this structure existed in the cobalt system. it would hardly be likely that solvent polarity would affect that part of the complex where chain growth occurs without a t least
5.2 6.8 6.9 7.w
indirectly affecting the structure of the complex responsible for polymer configuration. Proposed Mechanism of Catalysis
The polymerization of butadiene in the cobalt catalyst system to a soluble, high c i J - l , 4 elastomeric polymer requires two alkylaluminum compounds differing in Lewis acid acidity. T h e polymer can be made with a cocatalyst consisting of RzAlCl and 10 mole % (based on the RzAlCl) of water, or with any of the water substitutes discussed above, or with alkylaluminum sesquichloride (R?AlCl.RAlCl?), or with mixtures of R2AlCl and RA1C12 of limited mole ratios. This essential Lewis acid diffrrence suggests a n acid-base type of reaction between the alkylaluminum halides or the equivalent. A series of polymerizations was carried out using as cocatalysts with the standard amount of cobalt octoate equimolar amounts of all possible combinations of t\vo of the following alkylaluminum compounds : Et3Al, Et2AIC1, EtAlC12, and AICI,, stated in the order of increasing Lewis acidity. T h e
Table VI.
Polymer Microstructure at Different Amounts of Water in Catalyst (Et3A12C13)~,2/Cobaltoctoate = 20/0.04 mmoles phgm
60 %
Per Cent cis-1.4 at Max.
P . P .M , Hz0
tonuerston
conr'erszon
97.7 97.9
5 6 8 10
98.4 98.3 98.2 98.6 98.6 98.7 98.6 98.7 98.4
97 7
98.2 98.2 98.1 98.3 98.5 97.9
Table VII.
Polymerization Effects 6f Polar Solvents
Et2X1C1/M20/Cobalt octoate = 20/2.0/0.4 mmoles phgm Total solvent, 11.52 mmoles 7c
SolLment
B
Benzene Toluene o-Chlorotoluene m-Chlorotoluene Chlorobenzene o-Dichlorobenzene m-Dichlorobenzene
2.3 2.4 4.7 5.5 5.9 7.4 10.2
VOL. 4
Conn. 96.5 89.4 49.0 31.6 63.5 31 . 6 26.1
NO. 3
Per Cent cis- trans-
7,3 1:4 1,2 DSV
98 98 98 97 97 96 95
1 1 1 1 1
2 1 2 3 2
1
3
2
3
SEPTEMBER
5.5 4.3 2.4 3.1 4.0 4.2 1.4
1965
c /O Gel
0 0 0.7 0 0 0 0
165
Table VIII.
Effects Produced by Alkylaluminum Pairs Differing in Lewis Acidity Et,AlCl,/cobalt octoate 20/0.04 mmoles phgm c 5% /G Conu. cis- 1,4 DSV
+ ++ ++
EtaA1 EtzAlC1 e EtzA1’ Et3AlC1 Et3A1 EtAlClz F? Et2Al+ EtzAICln Et3Al AICI3 EtzA1 EtAlC13 EtzAlCl EtAlClZ e Et*Al+ Et.41a3 Et2AlCl AlC13 e Etzxl+ AlCl4 EtAlClz A1Ch F? EtAl+Cl A1C14
0
+ + + + ++ +
67.4
94.6
1.7
0
93.2
97.5
4.4
0
98.4
97.7
4.6
0
87.8 28.4
64.4
..,
90.0
, . .
67.5
Polymer could not be analyzed by infrared absorption.
The configuration of the primary butadiene unit remains fixed in the cis-1,4 form. The second step is the 1,4 addition to butadienylcobalt of the carbonium ion and its counterion. This second and subsequent steps represent the growth of the chain. The function of the cobalt, then, is twofold: (1) as a carrier of the cis-bound butadiene monomer, and (2) to confer increased polarizability upon the butadiene as the result of the distribution of its pi-electrons over all four carbon atoms. These reactions presuppose a butadienylcobalt complex which is not stable enough to prevent the initial 1,4 addition of the alkylaluminum ions nor of the carbonium ion and its counterion in the growth phase. Although the polymer growth has been proposed as occurring via a carbonium ion mechanisms, the growth may also occur a t the other end of the hydrocarbon if the double bond a t the beta carbon in R’ were to add some degree of stability to it in the form of a carbanion
~
alkylaluminum compounds shown in Table VI11 include the proposed acid-base interaction products. The alkylaluminum pair lowest in the Lewis acidity scale is noncatalytic with cobalt. Those pairs high in the scale give highly gelled polymers with loss in stereoregularity. The soluble polymer of high , ~ is produced by the pair conversion and highest C Z S - ~ content located in the middle of the scale or the pair composed of the compounds taken from each extreme. Stated more precisely, the ion pair Et2Al+Et2ZlCl is noncatalytic. The pair Et2A1+EtzAlClz leads to low conversion polymer of less than maximum cis-1,4 content. The Et2Al+EtAlClZ pair forms with cobalt the catalyst system yielding maximum conversion to a soluble polymer of a very high degree of stereoregularity. Finally, the AlC14 anion and its associated cations produce what appears as a Lewis acid type polymerization marked by a very fast rate and a polymer product which is highly gelled and nonstereoregular. Of the polymer-producing ion pairs, the middle pair, Et2AlfEtAlCI,, is the most desirable from the point of view of elastomer properties. Certain Group VI11 metals are capable of forming coordination complexes with dienes, many of which have been isolated. Platinum is known to form a complex with butadiene. There is evidence that butadiene forms a complex with iron carbonyl in which the pi-electrons are distributed over all four carbon atoms of the cisoid-bound butadiene (72). Assume that butadiene will complex with cobalt in a similar manner; then the stereoregular polyaddition of butadiene can be pictured as a two-step process, the first being the formation of the butadienylcobalt complex in which the monomer is bound in its cis configuration, followed by the 1,4 addition of the alkylaluminum ion pair to it with the simultaneous or subsequent expulsion of the cobalt atom. The result is a carbonium ion a t one end of the butenyl unit and R*R’Al, in which R’ is the monomer unit, a t the other end. The reaction is represented as follows :
R,AI
I
R,AI
t:
+
4 co
\ C II
+ + C
7
Rn’lCI, 166
l&EC
PRODUCT RESEARCH A N D DEVELOPMENT
R,AI
I
C
R~AI+ C
to which butadienylcobalt may add to a greater or lesser extent. This mechanism of the catalysis of butadiene accounts for certain features of the cobalt system. The exceedingly small amount of cobalt necessary to carry out the polymerization process is due to its function as monomer coordinator-carrier and release for re-use. The proposal that a butadienylcobalt containing a cisbound monomer participates as such in this mechanism finds support in certain observations: the very low vinyl content in the polymerized polybutadiene, the failure of the cobalt catalyst to polymerize olefins or nonconjugated diolefins, and that no olefin or nonconjugated diolefin complexes with any metal in the first row of Group VI11 are known. The Hallam and Pauson evidence of a n intact conjugated diene system in butadieneiron carbonyl (72) may also apply to the closely related cobalt. There is conflicting published evidence that the growing polymer chain is a carbonium ion, as shown by the presence of radioactivity in the polymer when C14H30Hwas added to the unexposed polymerizate (B), and that it is a carbanion, shown when radioactivity in the polymer resulted when methanol containing tritium in the hydroxyl group was used to stop the polymerization (9). Natta et al. (22) reported no C14activity in the polymer, and are therefore not in agreement with Childers. They did find tritium activity of a low order present, which supports the findings of Cooper. But they conclude that the tritium measurements are unreliable, and therefore caution against the use of CH30H3 termination as a method for determining active centers in diolefin polymerization in the aluminum-cobalt system. The Natta view of the catalyst as a n A1-Co complex containing a cobalt-monomer bond of the allylic type also makes his group doubtful that this bond is broken by CH30H3 with tritium bonded to the polymer. There are, then, divided opinions as to whether the mechanism is that of a carbonium ion or carbanion as based upon radioactive alcohol termination. The reports of both types of mechanisms are not actually contradictory in terms of this mechanism. As outlined here, the polymerization proceeds
(1)
carbanion and continuation of the polymerization with a reconstituted ion pair. A similar process of chain termination and continued polymerization can take place a t the carbanion end of the chain, if this were actually present. The products of the modifying action a t the carbonium ion end of the chain would be the terminated chain and the ion pair Hex+RA1C13 or H+RAlClS. Either of these may continue to add to butadienylcobalt and continue normal polymerization or, in the case of H+RAlCl,, interact to form AlC13 which \vi11 give the ion pair R 2 A + RA1C13 u.ith excess Et2AlCl. The products of the modifying action a t the carbanion end of the chain \vould be the terminated chain end and either Hex-R*Al- or H-R2A1+, which can either add to butadienylcobalt or, as would be more likely, form R3.41 with which excess RzAlCl cannot form a cocatalyst, according to the data in Table V I I I . I t has been shown (70) that the greater the ratio of hexane to benzene, the lower the polymer DSV without affecting the high polymer conversion.
(2)
Acknowledgment
(3)
The author expresses his appreciation to E. T. Handley, G. Alliger, and B. L. Johnson for their interest, to his colleagues
via a carbonium ion mechanism, and possibly via a carbanion a t the same time, depending perhaps on polymerization conditions. However, if the RZR’Al chain end were not in the ionic form, as suggested, the reaction of alcohol with trialkylaluminum would result in polymer radioactivity when treated with tritium-hydroxymethanol, according to the reaction R2R’Al
+ 3 CH30H*
+
R’H*
+ 2 R H * + Al(OCH3)
so that the effect would be the same as the reaction between this alcohol and a carbanion chain end. The increase in molecular weight of the polymer with a n increase in the H20/RqA1C1 ratio is the result of certain shifts in the several equilibrium reactions involved. T h e following series of reactions is postulated, in which HC1 is used in place of HzO. T h e qualitative effects of varying the amount of HC1 upon the type of ions formed correlate with the experimental data in Table V I I I : EtzAlCl (excess)
+ HC1+
EtAlCz
+ EtH
+ EtAlClz e Et2Al+ + EtAlC13 EtzAICl + s EtAlC1, + Et2A1C1, EtAICl, + HC1+ AlC14 + E t H EtAlCI
(4)
The extent to which any of these reactions participates depends on the amount of HC1 used in the catalyst. T h e first step is the conversion of a minor part of the Et2AlCl to the next higher Lewis acid alkylaluminum, as in Equation 1. This will combine with a corresponding amount of Et2AlC1 to give the ion pair of Equation 2. I n the presence of excess Et2A1C1 the EtXlC1, will be reduced to EtrklC1, according to Equation 3, the product EtAlC12 combining with available Et2AlC1 to provide more ion pairs as in Equation 2. The formation of EtzAlC12 according to Equation 3, which contributes to the polymerization of low conversion, loivered cis-1,4-content, and low DSV polymer (see Table V I I I ) , is suppressed by reducing the concentration of Et2AlC1. as by the addition of more HC1 (Equation 1). The increased concentration of HCl increases the EtAlCl3/Et&lCl? ratio \\.ith the consequent increase in the higher conversion, higher molecular weight polymer. When 50 mole % of HCl has been added, the ion products of Equation 2 should exist exclusively, in the equilibrium form shown. The addition of a slight excess of HCI over the 50 mole 7 0 causes the reaction of Equation 4 to begin to make its contribution with the type of polymer this ion pair has been found to cocatalyze. However, when H20 is used in place of HC1, the higher acid form EtAl(0H)Cl is assumed to result. Since OH is more electronegative than C1, it would be expected that the Et.41( 0 H ) C h anion is more acidic than the corresponding EtklCls. Therefore, at 50 mole % H20 the ion pair Et2Al+EtAl(OH)Cl? would cocatalyze polymers approaching those obtained \\.hen Et2AI+AlC14- is introduced into the mixed equilibria. It would then be necessary to cut back the amount of H?O, to decrease the ratio E t ~ l X ~ / E t z ~ ltoX some 2 value which is ideal in the case of H20. The decrease in polymer DSV when mixtures of hexane and benzene are used as polymerization solvent, compared to the DSV obtained in benzene solvent alone, may be the result of a chain termination with either a hydride ion or hexane
for their helpful discussions, and to The Firestone Tire & Rubber Co. for permission to publish this work. literature Cited
(1) Adema, E. H., Bartelink, H. J. M., Smidt, J., Rec. T ~ QChim. v. 80, 173 (1961). (2) Balas, J. G. (to Shell Oil Co.), U. S. Patent 3,067,189 (Dec. 4, 1967) - ,--
(3) Zbi;;, 3,111,510 ( N O ~19, . 1963). (4) Balas, J. G., Porter, L. M. (to Shell Oil Co.), Zbid.,3,040,016 (June 1 9 , 1962). (5) Bataafse Petroleum Maatschappij N.V., Brit. Patent 911,947 (Dec. 5, 1962). (6) Carlson, G. J., Dong, \V., Higgins, T. L., LVilcoxen, C. H. (to Shell Oil Co.), U. S. Patent 3,066,127 (Sov. 27, 1962). (7) Chemische ll-erke Hiils AG, Brit. Patent 905,001 (Sept. 5 , 1962). (8) Childers, C. \V., J . Am. Chem. Sac. 85, 229 (1963). (9) Cooper, l V . , Eaves, D. E., Vaughan, G., 1VMakramal. Chem. 67, 229 (1963). (10) Gippin, M., IND.EKG.CHEM.,PROD.RES. DEVELOP. 1, 32 (1962). (11) Goodrich-Gulf Chemicals Co., Brit. Patent 916,384 (Jan. 23, 1963). (12) Hallam, B. F., Pauson, P. L., J . Chem. Sac. 1958, p. 642. (13) Higgins, T. L., lVilcoxen, C. H. (to Shell Oil Co.), U. S. Patent 3,068,217 (Dec. 11, 1962). (14) Metal Hydrides, Inc., Tech. Bull., Sovember 1957. (15) Montecatini SocietL Generale per 1’Industria Mineraria e Chimica, Brit. Patent 916,643 (Jan. 23, 1963). (16) Ibid.,924,244 (April 24, 1963). (17) Ibid.,924,427 (April 24, 1963). (18) Ibid.,936,061 (Sept. 4, 1963). (19) Zbid.,Ital. Patent 587,976 (Feb. 16, 1958). (20) Ibid.,592,477 (Dec. 6. 1957). (21) Zbid.,594,618 (.April 24, 1958). (22) Natta, G., et a / . , .llaX-ramol. Chem. 71, 207 (1964). (23) Porter, I.. hl.,Balas, J. G. (to Shell Oil Co.), U. S. Patent 3,066,126 (Nov. 27, 1962). (24) Tucker, H. (to Goodrich-Gulf Chemicals, Inc.), Ibid., 3,094,514 (June 18, 1963). (25) Younsman, E. -4.(to Shell Oil Co.), Zbid., 3,066,128 (Nov. 27. 1962). (26) Youngman, E. A , S o z a k i , K., Boor, J . (to Shell Oil Co.), Ibid.,3,084,148 (.4pril 2, 1963).
RECEIVED for review November 9, 1964 ACCEPTEDMay 17; 1965
Division of Rubber Chemistry, 148th Meeting, ACS, Chicago, Ill., September 1964.
VOL.
4
NO. 3
SEPTEMBER
1965
167
