When bis(cyclopentadien)zirconium dichloride (Cp2ZrCl2) and MAO are used for ethylene polymerization, yields as high as 40×106 g of poly- ethylene (grams of zirconium×h)−1are obtained.
Every zirconium atom forms an active complex, as shown by Tait (103) and Chien and Wang (104), producing about 46,000 polymer chains per hour. (This efficient use of the zirconium atoms is contrasted to Ziegler–
Natta and Phillips polymerization, in which only a small fraction of the metal centers are active.) The insertion time of one ethylene unit is only 3×10−5s. The rates are comparable to those observed for some enzymes cat- alyzing synthesis reactions. The analogy to enzymes is manifested in others ways as well—for example, the influence of substitution, regioselectivity, and stereospecificity.
Unbridged, bridged, substituted, and half-sandwich complexes have been used as metallocenes for ethylene polymerization (Figs. 1 and 2). To com- pare the activities and molecular masses, the polymerizations are carried out under the same conditions (30◦C, 2 bar ethylene pressure, with toluene as a solvent) (105). Table IV shows the polymerization behavior of various met- allocene/alumoxane catalysts. Generally, zirconium-containing catalysts are
TABLE IV
Homopolymerization of Ethylene at 30◦C, 2.5 bar Ethylene Pressure, 6.25×10−6mol/Liter Metallocene Concentration, and Molar Ratio MAO/Metallocene=250a
10−4×activity (kg of PE) (moles 10−5× molecular Catalyst of metalloceneãhãCethylene)−1b mass (g/mole)
Cp2ZrCl2 6.09 6.2
Cp2TiCl2 3.42 4.0
Cp2HfCl2 0.42 7.0
Cp2TiMeCl 2.70 4.4
Cp2ZrMe2 1.40 7.3
Cp2TiMe2 0.12 5.0
Cp2ZrCl2/EAO 0.25 5.5
Cp2ZrCl2/iBAO 0.54 3.9
(C5Me5)2ZrCl2 0.13 15
(NmCp)2ZrCl2 1.22 10
(C5Me4Et)2ZrCl2 1.88 8.0
(O(SiMeCp)2)ZrCl2 5.78 9.3
(O(SiMe2tBuCp)2)ZrCl2 1.17 0.7
(En(Ind)2)ZrCl2 4.11 1.4
(En(Ind)2)HfCl2 0.29 4.8
(En(2,4,7)Me3Ind)2)ZrCl2 7.80 1.9
(En(IndH4)2)ZrCl2 2.22 10
(Me2Si(Ind)2)ZrCl2 3.69 2.6
(Ph2(Ind)2)ZrCl2 2.02 3.2
(Bz2Si(Ind)2)ZrCl2 1.22 3.5
(Me2Si(2,4,7Me3Ind)2)ZrCl2 11.19 2.5
(Me2Si(IndH4)2)ZrCl2 3.02 9.0
(Me2Si(2Me-4,6iPr-2Ind)2)ZrCl2 1.86 7.3
(Me2Si(2Me-4Ph-Ind)2)ZrCl2 1.66 7.3
(Me2Si(2Me-4,5BenzInd)2ZrCl2 0.76 4.5
(Ph2(Ind)(Cp))ZrCl2 0.333 0.18
(Me2C(Ind)(Cp))ZrCl2 0.155 0.25
(Me2(Ind)(3MeCp))ZrCl2 0.27 0.30
(Ph2C(Fluo)(Cp))ZrCl2 0.289 6.3
(Me2C(Fluo)(Cp))ZrCl2 0.20 5.0
(Me2C(Fluo)(Cp))HfCl2 0.089 5.6
(En(Fluo)2)ZrCl2 4.0 4.0
aEAO, ethylalumoxane; iBAO, triisobutylalumoxane (105).bCethyleneis ethylene concen- tration in moles/liters.
OLEFIN POLYMERIZATION CATALYZED BY METALLOCENES 111 more active than those containing hafnium or titanium (106). Partially sub- stituted bisindenyl systems and bridged bisfluorenyl zirconocenes (107, 108) show high activities, exceeding those of the sterically less hindered Cp2ZrCl2. In contrast, zirconocenes with bulky ligands such as neomentyl-substituted derivatives afford significantly lower productivities. This comparison indi- cates that electron-donating groups can enhance productivity, whereas steric crowding lowers it. Among the alumoxane cocatalysts, methylalumoxane is much more effective than ethyl- or isobutylalumoxane. The catalyst shows a long-lasting activity, even after more than 100 h of polymerization. The maximum activity is reached after 5–10 min; this period seems to be needed for the active site to form.
The concentrations of catalyst and cocatalyst and the partial pressure of ethylene influence the polymerization rate. A nearly linear dependence of the rate on the concentration of ethylene is observed. Polyethylenes produced by metallocene catalysts feature a molecular weight distribution (MWD) ofMw/Mn=2, and the product incorporates 0.9–1.2 methyl groups per 1000 carbon atoms. Polyethylenes formed with various metallocene cat- alysts differ in their molecular weights by a factor of more than 50. Catalysts with a substituent in the cyclopentadienyl or in the 2 position of an indenyl ligand give polymers with relatively high molecular weights. Polyethylene produced with bis(pentamethyl-cyclopentadienyl)zirconium dichloride has a molecular weight of 1.5 million. Similar results are observed with tetrahy- droindenyl compounds. The use of mixtures of more than one metallocene catalyst leads to bimodal distributions with values of MWDMw/Mnin the range of 5–10. The molecular weight is easily lowered by increasing the temperature, raising the metallocene to ethylene ratio, or by adding small amounts of hydrogen (0.1–2 mol%).
At a reaction temperature of 10◦C, polyethylene is formed by Cp2∗ZrCl2 with a molecular weight of 1.5×106 g/mol, whereas at 50◦C this value is reduced to 180,000 g/mol and at 90◦C to 90,000 g/mol. Raising the temper- ature to more than 100◦C yieldsα-olefins with an even number of carbon atoms (109).
The molecular weight decreases nearly linearly with increasing zirconium concentration. This observation leads to the conclusion that chain transfer occurs via a bimetallic mechanism. Usually aβ-hydrogen transfer from the growing polymer chain is responsible for the chain transfer. A metallocene hydride is formed along with polymer having a vinyl group at the chain end.
In some cases the hydrogen can be transferred to theπ-bonded ethylene, forming an ethyl–metallocene complex.
The molecular weight can also be influenced by the addition of hydrogen (110, 111). In contrast to Ziegler–Natta catalysis, in this case only traces of hydrogen are needed to lower the molecular weight over a wide range.
To reduce the molecular weight to about half the value obtained without hydrogen, the reactor needs to be fed with only 7.5 vol% H2.
B. COPOLYMERS
Metallocenes are useful for copolymerization of ethylene with other olefins. Propylene, 1-butene, 1-pentene, 1-hexene, and 1-octene have been investigated as comonomers, forming linear low-density copolymers with polyethylene (LLDPE) (97–101) (Table V). The major part of the como- nomer is randomly distributed over the polymer chain. The content of ex- tractables is much less than in polymers synthesized with Ziegler catalysts.
The copolymers have a great industrial potential and are formed at higher rates than the homopolymer. As a consequence of the short branches re- sulting from the incorporation of theα-olefin, the copolymers have lower melting points, lower crystallinities, and lower densities than the homopoly- mer. Films made of these materials more are flexible and more easily pro- cessed than those of polyethylene. Applications of the copolymers include packaging, shrink films with low steam permeabilities, elastic films that in- corporate a high comonomer concentration, cable coatings used in the med- ical fields because of the low concentrations of extractables, and foams, elastic fibers, adhesives, etc.
The copolymerization parameterr1expresses how much faster an ethy- lene unit is incorporated into the growing polymer chain than anα-olefin, provided that the last-inserted monomer was an ethylene unit and is be- tween 1 and 60, depending on the kind of comonomer and the catalyst. The
TABLE V
Copolymerization Parameters for Ethylene /α-Olefin Copolymerization with Various Metallocene/MAO Catalystsa
Metallocene catalyst Temperature (◦C) α-Olefin r1 r2 r1×r2
Cp2ZrMe2 20 Propylene 31 0.005 0.25
(En(Ind)2)ZrCl2 50 Propylene 6.61 0.06 0.40
(En(Ind)2)ZrCl2 25 Propylene 1.3 0.20 0.26
Cp2ZrCl2 40 Butene 55 0.017 0.93
Cp2ZrCl2 60 Butene 65 0.013 0.85
Cp2ZrCl2 80 Butene 85 0.010 0.85
(En(Ind)2)ZrCl2 30 Butene 8.5 0.07 0.59
(En(Ind)2)ZrCl2 50 Butene 23.6 0.03 0.71
Cp2ZrMe2 60 Hexene 69 0.02 1.38
(Me2Si(Ind)2)ZrCl2 60 Hexene 25 0.016 0.40
aTerms defined under Abbreviations (97).
OLEFIN POLYMERIZATION CATALYZED BY METALLOCENES 113 copolymerization parameterr2is the analogous ratio for theα-olefin. The productr1×r2is important in determining the distribution of the comonomer and is close to unity (indicating a randomly distributed comonomer) when C2-symmetric metallocenes are used as catalysts. This product is less than unity (r1×r2 = 0.1– 0.4), and the polymer has a more alternating structure, when the catalysts are Cssymmetric (118).
Leclerc and Waymouth (119) and, independently, Arndtet al.(120) syn- thesized alternating copolymers of ethylene and propylene with zirconocene catalysts. The ethylene/propylene (EP) copolymerizations were carried out at 30 and 60◦C for each of four metallocene catalysts (Me2C(3-RCp)(Flu)) ZrCl2(R=H, Me,isoPr,tertBu) (Fig. 10). As the size of the substituent in- creased, there were distinct changes in the copolymerization behavior and in the polymer microstructure.
However, the actual process of insertion could not be observed directly and has been the subject of speculation. Extensive studies of the reaction of the catalyst precursor with the cocatalyst showed that 14-electron species which have alkylzirconocenium ion character (Cp2Zr(polymeryl))+are rea- sonable models of the catalytically active species (121). Each of these com- plexes has a vacant coordination site suitable for olefin approach. Basically two types of mechanisms for insertion have been proposed (122, 123):
FIG. 10. Copolymerization diagrams for ethylene/propylene copolymerization carried out with (Me2C(3-tert-BuCp)(Flu))ZrCl2at 30 ( ) and 60◦C (). (120).
1. The alternating mechanism involves a “migration” of the growing poly- mer chain during insertion and therefore, in the case of metallocenes with stereogenic transition metal centers, inversion of the configuration (the po- sition formerly occupied by the polymer chain is available for the next olefin to approach, and the configuration of the metallocene alternates).
2. The retention mechanism in which the olefin always approaches the metallocene from the same direction and therefore, in the case of metal- locenes with stereogenic transition metal centers, the configuration at the transition metal is retained.
The highest degree of alternating structure was observed with (Me2C(3-isoPrCp)(Flu))ZrCl2(Fig. 11). The fraction of PEP trials (propene–
ethylene–propene combinations in the polymer chain), or (CH2– C H
(CH3)–
CH2–CH2–CH2– C H
(CH3)), measured by13C NMR spectroscopy is 40% at a molar fraction of 0.4 of ethylene in the copolymer. It is concluded that copolymerization proceeds via chain migratory insertion for R =H or Me,
FIG. 11. Calculated and experimental distribution of ethylene-centered triads for ethy- lene/propylene copolymerization carried to with (Me2C(3-tert-BuCp)(Flu))ZrCl2at 30◦C: , EEE;, EEP+PEE;♦, PEP. Copolymerization behavior fitted according to the terminal (−) and TSAM model (−) (120).
OLEFIN POLYMERIZATION CATALYZED BY METALLOCENES 115 whereas a retention mechanism was found for R = tertBu. For R = isoPr, especially at 60◦C, a defective alternating mechanism seems to be operative.
Under the same conditions, syndiospecific (Cs-symmetric) metallocenes are more effective in insertingα-olefins into an ethylene copolymer than isospecific (C2-symmetric) metallocenes or unbridged metallocenes. In par- ticular, hafnocenes are more efficient than zirconocenes. An interesting ef- fect is observed for the polymerization with ethylene(bisindenyl)zirconium dichloride and some other metallocenes. The catalytic activity for the ho- mopolymerization of ethylene is very high, and it increases when copoly- merization with propylene occurs (114) (Fig. 12). Mu ˜noz-Escalona et al.
(125) observed similar effects in the copolymerization of ethylene with 1-hexene.
The copolymerization of ethylene with other olefins is affected by the variation of the Al : Zr ratio, temperature, and catalyst concentration. These variables lead to changes in the molecular weight and the ethylene content.
Higher temperatures lead to increases in the ethylene content and low molec- ular weights. Investigations of ethylene copolymerization with 1-butene
FIG. 12. Rate constantkpof the ethylene/propylene copolymerization as a function of the ethylene concentration in the liquid phase at 37◦C: dashed line, calculated; solid line, measured (114).
catalyzed by Cp2ZrCl2/MAO indicated a decrease in the rate of polymer- ization with increasing comonomer concentration.
The copolymers of ethylene and propylene, with molar ratios in the range of 1 : 0.5–1 : 2, are of great industrial interest. These EP polymers have elastic properties; when the monomers are used together with 2–5 wt% of dienes as a third monomer, they produce elastomers (EPDM). Since it has no double bonds in the backbone of the polymer, this copolymer is less sensitive to oxi- dation reactions than polybutadiene. Ethylidene norbornene, 1,4-hexadiene, and dicyclopentadiene are also used as dienes. In most technological pro- cesses for the production of EP and EPDM rubber, soluble or highly dis- persed vanadium components have been used. Similar elastomers, which are less highly colored, can be obtained with metallocene/MAO catalysts that have much higher activities (126). The regiospecificity of the metallocene catalysts toward propylene leads exclusively to the formation of head-to-tail enchainments. Ethylidene norbornene polymerizes via vinyl polymerization of the cyclic double bond, and the tendency for branching is low. The molec- ular weight distribution is narrow (about 2) (127).
In polymerization at low temperatures, the time required to form one polymer chain is long enough to consume one monomer fully and allow the subsequent addition of another one. Thus, it becomes possible to synthesize block copolymers, provided that the polymerization (especially when it is catalyzed by hafnocenes) starts with propylene and, after the propylene is nearly consumed, continues with ethylene.
Branching, which is caused by the incorporation of long-chain olefins into the growing polymer chain, has been observed by Stevens (128), Shapiro et al.(129), Okudaet al.(130), and others (131–134) with a new class of silyl bridged amidocylopentadienyl titanium compounds (Fig. 3). These catalysts, used in combination with MAO or borates, incorporate oligomers with vinyl end groups which are formed during polymerization byβ-hydrogen transfer, resulting in long-chain branched polyolefins; in contrast, linear polymers are obtained when the reactions are catalyzed by other metallocenes. Copoly- mers of ethylene with 1-octene are very flexible materials, provided that the comonomer content is less than 10% (135). When this value reaches 20%, the long-branched polymers have elastic properties.