Catalytic Polymerization of Olefins and Acetylenes

Principles and Applications of Organotransition Metal Chemistry, Collman J.P., Hegedus L.S., Norton J.R. and Finke R.G., University Science Books

Catalytic Polymerization of Olefins and Acetylenes

11.1 Introduction

This chapter is concerned with polymerizations and oligomerizations of olefins and acetylenes, specifically those reactions which are catalyzed by transition -metal compounds. Both soluble and insoluble catalysts are important. We will focus on the homogeneous catalysts, since heterogeneous catalysts are less well-understood and are not a major thesis of this text.

Many familiar articles of commerce are derived from these important carbon-carbon-bond forming reactions. For example, polyethylene and polypropylene are produced in huge quantities (several million tons per year) by these catalysts. The most desirable physical properties of these polymers are related to their morphology, which has its roots in the stereoregular nature and molecular- weight range of these substances.

Measured by man-years of research and the number of books, papers, and patents, this chemistry is truly immense. It is furthermore controversial, especially with respect to the mechanism(s) of the carbon-carbon bond-forming polymer chain propagation step. Details of industrial processes are shrouded in mystery as trade secrets. In view of this complexity, we will present only a few principal examples, and our discussion of reaction mechanisms will not be comprehensive. Important reviews are available [1-6] and should be consulted for more detail.

These polymerization processes had their origin in the pioneering work of Ziegler and Natta; for their discoveries they were awarded the 1963 Nobel prize in chemistry.

11.2 Mechanism(s) of Monoene Polymerization/Oligomerization

The polymerization or oligomerization of simple monoenes involves three basic steps: (a) initiation, (b) propagation, and (c) termination (Equation 11.1). It is generally agreed that the catalytically active center at which polymerization or oligomerization occurs is a transition metal alkyl complex.

The initiation step (a) usually involves the generation of an unsaturated transition metal alkyl L_nM-R, often by reaction between a transition- metal halide and an electropositive alkyl such as AlR₃. Alternatively, an unsaturated hydride, L_nM-H, is first generated by an appropriate reducing agent; subsequent insertion of the alkene into this hydride then affords the catalyst center, L_nM-R.

There are two steps in the insertion reaction (b in Equation 11.1): olefin coordination, and the carbon-carbon bond-forming reaction. A vacant site is required for the olefin to coordinate before alkyl insertion can take place. For an effective polymerization the olefin must be a good ligand; among simple monoenes these polymerization reactions are usually limited to ethylene and other unhindered terminal olefins (ethylene > propylene > 1-butene). Internal olefins are usually not polymerized by these catalysts. Better binding olefins are competitive inhibitors of more weakly binding olefins. Many polymerization catalysts are notoriously sensitive to poisoning by impurities which act as competing ligands.

The second stage of the propagation process is controversial. The simplest possibility for this carbon-carbon bond-forming step is a migratory insertion of the coordinated alkyl to the coordinated olefinic substrate (“Cossee mechanism”) [7]. The regiochemistry and stereochemistry of this irreversible insertion control the stereoregular nature of the growing polymer chain. The overall rate of step (b) is important in controlling the molecular weight of the polymer; for high-molecular-weight polymers this step is typically very fast [2,8], often in the enzymatic range (as high as 10,000 turnovers per catalyst site per second!). These very fast rates complicate kinetic studies of polymer chain growth, since monomer access to the active site often becomes diffusion- limited as the active metal-alkyl site becomes enmeshed in the polymer chain [2].

For the chain termination step (c) there are several possibilities [6]. The simplest is the ubiquitous b-hydride elimination, which regenerates the starting hydride and produces a polymer or oligomer having a terminal olefin (Equation 11.2). Alternatively, the polymer chain can be terminated by the addition of hydrogen (hydrogenolysis of the metal-carbon bond; Equation 11.3). This process, which is sometimes used to control the molecular weight range, affords a saturated polymer [3].

A third type of polymer termination step, like b-hydride elimination, is intrinsic. This involves transfer of a b-hydride between the metal alkyl and the coordinated olefin monomer (Equation 11.4) [6]. In certain cases this step becomes more important with increasing temperature. Other chain transfer agents, such as metal alkyls, hydrogen halides, or halogenated hydrocarbons, may terminate the growing polymer chain.

The relative rates of the propagation and the termination steps, k_p and k_trespectively, determine the molecular weight of the product. Three situations arise. In the first case, k_p is much greater than k_t; high- molecular- weight polymers are formed. In the second case, k_p and k_t are similar; oligomers with a geometric molecular weight distribution (“Schultz/Flory” type) are formed. In the third case, k_t is much larger than

k_p; this results in the exclusive formation of dimers.

High- molecular- weight polymers are typically formed from catalysts derived from the earlier transition metals (groups IV, V, and VI); catalysts derived from group VIII metals favor b-hydride elimination and give oligomers or dimers [1]. Some modification of the k_p/k_t ratio can be achieved by changing the nature of the ligands. Donor ligands are said to increase and acceptor ligands to decrease the relative rate of chain propagation. With a given metal, increasing oxidation state is reported to decrease chain propagation [1].

It is usually considered that heterogeneous and homogeneous olefin polymerization catalysts function by similar mechanisms [3]; since the former are more difficult to investigate this premise remains speculative. Typically a relatively small number of active catalyst sites exist on the surface of heterogeneous catalysts; however, greater stereoregular control, especially for isotactic polymers, can be achieved with heterogeneous catalysts [2].

11.3 The Mechanism of the Chain Propagation Step

The carbon-carbon bond-forming, chain-propagation step in metal-catalyzed olefin polymerizations has long been controversial. From a range of proposals, two major pathways have been advanced: the four-center "migratory insertion" (Cossee [7]) and the carbene-to-metallacycle mechanism (Green [9]). These are shown as paths (a) and (b) respectively in Figure 11.1; both involve, at one point, an unsaturated metal alkyl. For most of the discussion in this chapter we have used the traditional insertion mechanism, which is better-supported by various model reactions. It is important to recognize that, for a particular catalyst or set of reaction conditions, either mechanism (or even some other pathway) might prevail.

The direct alkyl-olefin migratory insertion in the Cossee mechanism began to be questioned when it was recognized that there Eire many complexes in which olefin and alkyl ligands are cis to one another without undergoing insertion [11]. The first direct experimental model for an alkyl-olefin migratory insertion path was reported by Watson (101). Her results are summarized in Equation 11.5. A series of isotopic labeling NMR studies clearly established that a regiospecific alkyl-olefin insertion takes place, followed by two competing reactions: further olefin oligomerization, and b-hydride elimination. Note, however, that Green's metallacycle mechanism [9] requires a formal oxidative addition at the metal center [10]. The lutetium(III) complex could not readily undergo such a two-electron redox reaction.

Using an isotopically labeled, chelated (2,2-dimethyl-4-pentene-1-yl) platinum(II) complex, Flood was able to observe a reversible alkyl-alkene insertion (Equation 11.6) [11]. At 125º in CD₃NO₂ the rearrangement exhibits reversible first-order kinetics. This system is cleverly contrived to avoid competing b-hydride elimination. The intermediate cyclobutylcarbynyl derivative 1 is thermodynamically less stable and is not observed.

An important difference between the two suggested pathways is the involvement of an a-hydrogen migration in Green's carbene mechanism. A primary kinetic isotope effect (k_H/k_D) of about 3 is expected for this carbene-forming step. In view of this distinction, Grubbs [12] looked for a kinetic isotope effect in the Ziegler- Natta- type ethylene, perdeuteroethylene copolymerization shown in Equation 11.7. By determining the average number of deuterated units in the polymer chain, a precise isotope effect was measured for this reaction; k_H/k_D was found to be 1.04 ± 0.03. As Grubbs states, "these data strongly support an insertion mechanism that does not involve a hydrogen migration during the rate-determining step of propagation" [12]. However, this null result is in effect a "negative experiment" and is therefore not fully decisive.

Grubbs then devised a cunning stereochemical isotope experiment. The experiment shown in Figure 11.2 probes for an isotope effect on the stereochemistry of olefin insertion by employing a titanium alkyl, 2, with a pendant olefin. Insertion of an a-olefin into a metal-carbon bond which is monodeuterated at the a-position generates diastereomers, as shown in Figure 11.2. If prior to insertion activation of an a-hydrogen occurs (Green mechanism), the symmetry of the CHD would be broken and the ratio of diastereomers should reflect the ratio of C-H to C-D activation. However, direct migratory insertion should show no isotope effect. None was found. Using ²H NMR the ratio of cis/trans diastereomers in the organometallic and acid-quenched organic products was found to be 1.00 ± 0.005 [13a].

This system is further instructive in that addition of the Lewis acid cocatalyst EtAlCl₂ is necessary to induce insertion (as well as polymerization with added ethylene). Bipyridine quenches these reactions by sequestering the Lewis acid cocatalyst. The cationic complex Cp₂Zr(CH)₃(THF)⁺ polymerizes ethylene without a Lewis acid cocatalyst [13b].

Another experimental result is inconsistent with Green's metallacyclobutane hypothesis. When a ¹³C-enriched methyl complex was used as a catalyst, ¹³C NMR showed that none of the ¹³C label was in a methylene unit at the polymer terminus (Equation 11.8) [14].

There is at least one model system which demonstrates that the Green mechanism can polymerize olefins. Schrock [15] showed that a tantalum complex 3, which undergoes reversible insertion of its carbene ligand to form an alkyl complex and which shows evidence for metallacyclobutane formation, will produce a "living polymer" in the presence of ethylene (Equation 11.9).

Brookhart [16] has proposed that, whichever propagation mechanism is involved, there exists a parallel between “agostic” hydride-olefin complexes [17] and analogous alkyl-olefin complexes which should readily undergo the chain propagation reaction. The argument is schematically outlined in Figure 11.3. The agostic structure (4 in Figure 11.3) has a three-center, two-electron M-H-C bond, rather than the terminal hydride structure. This is an “arrested” insertion. Analogues of such systems in which an alkyl is in place of the hydride (5 in Figure 11.3) are proposed to have low activation energies for alkyl-olefin insertion and thus to be promising candidates for catalytic olefin polymerization. Brookhart [16] has illustrated this point by preparing complexes of the type (C₅Me₅)(P(OMe)₃)CoR⁺, which were shown to form living polymers upon reaction with ethylene.

Finally, it should be mentioned that three other metal-catalyzed carbon-carbon bond steps are well-documented for olefin polymerization or dimerization. These are metathesis (of cyclic olefins), electrophilic activation, and electrocyclic formation of metallacyclopentane intermediates. Each is discussed in a later section.

11.4 Polyethylene

Polyethylene is the largest volume plastic in the United States. There are two basic varieties of polyethylene: high-density (a linear polymer with a density of about 0.95 and a melting point around 135º), and low-density (a branched polymer with a density of about 0.92 and a wide melting point range) [3]. The high-density form is made by Ziegler catalysts. Originally, the low-density form was prepared by free-radical polymerization of ethylene under high pressures (up to 50,000 psi and 350º); however, Union Carbide has introduced a very efficient low-pressure (100-300 psi at < 100º) process using a coordination compound as the catalyst [3].

The “Ziegler” catalysts are typically prepared by combining titanium chlorides with an aluminum alkyl cocatalyst. The active catalyst site is proposed to be a titanium alkyl complexed with an aluminum derivative. Most of the current high-density polyethylene manufactured in the U.S. employs chromium complexes which are chemically bound to high-density supports such as silica. These catalysts may be prepared by several different procedures. An interesting example involves reaction between chromocene and silica (Equation 11.10). Similar catalysts are generated by reducing Cr(IV) sites with H₂ or CO. In these catalysts a chromium hydride such as 6 is speculated to be the initiation site. Polymer growth ensues by the reactions analogous to those mentioned earlier (Equation 11.11).

Chain growth can be terminated in two main ways. The metal-alkyl bond can be hydrogenolyzed, or b-hydride elimination can be induced by heating [3]. In either case a metal hydride site, capable of initiating a new polymer chain, is formed (Equation 11.12). Hydrogenolysis is a useful method for obtaining a narrow molecular-weight range and for removing the polymer from the silica support; this method is much more facile for Ti than for Cr catalysts. The b-hydride elimination method generates a terminal olefin which can reassociate with a growing polyethylene polymerization site. This results in branched, low-density polyethylene.

All commercial polyethylene (and polypropylene) polymerization catalysts are either heterogeneous or colloidal [2,3]. Removal of the catalyst from the polymer is expensive; it is undesirable to leave expensive or corrosive chloride ion catalyst components in the finished polymer. This problem is usually solved by employing extremely active, efficient catalysts such that >10⁴ kg of polyethylene (or polypropylene) are formed per g of catalyst [3,6]. In these circumstances the catalyst is left in the polymer as a trace component. Note that the silica-chromium catalysts do not have halide present, in contrast to the traditional Ziegler catalysts.

11.5 Polypropylene

The physical properties of polypropylene which are important in its technological applications are very sensitive to the stereoregularity and molecular weight of this polymer. The most desirable polymer has a regular head-to-tail sequence and is highly stereoregular (isotactic) [2,3].

Three types of polypropylene are recognized [3-5]: isotactic, 7, syndiotactic, 8, and atactic, 9. All are “head- to-tail” polymers. However, they have different relative orientations of the chiral carbons which occur at alternate positions along the polymer chain (Figure 11.4).

Idealized isotactic polypropylene, 7, has the same relative configurations at all of the chiral methine carbon centers. These are “pseudo asymmetric centers.” In actual practice, a few centers have the opposite handedness. This situation results in a crystalline, coiled structure which has each methyl group oriented along the outer surface of a helical coil. Typically, equal numbers of left and right-handed polymer coils are formed. These giant isotactic molecules are enantiomorphic coils. The isotactic polymer 7 is commercially the most important form of polypropylene; it is less soluble, higher melting, and greater in tensile strength than the other forms [2].

The other stereoregular “syndiotactic’ form, 8, has alternating configurations of adjacent chiral centers along the polymer chain; this substance is comprised of giant meso molecules. This syndiotactic form is lower melting and is not important commercially.

Atactic polypropylene 9 has a random orientation of the chiral centers along the chain; it is amorphous, low-melting, and commercially undesirable.

At present, all commercial polypropylene catalysts are heterogeneous [3]. Until recently, polypropylene with high degrees of isotactic structure could be obtained only by using heterogeneous Ziegler-Natta catalysts. Many studies of this stereospecific polymerization have been carried out. These results are consistent with a mechanism whereby polymerization occurs through insertion of a transition- metal alkyl into a cis-coordinated propylene at a chiral metal site [4]. Since the monomer propylene is prochiral, its complex with a chiral metal site should afford either of two diastereoisomers, which, of course, have different free energies. The more stable diastereoisomer is preferentially formed. An admittedly speculative description of this process is shown in Equation 11.13 where the metal center is chiral. Insertion of the alkyl into the olefin by the Cossee mechanism is proposed to take place in both a stereospecific (cis) and a regiospecific manner. This process generates a single (R or S) chiral center and a head-to-tail polymer. Subsequent insertions afford the same-handed center as the polymer chain grows. Each insertion is thought to form a primary metal-alkyl bond, engendering the head-to-tail structure. Note that these catalysts have equal numbers of R and S handed sites, so that the resulting isotactic polymer is a racemic mixture of left- and right-handed coils.

The stereospecificity of isotactic polypropylene catalysts is associated with details of the surface structure [2]. For instance, a-TiCl₃ produces polypropylene with high degrees of isotactic stereoregularity, but b-TiCl₃, which is also a very active polymerization catalyst for both propylene and ethylene, shows much less tendency to form isotactic polypropylene.

The above mechanism requires that the chirality of the centers in an isotactic polymer chain be controlled by the handedness of the active site, not by the configuration of the carbon atoms in the growing polymer chain [4,6]. There are several lines of evidence which support this hypothesis. For example, when a “mistake” occurs during polymer growth, the polymerization process soon returns to the original handedness; that is, the polymerization site is “self-correcting.” Furthermore, the insertion of ethylene units into a growing isotactic polypropylene chain does not affect the stereospecificity of the polypropylene sections.

The details of syndiotactic polypropylene formation are less certain [4,18]. The insertion of the alkyl has been proposed to, be stereospecifically cis, but the regiospecificity of the insertion is suggested to be the opposite of that of isotactic polypropylene; the growing chain is proposed to be a secondary metal-carbon bond (Equation 11.14). This regiospecificity decreases with increasing temperature.

Here steric control in the formation of each new chiral center is thought to arise from the chirality of the carbon atom in the last inserted propylene unit. This has been achieved with a soluble vanadium catalyst [18]. Syndiotactic polymers are not entirely regiospecific [6].

Until about 1984 homogeneous catalysts afforded the random atactic and occasionally syndiotactic rather than isotactic polypropylene. This situation has now changed [19]. A catalyst derived from a chiral ethylene-linked bis-indenyl zirconium dichloride 10 in combination with the cyclic oligomer, methylalumoxane ([Al(CH₃)O]_n), cocatalyst 11 affords highly isotactic polypropylene [8]. Since the racemate of 10 was used for this polymerization (Equation 11.15), racemic isotactic polypropylene was obtained. The activity of this homogeneous catalyst is very high, nearly as great as heterogeneous catalysts. A high molecular weight and a narrow molecular -weight distribution were obtained [8]. The high degree of isotacticity (>95%) was measured by high-resolution ¹³C NMR, which is exceptionally useful for deducing the pattern of stereochemical sequences in polypropylene. This discovery of a stereospecific homogeneous catalyst may eventually lead to a detailed mechanistic understanding of the stereospecific Ziegler-Natta polymerization and perhaps to a facile method for preparing chiral polyolefins. The same catalyst yields isotactic polybutene. The meso isomer of the titanium analogue of 10 affords a nearly ideal atactic polypropylene [19]. This result is consistent with the concept that coordination of the prochiral propylene monomer with a meso metal site could not result in diastereomers; thus there could be no stereochemical control of the polymerization by an achiral propagation center.

11.6 Ethylene- Propylene Copolymers

In contrast to the homopolymers, polyethylene and polypropylene, which are plastics, certain ethylene- propylene copolymers are elastic and can be used in place of rubber [3][21]. Most commercial ethylene- propylene elastomers are made with Ziegler catalysts derived from soluble vanadium compounds (for example, VOCl₃ + Al₂Et₃Cl₃ in heptane). A small amount of a diene such as dicyclopentadiene is usually incorporated into the polymer chain. This remaining less-reactive olefin is subsequently cross-linked by vulcanization with sulfur. The final, “curled” elastomer is free of double bonds and is thus more resistant than natural rubber to oxidative degradation, for example, with ozone.

The active catalyst site, which is thought to be a V(III) alkyl, is not very selective. Propylene insertions occur with both regiochemistries and a random sequence of the two principal comonomers is formed. Such a random microstructure is important for desired elastomeric properties. These monomers do manifest the different reactivities expected from their equilibrium binding affinities (ethylene > propylene > dicyclopentadiene). Recall that ideally they should be competitive inhibitors of one another as they compete for incorporation into the catalyst site. Their relative proportions in the polymer must therefore be controlled by controlling the relative concentration of these monomers during the polymerization.

11.7 Ring-Opening Olefin-Metathesis Polymerization

Cyclic olefins can be polymerized by olefin metathesis catalysts. This process involves simple ring opening by the chain-carrying metal-carbene metathesis catalysts using the mechanism discussed in Section 9.2. This sort of polymerization has been reviewed [22]. These metathesis/cyclic olefin reactions are far less well-studied than traditional Ziegler-Natta polymerization. Recent results illustrate the rich potential of metathesis polymerization.

Depending on the particular catalyst employed, high degrees of selectivity can be achieved. There are firm rules to predict the outcome of such reactions. For example, cyclopentene can be polymerized either to the all cis or to the all trans polymer (Equation 11.16). The latter compound is an elastomer and has commercial promise.

With the exception of unstrained cyclohexenes, most unsubstituted cyclic olefins can be polymerized by metathesis catalysts. Strained olefins are especially reactive (Section 9.1d). Thus, 1-methylcyclobutene reacts (Equation 11.17), even though trisubstituted olefins usually are unreactive [22,23].

The polymerization of norbornene in the presence of a titanium- carbene precursor (Equation 11.18) has been thoroughly studied by Grubbs [24], who showed that a “living polymer” is present. This permits the molecular weight to be controlled. Note that the cis orientation at the bridgehead carbons is retained in the polymer; however, a mixture of cis/trans olefin isomers is obtained.

A very interesting application of this chemistry involves the polymerization of 3,4-diisopropylidenecyclobutene, 12, using titanocene methylene precursors as the catalyst [25]. As shown in Equation 11.19, only the strained cyclic olefin reacts, affording the first known cross-conjugated polymer, 13. Oxidative "doping" of this polymer with iodine affords an unusual highly conductive molecular metal. The living polymer is quenched with methanol.

It should be noted that many olefin metathesis catalysts are inhibited by polar functional groups. Ring-opening polymerization of unsaturated lactones have been successfully carried out (Equation 11.20) [26].

11.8 Metal-Catalyzed Electrophilic Polymerizations

A classical method for polymerizing olefins involves cationic initiators, for example, from a proton source. Cationic polymerization of olefins and acetylenes may also be effected by the very electrophilic palladium complex, [Pd(CH₃CN)₄](BF₄)₂ [27].

An example is shown in Equation 11.21. Electron- deficient olefins and acetylenes do not polymerize with this catalyst. Both phosphines and halide ions deactivate this catalyst system. This chemistry takes advantage of the electrophilic polarization of coordinated olefins by Pd²⁺ (see Chapters 3 and 7).

A similar cationic palladium complex catalyzes the cooligomerization of CO and ethylene, affording a poly-1,4-diketone (Equation 11.22) [28]. Note that phosphine ligands are present and do not appear to deactivate this catalyst, but added halide ions do suppress this reaction. A plausible mechanism for this reaction is shown in Equation 11.23. An alkyl-palladium intermediate is thought to be generated in an unspecified reaction. Carbon monoxide insertion forms a reactive acyl which readily inserts ethylene but not CO (recall from Section 6.lg that acyls are resistant to CO migratory insertions). The polymer chain can be terminated by methanol, which forms a methyl ester by reaction with the palladium acyl (Section7.3). Note that ethylene insertion into a Pd-acyl bond is much more facile than simple ethylene polymerization. In this way a 1:1 copolymer is generated.

11.9 Oligomerization of Alkenes

As mentioned earlier, olefin oligomerization reactions occur by mechanisms similar to polymerization. The difference is that with oligomerization catalysts the chain termination step, b-hydride elimination, is faster. Both Ziegler-type and soluble late-transition-metal compounds (hydride precursors) can be employed as catalysts, These oligomerization reactions have been extensively reviewed [1]. The olefin reactivity ratios follow the relative coordination affinities for the catalytic center: ethylene >> propene > n-butene > n-pentene. Only a few branched or internal olefins are readily oligomerized. The commercially most interesting substrate is ethylene. This is discussed in the following section. For unsymmetric olefins the products are usually branched.

a. Shell Higher Olefin Process. The first stage in the "Shell Higher Olefin Process" (SHOP) [29,30] involves the oligomerization of ethylene by homogeneous nickel catalysts (Equation 11.24) [31]. The active catalyst is believed to be a nickel hydride which is generated in situ from precursors such as 14 and 15 (Equation 11.25). This catalyst is extremely selective for ethylene in the presence of other olefins. This selectivity, which may be related to the unusual bidentate phosphine carboxylate ligand, explains the formation of a statistical (Flory-Schultz) mixture of pure, linear a-olefins. A plausible mechanism for this catalytic oligomerization is shown in Figure 11.5. This catalytic reaction follows Michaelis -Menten saturation kinetics, from which the ethylene catalyst binding constant and the turnover frequency of the rate-limiting, chain-propagation step (0.8 sec^-1 at 75º) were determined [30]. The chain-termination step is b-hydride elimination. The average chain length can be influenced by added ligands such as tertiary phosphines.

The commercial SHOP technology involves additional stages [3]. Distillation separates the a-olefins (Equation 11.26) in three ranges: C₄-C₈, C₁₀-C₁₈ (which are marketed as such), and >C₁₉. The separate lighter and heavier fractions are isomerized to a mixture of internal olefins over a heterogeneous catalyst. The lighter and heavier fractions of the resulting internal olefins are then recombined over a heterogeneous metathesis catalyst (MoO₃/Al₂O₃) (Equation 11.27). The resulting product contains linear internal olefins in the C₁₀-C₁₈ range. These are then, hydroformylated (Equation 11.28), using a cobalt catalyst which simultaneously isomerizes the internal double bonds (see Section 12.3a for details). The resulting C₁₁-C₁₉ primary alcohols are used as plasticizers ("fatty alcohols"). This process thus converts inexpensive ethylene into valuable a-olefins and primary alcohols. A similar process uses AlEt₃ to oligomerize ethylene into a statistical mixture of a-olefins [3]. Since transition metals are not involved, this process is out of the scope of this text.

b. Dimerization of Alkenes by Insertion into Metal-Carbon Bonds. Two major mechanisms, the traditional metal-alkyl olefin insertion and a metallacyclic process, are recognized for alkene dimerization. The former involves the same catalytic cycle as olefin polymerization, except that the rate of b-hydride chain termination is much greater than the chain- propagation step. This ensures that dimers; are formed nearly exclusively. This subject has also been reviewed [1].

Ethylene dimerization to butenes is catalyzed by many transition- metal compounds, especially those of group VIII metals. These reactions are not commercially interesting, because butenes are generally cheaper than ethylene [3].

Propylene dimerization usually affords a mixture of C₆ products which are the result of different competing regioselective insertion steps. Four skeletal isomers of the intermediate metal alkyl can arise from the two different directions of M-H insertion, followed by two different modes of M-R insertion (Equation 11.29). Both types of insertion can yield isomers.

An example of this is the dimerization of propylene with Ziegler-type nickel catalysts (Equation 11.30) [3]. The distribution of isomeric dimers depends on the nature of the phosphine used with this catalyst. Bulky phosphines yield more branched-chain olefins. Note that these products are formed as a mixture of' olefinic isomers. This ligand effect is largely due to steric control of the second propene insertion, rather than the initial hydride insertion. These reactions can be extremely fast. For example, a complex of NiBr(h³-C₃H₅)(PCy₃) and EtAlCl₂ in chlorobenzene at 25º catalyzes propene dimerization at a rate of 60,000 turnovers per second [32].

Catalytic dimerization of substituted olefins has been observed [1]. Such reactions offer an interesting route to polymer intermediates. An example is the palladium-catalyzed dimerization of methyl acrylate to a predominantly linear dimer (Equation 11.31) [33].

Asymmetric catalytic codimerization of two different olefins has been observed in several instances [34]. A particularly notable example, discovered by Bogdanovic and Wilke [35], is the codimerization of ethylene and norbornene catalyzed by a (p-allyl)nickel Ziegler-type catalyst (Equation 11.32). In the presence of chiral phosphines (L*) such as dimenthyl(isopropyl)phosphine, up to 80% ee was achieved at low temperatures. Such reactions have not been very widely studied, in spite of the fact that these offer attractive chiral building blocks for pharmaceutical manufacture.

c. Dimerization of Alkenes by a Metallacyclic Mechanism. Certain low-valent early - transition- metal complexes dimerize propene selectively to 2,3-dimethyl-1-butene. Such selective dimerization signals a completely different mechanism, one involving a metallacyclic intermediate (Equation 11.33) [36, 37]. This high head-to-head selectivity occurs because of the predominant formation of a metallacyclopentane having less hindered primary metal-carbon bonds. On the other hand, if the methyl groups are replaced by bulky neopentyl substituents, head-to-tail dimerization is observed, since two very bulky alkyl groups cannot be placed on adjacent carbons [38]. These metallacyclic intermediates have been characterized spectroscopically. As discussed in Section 9.4a, such metallacyclopentanes result from the electrocyclization of two olefins on certain reactive metal centers in a sort of oxidative addition process. The metallacycle probably breaks down to the olefin by a two-step process: b-hydride elimination followed by reductive elimination (Section 9.4b). This metallacyclic olefin dimerization mechanism is probably fairly common for non-Ziegler-type transition-metal catalyst systems having no aluminum cocatalyst.

11.10 Oligomerization and Polymerization of Conjugated Dienes [39]

Transition-metal-catalyzed oligomerization and polymerization of' conjugated dienes is an industrially important process for the preparation of materials ranging from synthetic rubbers (e.g., all-cis polybutadiene and polyisoprene) to raw materials for the production of nylon (e.g., cyclododecatriene). Two classes of catalysts are used to effect this process: Co-based and Ti-based Ziegler-type catalysts (see above), and (p-allyl)nickel and palladium catalysts. Although the former are more extensively used (at least for polymerization), the latter are much better understood mechanistically, and will form the basis of this section.

Regardless of catalyst, virtually all polymerizations and oligomerizations of conjugated dienes are thought to proceed through (h³-allyl)metal complex intermediates. These h³-allyl complexes can be formed from conjugated dienes in two ways. In catalyst precursors involving metal hydrides or metal alkyls, h³-allyl complexes can be produced by diene insertion into a metal-hydrogen or metal-alkyl bond (Equation 11.34). The equilibrium between the h³ and h¹ forms is strongly dependent on the nature of the other ligands, and is a major factor in determining the further course of the reaction (see below).

Alternatively, many low-valent metals, particularly Ni(0) and Pd(0), can reductively dimerize butadiene to the corresponding bis-(h³-allyl) complex (Equation 11.35). This process is important in many cyclooligomerization reactions of dienes (see below). Further insertion reactions of these h³-allyl intermediates with more diene or monoene results in the oligomerization and polymerization reactions discussed in this section.

a. Polymerization of 1,3-Dienes. Ziegler-type catalysts are most commonly used for the commercial production of polybutadiene and polyisoprene. The selectivity of the process is strongly dependent on the particular transition- metal catalyst used (Table 11.1) [40]. Although these differences in reactivity have not been rationalized, it is evident that (h³-allyl)metal complexes are the active species, and that the structure of the product is determined by the stereochemistry of the butadiene insertion step. A general mechanism is shown in Figure 11.6 [41]. The key insertion step is illustrated as if it occurs from the h¹-allyl complex for purposes of clarity, although direct insertion from the h³-allyl complex is equally likely.

Table 11.1 Stereoselective Polymerization of Butadiene

Catalyst	trans-1,4	cis- 1,4	1,2
CoCl₂ / AlEt₂Cl	1	98	1
MoO₂(OR)₂ / AlEt₃	1	3-6	92-96 (syndiotactic)
Cr(acac)₃ / AlEt₃	1-2	0-3	97-99 (isotactic)
VC1₃(THF)₃ / AlEt₂Cl	99	0	1

Nickel catalysts [42] are used less frequently than Ziegler-type catalysts for the polymerization of conjugated dienes, but much more is known about these catalysts because of extensive studies, especially by Wilke and coworkers [42b]. Preformed (h³-allyl)nickel complexes are themselves active catalysts for this process, but more commonly the active catalyst is formed in situ from a nickel(II) salt and an alkylaluminum reagent. Again, polymerization occurs by insertion of butadiene into the (h³-allyl)nickel complex, and the stereochemistry of the product is determined by the geometry (syn or anti) of the active h³-allyl complex (Equation 11.36).

Polymerization of other 1,3-dienes, and copolymerization of different 1,3-dienes, proceed by similar mechanisms.

b. Oligomerization and Cooligomerization of Conjugated Dienes. The catalytic dimerization and oligomerization of 1,3-dienes can lead to a very large number of linear or cyclic products, by mechanistically very similar paths. The specific (major) product formed depends strongly on both the metal and the auxiliary ligands, and much is known about controlling the product distribution (see below). In this section, a few mechanistically well -characterized systems will be discussed. For more details see reviews on the subject [39].

Linear Oligomerization of Dienes. Although many catalyst systems effect the linear dimerization of butadiene, one of the best-understood and most specific systems is the nickel(0)-triethylphosphite-morpholine catalyst developed by Heimbach [43] (Figure 11.7). Reductive dimerization of butadiene by Ni(0) produces the bis-(h³-allyl) nickel complex. Protonation of an internal position of one of the h³-allyl groups by morpholine, followed by proton abstraction a to the other h³-allyl group, produces the observed butadiene dimer and regenerates the catalyst.

With unsymmetric dienes such as isoprene, four different dimers (head-head, head-tail, tail-head, tail-tail) are possible, and the trick is to maximize production of the desired one (usually head-to-tail to produce acyclic terpenes). Appropriate palladium complexes in the presence of formic acid (a hydride source) catalyze such a process (Figure 11.8). Again h³-allyl complexes are key intermediates in this system, although they are formed by Pd-H addition to the diene rather than by reductive diene dimerization. Note that insertion of the more substituted double bond of isoprene into the Pd-H bond, and of the less -substituted double bond into the Pd-C bond, are required to give the observed products. The reasons for the specificity of this system are unclear at the present time.

In the absence of added ligands, many palladium catalysts convert butadiene into linear trimer dodecatetraenes (Figure 11.9). Excellent evidence for the mechanism shown has been obtained by Jolly [44], who was able to isolate and characterize complex 17 and the diphos analogue of 18, and to show that these convert to dodecatetraenes. In the presence of added ligands only dimers are obtained, probably because the ligand prevents coordination of the third diene necessary to produce trimers.

Alternatively, Wilke has proposed a bimetallic mechanism (Figure 11.17) based on the observation that {Ni(COT)}₂ is itself a catalyst for this process and on structural studies from related chromium complex chemistry [56]. In this process, two adjacent nickelacyclopentadiene moieties, stabilized by coordination to cyclooctatetraene, combine to form a dinickel species by coupling of their respective C₄ units. The C₈ chain in this species is bonded in the manner observed for the chromium species just mentioned, [Cr(h-C₅H₅)]₂(C₈H₈). Coupling of the two ends of the C₈ chain produces {Ni(COT)}₂, which reenters the cycle.

Although this discussion has been restricted to “self” or “homo”" oligomerization processes, alkynes cooligomerize with different alkynes, as well as with 1,3-dienes and olefins. The number of different possible products is truly staggering; however the processes by which they form are likely to be simple combinations of the stepwise processes discussed in this chapter.

Notes

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F.G.A.; Abel, E.W., Eds.; Pergamon: New York, 1982; Chapter 52, p 371.

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5. Boor, J. Ziegler-Natta Catalysis and Polymerizations; Academic: New York, 1979.

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507.

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