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Constrained geometry and other single site metallocene polyolefin catalysts: A revolution in olefin polymerization
J.W. Hightower, W.N. Delgass, E. Iglesia and A.T. Bell (Eds.) 11th International Congress on Catalysis - 40th Anniversary
Studies in Surface Science and Catalysis, Vol. 101 1996 Elsevier Science B.V.
11
C o n s t r a i n e d G e o m e t r y and O t h e r Single Site Metallocene Polyolefin Catalysts: A Revolution In Olefin P o l y m e r i z a t i o n James C. Stevens Polyolefins and Elastomers Research and Development Laboratories, The Dow Chemical Company, 2301 Brazosport Boulevard, Freeport, TX 77541 1. ABSTRACT The polyolefins industry is at a crossroads. A new generation of single-site catalyst technology promises to revolutionize this multi-billion pound per year industry. Single-site catalysts have recently moved from laboratory curiosities to commercial success. Newly developed single-site c a t a l y s t s allow u n p r e c e d e n t e d control of polymer molecular a r c h i t e c t u r e , which yields products having improved properties. The consistency and control of polymer structure is allowing new discoveries to be made in f u n d a m e n t a l polymer research. This paper will touch on all aspects of single-site catalyst technology, including m e t a l l o c e n e m e t a l complexes, a c t i v a t i n g c o c a t a l y s t s (e.g., alumoxanes), cationic catalysts, as well as single site polymers, focusing on recent developments at Dow Plastics. 2. INTRODUCTION The metal catalyzed production of polyolefins such as high density polyethylene (HDPE), linear low d e n s i t y polyethylene (LLDPE) a n d polypropylene (PP) has grown into an enormous industry. Heterogeneous transition metal catalysts are used for the vast majority of PE and all of the PP production. These catalysts fall generally within two broad classes. Most commercial PP is isotactic and is produced with a catalyst based on a combination of titanium chloride and alkylaluminum chlorides. 1 HDPE and LLDPE are produced with either a titanium catalyst or one based on chromium supported on silica. 2 Most commercial t i t a n i u m - b a s e d PE catalysts are supported on MgC12. One of the most exciting developments in the polyolefins industry in recent years has centered on the development of commercial homogeneous single-site catalysts. These single-site catalysts produce olefin polymers with properties that are different when compared with traditional thermoplastic polyolefins. Homogeneous single-site catalysts based on bis-cyclopentadienyl derivatives of titanium have been known since the 1950's, although the catalytic activity of these early catalysts was too low for commercial practicality. 3 The key discovery by K a m i n s k y and Sinn t h a t m e t h y l a l u m i n o x a n e (MAO, [MeA10]n ) i n conjunction with Cp2TiMe2 and Cp2ZrC12 afforded extremely active catalysts for
12 PE and atactic PP lead to the recent explosion of interest in single-site catalysts.4, 5 The most valuable feature of single-site catalysts is the ability to logically control the structure of the polymer from the design of the catalyst. The most commonly used families of single-site catalysts are based on metal complexes shown in Figure 1. The polymer that is produced with these catalysts is strongly influenced by the catalyst structure. Catalysts with structure 1, having C2v symmetry produce atactic PP. By adding a bridging group between the cyclopentadiene ligands, catalysts having chiral C2 symmetry such as 3 produce isotactic pp.6,7 Linking the cyclopentadienes together to give a catalyst having Cs symmetry as in 4 produces syndiotactic PP. These relationships have led to the development of catalysts which produce isotactic-b-atactic PP by rotation of a substituted indenyl catalyst through C2v and C2 symmetry.8
R
2
~MX2 ~'~R
TiX3
HDPE, LLDPE Atactic PP
Syndiotactic PS
Isotactic PP
1
2
3
MX 2
i
Syndiotactic PP 4
Isotactic-b-atactic PP 5
F i g u r e 1. Single-site catalysts for olefin polymerizations. The bis-cyclopentadienyl-based, or metallocene, single-site catalysts are generally activated with MAO in relatively large molar amounts. The catalytic activity increases with increasing A1 : M ratio. Typically, at least 500 - 1000 molar equivalents of aluminum are required for acceptable activity. The high levels of MAO are a problem commercially, due to the relatively high cost of MAO. In addition, very high levels of MAO leave a large amount of aluminumcontaining "ash" in the polymer which can affect the product properties. The
13 catalytic activity is also a function of the transition metal. In general, the order of activity is Zr > Hf > Ti. Polymer molecular weight is also a function of the transition metal, generally following the order Hf > Ti > Zr. The ability to control the polymer from the design of the catalyst, coupled with high catalytic efficiency has led to an explosion of commercial and academic interest in these catalysts. Exxon started up a 30 million lb/yr ethylene copolymer demonstration plant in 1991 using a bis-cyclopentadienyl zirconium catalyst of structure 1. The Dow Chemical Company (Dow) began operating a 125 million lb/yr ethylene/1-octene copolymer plant in 1993 and has since expanded production capacity to 375 million lb/yr. This paper will focus on the structure / property relationships of the catalysts used by Dow to produce single-site ethylene a-olefin copolymers.
3. C o m m ~
Polyethylene
Commercial polyethylene falls within three general classes, as shown in Figure 2. LDPE is a highly branched dendritic polymer containing a range of short and long-chain b r a n c h e s , which r e s u l t from v a r i o u s r a d i c a l recombination processes in a high temperature (250 - 300 ~ high pressure process of up to about 45,000 psi. The numerous long-chain branches impart high melt strength and excellent processability to the polymer. In contrast, linear HDPE and LLDPE are produced using coordination catalysts and are characterized by a linear backbone containing no long-chain branching. LLDPE is primarily produced as a copolymer of ethylene with C4-C8 a-olefins up to about 15 weight percent. As a result, the linear molecules impart good toughness and strength properties, but are relatively more difficult to process than LDPE. Processability can be improved by broadening the molecular weight distribution, which tends to increase the number of low molecular weight molecules. The increased processability is generally achieved at the expense of strength and other physical properties due to the reduced number of high molecular weight molecules. Narrow molecular weight linear polyethylenes are relatively difficult to process. Focusing on LLDPE, traditional chromium or titanium-based Ziegler/Natta catalysts produce a product with a broad distribution of individual polymer molecules, each of which contributes to the overall properties of the resin. It is believed that the active catalysts contain active sites of various oxidation states and coordination environments, each of which exhibits different rates of propagation, termination, and comonomer reactivity. 9 The catalyst sites which incorporate comonomer have higher rates of chain t e r m i n a t i o n , and consequently the polymer molecules which contain more comonomer are lower molecular weight. A significant fraction of the polymer contains little if any comonomer and is generally of high molecular weight. The "mixture" of polymer molecules which results represents a limitation of conventional heterogeneous polyolefin catalysts in that the ability to control the individual component polymer molecules is rather restricted.
14
radicals H2C---CH 2 high temperature
I~PE
Z/N Catalysts H2C---CH 2
n ttDPE
H2C---CH 2
R Z/N Catalysts
+
LLDPE F i g u r e 2. Classes of commercial polyethylene. Metallocene catalyzed LLDPE is characterized by polymer molecules t h a t are the result of a single active catalytic site. As a result, all of the polymer molecules can in theory be made with statistically the same comonomer distribution. The molecular weight distribution is quite narrow, in the range of about 2.0 Mw/Mn. As a result of the ability to tailor the individual polymer chains, material scientists now have the ability to produce targeted polymer species and control the properties of the resin to a high degree. Great advances in such areas as product strength, clarity, toughness, and melting behavior can be commercially realized using metallocene catalysts. Single site polymers having a uniform comonomer and molecular weight distribution produced with low efficiency v a n a d i u m catalysts have been commercially available from Mitsui (Tafmer resins) since the 1980's. However, these polymers are relatively expensive and the ability to tailor the product is limited.
15 4. Constrained Geometry Catalysts Dow has developed a new family of ethylene-based polyolefins using constrained geometry catalyst technology. The catalyst and process technology has been commercialized under the tradename INSITE TM. INSITE Technology utilizes a family of new constrained geometry catalysts t h a t allows the production of unique polyolefin polymers in a relatively low pressure solution process. An important feature of the solution process is the need to operate the polymerization reaction above the melting point of the polymer in order to keep the product in solution. Solution LLDPE processes generally run at very high ethylene conversion and with a short reactor residence time of only a few minutes. Unfortunately, metallocene catalysts such as structures 1-5 produce low molecular weight products under such conditions, due to relatively facile ~hydride elimination. Constrained geometry catalysts, on the other hand, allow for the production of high molecular weight ethylene copolymers in a high temperature solution process. The key catalyst features are shown in Structure 6. The catalysts are monocyclopentadienyl Group 4 complexes with a covalently attached amide donor ligand. The amide ligand stabilizes the metal electronically, while the short bridging group (B) has the effect of sterically opening up one side of the complex, producing a stable but highly open and reactive active site upon activation with a variety of cocatalysts.
Rn N.-MX2 ]
R M = Ti, Zr, H f B = SIR2, C2H4, etc. R = alkyl, aryl, etc. X = halide, CH3 S t r u c t u r e 6. General structure of Constrained Geometry Catalysts.
In general, the open nature of the catalytic site in the constrained geometry catalysts does not allow for much steric control of the polymerization reaction, and homopoly a-olefins are generally atactic, although it has been reported that a small degree of tacticity can be introduced in polypropylene using such catalysts by selection of the substituents and conducting the polymerization at r e l a t i v e l y low t e m p e r a t u r e s . 1~ The degree of tacticity obtained under commercially useful conditions is so low that the catalysts can be considered to be atactic. The s t e r i c a l l y u n e n c u m b e r e d c a t a l y s t active site allows the copolymerization of a wide variety of olefins with ethylene. Conventional heterogeneous Ziegler/Natta catalysts as well as most metallocene catalysts are much more reactive to ethylene than higher olefins. With constrained geometry catalysts, a-olefins such as propylene, butene, hexene, and octene are readily incorporated in large amounts. The kinetic reactivity ratio, rl, is approximately
16 4 for the copolymerization of ethylene with 1-octene, which is approximately 2 orders of m a g n i t u d e more reactive towards octene t h a n some MgC12-supported heterogeneous catalysts. In addition, non-traditional olefins such as s t y r e n e can be incorporated in high levels. Styrene / ethylene copolymers containing significant a m o u n t s of styrene and having a high molecular weight have not been available in the past, as conventional polyolefin catalysts will e i t h e r not copolymerize ethylene with styrene to any appreciable extent, or the molecular weight is too low to be u s e f u l .
5. C o ~ a t a l y s t s and Polymerization Behavior C o n s t r a i n e d geometry complexes and metallocenes in general r e q u i r e the addition of a cocatalyst in order to become catalytically active. When activated with a large excess of MAO, catalyst efficiencies between 150,000 and 750,000 g of polymer per g r a m of metal are obtained, depending on the reactor t e m p e r a t u r e , specific catalyst, MAO level and other process variables. In general, these efficiencies a r e lower t h a n can be o b t a i n e d w i t h b i s - c y c l o p e n t a d i e n y l m e t a l l o c e n e s a n d MAO. In solution p o l y m e r i z a t i o n s , however, h i g h Mw polymers are obtained, even at t e m p e r a t u r e s as high as 160 ~ with 1-octene as comonomer. The Mw decreases with i n c r e a s i n g t e m p e r a t u r e , as shown in Figure 3, as a result of increasing ~-hydride elimination to give u n s a t u r a t e d chain ends. The Mw is sufficiently high u n d e r solution conditions t h a t H2 can be used as a Mw control, giving two i n d e p e n d e n t controls over m o l e c u l a r weight. 200000 , .
150000-
D
100000 -
50000I
I
I
I
I
I 0 t""
0
Reactor T, ~ F i g u r e 3. Mw data for ethylene 1-octene copolymerization. Catalyst = [(C5Me4)SiMe2N(t-Bu)TiC12 / MAO, 450 psi ethylene, 10 minute reaction time. 11
17 The open nature of the catalytic site allows for the incorporation of extremely high levels of comonomer, producing elastomers with over 20 weight % 1-octene comonomer. Increasing levels of comonomer depress the density, leading to ultra-low density elastomers. High Mw elastomeric ethylene/octene resins with densities between 0.87 and 0.85 g/mL can be obtained with high efficiencies. Figure 4 shows the relationship between density and weight % 1octene, determined using 13C NMR for such elastomers. Prior to the advent of metallocene catalysis, such extremely low density copolymers were not commercially accessible at low cost.
0.88
0.87
[]
[]
0.86
0.85 30
40
50
60
Weight % Octene
Figure 4. Ethylene-co-l-octene density as a function of 1-octene content for elastomeric copolymers. Comonomer incorporation, molecular weight, and catalytic efficiency are sensitive to the nature of the group bridging the Cp ring and the substituent on the amide ligand. Shorter bridges constrain the cyclopentadienyl ligand and amide group to adopt a particularly open and reactive catalytic environment. 12 Unlike the bis-Cp metallocenes, the titanium constrained geometry catalysts generally show the highest activity, comonomer incorporation, and Mw. While the constrained geometry catalysts exhibit many unique properties, the catalytic efficiency using MAO cocatalyst is relatively low for commercial applications, on the order of 104 - 10 5 g polymer per g of Ti. In contrast, a variety of cationic constrained geometry catalysts can be prepared which show extremely high activity, exceeding 10 7 grams of polymer per gram of transition metal. Cationic catalysts can be prepared with ammonium salts (Figure 5a) 13, oxidation of a corresponding Ti(III) complex (Figure 5b) 14, or abstraction of a hydrocarbyl group using B(C6F5)3 (Figure 5c). 15
18
[R3NH] [B(C6F5)4] A)
e2SiNN~T1Me2
NaN ~
-
CH 4
t_B/
[Cp2Fe][B(C6F5)4] ...... ~
~
t.Bu]
.
~ 1 ,CH Me2Si\ iTi'" \2
Me2SiXNITi.
[B(CGF5)4] G Me
/
B)
t-Bu~ M e 2 ~ N ~
\
C)
~ ~ " Me2Si\NIT1Me 2 t'Bu]
/
Cp2Fe Me2Si( ~~.~,,,C.H2 [B(C6F5)47
N-'"[
B(C6F5)3 S. ~ / ~ ~ ~ Me2 1\NIT1 ~ t-Bu/
| [CH3B(C6F5)3]
Me
Figure 5. Formation of Cationic constrained geometry catalysts: The use of B(C6F5)3 as a cocatalyst is particularly useful for solution polymerization, as this cocatalyst is soluble in the hydrocarbon polymerization solvent. The polymers produced in a continuous solution polymerization using these constrained geometry catalysts possess the expected properties of narrow molecular weight distribution and uniform comonomer distribution across the entire molecular weight range. In general, a narrow molecular weight and comonomer distribution would be expected to improve physical properties at the expense of processability. The polyolefins produced using INSITE Technology in a continuous solution polymerization process at high temperatures and high ethylene conversion give a polymer with high shear sensitivity, low melt fracture, high melt strength, and easy processability. The unusual properties of these INSITE polyolefins is the result of small but significant levels of longchain branching in an otherwise linear molecule. 16 These long-chain branches are postulated to result from the reincorporation of vinyl-terminated polymer molecules according to Scheme 1. The conditions of high reactor temperature
19 which leads to high vinyl termination, high comonomer reactivity, and high conversion with the concomitant high polymer concentration and low ethylene and comonomer concentrations in the continuous solution process produces the conditions favorable to long-chain branch formation.
Ti--CH2CH2--polymer A
Ti
+
Polymer--CH-CH 2 +
TimH
+
Polymer--CH--CH 2
monomers
PolymermCH--Polymer I
Polymer S c h e m e 1. Mechanism for formation of long-chain branching: Ti = active constrained geometry catalyst. 6. Conclusions Metallocene catalysts allow for the targeted control of polymer molecular structure to a degree which has not been possible previously. Rational structure-property relationships of these homogeneous catalysts are allowing new polymers to be produced which are finding large commercial markets. The high catalytic productivity possible with metallocene catalysts enable the production of polymers at competitive prices, even though the catalysts and cocatalysts are complex and relatively expensive. For ethylene/a-olefin copolymers, constrained geometry catalysts allow the production of a unique family of olefinic polymers. The proper selection of the metal, bridging group, and other substituents allows the control of product properties in a high temperature solution process. With the proper selection of catalyst variables, products ranging from high molecular weight elastomers to high density polyethylene can be produced.
References
1. J. Boor, Jr., Ziegler-Natta Catalysts and Polymerizations, Academic Press, 1979, New York, p. 108-129. 2. Ibid, p. 8.
20 3. D. S. Breslow and N. R. Newberg, J. Am. Chem. Soc., 1959, 81, 81. 4. H. Sinn,and W. Kaminsky, Adv. Organomet. Chem. 1980, 18, 99. 5. W. Kaminsky, M. Miri, H. Sinn, R. Woldt, Makromol. Chem., Rapid Commun. 1983, 4, 417. 6. o
8. o
J.A. Ewen, J. Am. Chem. Soc. 1984, 106, 6355. W. Kaminsky, K. Kiilper, H. H. Brintzinger, F. R. W. P. Wild, Angew. Chem., Int. Ed. Engl. 1985, 24, 507. G.W. Coates and R.M. Waymouth, Science 1995, 267, 217-218. J. Boor, Jr., Ziegler-Natta Catalysts and Polymerizations, Academic Press, 1979, New York, p. 262-269.
10. J. A. Canich, U.S. Patent 5,026,798 (1991). 11. J. C. Stevens, et al., European patent application 416,815 (1991). 12. J. C. Stevens, Stud. Surf. Sci. and Catal. 1994, 89, 277-284. 13. J. C. Stevens and D. R. Neithamer, US patents 5,064,802 (1991); 5,132,380 (1992). 14. R. E. LaPointe, et al., US patent 5,189,192 (1993). 15. R. E. LaPointe, et al., European patent application 520,732 (1991). 16. S.Y. Lai, et al., U. S. patent 5,272,236 (1993); 5,278,272 (1994).
Studies in Surface Science and Catalysis, Vol. 101 1996 Elsevier Science B.V.
11
C o n s t r a i n e d G e o m e t r y and O t h e r Single Site Metallocene Polyolefin Catalysts: A Revolution In Olefin P o l y m e r i z a t i o n James C. Stevens Polyolefins and Elastomers Research and Development Laboratories, The Dow Chemical Company, 2301 Brazosport Boulevard, Freeport, TX 77541 1. ABSTRACT The polyolefins industry is at a crossroads. A new generation of single-site catalyst technology promises to revolutionize this multi-billion pound per year industry. Single-site catalysts have recently moved from laboratory curiosities to commercial success. Newly developed single-site c a t a l y s t s allow u n p r e c e d e n t e d control of polymer molecular a r c h i t e c t u r e , which yields products having improved properties. The consistency and control of polymer structure is allowing new discoveries to be made in f u n d a m e n t a l polymer research. This paper will touch on all aspects of single-site catalyst technology, including m e t a l l o c e n e m e t a l complexes, a c t i v a t i n g c o c a t a l y s t s (e.g., alumoxanes), cationic catalysts, as well as single site polymers, focusing on recent developments at Dow Plastics. 2. INTRODUCTION The metal catalyzed production of polyolefins such as high density polyethylene (HDPE), linear low d e n s i t y polyethylene (LLDPE) a n d polypropylene (PP) has grown into an enormous industry. Heterogeneous transition metal catalysts are used for the vast majority of PE and all of the PP production. These catalysts fall generally within two broad classes. Most commercial PP is isotactic and is produced with a catalyst based on a combination of titanium chloride and alkylaluminum chlorides. 1 HDPE and LLDPE are produced with either a titanium catalyst or one based on chromium supported on silica. 2 Most commercial t i t a n i u m - b a s e d PE catalysts are supported on MgC12. One of the most exciting developments in the polyolefins industry in recent years has centered on the development of commercial homogeneous single-site catalysts. These single-site catalysts produce olefin polymers with properties that are different when compared with traditional thermoplastic polyolefins. Homogeneous single-site catalysts based on bis-cyclopentadienyl derivatives of titanium have been known since the 1950's, although the catalytic activity of these early catalysts was too low for commercial practicality. 3 The key discovery by K a m i n s k y and Sinn t h a t m e t h y l a l u m i n o x a n e (MAO, [MeA10]n ) i n conjunction with Cp2TiMe2 and Cp2ZrC12 afforded extremely active catalysts for
12 PE and atactic PP lead to the recent explosion of interest in single-site catalysts.4, 5 The most valuable feature of single-site catalysts is the ability to logically control the structure of the polymer from the design of the catalyst. The most commonly used families of single-site catalysts are based on metal complexes shown in Figure 1. The polymer that is produced with these catalysts is strongly influenced by the catalyst structure. Catalysts with structure 1, having C2v symmetry produce atactic PP. By adding a bridging group between the cyclopentadiene ligands, catalysts having chiral C2 symmetry such as 3 produce isotactic pp.6,7 Linking the cyclopentadienes together to give a catalyst having Cs symmetry as in 4 produces syndiotactic PP. These relationships have led to the development of catalysts which produce isotactic-b-atactic PP by rotation of a substituted indenyl catalyst through C2v and C2 symmetry.8
R
2
~MX2 ~'~R
TiX3
HDPE, LLDPE Atactic PP
Syndiotactic PS
Isotactic PP
1
2
3
MX 2
i
Syndiotactic PP 4
Isotactic-b-atactic PP 5
F i g u r e 1. Single-site catalysts for olefin polymerizations. The bis-cyclopentadienyl-based, or metallocene, single-site catalysts are generally activated with MAO in relatively large molar amounts. The catalytic activity increases with increasing A1 : M ratio. Typically, at least 500 - 1000 molar equivalents of aluminum are required for acceptable activity. The high levels of MAO are a problem commercially, due to the relatively high cost of MAO. In addition, very high levels of MAO leave a large amount of aluminumcontaining "ash" in the polymer which can affect the product properties. The
13 catalytic activity is also a function of the transition metal. In general, the order of activity is Zr > Hf > Ti. Polymer molecular weight is also a function of the transition metal, generally following the order Hf > Ti > Zr. The ability to control the polymer from the design of the catalyst, coupled with high catalytic efficiency has led to an explosion of commercial and academic interest in these catalysts. Exxon started up a 30 million lb/yr ethylene copolymer demonstration plant in 1991 using a bis-cyclopentadienyl zirconium catalyst of structure 1. The Dow Chemical Company (Dow) began operating a 125 million lb/yr ethylene/1-octene copolymer plant in 1993 and has since expanded production capacity to 375 million lb/yr. This paper will focus on the structure / property relationships of the catalysts used by Dow to produce single-site ethylene a-olefin copolymers.
3. C o m m ~
Polyethylene
Commercial polyethylene falls within three general classes, as shown in Figure 2. LDPE is a highly branched dendritic polymer containing a range of short and long-chain b r a n c h e s , which r e s u l t from v a r i o u s r a d i c a l recombination processes in a high temperature (250 - 300 ~ high pressure process of up to about 45,000 psi. The numerous long-chain branches impart high melt strength and excellent processability to the polymer. In contrast, linear HDPE and LLDPE are produced using coordination catalysts and are characterized by a linear backbone containing no long-chain branching. LLDPE is primarily produced as a copolymer of ethylene with C4-C8 a-olefins up to about 15 weight percent. As a result, the linear molecules impart good toughness and strength properties, but are relatively more difficult to process than LDPE. Processability can be improved by broadening the molecular weight distribution, which tends to increase the number of low molecular weight molecules. The increased processability is generally achieved at the expense of strength and other physical properties due to the reduced number of high molecular weight molecules. Narrow molecular weight linear polyethylenes are relatively difficult to process. Focusing on LLDPE, traditional chromium or titanium-based Ziegler/Natta catalysts produce a product with a broad distribution of individual polymer molecules, each of which contributes to the overall properties of the resin. It is believed that the active catalysts contain active sites of various oxidation states and coordination environments, each of which exhibits different rates of propagation, termination, and comonomer reactivity. 9 The catalyst sites which incorporate comonomer have higher rates of chain t e r m i n a t i o n , and consequently the polymer molecules which contain more comonomer are lower molecular weight. A significant fraction of the polymer contains little if any comonomer and is generally of high molecular weight. The "mixture" of polymer molecules which results represents a limitation of conventional heterogeneous polyolefin catalysts in that the ability to control the individual component polymer molecules is rather restricted.
14
radicals H2C---CH 2 high temperature
I~PE
Z/N Catalysts H2C---CH 2
n ttDPE
H2C---CH 2
R Z/N Catalysts
+
LLDPE F i g u r e 2. Classes of commercial polyethylene. Metallocene catalyzed LLDPE is characterized by polymer molecules t h a t are the result of a single active catalytic site. As a result, all of the polymer molecules can in theory be made with statistically the same comonomer distribution. The molecular weight distribution is quite narrow, in the range of about 2.0 Mw/Mn. As a result of the ability to tailor the individual polymer chains, material scientists now have the ability to produce targeted polymer species and control the properties of the resin to a high degree. Great advances in such areas as product strength, clarity, toughness, and melting behavior can be commercially realized using metallocene catalysts. Single site polymers having a uniform comonomer and molecular weight distribution produced with low efficiency v a n a d i u m catalysts have been commercially available from Mitsui (Tafmer resins) since the 1980's. However, these polymers are relatively expensive and the ability to tailor the product is limited.
15 4. Constrained Geometry Catalysts Dow has developed a new family of ethylene-based polyolefins using constrained geometry catalyst technology. The catalyst and process technology has been commercialized under the tradename INSITE TM. INSITE Technology utilizes a family of new constrained geometry catalysts t h a t allows the production of unique polyolefin polymers in a relatively low pressure solution process. An important feature of the solution process is the need to operate the polymerization reaction above the melting point of the polymer in order to keep the product in solution. Solution LLDPE processes generally run at very high ethylene conversion and with a short reactor residence time of only a few minutes. Unfortunately, metallocene catalysts such as structures 1-5 produce low molecular weight products under such conditions, due to relatively facile ~hydride elimination. Constrained geometry catalysts, on the other hand, allow for the production of high molecular weight ethylene copolymers in a high temperature solution process. The key catalyst features are shown in Structure 6. The catalysts are monocyclopentadienyl Group 4 complexes with a covalently attached amide donor ligand. The amide ligand stabilizes the metal electronically, while the short bridging group (B) has the effect of sterically opening up one side of the complex, producing a stable but highly open and reactive active site upon activation with a variety of cocatalysts.
Rn N.-MX2 ]
R M = Ti, Zr, H f B = SIR2, C2H4, etc. R = alkyl, aryl, etc. X = halide, CH3 S t r u c t u r e 6. General structure of Constrained Geometry Catalysts.
In general, the open nature of the catalytic site in the constrained geometry catalysts does not allow for much steric control of the polymerization reaction, and homopoly a-olefins are generally atactic, although it has been reported that a small degree of tacticity can be introduced in polypropylene using such catalysts by selection of the substituents and conducting the polymerization at r e l a t i v e l y low t e m p e r a t u r e s . 1~ The degree of tacticity obtained under commercially useful conditions is so low that the catalysts can be considered to be atactic. The s t e r i c a l l y u n e n c u m b e r e d c a t a l y s t active site allows the copolymerization of a wide variety of olefins with ethylene. Conventional heterogeneous Ziegler/Natta catalysts as well as most metallocene catalysts are much more reactive to ethylene than higher olefins. With constrained geometry catalysts, a-olefins such as propylene, butene, hexene, and octene are readily incorporated in large amounts. The kinetic reactivity ratio, rl, is approximately
16 4 for the copolymerization of ethylene with 1-octene, which is approximately 2 orders of m a g n i t u d e more reactive towards octene t h a n some MgC12-supported heterogeneous catalysts. In addition, non-traditional olefins such as s t y r e n e can be incorporated in high levels. Styrene / ethylene copolymers containing significant a m o u n t s of styrene and having a high molecular weight have not been available in the past, as conventional polyolefin catalysts will e i t h e r not copolymerize ethylene with styrene to any appreciable extent, or the molecular weight is too low to be u s e f u l .
5. C o ~ a t a l y s t s and Polymerization Behavior C o n s t r a i n e d geometry complexes and metallocenes in general r e q u i r e the addition of a cocatalyst in order to become catalytically active. When activated with a large excess of MAO, catalyst efficiencies between 150,000 and 750,000 g of polymer per g r a m of metal are obtained, depending on the reactor t e m p e r a t u r e , specific catalyst, MAO level and other process variables. In general, these efficiencies a r e lower t h a n can be o b t a i n e d w i t h b i s - c y c l o p e n t a d i e n y l m e t a l l o c e n e s a n d MAO. In solution p o l y m e r i z a t i o n s , however, h i g h Mw polymers are obtained, even at t e m p e r a t u r e s as high as 160 ~ with 1-octene as comonomer. The Mw decreases with i n c r e a s i n g t e m p e r a t u r e , as shown in Figure 3, as a result of increasing ~-hydride elimination to give u n s a t u r a t e d chain ends. The Mw is sufficiently high u n d e r solution conditions t h a t H2 can be used as a Mw control, giving two i n d e p e n d e n t controls over m o l e c u l a r weight. 200000 , .
150000-
D
100000 -
50000I
I
I
I
I
I 0 t""
0
Reactor T, ~ F i g u r e 3. Mw data for ethylene 1-octene copolymerization. Catalyst = [(C5Me4)SiMe2N(t-Bu)TiC12 / MAO, 450 psi ethylene, 10 minute reaction time. 11
17 The open nature of the catalytic site allows for the incorporation of extremely high levels of comonomer, producing elastomers with over 20 weight % 1-octene comonomer. Increasing levels of comonomer depress the density, leading to ultra-low density elastomers. High Mw elastomeric ethylene/octene resins with densities between 0.87 and 0.85 g/mL can be obtained with high efficiencies. Figure 4 shows the relationship between density and weight % 1octene, determined using 13C NMR for such elastomers. Prior to the advent of metallocene catalysis, such extremely low density copolymers were not commercially accessible at low cost.
0.88
0.87
[]
[]
0.86
0.85 30
40
50
60
Weight % Octene
Figure 4. Ethylene-co-l-octene density as a function of 1-octene content for elastomeric copolymers. Comonomer incorporation, molecular weight, and catalytic efficiency are sensitive to the nature of the group bridging the Cp ring and the substituent on the amide ligand. Shorter bridges constrain the cyclopentadienyl ligand and amide group to adopt a particularly open and reactive catalytic environment. 12 Unlike the bis-Cp metallocenes, the titanium constrained geometry catalysts generally show the highest activity, comonomer incorporation, and Mw. While the constrained geometry catalysts exhibit many unique properties, the catalytic efficiency using MAO cocatalyst is relatively low for commercial applications, on the order of 104 - 10 5 g polymer per g of Ti. In contrast, a variety of cationic constrained geometry catalysts can be prepared which show extremely high activity, exceeding 10 7 grams of polymer per gram of transition metal. Cationic catalysts can be prepared with ammonium salts (Figure 5a) 13, oxidation of a corresponding Ti(III) complex (Figure 5b) 14, or abstraction of a hydrocarbyl group using B(C6F5)3 (Figure 5c). 15
18
[R3NH] [B(C6F5)4] A)
e2SiNN~T1Me2
NaN ~
-
CH 4
t_B/
[Cp2Fe][B(C6F5)4] ...... ~
~
t.Bu]
.
~ 1 ,CH Me2Si\ iTi'" \2
Me2SiXNITi.
[B(CGF5)4] G Me
/
B)
t-Bu~ M e 2 ~ N ~
\
C)
~ ~ " Me2Si\NIT1Me 2 t'Bu]
/
Cp2Fe Me2Si( ~~.~,,,C.H2 [B(C6F5)47
N-'"[
B(C6F5)3 S. ~ / ~ ~ ~ Me2 1\NIT1 ~ t-Bu/
| [CH3B(C6F5)3]
Me
Figure 5. Formation of Cationic constrained geometry catalysts: The use of B(C6F5)3 as a cocatalyst is particularly useful for solution polymerization, as this cocatalyst is soluble in the hydrocarbon polymerization solvent. The polymers produced in a continuous solution polymerization using these constrained geometry catalysts possess the expected properties of narrow molecular weight distribution and uniform comonomer distribution across the entire molecular weight range. In general, a narrow molecular weight and comonomer distribution would be expected to improve physical properties at the expense of processability. The polyolefins produced using INSITE Technology in a continuous solution polymerization process at high temperatures and high ethylene conversion give a polymer with high shear sensitivity, low melt fracture, high melt strength, and easy processability. The unusual properties of these INSITE polyolefins is the result of small but significant levels of longchain branching in an otherwise linear molecule. 16 These long-chain branches are postulated to result from the reincorporation of vinyl-terminated polymer molecules according to Scheme 1. The conditions of high reactor temperature
19 which leads to high vinyl termination, high comonomer reactivity, and high conversion with the concomitant high polymer concentration and low ethylene and comonomer concentrations in the continuous solution process produces the conditions favorable to long-chain branch formation.
Ti--CH2CH2--polymer A
Ti
+
Polymer--CH-CH 2 +
TimH
+
Polymer--CH--CH 2
monomers
PolymermCH--Polymer I
Polymer S c h e m e 1. Mechanism for formation of long-chain branching: Ti = active constrained geometry catalyst. 6. Conclusions Metallocene catalysts allow for the targeted control of polymer molecular structure to a degree which has not been possible previously. Rational structure-property relationships of these homogeneous catalysts are allowing new polymers to be produced which are finding large commercial markets. The high catalytic productivity possible with metallocene catalysts enable the production of polymers at competitive prices, even though the catalysts and cocatalysts are complex and relatively expensive. For ethylene/a-olefin copolymers, constrained geometry catalysts allow the production of a unique family of olefinic polymers. The proper selection of the metal, bridging group, and other substituents allows the control of product properties in a high temperature solution process. With the proper selection of catalyst variables, products ranging from high molecular weight elastomers to high density polyethylene can be produced.
References
1. J. Boor, Jr., Ziegler-Natta Catalysts and Polymerizations, Academic Press, 1979, New York, p. 108-129. 2. Ibid, p. 8.
20 3. D. S. Breslow and N. R. Newberg, J. Am. Chem. Soc., 1959, 81, 81. 4. H. Sinn,and W. Kaminsky, Adv. Organomet. Chem. 1980, 18, 99. 5. W. Kaminsky, M. Miri, H. Sinn, R. Woldt, Makromol. Chem., Rapid Commun. 1983, 4, 417. 6. o
8. o
J.A. Ewen, J. Am. Chem. Soc. 1984, 106, 6355. W. Kaminsky, K. Kiilper, H. H. Brintzinger, F. R. W. P. Wild, Angew. Chem., Int. Ed. Engl. 1985, 24, 507. G.W. Coates and R.M. Waymouth, Science 1995, 267, 217-218. J. Boor, Jr., Ziegler-Natta Catalysts and Polymerizations, Academic Press, 1979, New York, p. 262-269.
10. J. A. Canich, U.S. Patent 5,026,798 (1991). 11. J. C. Stevens, et al., European patent application 416,815 (1991). 12. J. C. Stevens, Stud. Surf. Sci. and Catal. 1994, 89, 277-284. 13. J. C. Stevens and D. R. Neithamer, US patents 5,064,802 (1991); 5,132,380 (1992). 14. R. E. LaPointe, et al., US patent 5,189,192 (1993). 15. R. E. LaPointe, et al., European patent application 520,732 (1991). 16. S.Y. Lai, et al., U. S. patent 5,272,236 (1993); 5,278,272 (1994).