Polymerizations with Coordination Catalysts

Polymerizations of many internal olefins, like 2-butene, 2-pentene, 3-heptene, 4-methyl-2-pen-tene, 4-phenyl-2-butene, and others, with Ziegler-Natta catalysts, are accompanied by monomer rearrangements. The isomerizations take place before insertions into the chains. The double bonds migrate from the internal to the a-positions  

The products consist exclusively of poly(l-olefin) units. Polymerizations of 2,3 and 4 octenes with TiCl3/Al(C2H5)3 at 80 C, for instance, result in the same high molecular weight homopolymer, poly(l-octene).  

Recently, it was reported that addition of some transition metal compounds of Group Vm to Ziegler-Natta catalysts enhances isomerization of 2-olefins. When nickel compounds are added to llCl3-(ci5)3Al, they react and form IlCl3-NiX2-(C2H5)3Al. X can be a chloride, an acetylacetonate, or a dimethyl glyoximate. The product is an efficient isomerization catalyst. 

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Mathematical models for olefin polymerization with coordination catalysts are usually classified into microscale. 

The kinetic steps shown in Tables 5.2 and 5.3 are widely accepted and are commonly used to develop polyolefin microstructural models, but they do not describe all aspects of olefin polymerization with coordination catalysts ... 

Alternatively, the complete population balance can be solved dynamically using efficient ODE solvers [70, 71], The versatile commercial software PREDICI can solve population balances that describe polymerizations with coordination catalysts and many other polymerization mechanisms [72]. In this approach, the complete microstructural distributions are modeled, leading to a detailed description of the polymer microstructure. 

Figure 2.19 illustrates Cossee s mechanism for polymerization with coordination catalysts. The active site is depicted as having a coordination vacancy that attracts the electrons in the olefin rr-bond. Coordination is followed by insertion into the polymer chain (R) and the re-establishment of the coordination vacancy for further monomer insertion. This figure also shows an important characteristic of coordination polymerization that makes it very different from free-radical polymerization the monomer is inserted between the carbon-metal bond. As a consequence, the electronic and steric environment surrounding the transition metal has a huge influence on the kinetics of polymerization. This is why... 

We will start with a very simple homopolymerization model that includes only initiation, propagation, transfer to hydrogen, -hydride elimination and imimolecular catalyst deactivation, as depicted in Table 2.4. From our previous discussion of the standard model for polymerization with coordination catalysts, it is known that several steps are not included in Table 2.4. It will be shown, however, that general expressions for population balances and the methods of moments starting with this simplified mechanism can be developed and later they can be extended, rather easily, to include more polymerization steps. 

Suspension and emulsion polymerization are the main commercial processes for the manufacture of FIFE. For industrial appHcations, these processes are described in several patents [652 676]. Suitable initiators for polymerization are peroxy compounds. The polymerization with coordination catalysts of the Ziegler-Natta type, the photoinitiation by metal carbonyls, combination of metal organyls with hydrogen peroxide, or the radiation-induced polymerization in solution or emulsion has been used extensively on a laboratory scale [677-685]. The polymerization of TFE is very exothermic (172kJ/mol) [653]. 

Polymerization with coordination catalysts proceeds via two main steps monomer coordination to the active site, and monomer insertion into the growing polymer chain, as illustrated in Figure 8.9. Before insertion, the double bond in the olefin monomer coordinates to the coordination vacancy of the transition metal. After the olefin is inserted into the growing polymer chain, another olefin monomer can coordinate to the vacant site thus the process of insertion is repeated to increase the size of the polymer chain by one monomer unit at a time until chain transfer takes place [17]. In the case of copolymerization, there is a competition between the comonomers to coordinate to the active sites and to be inserted into the growing polymer chains. Different rates of coordination and insertion of comonomers determine the final chemical composition of the copolymer chain. 

The catalytic cycle is a convenient graphical way to describe the central role played by the active site in the mechanism of polymerization. Changes in the nature of the active site will affect the catalytic mechanism and consequently the activity and the selectivity of the polymerization. Changes in the polymerization reactor conditions, such as temperature and monomer concentration, play a vital role in the catalyst mechanism because they affect the rate constants of each of these steps. Figure 8.13 shows a catalyst cycle for olefin polymerization with coordination catalysts. 

Al—Ti Catalyst for cis-l,4-PoIyisoprene. Of the many catalysts that polymerize isoprene, four have attained commercial importance. One is a coordination catalyst based on an aluminum alkyl and a vanadium salt which produces /n j -l,4-polyisoprene. A second is a lithium alkyl which produces 90% i7j -l,4-polyisoprene. Very high (99%) i7j -l,4-polyisoprene is produced with coordination catalysts consisting of a combination of titanium tetrachloride, TiCl, plus a trialkyl aluminum, R Al, or a combination of TiCl with an alane (aluminum hydride derivative) (86—88). 

Isomerization polymerizations can be associated with coordination catalyst systems, ionic catalyst systems, and free radical systems. The cationic isomerization polymerization of 4-methyl-1-pentene is of interest because the product can be viewed as an alternating copolymer of ethylene and isobutylene. This structure cannot be obtained by conventional... 

Predominantly cis-1,4-polybutadiene is produced by coordination polymerization with mixed catalysts.187,487,488 Three catalyst systems based on titanium, cobalt, or nickel are used in industrial practice. Iodine is an inevitable component in titanium-alkylaluminum sytems to get high cis content. Numerous different technologies are used 490,491 A unique process was developed by Snamprogetti employing a (Tr-allyl)uranium halide catalyst with a Lewis acid cocatalyst.492-494 This catalyst system produces poly butadiene with 1,4-ris content up to 99%. 

Lithium catalysts are far more stereospecific than the other alkali metal catalysts and are effective even in homogeneous system because of the strong coordinating ability of the lithium. Bawn and Led with (192) have included in their review an excellent discussion on the mechanism of polymerization with lithium catalysts. The key feature is the coordination of the olefinic zr-electrons with vacant s- or -orbitals in the lithium prior to an intramolecular rearrangement involving migration of the carbanion to the more electrophilic carbon of the polarized monomer. 

Based on the proposed model and on the experimental observation that the polymerization rate is proportional to the monomer concentration, it has been concluded that olefin coordination constitutes the rate determining step. However, the variety in preparation and performance of Mg/Ti catalysts for ethylene polymerization is such that it is impossible to reduce it to a single kinetic scheme, even though the active principle may be, in any case, constituted by MgCl2 89-90-91>. Ethylene polymerization with these catalysts has recently been reviewed 89) and will not be dealt with in detail here. However, it is important to underline that Mg/Ti catalysts for... 

There are relatively few detailed kinetic investigations on coordination polymerization from which general conclusions can be drawn, but there have been many studies which although limited in scope give indications of overall monomer and catalyst reactivity. There seems to be merit, therefore, in reviewing the general polymerization behaviour of some of the more important monomers with coordination catalysts. Rate data are frequently given, albeit sometimes in a form which is difficult to transform into standard units, but useful information on the dependence... 

This monomer is usually obtained as a mixture of the cis and trans isomers both of which have been polymerized with coordination type catalysts. Polymerization of the cis form is considered to be preceded by isomerization, since those catalysts which do not isomerize the cis monomer (e.g. cobalt salt—organo aluminium halide) selectively polymerize the trans isomer. A kinetic study of the polymerization of cis 1,3-pentadiene using Ti(OBu-n)4/AlEt3 (Al/Ti = 1.3—6) as catalyst has been published [267]. This gives a polymer containing ca. 73% cis 1,4 15—16% trans 1,4 and 11—12% 3,4 microstructure. 

The polymerization mechanisms for vinyl chloride and acrylonitrile (and also styrene) with coordination catalysts are also uncertain [222] and the copolymerization of butadiene/acrylonitrile (q.v.) also shows some features suggesting the formation of free radicals (or possibly radical-ions from charge transfer complexes). As these polar monomers can react, or form strong complexes, with the organo-metal compound it is likely that the kinetic schemes will be complex. As with styrene there is a good deal of scatter in the experimental kinetic data with these monomers which detracts from the certainty of the deductions, and much work will be required to put their polymerization by coordination catalysts on a sound mechanistic and kinetic basis. 

Chain-reaction polymerization may be induced by various catalysts. For example, vinyl polymerization (Reaction 5) occurs with free-radical, anionic, or cationic catalysts, as well as with coordinate catalysts (which may be of the anionic or the cationic type) (30, 31). Poly(a-amino acid) formation (Reaction 8) may be carried out with basic catalysts. 

Galli, P. Luciani, L. Cecchin, G. Advances in the polymerization of polyolefins with coordination catalysts. Angew. Makromol. Chem. 1981, 94, 63-89. 

Chain-growth polymerization involves the sequential step-wise addition of monomer to a growing chain. Usually, the monomer is unsaturated, almost always a derivative of ethene, and most commonly vinylic, that is, a monosubstituted ethane, 1 particularly where the growing chain is a free radical. For such monomers, the polymerization process is classified by the way in which polymerization is initiated and thus the nature of the propagating chain, namely anionic, cationic, or free radical polymerization by coordination catalyst is generally considered separately as the nature of the growing chain-end may be less clear and coordination may bring about a substantial level of control not possible with other methods. ... 

To describe the kinetics of olefin polymerization with heterogeneous catalysts, kinetic models based on adsorption isotherm theories have been proposed [7-10], The most accepted two-step mechanism of ZN polymerization, proposed by Cossee [10-12], includes olefin coordination and migratory insertion of coordinated monomer into a metal-carbon bond of the growing polymer chain. 

Section 2.3 describes phenomenological models for polymerization kinetics with coordination catalysts. Molar and population balances will be derived using what we like to call the standard model for olefin polymerization kinetics with coordination catalysts. Also how molecular weight averages can be modeled in batch, semibatch and continuous reactors are shown using the method of moments and the method of instantaneous distributions. Unfortunately, the kinetics of olefin polymerization is complicated by several factors that are not included in the standard model some of these effects and possible solutions and model extensions are mentioned briefly at the end of Section 2.2. 

The polymerization mechanism with coordination catalysts has been studied extensively since the discovery of Ziegler-Natta and Phillips catalysts. Some of the steps in this mechanism are very well known and constitute what we will call the standard model for polymerization in this chapter. Some important phenomena are not included in the standard model because, even though they are commonly observed experimentally, there is no... 

Even though the discussion has been mainly on homopolymerization, the same polymerization mechanism steps are valid for copolymerization with coordination catalysts. In this case, for a given catalyst/cocatalyst system, propagation and transfer rates depend not only on the type of coordinating monomer, but also on the type of the last monomer attached to the living polymer chain. It is easy to understand why the last monomer in the chain will affect the behavior of the incoming monomer as the reacting monomer coordinates with the active site, it has to be inserted into the carbon-metal bond and will interact with the last (and, less likely, next-to-last or penultimate) monomer unit inserted into the chain. This is called the terminal model for copolymerization and is also commonly used to describe free-radical copolymerization. In the next section it will be seen that, with a proper transformation, not only the same mechanism, but also the same polymerization kinetic equations for homopolymerization can be used directly to describe copolymerization. 

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