Ethylene polymerization with Ziegler catalysts

L. L. Bohm (2003) Angewandte Chemie International Edition, vol. 42, p. 5010 - The ethylene polymerization with Ziegler catalysts fifty years after the discovery . 

Mitsui Chemicals, Inc. Ethylene-propylene rubber Ethylene, propylene, termonomer EPM/EPDM process uses solution polymerization with Ziegler catalyst 1 1994... 

Derivation Ethylene polymerized by Ziegler catalysts at 1-100 atm (15-1500 psi) at from room temperature to 200F. Catalyst is a metal alkyl, e.g., triethylaluminum plus a metallic salt (TiCl4) dissolved in a hydrocarbon solvent. A vapor-phase modification of this process was developed in 1965. Another method uses such metallic catalysts as Cr203at 100-500 psi with solvents such as cyclohexane or xylene. 

The first example of homogeneous transition metal catalysis in an ionic liquid was the platinum catalyzed hydroformylation of ethene in tetraethylammonium trichlorostannate (mp. 78 °C), described by Parshall in 1972 (Scheme 5.3-l(a)) [1]. In 1987, Knifton reported the ruthenium- and cobalt-catalyzed hydroformylation of internal and terminal alkenes in molten [Bu4P]Br, a salt that falls within the now accepted definition for an ionic liquid (see Scheme 5.3-l(b)) [2]. The first applications of room-temperature ionic liquids in homogeneous transition metal catalysis were described in 1990 by Chauvin et al. and by Wilkes et al. Wilkes et al. used weakly acidic chloroaluminate melts and studied therein ethylene polymerization with Ziegler-Natta catalysts (Scheme 5.3-l(c)) [3]. Chauvin s group dissolved nickel catalysts in weakly acidic chloroaluminate melts and investigated the resulting ionic catalyst solutions for the dimerization of propene (Scheme 5.3-l(d)) [4]. 

Recently, many titanium compounds have been studied as polymerization catalysts of olefins [5 la-5 If). In these studies, the active center of ethylene polymerization with Ziegler-Natta type catalysts by using a square-pyramidal model is thought to be a weak 7t-bond between ethylene and titanium. The bond length and bond energy are calculated 2.80 A and 7—11 kcal/mole, respectively [51a]. 

Tureu, A. X, Toader, M., Boborodoa, C., and lloisou, E, Oligomers in the ethylene polymerization with Ziegler-Natta catalysts, Materiale Plastics (Bucharest), 17, 153, 1980 (Romanian). 

Table 6. Values of chain transfer constants for ethylene or propylene polymerization with Ziegler-Natta catalysts... 

Organometallic mixed catalysts for ethylene polymerization, discovered (by serendipity) when nickel-contaminated autoclaves were used to carry out an Aufhaureaktion (reaction of A1(C2H5)3 with ethylene). The nickel effect lead to the zirconium-catalyzed ethylene polymerization in Ziegler s laboratory on October 26, 1953, see F. M. MacMillan, The Chain Straighteners, The MacMillan Press Ltd., p. 62f., London, 1979. 

One first assumed that polymerization with Ziegler-Natta catalysts, such as aluminum-alkyls plus halides, works by a simple ionic mechanism. Since single aluminum alkyls normally cause anionic and titanium halides a cationic chain reaction (Chapter 8), the two components of the initiator should neutralize each other and only the excess one over the other should be active. If this were true, then either one of the components alone should be able to initiate the polymerization of ethylene or propylene, but this is not the case. A simple anionic or cationic mechanism can therefore not explain the polymerization with Ziegler-Natta catalysts. 

The stirred flow reactor is frequently chosen when temperature control is a critical aspect, as in the nitration of aromatic hydrocarbons or glycerine (Biazzi-process). The stirred flow reactor is also chosen when the conversion must take place at a constant composition, as in the copol3rmerization of butadiene and styrene, or when a reaction between two phases has to be carried out, or when a catalyst must be kept in suspension as in the polymerization of ethylene with Ziegler catalyst, the hydrogenation of a-methylstyrene to cumene, and the air oxidation of cumene to acetone and phenol (Hercules-Distillers process). 

The low-pressure polymerization of ethylene with Ziegler catalysts (Ti com-poimds/Al alkyls) is depicted in Scheme 3-9. Explain the mechanism of the polymerization. 

FIGURE 5.2. Countercurrent-flow reactor for slurry polymerization of ethylene with Ziegler catalysts (as illustrated in Koppeis Co. Inc., British Patent 826,563). 

Coordinated anionic polymerizations with Ziegler-Natta catalysts yield similar polymers that range from viscous liquids to rubbery solids. At 0 °C, a catalyst with a 1 16 Ti to A1 molar ratio yields a polymer with a molecular weight of 5000-6000. The molecular weight, however, is dependent upon the reaction time. This contrasts with polymerizations of ethylene, propylene, and 1-butene by such catalysts, where the molecular weights of the products are independent of the reaction time. In addition, there are some questions about the exact molecular structures of the products. ... 

Propylene, CH2=CHCH3, is obtained as a by-product of ethylene production from the cracking of petroleum fractions. Free radical polymerization yields only low-molecular-weight oils consisting of branched, atactic molecules. Isotactic poly (propylenes) were first made possible and came into commercial use through Natta s work with Ziegler catalysts. 

Lohrenz, J. C. W. Woo, T. K. Ziegler, T. A density functional study of the origin of the propagation barrier in the homogeneous ethylene polymerization with Kaminsky-type catalysts. J. Am. Chem. Soc. ms, 117, 12793-12780. 

Random copolymers are often formed by chain polymerizations when two or more monomers are polymerized together. Many commercial polymers belong to this group, e.g. styrene/acrylonitrile (SAN), polyvinyl chloride/ polyvinylidene dichloride (Saran film), polyvinylidene difluoride/polyhexa-fuoropropene (Viton) which are all produced using free radical initiators (section 1.8.1). Ethylene/propylene elastomers are random copolymers (section 1.15.1.4) and they are obtained with Ziegler catalysts. 

Actinide, lanthanide, and yttrium-based catalyst systems showing characteristics of reversible chain transfer in ethylene polymerization are summarized in Table 3. Samsel and Eisenberg claimed to observe the characteristics in ethylene polymerization with several metallocenes of actinides, such as the bis(pentamethylcyclopentadienyl) thorium complex 5 in combination with aluminum alkyl reagents. These systems catalyze the production of aluminum alkyl chain growth products at lower temperatures than those required by the uncatalyzed Ziegler process. The systems were limited to production of low-molecular-weight PE oligomers. 

The second type of solution polymerization concept uses mixtures of supercritical ethylene and molten PE as the medium for ethylene polymerization. Some reactors previously used for free-radical ethylene polymerization in supercritical ethylene at high pressure (see Olefin POLYMERS,LOW DENSITY polyethylene) were converted for the catalytic synthesis of LLDPE. Both stirred and tubular autoclaves operating at 30—200 MPa (4,500—30,000 psig) and 170—350°C can also be used for this purpose. Residence times in these reactors are short, from 1 to 5 minutes. Three types of catalysts are used in these processes. The first type includes pseudo-homogeneous Ziegler catalysts. In this case, all catalyst components are introduced into a reactor as hquids or solutions but form soHd catalysts when combined in the reactor. Examples of such catalysts include titanium tetrachloride as well as its mixtures with vanadium oxytrichloride and a trialkyl aluminum compound (53,54). The second type of catalysts are soHd Ziegler catalysts (55). Both of these catalysts produce compositionaHy nonuniform LLDPE resins. Exxon Chemical Company uses a third type of catalysts, metallocene catalysts, in a similar solution process to produce uniformly branched ethylene copolymers with 1-butene and 1-hexene called Exact resins (56). 

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