The properties of polypropylenes depend mainly on the regio- and stereo- specificity of the inserted propylene units which influence the microstruc- ture. The microstructure of polypropylene in terms of the enchainment of
OLEFIN POLYMERIZATION CATALYZED BY METALLOCENES 117 the monomer units and their configuration is determined by the regio- and stereospecificity of the insertion of the monomer. Depending on the orien- tation of the monomer during insertion into the transition metal–polymeryl bond, primary (1,2) and secondary (2,1) insertions are possible (Fig. 13).
Consecutive regiospecific insertion results in regioregular head-to-tail en- chainment (1,3-branching) of monomer units, whereas regioirregularities cause the formation of head-to-head (1,2-branching) and tail-to-tail (1,4- branching) structures (136, 137).
Generally, metallocenes favor consecutive primary insertions as a conse- quence of their bent sandwich structures. Secondary insertion also occurs to an extent determined by the structure of the metallocene and the exper- imental conditions (especially temperature and monomer concentration).
Secondary insertions cause an increased steric hindrance to the next pri- mary insertion. The active center is blocked and therefore regarded as a resting state of the catalyst (138). The kinetic hindrance of chain propagation by another insertion favors chain termination and isomerization processes.
One of the isomerization processes observed in metallocene-catalyzed poly- merization of propylene leads to the formation of 1,3-enchained monomer units (Fig. 14) (139–142). The mechanism originally proposed to be of an elimination–isomerization–addition type is now thought to involve transi- tion metal-mediated hydride shifts (143, 144).
FIG. 13. Primary (1,2) and secondary (2,1) insertion in propylene polymerization.
FIG. 14. Elimination, isomerization, and addition mechanism for the formation of 1,3- enchained propylene.
B. MICROSTRUCTURES OFPOLYPROPYLENES
Another type of steric isomerism observed in polypropylene is related to the facts that propylene and otherα-olefins are prochiral and that poly- mers have pseudochiral centers at every tertiary carbon atom of the chain.
The regularity of the configuration of successive pseudochiral centers deter- mines the tacticity of the polymer. If the configuration of two neighboring pseudochiral centers is the same, this “diad” is said to have a meso arrange- ment of the methyl groups. If the pseudochiral centers are enantiomeric, the diad is calledracemic. A polymer containing only meso diads is called isotactic,whereas a polymer consisting of racemic diads only is calledsyn- diotactic. Polypropylene in which meso and racemic diads are randomly distributed is called atactic (Fig. 15). The tacticity has a major influence on the properties of the polymer. Atactic polypropylene is a liquid with a high viscosity, whereas isotactic and syndiotactic polypropylenes are solids and partially crystalline. Each of these microstructures can be produced in a
FIG. 15. Microstructures of polypropylenes of various tacticities; modified Fischer pro- jections.
OLEFIN POLYMERIZATION CATALYZED BY METALLOCENES 119 high purity by metallocene catalysis, whereas Ziegler–Natta catalysts give mixtures.
A single step of the polymerization is analogous to a diastereoselective synthesis. Thus, to achieve a certain level of chemical stereocontrol, chiral- ity of the catalytically active species is necessary. In metallocene catalysis, chirality may be associated with the transition metal, the ligand, or the grow- ing polymer chain (e.g., the terminal monomer unit). Therefore, two basic mechanisms of stereocontrol are possible (145, 146): (i) catalytic site con- trol (also referred to as enantiomorphic site control), which is associated with the chirality at the transition metal or the ligand; and (ii) chain-end control, which is caused by the chirality of the last inserted monomer unit.
These two mechanisms cause the formation of microstructures that may be described by different statistics; in catalytic site control, errors are corrected by the (nature (chirality) of the catalytic site (Bernoullian statistics), but chain-end controlled propagation is not capable of correcting the subse- quently inserted monomers after a monomer has been incorrectly inserted (Markovian statistics).
1. Atactic Polypropylenes
Atactic polypropylenes are produced in catalysis by C2v-symmetric metal- locenes that are achiral, such as Cp2MCl2or (Me2Si(FLu)2)ZrCl2. The only stereocontrol observed is both of the chain-end type and low because the chiral center of the terminal monomer unit of the growing chain is in the β position as a consequence of the 1,2 insertion of the monomers. A signif- icant influence on the tacticity is observed only at low temperatures, being much more pronounced for titanocenes and hafnocenes than zirconocenes as a consequence of their shorter M–Cαbonds, bringing the chiralβ-carbon closer to the active center (147, 148).
2. Isotactic Polypropylenes
The first metallocene/MAO catalysts for the isotactic polymerization of propylene were described in 1984. Ewen (146) found that Cp2TiPh2/MAO produced isotactic polypropylene at low temperatures by the chain-end con- trol mechanism (giving a stereoblock structure). When he used a mixture of racemic and meso (En(Ind)2)TiCl2 in combination with MAO, he ob- tained a mixture of isotactic and atactic polypropylene, with the isotactic polymer having a microstructure in accord with catalytic site control (an isoblock structure). The use of pure racemic (En(Ind)2)ZrCl2yielded the first pure isotactic polypropylene formed by metallocene/MAO catalysis (149–151). These investigations were the beginning of rapid development in the area of metallocene-catalyzed polymerization of propylene, resulting
in the invention of tailor-made metallocenes for different microstructures based on the mechanistic understanding of stereocontrol.
Depending on the structure of the metallocene, different polymer mi- crostructures are formed. Generally, among the rigid metallocenes, different structures may be distinguished (152, 153), and there are also metallocenes that have fluctuating structures.
Table VI provides a comparison of the catalytic activities of various zir- conocenes and one hafnocene measured under the same conditions, along with the product molecular weight, isotacticity, and melting point. The bisin- denyl compounds show high activity, but the activities of the hafnocene and the mixed (indenyl)cyclopentadienyl compounds are significantly lower.
The ligand in the bridge is also influencial. Replacement of the methyl group with a phenyl increases the activity, whereas replacement with benzyl
TABLE VI
Polymerization of Propylene with Metallocene Catalysts in Toluenea
Activity [kg PP/(moles of Molecular weight Isotacticity Catalyst metallocene×h×Cmonomer)]b [g(mol)−1] (mmmm%)
Cp2ZrCl2 140 2,000 7
(NmCp)2ZrCl2 170 3,000 23
(C5Me4Et)2ZrCl2 290 200 5
(O(SiMe2Cp)2)ZrCl2 230 300 24
(En(Ind)2)ZrCl2 1,690 32,000 91
(En(Ind)2)HfCl2 610 446,000 85
(En(2,4,7Me3Ind)2)ZrCl2 750 418,000 99
(En(IndH4)2)ZrCl2 1,220 24,000 90
(Me2Si(Ind)2)ZrCl2 1,940 79,000 96
(Ph2Si(Ind)2)ZrCl2 2,160 90,000 96
(Bz2Si(Ind)2)ZrCl2 270 72,000 86
(Me2Si(2,4,7Me-3Ind)2)ZrCl2 3,800 192,000 95
(Me2Si(IndH4)2)ZrCl2 7,700 44,000 95
(Me2Si(2Me-4,6iPr-2Ind)2)ZrCl2 6,100 380,000 98
(Me2Si(2Me-4PhInd)2)ZrCl2 15,000 650,000 99
(Me2Si(2Me-4,5BenzInd)2ZrCl2 6,100 380,000 98
(Ph2C(Ind)(Cp))ZrCl2 170 2,000 19
(Me2C(Ind)(Cp))ZrCl2 180 3,000 19
(Me2(Ind)(3MeCp))ZrCl2 400 4,000 34
(Ph2C(Fluo)(Cp))ZrCl2 1,980 729,000 0.4
(Me2C(Fluo)(Cp))ZrCl2 1,550 159,000 0.6
(Me2C(Fluo)(Cp))HfCl2 130 750,000 0.7
(Me2C(Fluo)(3-t-BuCp)ZrCl2 1,045 52,000 89
(Ph2SiH(2,3,5Me3-Cp)ZrCl3 150 5,600 6
aUnder the following conditions: temperature, 30◦C; propylene partial pressure, 2.5 bar;
metallocene concentration, 6.25×10−6mol/liter; MAO/metallocene molar ratio, 250 (24).
bCmonomeris monomer concentration (mol/liter).
OLEFIN POLYMERIZATION CATALYZED BY METALLOCENES 121 decreases it. The (indenyl)zirconocene is more active than the (tetrahy- droindenyl)zirconocene. The activity increases with increasing temperature.
The molecular weights of the polymers catalyzed by the Cs-symmetric com- pounds are high, especially when there are phenyl groups in the bridge.
Spaleck et al. (154) reported a large number of chiral zirconocenes with different bridges and substitutions on the indenyl ligand (Table VII).
Some C2-symmetric metallocenes give polypropylenes with a high melting point (162◦C) and tacticities (mmmm pentades) of 97–99%, measured by13C-NMR spectroscopy (155, 156).
Systematic investigations of bis(indenyl)zirconocenes as catalysts showed that the main chain termination reaction is β-hydrogen transfer with the monomer (157, 158). This reaction is very effectively suppressed by sub- stitutents (Me and Et) in position 2 of the indenyl ring (159, 160). Sub- stituents in position 4 also cause an enhancement in molecular weight by reducing 2,1 misinsertions, which preferentially result in chain termination byβ-hydrogen elimination. Because primary insertion is sterically hindered after a regioerror occurs, and therefore the catalyst is in a resting state after a 2,1 insertion, suppression of this type of misinsertion also leads to en- hanced activities. Aromatic substitutents in position 4 result in additional electronic effects. Thus, the most active catalysts incorporate a methyl or ethyl group in position 2 and an aromatic group in position 4 of the indenyl rings.
TABLE VII
Comparison of the Productivity, Molecular Weights, Melting Point, and Isotacticity Obtained in Polymerization Experiments with Various Zirconocene /MAO Catalystsa
Productivity 10−3×
[kg of PP (mmol molecular Melting Isotacticity Metallocene catalyst of Zr×h)−1] weight (g mol−1) point (◦C) (% mmmm)
(En(Ind)2)ZrCl2 188 24 132 78.5
(Me2Si(Ind)2)ZrCl2 190 36 137 81.7
(Me2Si(2Me-Ind)2)ZrCl2 99 195 145 88.5
(Me2Si(2Me-4iPr-Ind)2)ZrCl2 245 213 150 88.6
(Me2Si(2Me-4,5BenzInd)2)ZrCl2 403 330 146 88.7
(Me2Si(2Me-4Ph-Ind)2)ZrCl2 755 729 157 95.2
(Me2Ge(2Me-4Ph-Ind)2)ZrCl2 750 1135 158
(Me2Si(2Me-4Naph-Ind)2)ZrCl2 875 920 161 99.1
(Me2Si(2Et-4Naph-Ind)2)ZrCl2 825 990 162
(Me2C(Fluo)(Cp))ZrCl2 180 90 0.82
(Ph2C(Fluo)(Cp))ZrCl2 3138 478 133 0.87
aConditions: bulk polymerization in 1 liter of liquid propylene at 70◦C, Al /Zr molar ratio= 15000. The results illustrate the broad range of attainable product properties (155).
Besides the bis(indenyl) ansa compounds, C2-symmetric bridged bis (cyclopentadienyl) metallocenes of zirconium and hafnium were found to produce isotactic polypropylene (161). The keys to high isotacticity are sub- stituents in positions 2, 4, 3, and 5, which surround the transition metal similar to the one in bis(indenyl) metallocenes. In this type of metallocenes the chirality is a consequence of the chirality of the ligand, and the two chlorine atoms (e.g., the position of the growing chain and the coordinating monomer) are homotopic. According to a model of Pinoet al.(162–165), and Corradiniet al.(166–171), the conformation of the growing polymer chain is determined by the structure of the incoming monomer and is forced into a distinct orientation by steric interactions of its side chain with the polymer chain (Fig. 16) (the relative topicity of this reaction was found to be similar). As a consequence of the C2symmetry and the homotopicity of the coordination vacancies, isotactic polymer is produced (Fig. 17).
Isotactic polypropylene synthesized by metallocene catalysis has proper- ties different from those of conventional polypropylene produced by Ziegler–Natta catalysis. With approximately the same melting points and molecular weights, the metallocene PP shows a 30% higher E-modulus (Young’s modulus), a much more narrow molecular weight distribution, and only a 10th of the content of extractables, which could be important in pack- aging or medicinal applications. The differences arise from the single-site character of the metallocene catalysts relative to the multisite character of the Ziegler–Natta catalyst.
3. Syndiotactic Polypropylene
In 1988, Ewen and Razavi developed a catalyst for the syndiotactic poly- merization of propylene based on Cs-symmetric metallocenes (Table VI)
FIG. 16. Origin of the stereospecificity of C2 symmetric bis(indenyl) zirconocene catalysts.
The orientation shown on the right is favored over the one shown on the left because of nonbondig interaction of the approaching monomer and the ligand.
FIG.17.Mechanismoftheisotacticpolymerizationofpropyleneinvolvinganalkylzirconoceniumiongenerated fromaC2symmetricbis(indenyl)zirconocene.
123
(172, 173). The syndiotactic polypropylene has a melting point up to 133◦C (174). (Me2C(Flu)(Cp))ZrCl2and similar metallocenes, in combination with MAO or other perfluorinated borates, can produce chiral metallocenium ions in which chirality is centered at the transition metal. Because of the flip- ping of the polymer chain, the metallocene alternates between the two enan- tiomeric configurations and produces a syndiotactic polymer (172, 175). It had not previously been possible to produce such pure syndiotactic polymer.
Modifications of Cs-symmetric metallocenes may lead to C1-symmetric metallocenes (Fig. 8). If a methyl group is introduced at position 3 of the cyclopentadienyl ring, stereospecificity is disturbed at one of the reaction sites so that every second insertion is random; a hemiisotactic polymer is produced (176, 177). If steric hindrance is greater (e.g., if at-butyl group replaces the methyl group), stereoselectivity is inverted, and the metallocene catalyzes the production of isotactic polymers (178–180).
The microstructures described previously are associated with chain migra- tory insertion. Although in the case of a C2- or C2v-symmetric metallocene, as a consequence of the homotopic nature of the potentially active sites chain stationary insertion or migratory insertion followed by site isomerization would result in the same microstructure as chain migratory insertion, in the case of Cs-symmetric catalysts the result is isotactic blocks. C1-symmetric metallocenes are able to produce new microstructures if consecutive inser- tions take place on the same active site in addition to chain migratory in- sertion. Polypropylenes containing blocks of atactic and isotactic sequences are produced, with the block lengths depending on the rate of chain station- ary insertion or site isomerization vs chain migratory insertion (181–185).
Thus, hemiisotactic polypropylene represents a special case, having a chain stationary:chain migratory ratio of 1 : 1.
4. Stereoblock Polypropylenes
Polymers with atactic and isotactic blocks in the same chain are obtained if unbridged substituted metallocenes having a significant rotational iso- merization barrier are used as catalysts (Fig. 18). Early attempts focused on substituted cyclopentadienyl and indenyl compounds (186–188). Efforts by Coates and Waymouth (189) and others (190, 191) have shown that 2-phenylindenyl groups are well suited for this purpose. They oscillate be- tween the enantiomeric and meso arrangements, giving rise to a stereoblock polypropylene containing atactic (produced by the meso rotamer) and iso- tactic (produced by the chiral rotamer) sequences. The block length is strongly dependent on the temperature. The polymer has elastomeric prop- erties. Only metallocenes offer the possibility for tailoring the microstructure
OLEFIN POLYMERIZATION CATALYZED BY METALLOCENES 125
FIG. 18. Oscillating metallocene; by rotation of the cyclopentadienyl rings, the metallocene epimerizes.
of the polypropylenes by changing the ligands in the aromatic rings of the sandwich compound catalysts.