Table IV.
Tensile Properties with Selected Formulations Cure cycle: 40 minutes at 310’ F.
EPR 100 100 HAF black 50 50 Dicumyl peroxide, p.h.r. 3,5 2.5 .,. 3.0 Divinyl adiphate, p.h.r. Triallyl cyanurate, p.h.r. , . . ... Sulfur, p.h.r. ... ... 300% modulus, p.s.i. 1350 1350 Tensile strength, p.s.i. 2260 2100 Elongation at break, % 460 375
100 100 100 50 50 50 2.5 3.5 2.5 ... ... 1 .o 1.5 , . . ... , . . 0.9 0.5 1350 1350 1350 2100 2800 3180 425 510 530
literature Cited
(1) Amberg, L. O., Robinson, A. E., IND.ENG.CHEM.53, 368 (1961) ; Rubber Plastics Age 42, 875-9 (July 1961). (2) . , Am. SOC.Testing Materials, Philadelphia, Pa., ASTM D297-60T, Sect. 18-20. (3) Davis, W. E., Applied Mathematics Division, Hercules Powder Go.. Wilmington. Del.. derivation based on W. Feller. “Introduction to Probvability Theory and its Application,” ’Vol. I, pp. 56-9. Wilev. New York. 1950.
(7) Rehner, J., Jr., Wanless, G. G., Wei, P. E . , ference on Elastomers, 1961. lower the peroxide requirement but did significantly enhance tensile strength. The combination of sulfur and divinyl adipate had the additive effect of both lowering the peroxide requirement and enhancing strength.
RECEIVED for review September 27, 1961 ACCEPTED April 6, 1962 Division of Petroleum Chemistry. 140th Meeting, ACS, Chicago, Ill., September 1961.
CATALYSTS FOR T H E POLYMERIZATION OF
IS0 PR EN E T O fr~n~=l,4=PO LY IS0 PR EN E (SY NTH ET IC BALATA) J. S.
L A S K Y , H. K . G A R N E R , A N D R . H. E W A R T
Research Center, United States Rubber Go., Wayne, iV. J .
A catalyst prepared from aluminum triethyl and vanadium trichloride has been reported in the literature to polymerize isoprene to frons- 1,4-poIyisoprene. The efficiency of this heterogeneous catalyst is increased by a factor of 10 or more by supporting the vanadium trichloride on clay to increase its surface area. A further and even more substantial improvement in efficiency was achieved by adding a tetraalkyl titanate as a third catalyst component. This gave a highly active soluble catalyst system for the preparation of synthetic balata. Properties of synthetic balata are compared to those of natural balata.
t r a w l ,4-polyisoprene, from natural sources has been known for many years. I t is a moderately hard, tough, crystalline thermoplastic material whose major use today is in golf ball covers. A catalyst prepared from aluminum triethyl and vanadium trichloride for the polymerization of isoprene to synthetic balata has been disclosed ( 4 , 5 ) . The authors’ work with this catalyst has led to substantial improvements ( 3 ) ,which are reported on here. A new, highly efficient soluble catalyst system for the preparation of synthetic balata and the properties of the polymer so produced are also discussed. ALATA,
Supported Vanadium Trichloride
After extensive work with the aluminum trialkyl-vanadium trichloride catalyst described by Natta, the best efficiencies in the conversion of isoprene to synthetic balata were only 1.30 to 1.60 grams of polymer per gram of VC13 per hour. The catalyst was heterogeneous, and the limiting factor seemed to be the surface area available. Since VC14 is a liquid which is soluble in organic solvents, it seemed likely that a more finely divided catalyst could be obtained, using this as a starting material in place of the solid, crystalline, insoluble VCl,. Furthermore, Natta (6) had found 82
I&EC P R O D U C T RESEARCH A N D DEVELOPMENT
VC1, to be lar more efficient than VCl, in a catalyst for conversion or butadiene to the all-trans polymer. I n contrast, aluminum trialkyl-vanadium tetrachloride catalysts proved in extensive testing to be less efficient (maximum efficiency, 20 grams of polymer per gram of VC14) than aluminum trialkylvanadium trichloride catalysts (maximum efficiency, 80 to 100 grams of polymer per gram of VCl,). Furthermore, the polymer prepared using VC14 had a higher 3,4- content and a lower trans content than that prepared using Vel,, and also conlained gelled material that could not be broken down by milling. This illustrates again the major difference in polymerization behavior that is often observed with minor changes in monomer structure when stereospecific catalysts are used. I n an effort to increase the active surface, supports were used for the vc13. The VClB could be deposited on the surface of a nonporous inert carrier such as titanium dioxide or kaolin by heating a slurry of the supporting material in a benzene solution of vanadium tetrachloride at reflux for 1 to 2 hours. The VC14 smoothly decomposed into VCI, and chlorine, and the VC1, deposited on the surface as formed. When the reaction was complete, the supported VCl, was filtered off in an inert atmosphere, washed, and dried, and was then ready for use. T h e yield of VC13 based on VClr usually ran between 80 and
90%.
Table 1.
Table II.
Effect of AI/V Ratio and Coverage on Efficiency
Degree o j Coverage
AI/V Ratio
VCIa, wt. %
Eficiency , Polymer/ G. VClB
25 25 25 25 25 1.5 3.2 8.3 14.7 19 28.2 33
2.7/1 5.5/1 11/1 16'. 5 /1
18.3 18.3 18.3 18.3 18.3 1. I 2.3 6.0 10.7 13.7 20.4 24
57 41 8 31 9 264 257 0 178 280 470 420 405 300
5/1 5/I
An attempt was made to prepare supported catalyst directly by reacting aluminum trialkyl with VCld in the presence of a support. This, however, led to no improvement in efficiency over similar catalysts prepared in the absence of support. Using the supported Vc13, polymerization efficiencies could be increaspd by a factor of 10 or more over those previously obtained. Table I shows the effect of the molar ratio of aluminum to vanadium on the efficiency, as well as the effect of the degree of coverage of the support by vanadium trichloride. T h e latter quantity was calculated using the equation given by Innes (2) and is expressed as the number of layers of vanadium trichloride laid down on the surface. All of the experiments of Table I were run using T i 0 2with a surface area of 5.5 sq. meters per gram as the support. The polymerizations were run using the following recipe : Benzene, 350 grams VCla on support, an amount equivalent to 70 to 100 mg. of VCl, Aluminum triisobutyl, to give a ratio as indicated Isoprene, 80 grams Temperature, 50" C. Table I shows that the efficiency rises to a maximum a t A1 to V ratios around 5 to 1 and then falls off. This behavior is not uncommon in these organometallic catalyst systems. However, unlike most other catalyst systems of this type for the polymerization of dienes, the structure of the polymer produced is insensitive to ratio changes, trans-polyisoprene being produced over the whole range. T h e lower portion of Table I shows the efficiency rising to a maximum a t coverages equivalent to about 14 layers of \XI3, then holding fairly steady as coverage increases and finally decreasing a t around 30 layers of VC13. These data are all for one support, TiOs. Table I1 shows that this phenomenon of the efficiency rising, holding steady, and then falling is related to the degree of coverage and not to the chemical nature of the support or the weight per cent of Vcl3 present. T h e phenomenon of a maximum in efficiency with degree of coverage is probably connected with the manner in which the VC13 is deposited on the support surface. In the calculation of degree of coverage, the Vc13 was assumed to have been laid down uniformly. Since the support surface itself is not uniform, it seems more likely that vc13 is initially deposited preferentially on favored sites, and that these initial deposits grow and spread as additional Vcl3 is laid down. If it is assumed that a minimum thickness of VC13 is necessary a t any point for the subsequent formation there of a n active polymerization site, the VC13 surface area available for formation of these sites per gram of Vc13 will increase as additional areas are
Support None AlzOz Mgo Mg.0 Ti82 MgO Dicalite 1 Mgq Kaolin Kaolin Ti02 Ti02 Kaolin
Sic
Effect of Support on Efficiency
Surface Area, Sq. M./G.
... 400 125 125 5.5 25 20 25 10 10 5.5 5.5 10
1 (max.)
VCIB , Wt. % 100 11.3 19.6 41 . O
Eficiency, G. PolyDegree of Coverage ... 0.33 1.37 2.9
15.5 10.7 11.0 20 8
11.8 14.7 15.2 15.8
23.0
58 (min.)
mer/G. VClr 85 0 65 145
382 470 474 42 5
74
built up to the minimum thickness. This represents the range of increasing efficiency. A range of constant efficiency is reached when part of the VC13 is being used to build up additional areas, but an equal amount is merely increasing the thickness of areas already of the size necessary for activity. Finally, the efficiency begins to drop, as additional VC13 merely increases the thickness of the coating on the already completely covered support particles without forming new active \'c13 surface. An optimum surface area for the support is also noted. An increase in efficiency with increasing area is of course expected. The decrease a t very high surface areas is undoubtedly due to a large part of the surface area of these supports being contained in pores too small to allow the build-up of Vc13 particles to a size that will form potential polymerization sites. T h e use of supports sucn as T i 0 2 or kaolin at coverages between 12 and 25 layers of vanadium trichloride leads to highly efficient catalyst systems. T h e maximum rate observed was 20 grams of polymer per gram of vanadium trichloride per hour. This compares to 1.6 grams of polymer per gram of VC13 per hour for the unsupported catalyst.
Soluble Catalyst for Synthetic Balata T h e catalysts discussed so far are all heterogeneous and have been shown to contain no soluble material capable of initiating polymerization. At the very best, systems of this sort make inefficient use of the catalyst raw materials, since only the surface is used and a major portion of the material remains buried within the particles. A soluble catalyst system of comparable activity and stereoselectivity would theoretically be expected to show major improvements in polymerization rate and catalyst raw material utilization. The problem was to find a practical means of providing such a catalyst system. T h e addition of a titanate ester, Ti(OR)4, or a n alkyl titanium trialkoxide, R ' T i ( 0 R ) 3, to the aluminum trialkylvanadium trichloride catalyst system results in a n active soluble catalyst for the polymerization of isoprene to trans-l,4polyisoprene. Marked increases in rate and efficiency have been observed. With either the supported or the unsupported VC13, the efficiency in grams of polymer per gram of VCla per hour can be increased 50-fold over that obtained with the same catalyst in the absence of the titanium ester. I n other words, VOL 1
NO. 2
JUNE 1962
83
Table 111. vc13 f AIR3
- -
Formation of Soluble Balata Catalyst
supernatant liquid
Vc13
Ti(0R)r
no balata
(1)
no balata
(2)
balata
(3)
balata
(4)
+ Ti(ORja
AIR3
supernatant liquid
vc13 f T i ( 0 R ) i
+ AlR3 supernatant liquid
AlRI 4- T i ( 0 R ) r
supernatant liquid
VC18
500-fold increases in efficiency over the original aluminum trialkyl-VC13 catalyst of Natta can be readily obtained. The supported Vc13 still shows the same advantage over the unsupported Vc13 that was observed in the absence of titanium esters. The reasons for this are not completely clear, but are probably connected with the rate of formation of the catalyst and its stability. I n any event, the greater surface area of the supported Vcl3 leads to a more efficient conversion to active catalyst by reaction with other components. I n either case the polymer produced by the catalyst containing titanium appears to be identical with that produced by aluminum trialkyl and VC13 alone. A typical recipe is given below. Generally, 35 or 70 mg. of vanadium trichloride was used supported on clay, with benzene as the solvent. Aluminum triisobutyl and tetra(2-ethylbuty1)titanate were used as the other components of the catalyst unless otherwise noted. Recipe VCl3 on clay, an amount equal to 70 mg. of vc13 AIR3 f T i ( 0 R ) r to give ratios of Al/V = 10/1 and V/Ti = 2 11 Benzene, 120 grams Isoprene, 100 grams
Polymerization temperature, 50" C. After 1.5 hours the polymerization is stopped and the polymer worked up in standard fashion. Yields are generally 70 to 80 grams of trans-l,4-polyisoprene. A mixture of aluminum trialkyl and a titanate ester gives a catalyst that polymerizes isoprene to the 3,4 polymer (7). No increase was found in 3,4 content in the polymer over that in the absence of titanate nor was there any evidence of the presence of 3:4 polymer mixed with the 1,4 polymer, in spite of the fact that the ratios of aluminum to titanium found optimum are in the range that gives 3,4 polyisoprene in the absence of VC13.
Table IV. 1. CsHjTi(0R)I
-
Experiments with C6H6Ti(OR)3 > no balata
5.
+ .AIR3 + VC13 CeHsTi(OR)3 + VClp Supernatant from 4 + AlR3
6.
A1R3 f VCla
7.
A1R3 4- VC13 f CsHjTi(0R)s supernatant liquid
2.
CsHjTi(OR)3
3.
Supernatant from 2
4.
__+
balata
no balata no balata Table V.
C6H5Ti(OR)s
supernatant liquid
84
no balata
The fact that this is a soluble catalyst species and that all three constituents of the system must be present in the solution i? shown by Table 111. These results both hold for supported and unsupported vc13. A series of experiments based on Equation 4 in Table 111 is of interest, since it sheds considerable light on the mode of action of the titanate ester. If the AlR3 and Ti(0R)d are mixed and aged for different periods of time before contacting with vCl3, a regular decrease in efficiency with aging time is found. If the logarithm of the yield of polymer in a set polymerization time is plotted against the aging time, a straight line is obtained. At long aging times the yield decreases to that obtained in the complete absence of the titanate ester. This, in conjunction with the other data of Table 111, suggests that the solubilizing component is an unstable intermediate formed from the reaction of aluminum alkyl and alkyl titanate. Herman and Selson (7) have shown that the first step in the reaction of a titanate and an alkylating agent such as an alkyl lithium is the formation of the alkyl titanium trialkoxide. This compound subsequently breaks down to a free radical and the corresponding trivalent titanium species. The decomposition would be expected to show first-order kinetics. suggesting that the observed decrease in yield with aging time mentioned above is connected in some way with the disappearance of the alkylated titanium compound. The relatively stable aryl titanium trialkoxide, phenyl titanium triisopropylate, acts in exactly the same fashion as does a titanate ester in increasing the efficiency of the balata polymerization. If aluminum triphenyl is used in place of aluminum triisobutyl with either a titanate ester or phenyl titanium triisopropylate. yield of polymer does not decrease with aging of the aluminum-titanium mixture. However, if phenyl titanium triisopropylate is aged with aluminum triisobutyl before use in a catalyst preparation, the catalyst efficiency decreases with aging time. Also, if a solution of phenyl titanium triisopropylate is treated at 100' C. for 18 hours to decompose it before use as a catalyst component, the efficiency of polymerization is the same as if no titanium compound were present. All of these observations are consistent with the assumption that the titanium compound which is essential for the formation of the soluble catalyst is an alkyl or aryl titanium trialkoxide. This may be either added as such or formed in situ by the reaction of an aluminum trialkyl or triaryl with a titanium tetraalkoxide. Correlation of the stability of the alkyl or aryl titanium trialkoxide with the conditions and aging times during catalyst preparation points conclusively to the direct participation of these compounds. . Incidentally, phenyl titanium triisopropylate has been isolated in high yield from the reaction of aluminum triphenyl and tetraisopropyl titanate. This appears to be a more straight. forward and efficient srnthesis of this compound than that reported by Herman and Nelson (7). Table I V shows results of some experiments using phenyl titanium trialkoxide. These results hold for both supported and unsupported Vc13.
no balata balata
l & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
Al/V
5/1 10/1 20/1
-
Optimum Ratios of Catalyst Components V/ Ti Al/Ti 10/1 20/1 2/1 4/1 10/1 A 20/1 1/1 211 1/1 10/1 A 20/1 1/2 __f
__f
FVhile the structure of the soluble catalyst is not known, these experiments d o shed some light on the type of compound it must be. I n the first place, it seems likely that it contains all three metals, ,41, V, and Ti, since all three must always be present in the final catalyst solution. The organotitanium compound acts chiefly as a solubilizing agent, while the vanadium seems to be responsible for the polymerization as judged by the structure of the polymer produced. Three mechanisms of catalyst formation appear possible. Preliminary reaction of the organotitanium compound with to give a n insoluble complex which reacts with AlR3 to give the soluble catalyst. Preliminary reaction of the organotitanium compound with AIR3 to give a soluble complex which can then react with and dissolve vc13 to give soluble catalyst. Preliminary reaction of AIR3 and vc13 to give an insoluble catalyst complex, which then reacts with the organotitanium compound and becomes soluble.
vc13
,4n unambiguous choice among these cannot be made on the basis of current data. One piece of evidence, however, indicates that the third possibility, primary reaction between AIR3 and ’L’c13,may be correct. Small but real increases tvere observed in rate of polymerization with increasing aging time when aluminum trialkyl and vanadium trichloride are combined and aged, and then treated with phenyl titanium triisopropylate. \Vhile the other routes are not excluded b>this? it does indicate that a t least part of the catalyst is formed in this way. IVhile the molar ratios of the aluminum to vanadium to titanium in the catalyst are not highly critical, optimum ranges were observed. Table V summarizes the findings on the best ratios to use. T h e A1,’Ti ratios are consistent with the statements concerning the organotitanium compound. If the Al/Ti ratio is very high-e.g., 200/1-no effect on efficiency is observed. since there is not enough titanium present. If it is low-e.g., 2 ‘I-no formation of balata is observed, since the aluminum alkyl is used up in alkylation of the titanate and not enough is available to complete the formation of active catalyst by reaction with vanadium halide. Table V I summarizes the efficiencies of the various catalyst systems for balata based on VC13.
Table VI.
Efficiencies of Balata Catalyst Systems Eflciency, G. Polymer /G. Catalyst System VC/~/HOU~
+ VCIs, unsupported ++ VClr, supported on clay VCL, unsupported, + T i ( 0 R ) r + VCI,, supported on clay, + Ti(0R)a
AIR3 AIR3 AIRr AIR%
Table VII.
Melting points, tometric)
O
1.3-1,6 15-20 40-50 600-700
Physical Properties of Synthetic Balata Synthetic Natural Balata Balata C. (dila56 and 63 56 and 62 (prepared
ML-4 at 212’ F. After 4-min. milling at 305’ F. Tensile, R.T., p.s.i. Elongation, yo Tensile, 212’ F., p.s.i. Elongation, % Impact notched Izod at 30’ C. S-300, p.s.i. Shore D Torsional modulus: p.s.i. 50’ C. 25” C.
oa c.
-30” C.
20 to 30
...
in benzene) 51 and 61 (prepared in heptane) Over 150
0
24 to 36 4,400 480 0 320
No break 2,250 43
No break 2,200 40
666 37,000 49,800 79,500
384 33,700 44,500 61,200
3,860 440 0
lyst in an aliphatic solvent is about one third the rate observed in benzene. This may be due to lower solubility of the highly polar catalyst species in the aliphatic solvent. With regard to the usual physical properties, natural balata is a little stiffer than the synthetic, although the spread of torsional moduli between the natural and the synthetic comes very close to the spread observed between different lots of natural balata. In tensile strength and elongation, the synthetic balata is superior to natural balata. Rather extensive evaluation of synthetic balata has been made in golf ball covers. I t appears to be a satisfactory substitute for the natural material in this application.
Properties of Synthetic Balata
The polymer produced by these systems, except for differences in molecular weight, is very close to natural balata in properties. T h e infrared spectra and x-ray diffraction patterns of synthetic and natural balata are identical. The per cent crystallinity of synthetic balata as determined by x-ray methods may be slightly higher than that of natural balata. Table VI1 summarizes some pertinent data comparing natural and synthetic balata. The molecular weight of the synthetic balata as it comes from the reactor is far higher than that of natural. However, the Mooney viscosity of the synthetic can be reduced to that of natural by a few minutes on the mill a t 300’ F. Balata, both natural and synthetic, exists in t\vo crystalline modifications. The melting points of the synthetic material prepared in benzene are equal to those of natural. If the polymer is prepared in an aliphatic solvent, the melting point of the lokver melting crystal modification is somewhat lower, probably indicating a reduced trans content in these polymers. T h e rate of polymerization using the titanate-containing cata-
Acknowledg men1
The authors thank R . S . DeKure for his assistance with the experimental portion of this work. literature Cited (1) Herman, D. F., Nelson, W. K., J . Am. Chem. SOC.75, 3877
11933).
(2)’ Inn&, \V. B., in “Catalysis,” Vol. I, P. H. Emmett, ed., p. 258, Reinhold. Ne# York, 1954. (3) Lasky, J. S. (to U. S. Rubber Co.), Belg. Patent 587,628; Australian Patent Appl. 57863/60. (4) Natta, G., Corradini, P., Morero, D., Chim. e 2nd. 40, 362
(1958). (5)’ Naha, G.. Porri, L., Mazzanti, G., Belg. Patent 545,952 (March 10, 1956). (6) Natta, G.. Porri. L., Mazzei. A.. Chzm. e ind. 41, 116 (1959). (7) Ziegler, K., Ger. Patent 1.039,055 (Sept. 18, 1958). RECEIVED for review September 27, 1961 ACCEPTEDApril 3, 1962 Division of Petroleum Chemistry, Symposium on Synthetic Elastomers from Petroleum Hydrocarbons, 140th Meeting, ACS, Chicago, Ill., September 1961. Contribution No. 208 from the Research Center, United States Rubber Co., TYayne, N. J. VOL.
1
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85
