back to the metal sites, where they are hydrogenated to isoparaffins.
metal
acidic
Saturate _ _ _ j unsaturate 4 rearranged
7 unsaturate
metal
C--
site
rearranged saturate
In this mechanism every paraffin molecule that is isomerized must be dehydrogenated prior to isomerization. literature Cited (1) Bloch, H. S., Pines. H., Schmerling, L., J . Am. Chem. SOC. 68,
153 (1946). (2) Ciapetta, F.G., IND. ENG.CmM. 45, 159-65 (1953). (3) Leftin, H. P., W. K. Hall, Proceedings of Second International Congress on Catalysis, Paris, France, July 1960.
(4) Mavity, J. M., Pines, H., Wackher, R. C., Brooks, J. A., IND. ENG.CHEM.40, 2374-9 (1948). (5) . , Meisel, S. L., Koft, E., CiaDetta. F. G.. Division of Petroleum Chemistry, 138th Meeting, ACS, New York, 1957. (6) M i l k G. A.7 Heinemam, H.9 Milliken, T. H.3 Oblad, A. G.9 IND.ENG.CHEM.45, 134 (1953). (7) Peri, J. B., Proceedings of Second International Congress on Catalysis, Paris, France, July 1960. (8) Pines, H., Haag, W. O., J . Am. Chem. SOC.82, 2471 (1960). (9) Pines, H., Wackher, R. C., Ibid., 68,595 (1946). (10) Thomas, 0. H., and Mooi, J. (to Sinclair Refining Co.), U. S. Patent 2,952,721 (September 1960). (11) Webb, A. N., Division of Petroleum Chemistry, 135th Meeting, ACS, Boston, Mass., April 1959, Preprints, p. '2-171. (12) Weisz, P. B., Science 123, 887 (1956). (13) jt'eisz, P. B., Swegler, E. W., Ibid., 126, 31-2 (1957). RECEIVED for review February 10, 1961 ACCEPTED November 15, 1961
SOLUTION POLYMERIZATION OF PROPYLENE A T HIGH TEMPERATURES DONALD F. H O E G ' A N D SAMUEL LIEBMAN W. R. Grace & Go., Clarksuille, Md.
A study of propylene polymerization with TiC12- and TiCI3- RBAl systems above 100" C. gave data consistent with heterogeneous catalysis. Arrhenius activation energies for the over-all rate of polymerization from 120" to 160" C. were smaller than reported previously a t lower temperatures, partly because of catalyst instability. Catalyst specificity is maintained near-constani over a very wide temperature range. These data can best b e interpreted in terms of isolated sites on the catalyst surface, presenting varying stereospecificities. In the solution polymerization of propylene ( > l o o " C.),a higher over-all activation energy for chain transfer than for chain growth, was observed. Variation in molecular weight with monomer concentration showed a major transfer process operating which did not involve monomer - probably "spontaneous dissociation" of the polymer-catalyst complex, leading to a regenerated active center ("active hydride") and a vinylidene-terminated polymer chain.
INCE
the first discoveries of the synthesis of stereoregulated
S polymers from the simple a-olefins, a great interest has
been aroused in the mechanism of these sterospecific polymerizations (6, 76, 77, 79, 27-23). Several studies have been published, largely by Natta and coworkers. on the stereospecific polymerization of propylene, and the influence of the various reaction parameters on the synthesis of isotactic polypropylene (73, 74, 78). These studies provided much of the fundamental information needed in efforts to understand the origins of the remarkable specificity of these heterogeneous Ziegler-Natta catalysts. In almost all of these detailed studies, however, the maximum temperature explored was of the order of 80" or 90' C . Under these conditions the polypropylene, or a t least the predominantly isotactic portion, precipitates out from the hydrocarbon reaction solvent partially crystallized as it is formed. Thus the catalyst is partly buried in a solid polymer matrix for a significant portion of the polymerization reaction. Shortly after we began our studies, our initial results prompted us to explore higher temperatures, particularly 1 Present address, Roy C. Ingersoll Research Center, BorgWarner Gorp., Wolf Br Algonquin Roads, Des Plaines, Ill.
120
I & E C PROCESS D E S I G N A N D DEVELOPMENT
above about 100" C. In this case, the polymer remains totally dissolved in the hydrocarbon vehicle. Thus, neither the crystallization of the polymer during the course of the polymerization nor the presence of a solid semicrystalline polymer phase could be a factor in the stereochemistry of the propagation reaction. Nor could either influence any of the kinetic features of chain growth or chain transfer. Although the catalyst is still present as a solid insoluble phase, we have called these systems solution polymerizations. The mechanism of chain growth and chain transfer operating in the solution polymerization of propylene has been studied, using catalysts derived from : (1) TiClt-triisobutylaluminum (TIBA), (2) TiC1,-Triisobutylaluminum, and (3) T i c & triethylaluminum (TEA). Experimental
Materials. Titanium dichloride was supplied by the New Jersey Zinc Co. The material as obtained was black, crystalline, and only mildly pyrophoric, and analyzed 39.0370 T i and 59.5970 C1 (98.470 TiCl, Titanium trichloride was supplied by the New Jersey Zinc Co. and the Stauffer Chemical Co. As obtained, Tic13 was
red-violet in color and extremely fine, and fumed violently on contact with moist air. A typical analysis ran 31.3270 T i and 68.l2yO C1 (TiClln.,s or 99.4470 TiC13). T h e x-ray diffraction pattern was that of a-TiC13. I n these experiments, TiClz and Tic13 were ground in stainless steel ball mills under argon for a t least 15 hours prior to use. Triisobutylaluminum was supplied by the Hercules Powder Co. and used without further purification. Triethylaluminum was supplied by the Ethyl Corp. and used without further purification. Cyclohexane was obtained from the Phillips Petroleum Co. T h e solvent was purged with nitrogen and approximately 570 of the lower boiling fraction removed via vacuum distillation. T h e solvent was tramferred to the reactor under nitrogen pressure through a drying tower containing activated alumina and Linde 4A Molecular Sieves. Propylene (Matheson Co. C.P. grade) was used throughout these experiments. Typical analysis showed the gas contained approximately 3y0 propane and less than 50 p.p.m. acetylene derivatives. T h e propylene was passed through drying towers (18 X 2 inches,) of activated alumina and Linde 4A Molecular Sieves. General Electric “laimp grade” nitrogen was used as inert carrier gas. I n the transfer system for the trialkylaluminum, the nitrogen was further purified using Ascarite, pellets of NaOH, and P z Oon ~ glass beads. Polymerization Proc:edures. Polymerizations were carried out in 1-liter stainless steel stirred reactors. Heat was supplied externally by an electric heating mantle. Temperature control was maintained with a COz-pressured water jacket. All experiments were carried out a t constant monomer concentration throughout the run. The propylene was supplied a t the stated pressure on a demand basis. T h e solid cocatalyst component (Tic12 or TiC13) was transferred to the reactor under a nitrosen “blanket” and then 300 grams of cyclohexane was weighed in through a drying tower. Subsequently, the organometallic compound was injected into the reactor a t the temperature of the experiment. Propylene was admitted immediately. After a given time the propylene feed was halted, unreacted monomer discharged, a!nd the reactor cooled with circulating cold water. T h e polymer-solvent-catalyst mass was transferred from the reactor into a solution of isopropyl alcohol containing 10 volume (% concentrated hydrochloric acid and lyOacetylacetone. Rate data were obta.ined by isolating the polymer formed after varying times; thus each point on the curves presented
represents a single experiment. Rates have been expressed as grams of polymer produced per gram of Tic12 or Tic13 used per hour of reaction time. This slurry was then finely divided in a Waring laboratory blender. T h e polymer was filtered from the solvent containing dissolved catalyst and thoroughly washed with isopropyl alcohol. T h e purified polymer was then vacuum-dried and weighed. This procedure was satisfactory for removing catalyst residues which were undesirable for polymer property evaluation. Ash contents (per cent by weight oxide residues after ignition of polymer) were generally less than 0.05 weight yo. n-Heptane Extractions. As a preliminary index of stereoregular (isotactic) content, the fraction of the polymer unextractable by n-heptane was measured. It has been our experience that this is a reliable relative index of isotacticity for high molecular weight polypropylene-generally for polymers whose reduced specific viscosities were above about 1.5 dl. per gram. T h e extractions were carried out in Soxhlet extraction apparatus for 15 hours. Exactly 2 grams of the purified dry polymer were added to the extraction thimble and approximately 0.1% (by weight of heptane used) of a volatile antioxidante.g., Ionol-added to the heptane. The weight of the residue was determined after a 15-hour extraction. This period accounts for almost quantitative removal of the readily soluble fraction; a n additional 15-hour extraction removed only 2% more of the polymer. The soluble fraction was isolated by precipitation by addition of a 50/50 volume mixture of methanol-isopropyl alcohol. Viscosities. Solution viscosities of these polymers were measured in Decalin a t 135” C. (0.1 gram per 100 ml. of solution) in modified Ubbelohde viscometers. Intrinsic viscosities were calculated from Chiang’s data for Martin’s constant in the equation: log
where k
=
(VSPlC)
= log [SI
i k!vlc
0.18.
T h e weight-average molecular weights were calculated from the intrinsic viscosities from Chiang’s equation : [ q ] = 1.00
x
10-4[M,]o8
in Decalin ( 3 ) .
l5O
200
t
15’ 2
I
1
0
Figure 1 . X-ray diffraction pattern of polypropylene prepared with Tic12 as catalyst Temp. 50’ C.
0 TEA -TiCI, (SAMPLE B) @TIBA-TiC13(SAMPLE A)
Figure 2.
I
I
I
2 3 4 MOLES OF TiC13x103
I
I
5
6
I
Apparent first-order dependence of rate
of polymerization on amount of Tic13 used VOL. 1
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121
Results and Discussion
in the active center of chain growth in the conventional Natta-Ziegler catalyst system. TiC12-Triisobutylaluminum System. PRELIMINARY The second reason this system is discussed first was the followSTUDY. The first studies were carried out a t low temperaing. As the temperature of polymerization was raised, a very tures, where the polymer precipitated out of the solvent, noticeable drop in the molecular weight of the polymer ocusing as catalyst TiClz and TIBA. Results were similar to curred. However, the isotacticity of the polymer remained those Natta has reported for Tic13 and TEA. The over-all constant. Even a t 120’ C., in which case the polymer was rate of polymer formation is proportional to the amount of totally dissolved in the hydrocarbon solvent (in this case cycloTic12 used and monomer concentration (for low propylene hexane), the isotactic content was the same as a t lower tempressures), peratures. Thus it was concluded that neither the crystallizaO n the other hand, the rate of polymerization is largely tion of the polymer nor the presence of a crystalline polymer independent of the amount of trialkylaluminum used above ‘.template” was a factor in the stereochemistry of the propagaan indefinite low concentration. Similarly, the stereotion reaction. These results prompted a study in greater detail specificity of the catalyst is independent of the amount of of this area of “solution polymerization.” I t was also decided trialkylaluminum used. Thus a small amount of TIBA is to concentrate on the more readily available and commercially sufficient to produce a constant number of active centers for more significant TiC13. polymer growth on the Tic12 surface. These are essentially TiC13-TEA a n d TiC1,-TIBA Systems. .4gain a large the conclusions reached by Natta and coworkers with Tic13 amount of data can be summarized. In the solution polyand TEA. merization of propylene, with TiCl~-trialkylaluminum as One point about the TiC11-based system, however, of catalyst, the results obtained are in substantial agreement with particular interest is this: I n the complete absence of TIBA, that the catalysis is those of Natta and coworkers and Bier ( 2 ) > finely divided, freshly ball-milled Tic12 is a stereospecific completely heterogeneous. The rate of polymerization at catalyst for the polymerization of propylene. The rate of 120’ C . follo\ved a first-order rate law based on the amountIof polymerization. to be sure, is very slow. Only about 1 gram Tic13 used (for low Tic13 concentrations) and based on the of polymer per gram of Tic12 per hour was obtained. The propylene concentration, or pressure, at least for low propylene polymer was also highly swelled by the solvent and had to be pressures. Further, at 120’ C. the stereospecificity is only coagulated by addition of methanol. After drying. it was mildly affected by variations in the concentration of the alkylalmost 30% insoluble in boiling heptane. Further. the x-ray aluminum (Figures 2 and 3). The rate of polymerization diffraction pattern (Figure 1) indicated that at least a portion sho\vs a slight dependence on the amount of alkylaluminum of the polymer was isotactic crystalline polypropylene (72). present. As \vas found previously with the TiC12-TIBA Thus, the crystalline surface of Tic12 in itself provides the catalyst system, the isotacticity of the polypropylene formed essential requirements for stereospecificity ( 4 ) . in solution a t 120°C. was identical to that obtained at lower However. the polymer obtained with Tic12 alone is only temperatures under conditions of suspension polymerization 227, crystalline in spite of its low molecular weight. Upon (Figure 4). the addition of small amounts of TIBA. not only does the rate These results indicate that the presence of a solid polymer of polymerization rise rapidly (by at least two powers of lo), phase in the low temperature studies of S a t t a and coworkers but the specificity of the system more than doubles (based on and more recently Bier and others was not a factor in the x-ray crystallinity and solubility of the polymer in heptane). regulation of chain growth. Isotacticity LIS. Temperature. Natta and coworkers reThese data, taken with the variations in specificity noted as ported that the isotactic content of the polymer was not the structure of the organometallic compound is changed, suginfluenced by the temperature of polymerization from 30 to gest a very intimate association of the organometallic compound
w 2 U
t Y
z
W
z
3 4
75
z c
r
i,W50u m
@ TIBA-Tic13
LB
0 TEA-TICIS
K3
@ TIBA-TiCl3 0 TEA-TiClg
c l 25I 2
5
2
4
6
8
10
A L / T I MOLAR RATIO
Figure 3. Dependence of catalyst stereospecificity on alkylaluminum concentration a t 120” C. Reoction time, 2 hours. 0.50 gram Tic13 per 300 grams cyclohexane. C3Hs 0.1 5 mole fraction in cyclohexane 122
75-
I&EC PROCESS DESIGN A N D DEVELOPMENT
I
I
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I
I
70" C. with Tic13 and TEA.
As is shown in Figure 4, with both TEA and TIBA the percentage of the polymer insoluble in heptane is almost constant up to a t least 125' C. With higher temperatures a gradual increase in heptane extractability occurs with both catalyst systems, but the lower molecular weights formed a t these high temperatures may be a contributing factor. Even the purely isotactic polymer becomes soluble in heptane a t sufficiently low molecular weights. O n the other hand, the crystallinity of the polymer prepared at 180' C.-that is, above the crystalline melting point of the isotactic polymer-is still of the order of 607,. These catalysts are still highly specific even at 180" C. The isotactic helical composition of these polymers was also measured by the technique of Ashby and Hoeg (7). Here again the '.isotacticity" appears constant u p to about 120' C. At higher temperatures of reaction, there was gradual drop in specificity. But again, even a t 180" C., a high level of specificity was indicated. It has been suggested that one driving force for stereospecific growth might be the formation of the helix in the isotactic placement (20). Atactic and isotactic polypropylene, however: exhibit hardly distinguishable dilute solution viscosities, which suggests that both structures in solution exist as random coils (5). T h e high level of specificity which has been found in the solution polymerization of propylene (particularly above 180" C.) suggests that for polypropylene the formation of the helix is not a major factor influencing stereospecificity. Of course, neither catalyst system possesses absolute specificity. The synthesis of isotactic polymer is always accompanied by some atactic material. I n fact, under all conditions, the TIBA-Tic13 system is slightly less stereospecific than the TEA-Tic13 system. Yet both catalyst systems maintain their imperfect specificity over an enormous temperature range and over widely divergent reaction conditions. The activation energy for the isotactic growth process is virtually identical to that for the atactic synthesis. These results are believed to be most readily interpretesd in terms of isolated active sites on the catalyst surface presenting varying specificities, as originally proposed by Katta and coworkers. Molecular Weight ZJS. Temperature. The increase in the temperature of polymerization is also accompanied by a very
significant drop in the molecular weight of the polymer (Figure 5). I n the previous studies by Natta and his group, the molecular weight was not significantly affected by the temperature of polymerization. They thus concluded that the over-all activation energy for the chain-breaking processes was of magnitude similar to the activation energy for chain growth. In this case, however, a significantly higher activation energy was found for the chain-breaking steps than for chain growth. This may indicate that a change has occurred in the important or major mode of chain transfer. One other important point may be made about the data shown in Figure 5. The decrease in the molecular \\.eight of the predominantly isotactic fraction with temperature is more rapid than that of the less stereoregular fraction-at least u p to 130' C., where the heptane extractability of the polymer is constant, and hence the comparison is fair-indicating that the activation energy for
loo
I
0 PROPYLENE ADMITTED IMMEDIATELY (3 I HOUR DELAY BEFORE
1
ADDING PROPYLENE
I
I
I
I
I
60
80
100
120
140
TEMPERATURE,'C.
Figure 6.
Aging of TiC13-TIBA catalyst
TEA/TICI,
12.0 -
2
4
MOLAR RATIO
6
8
10
12
0 TEA-SAMPLE B
50t
\
25p-;-s
75
100
125
150
175
TEMPERATURE OF POLYMERIZATION,
OC
Figure 5. Effect of temperature of polymerization on molecular weight of polymer TIBA-Tic13 system
60'C.
1
2 3 4 5 i B A / T i C l r M O L A R RATIO
180' . 6
Figure 7. Dependence of catalyst activity on trialkylaluminum concentration a t various temperatures VOL. 1
NO. 2
APRIL 1962
123
chain transfer involving the isotactic chains is higher than that for the atactic chains. Thermal Stability of Catalyst System. An additional complicating feature of these studies concerns the question of how the over-all rates of polymerization change with temperature. Simply stated, these catalysts are thermally unstable. They decompose irreversibly a t high temperatures. If stored above 120" C., the catalysts (TIBA and TEA) show progressively lower over-all polymerization rates (Figure 6). O n the other hand, if propylene is added immediately after addition of the alkylaluminum, the rate of the observed polymerization increases with temperature. At very high temperatures this decomposition of the catalyst is accelerated by alkylaluminum (Figure 7 ) . It is presumed that we are dealing with a reduction of trivalent titanium to lower valence states. Activation Energies. Arrhenius plots of the rate data collected in the range of 120 to 160 C. have been made for A1-Ti molar ratios of about 3 to 1 and below (for 1 to 2 hours' reaction time and below 8-atm. partial pressure of propylene). The over-all activation energies calculated are shown in Table I with those of previous studies. The activation energies observed at these high temperatures are considerably smaller than those observed by Natta and coworkers. Previous data indicate that this is due to the instability of these catalysts a t high temperatures. Because of these complications most of the work was focused on polymerizations a t about 120' C. Mechanisms of Chain Transfer. Since the chain-breaking step does not consume the active center-that is, we are dealing with catalysts and not initiators-these chain-breaking events should be considered as chain transfers. An additional point should be made concerning the effect of the reaction time. \$'hen ball-milled Tic13 is used, an induction period is not observed. The polymerization begins immediately a t a high rate. There is an initial adjustment period, but after several minutes the molecular weight and isotacticity of the polymer assume a fairly steady value. Thus the average lifetime of the growing chains is relatively short-certainly not more than a few minutes at 120' C. The rate of polymerization falls slightly over several hours of reaction but may be considered approximately time-independent up to 2 hours and possibly longer. At low temperatures-i.e., below 80' C.-Natta and coworkers found that the molecular weight of the polymer was not
(1) "SPONTANEOUS DISSOCIATION" C ~ - 1 C H 2 - ~ ~ c h+ a i nC
F
Table 1.
Terne Source Range, C. Natta et d . ( 9 , 70) 30-71 hTattaet al. 30-71
+
or-TiCla TEA or-TiCls (ball-milled) TEA cu-TiCla(ball-milled) TEA cu-TiCla (ball-milled) TIBA
120-1 60
1, 8 3.2
a
significantly affected by changes in the propylene concentration, a t least above 1-atm. pressure (74). Since in this range they found chain growth first-order in monomer, they concluded that the major mode of chain transfer must be of the same molecularity with regard to monomer as the chaingrowth step. Thus, the major mode of chain transfer appeared to be first-order in monomer, a process which might be considered chain transfer with monomer. This is schematically represented as Reaction 2 (Figure 8). The mechanism involves hydrogen transfer (2-position) to the monomer, resulting in vinylidene end groups in the polymer chain; this vinylidene unsaturation was detected in the infrared spectra of low molecular weight fractions ( 7 7 ) . Several other less important modes of chain transfer were also detected : spontaneous dissociation to .'active hydride," transfer with alkylaluminum, and transfer with soluble compounds derived from TiC13. The number-average degree of polymerization (DE',) was found to be adequately described by the equation shown (Figure 8), where the K's are the velocity coefficients of the corresponding numbered reactions. The question then is: Are these same mechanisms operating in the solution polymerization? .4nd what is responsible for the high activation energy observed for chain transfer in the solution polymerization? We have been unable to detect a chain-transfer process in which alkylaluminum plays a role. At 120' C. the molecular weight of the polymer was not influenced by variations in the concentration of the alkylaluminum. The results obtained with TIBA are shown in Figure 9, compared with those ob-
-
0
50
-x
H
EA" I Kca1.l Mole 9 5 7.5
Catalyst
+ + This study 120-1 60 + Does not include AH of solution f o r propjlene. This study
N
CH3 H t CH,=k-choin
Arrhenius Activation Energies at Various Temperatures
NATTA'S DATA
t
(2) TRANSFER WITH MONOMER
/ AT 70'C.
0
30 c u
(3) TRANSFER WITH ALUMINUM ALKYL w-)CH$Lcl-& H
+
R,AI -+ P
R
+
FH3
R,AI-CHfC-chain
k
(4) TRANSFER DEPENDENT on "TiCl:
5 20
't
l0L 1
-=
KI + K&,M,+
DPN
MAL + bpc3M6& KPPC3W6
Figure 8.
Proposed chain transfer mechanisms N a t t a and coworkers
124
l&EC
PROCESS DESIGN A N D DEVELOPMENT
I
2
I
4
I
6
I
8
I
10
I
12
I
14
I
16
I
MOLES OF T l B A X 1 0 3 300 ML. OF SOLVENT
Figure 9. Relation of molecular weight (of preponderantly isotactic fraction) to trialkylaluminum concentration in solution polymerization of propylene
7.0
i
6.0
7.0
125
1
- 5.0
5.0
A
m 0
n
- 4.0
-
VI
b 4.0 X
3
c
32 mo
3.0
-07
- 3.0 2;
0 HEPTANE INSOLUBLE FRACTION 0 HEPTANE SOLUBLE FRACTION
2.0
Ul
n 0
.1
3
1.
1 .o
j2.0
2.0 I
2
l
4
l
6
l
8
l
1
l
l
l
l
l
l
l
l
0
2 4 6 8 1 0 1 2 PROPYLENE PARTIAL PRESSURE, ATM.
Figure 10. Dependence of molecular weight of preponderantly isotactic fraction and catalyst activity on monomer concentration a t 120’ C. TIBA-Tic13 catalyst
tained by Natta and coworkers a t lower temperatures. T h e marked invariance of molecular weight is apparent. Thus no distinguishable transfer process involving trialkylaluminum has been found in the solution polymerization of propylene (at the concentration levels indicated), If a transfer process does occur, it is independent of dissolved alkylaluminum. I t has been suggested that in the solution polymerization of propylene, the vicinity of the active center is saturated with regard to alkylaluminum and transfer is independent of the concentration of alkylaluminum in solution. There is another important difference between the solution polymerization of propylene and the suspension polymerization. Molecular weight is influenced by monomer concentration. When the over-all rate of polymerization is affected by monomer concentration, so is the molecular weight; when the rate of chain growth becomes independent of monomer concentration, so does the molecular weight. Some typical data obtained for the TiCl3-TIBA system a t 120’ C. are shown in Figure 10. Thus it is concluded that a major transfer process occurs a t high temperatures in which monomer plays no part. This is believed to be spontaneous dissociation (Reaction 1). This process apparently becomes more important a t high temperatures. T h e high activation energy observed is also consistent with this reaction’s playing an important role in chain transfer, since we would expect it to be high energy process. There was also detected a transfer process dependent on the amount of Tic13 used, as observed at lower temperatures. Yet the evidence supports the position that these catalysts are completely heterogeneous-that all the chain growth occurs a t the surface. Still, as the concentration of Tic13 is raised, lower molecular \veights are obtained (Figure 11). These results have been interpreted by Xatta and coworkers i n terms of hydrocarbon-soluble compounds in the Tic13 which can serve as chain-transfer agents. This effect has been demonstrated by repeated washings of the Tic13 with anhydrous heptane (Table 11). The molecular weight is increased without significantly altering the stereospecificity or the catalyst activity a t 120’ C. T h e chain-transfer process dependent on Tic13 does appear to be approximately first-order i n Tic13 a t 120’ C. (Figure 1I).
I
1.0
I
I
I
I
I
I
I
3.0 4.0 5.0 6.0 7.0 MOLES OF TiC13xlO’ ( 3 6 0 m l . SOLVENT) 2.0
Figure 1 1 . Influence of titanium trichloride concentration on molecular weight of polymer TEA-Tic13 system TEA-Tic13 molar ratio. Temp. 1 2 0 ’ C.
2 to 1
Reaction time. 1 hour C3Hs. 0.1 5 mole fraction in cyclohexane
I t also appears that a n additional portion of the catalyst is “solubilized” by TEA. The radiochemical data obtained by Natta et al. suggest that the soluble compounds derived from Tic13 which act as transfer agents are associated with alkyl groups derived from the trialkylaluminum compound (70, 75, 76). Preliminary Infrared Data. Chain transfer with monomer and by “spontaneous decomposition” to active hydride results in vinylidene end groups. In the infrared absorption spectra of the low molecular weight fractions of the polymers prepared a t 120’ C. and above, vinylidene absorption (shoulder a t 890 cm.?) was detected (8, 77). I n the polymers prepared with the TiCl,-TEA system, propyl (740 cm.-I) and ethyl groups (770 cm.?) were also present, but only a weak isopropyl (920 crn.-I) absorption has been observed (7>9 ) . Stable metal-polymer bonds of the type, CH3 Me-CH2-CH
I
---
C3H7
which could be primary products of a chain-transfer step with metalalkyl, thus are not present in very large quantity, since these would produce isopropyl groups when the polymer was treated with isopropyl alcohol-HC1 a t the end of the run. If these form, subsequent decomposition to vinylidene and metal
~~
Table II.
Evidence of Heptane-Soluble Transfer Agents in TiCI3 Active at 120’ C.
Tiel3 Preparation 1. Powdered a-TiCla 2. Powdered a-TiCl3 “washed“ with anhydrous heptane 3. Powdered a-TiClj with TE.4 (1 mole TiC13,’O.l mole TEA), then washed with anhydrous heptane
VOL. 1
% nHepane-
eoCatalyst
vap/c
Insol.
TEA
3.07
69
TEA
3.91
70
TEA
4.96
72
NO. 2
APRIL 1 9 6 2
125
hydride is indicated. The ethyl groups are presumed to be chain end groups derived from the triethylaluminum. These end groups have not yet been measured quantitatively. An attempt was made to evaluate the relative importance of the various chain-transfer processes operating a t these high temperatures, but with limited success. At low Tic13 concentrations the reproducibility of the data has been poor, and long extrapolations had to be made to estimate the values of chain-transfer processes independent of TiC13. So the exact nature of the chain-transfer process dependent on Tic13 is not known; therefore, the exact contribution of each chaintransfer mechanism is not known with certainty. Leaning heavily on Natta and coworkers’ studies, however, it appears that a t high temperatures the main mechanisms of chain transfer are transfer with monomer, transfer with TiC13derived compounds, and spontaneous dissociation. Conclusions
Kinetic studies of heterogeneous catalysts are generally complicated. This difficulty has become increasingly severe a t the high temperatures of these solution polymerizations, where several competing reactions appear to occur. Yet all of the evidence supports the main conclusions of Natta and coworkers concerning the nature of chain growth. The catalysis appears completely heterogeneous-that is, growth occurs a t the surface of TiCls. The organoaluminum compound is believed to play a n intimate role in the active center of chain growth. Crystallization of the isotactic polymer was not found to be a factor in the stereochemistry of the propagation reaction. At the higher temperatures of these solution polymerizations, spontaneous dissociation of the polymer from the active center assumes a more important role as a mechanism by which chain transfer occurs. I t is also suspected that the high activation energy noted for the over-all chain-transfer processes is probably due to the more important role of this reaction. The most striking feature of the published studies has been the almost constant stereospecificity of these catalysts of Tic12 and Tic13 with trialkylaluminum over extreme temperature ranges. Although a slight difference exists in the stereospecificity of the catalysts based on TIBA and TEA, both systems possess imperfect specificity. Yet this imperfect specificity is maintained a t a near-constant level over a wide temperature range, and over widely divergent reaction conditions. These data are believed to be best interpreted in terms of isolated sites on the catalyst surface presenting different
126
I & E C P R O C E S S DESIGN A N D D E V E L O P M E N T
degrees of specificity, as originally proposed by Natta and coworkers. Acknowledgment
The authors thank the many members of the Research Center who have contributed helpful criticism during the course of this work, particularly F. X. Werber, R. S. Gregorian, C. J. Benning, A. D. Ketley, and M. C. Harvey. They also acknowledge many helpful discussions with Charles Overberger of Brooklyn Polytechnic Institute and Paul Emmett of Johns Hopkins University. X-ray data were obtained by G. E. Ashby, whose valuable assistance is appreciated. Polymerization work was carried out by R. V.Hoenes?Steven Rabel, and Ronald Baklarz. literature Cited
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(14) Natta, G.: Pasquon, I., Giachetti, E., Makromol. Chem. 24, 258 11957). (15) -Natta,’G., Pasquon, 1.: Giachetti. E., Pajaro, G., Chim. e 2nd. 40, 267 (1958). (14) Natta, G., Pasquon, I.. Pajaro, G., Giachetti, E., Ibid., 40, 356 (1958). (17) Natta: G., Pino, P., Corradini, P.! Danusso, F., Mantica, E., Mazzanti, G., Moraglio, G., J . Am. Chem. Soc. 77,1708 (1955). (18) Natta, G., Pino, P., Corradini, P., Danusso, F., Mantica, E., Mazzanti, G., Moraglio, G., J . Polymer Sci. 77, 1710 (1959). (19) Natta. G., Pino, P., Mazzanti, G., Longi, P., Gatz. chim. ital. 87, 570 (1957). (20) Szwarc, M., Chem. andInd. (London) 1958, 1589. (21) Tobolsky, A. V., Am. Scientist 45, 34 (1957). (22) Werber, F. X., Hoeg, D. F., Chem. Eng. News 37, 58 (March 23, 1959). (23) Ziegler, K., Holzkemp, E., Breil. H., Martin, H. Angew. Chem. 67, 541 (1955). RECEIVED for review August 9, 1961 ACCEPTEDDecember 28, 1961 Division of Petroleum Chemistry, Symposium on Monomers and Polymers from Petroleum, 139th Meeting, ACS, St. Louis, Mo.? March 1961. Publication 34, LV. R. Grace & Co.
