Transition metal -complexes, stereospecific polymerization

The examples discussed so far are all transition metal complexes. As we will see later (Chapters 4-9), most homogeneous catalytic processes are indeed based on transition metal compounds. However, catalytic applications of rare earth complexes have also been reported, although so far there has not been any industrial application. Of special importance are the laboratory-scale uses of lanthanide complexes in alkene polymerization and stereospecific C-C bond formation reactions (see Sections 6.4.3 and 9.5.4). 

Olefin polymerization by transition metal complexes such as those in the catalyst systems of Ziegler and Natta is remarkably stereospecific. A mixture of an alkylaluminum halide and TiCl4 polymerizes ethylene at low pressure to crystalline linear polyethylene 184) with a relatively high density (0.96) and melting point (132° C). These properties contrast sharply... 

Since these catalysts are transition metal complexes, the mechanism of stereospecific polymerization most likely involves transitory 7r-complex formation between the olefin and the catalyst. 

The most efficient enantioface discriminating agents seem to be transition metal complexes covalently bound to the growing chain end, which are also able to achieve a very high regio-selectivity in the attack to the double bond. Unfortunately, the type of monomers which are polymerized stereospecifically with this type of catalysts are mainly unsaturated hydrocarbons. Propylene (14) and butadiene (46) can be polymerized by the above catalysts both to isotactic and syndiotactic polymers. 

Complexes of type III and also traces of soluble halides of strongly electropositive transition metals, being able to form complexes with metallorganic compounds of the type I, II, increase the activity of the stereospecific catalysts formed by the action of metallorganic compounds on crystalline substrates 10,11). They can also polymerize in a stereospecific way in the presence of crystalline substrates of transition metals (for instance, CoCU) which are not by themselves sufficiently electropositive, (when used in the presence of metallorganic compounds) to polymerize the a-olefins 10, 11). 

Only when such a complex is chemisorbed or lies on the surface of a crystalline lattice made of a compound of a transition metal does the catalyst act in a stereospecific way in the polymerization of a-olefins. 

Well-controlled polymerization of substituted acetylenes was also reported. A tetracoordinate organorhodium complex induces the stereospecific living polymerization of phenylacetylene.600 The polymerization proceeds via a 2-1 -insertion mechanism to provide stereoregular poly(phenylacetylene) with m-transoidal backbone structure. Rh complexes were also used in the same process in supercritical C02601 and in the polymerization of terminal alkyl- and arylacetylenes.602 Single-component transition-metal catalysts based on Ni acetylides603 and Pd acet-ylides604 were used in the polymerization of p-diethynylbenzene. 

It is not necessary to incorporate the concept of macrosurfaces nor of olefinic coordination complexes of the metal in order to explain stereospecific polymerization. Simple 4 and 6 membered cyclic transition states account for steric control. 

Stereospecific emulsion polymerization of butadiene has been achieved in the presence of soluble transition metal salts 350, 351). Polymer microstructure was controlled by varying the transition metal ion and its ligands. Although the initiation mechanism has not been determined, it is most likely to be of the coordinated radical type with steric control arising from the transition metal-diene complexes. 

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. 

Towards the end of the second millennium, studies of the transition elements continued to make major contributions to chemical science and technology. The development of new catalysts and reagents represents one area of activity. Examples are provided by the activation of saturated hydrocarbons by rhodium or lutetium complexes, new syntheses of optically active products in reactions which employ chiral metal compounds, and transition metal compounds which catalyse the stereospecific polymerization of alkenes. The ability of transition metal centres to bind to several organic molecules has been exploited in the construction of new two- and three-dimensional molecular architectures (Figure 1.4). New materials containing transition elements are being developed, one... 

At the present time, the most likely concept of the mechanism of a heterogeneous polymerization catalyzed by a Ziegler-Natta catalyst involves a complex in which the organometallic component and the transition metal component—i.e., the A1 and Ti atoms—are joined by electron-deficient bonds. Natta, Corradini, and Bassi (13) have reported such a structure for the active catalyst prepared from bis (cyclopentadienyl) titanium dichloride and aluminum triethyl. Natta and Pasquon (14), Patat and Sinn (18), and Furukawa and Tsuruta (2) have proposed mechanisms for the stereospecific polymerization of a-olefins in terms of such electron-deficient complexes. 

Precatalyst 4(Sm) was utilized as a standard system [60]. The mechanism follows a coordination anionic polymerization via an eight-membered transition state (Scheme 3, see p. 985). Formation of a metal enolate turned out to be essential for the initiation of the MMA polymerization and was confirmed by the initiation activity of the enolate complex [(C5H4SiMe3)2Y(OCH=CH2)]2- The rate of polymerization is directed by steric factors depending on the metal (Sm > Y > Yb > Lu) and the auxiliary ligand (Cp > Cp ). Ethyl, isopropyl and f-butyl methacrylates are also stereospecifically polymerized, but the rate of poly-... 

Ziegler catalyst. A type of stereospecific catalyst, usually a chemical complex derived from a transition metal halide and a metal hydride or a metal alkyl. The transition metal may be any of those ingroups IV to VIII of the periodic table the hydride or alkyl metals are those of groups I, II, and III. Typically, titanium chloride is added to aluminum alkyl in a hydrocarbon solvent to form a dispersion or precipitate of the catalyst complex. These catalysts usually operate at atmospheric pressure and are used to convert ethylene to linear polyethylene and also in stereospecific polymerization of propylene to crystalline polypropylene (Ziegler process). 

Complexes, catalyst- cocatalyst n. Stereospecific chemical complexes, usually derived from a transition metal halide and a metal hydride or a metal alkyl. An example is in stereospecific polymerization of propylene to crystalline polypropylene. 

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