Polymerization transition metals, vanadium

Other catalysts contain the transition metals vanadium, chromium, cobalt, and nickel as their main components [194-202]. The polymerization activity is usually far lower than in the synthesis of cis polymers (see Table 2). Addition of a donor such as tetrahydrofuran, which directs the bonds into a tra 5-position to the catalyst of titanium tetraiodide and triethylaluminum, results in the formation of a polybutadiene with 80%... 

However, explanations for growth limitation based on repulsion of metal ions may be somewhat oversimplified. Elements other than vanadium, niobium, tantalum, molybdenum, and tungsten do not form isopoly anions. Other ions which have appropriate radii (e.g., Al,+, 67 pm Oa5+, 76 pm I7"1. 67 pm) for discrete isopoly anion formation instead form chains, sheets, or three-dimensional frameworks. Why does polymerization stop for isopoly anions An oxygen atom in a terminal position in an isopoly anion is strongly n bonded to a transition metal such as Mo(V ) or W (VI). These terminal oxygen atoms are never found trans to one another because they avoid... 

In the preceding chapter it has been shown that the DFT methods currently available can be used to reproduce relative trends in both reactivities and transition-metal NMR chemical shifts. Thus, NMR/reactivity correlations can be modeled theoretically, at least when relative reactivities are reflected in relative energies on the potential energy surfaces (activation barriers, BDEs). It should in principle also be possible to predict new such correlations. This is done in the following, with the emphasis on olefin polymerization with vanadium-based catalysts. 

The Ziegler-Natta catalysts are mixtures of solid and liquid compounds containing a transition metal such as titanium or vanadium (16). The first catalyst used by Ziegler and coworkers for the polymerization of ethylene was a mixture of TiCLt and AlEt3. Each of these is soluble in hydrocarbon solvents, but when they are combined an olive-colored, highly sensitive... 

Polymerization activity was obtained with a variety of catalyst compositions. The best stereospecific catalyst was the split pretreated type (357) in which one mole of VC14 was reduced by a stoichiometric amount of an alkyl metal (0.34 mole AlEt3) in heptane at room temperature and heated 16 hours at 90° C. to obtain the purple crystalline VC13-1/3 A1C13. This reduced transition metal component was then treated with two moles of (i-Bu)3Al tetrahydrofuran complex for 20 hours at room temperature to obtain a chocolate-brown catalyst consisting predominantly of divalent vanadium with 0.21 Al/V and 1.4 i-Bu/Al. Polymerizations at 30° C. gave crystalline polymers from methyl, ethyl, isopropyl, isobutyl, tert.-butyl, and neopentyl vinyl ethers. 

For instance, in the field of elastomers, alkyllithium catalyst systems are used commercially for producing butadiene homopolymers and copolymers and, to a somewhat lesser extent, polyisoprene. Another class of important, industrial polymerization systems consists of those catalyzed by alkylaluminum compounds and various compounds of transition metals used as cocatalysts. The symposium papers reported several variations of these polymerization systems in which cocatalysts are titanium halides for isoprene or propylene and cobalt salts for butadiene. The stereospecificity and mechanism of polymerization with these monomers were compared using the above cocatalysts as well as vanadium trichloride. Also included is the application of Ziegler-Natta catalysts to the rather novel polymerization of 1,3-pentadiene to polymeric cis-1,4 stereoisomers which have potential interest as elastomers. 

The polymerization of conjugated dienes to products with a controlled structure usually occurs in the presence of alkylaluminium compounds. The choice not only the transition metal but also of its ligands is of importance. Some systems produce a certain kind of stereochemical structure irrespective of external conditions. So, for example, vanadium compounds yield predominantly the trans-1,4 structure whereas cobalt salts yield the c -1,4 structure. Other catalysts are very sensitive, and a small external effect completely changes their stereochemical activity [267b] [e. g. Cr(acetylacetona-te)3-R3Al]. Examples of several catalytic systems are summarized in Table 7. 

The chemistry of Lewis acids is quite varied, and equilibria such as those shown in Eqs. (28) and (29) should often be supplemented with additional possibilities. Some Lewis acids form dimers that have very different reactivities than those of the monomeric acids. For example, the dimer of titanium chloride is much more reactive than monomeric TiCL (cf., Chapter 2). Alkyl aluminum halides also dimerize in solution, whereas boron and tin halides are monomeric. Tin tetrachloride can complex up to two chloride ligands to form SnCL2-. Therefore, SnCl5 can also act as a Lewis acid, although it is weaker than SnCl4 [148]. Transition metal halides based on tungsten, vanadium, iron, and titanium may coordinate alkenes, and therefore initiate polymerization by either a coordinative or cationic mechanism. Other Lewis acids add to alkenes this may be slow as in haloboration and iodine addition, or faster as with antimony penta-chloride. 

Accordingly, a broadening of MWD should be possible only by creating inhomogeneity in the active centre valences so as to promote different capabilities of monomer coordination and insertion and, thus, different propagation constants. The correspondence between narrow MWD and a unique oxidation state of the transition metal has been also pointed out by Christman for the ethylene polymerization with vanadium compounds-aluminum alkyls homogeneous systems. In this case, addition of a promoter causes re-oxidation of the deactivated sites (V") to the same identical initial ones (V "). 

The catalytic polymerization process is carried out in hydrocarbon diluents (diesel oil, hexane, isobutane) or the bulk monomers (ethylene, propylene, norbomene, styrene). The transition metal catalysts are usually insoluble and thus act heterogeneously, except for some homogeneously active vanadium compounds [2] and the metallocenes [20] (cf Section 2.3.1.1). Over a wide tempera-... 

Homogeneous catalysis takes place at transition metal complexes dissolved in the reaction medium. For the oligomerization or polymerization of olefins, such transition metal complexes are provided by some vanadium compounds and the metallocenes, both activated with soluble organoaluminum compounds. The polymers can be dissolved (solution process) or precipitated (slurry process). 

Karol, F.J. Cann, K.J. Wagner, B.E. Developments with high-activity titamium, vanadium, and chromium catalysts in ethylene polymerization. In Transition Metals and Organome-tallics as Catalysts for Olefin Polymerization, Kaminsky, W., Sinn, H.,, Eds. Springer-Verlag Berlin, 1988 149-161. 

In the broadest sense, Ziegler Natta catalysts may be regarded as combinations of Group IV-VIII transition metal compounds with Group I-III organometallics which can effect polymerization of olefins and dienes under relatively mild conditions of temperature and pressure [82,83]. Titanium compounds and aluminum alkyls are used most frequently in commercial polyolefin processes. Both titanium and vanadium compounds are used in conjunction with aluminum alkyls in catalyst systems for synthetic rubber/elastomers. 

The electrolytic cell is connected to a polymerization vessel in which the electrolysis products are mixed with the olefins in the presence of a suitable solvent. The transition metals employed as anodes were vanadium and manganese. The examples quoted in the patent are related only to the polymerization of ethylene when vanadium was employed, a polymer containing a residue of this metal not exceeding 8 p.p.m., which is a sufficiently low level for many commercial applications, was obtained. 

As summarized in Table 7, the yields in solid polyethylene (mp initial 132 °C, final 135 °C, by differential thermal analysis (DTA) cristallinity 85-90%) are very high. The transition metals proposed for this polymerization are Ti, V, Cr, Mn, Fe, Co, Ni, Cu. The results quoted in the patent related to the use of these metals38 are reported in Table 8 chromium and vanadium give the highest yield in solid polyethylene, nickel gives quantitative dimerization to 1-butene, as expected from the bibliographic data related to the chemical activity of these metals. 

Before the 1970s, Ziegler-Natta catalysts for a-olefin production were normally prepared from certain compounds of transition metals of Groups IV-VI of the periodic table (Ti, V, Cr, etc.) in combination with an organoraetallic alkyl or aryl (Table I). Practically all subhalides of transition metals have been claimed as catalysts in stereoregular polymerization. Only those elements with a first work function <4 eV and a first ionization potential <7 V yield sufficiently active halides, that is, titanium, vanadium, chromium, and zirconium (7, Only titanium chlorides have gained widespread acceptance in crystalline polyolefin production. 

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