Titanium complexes Ziegler-Natta polymerization

Catalysts used in the polymerization process. For example, commercial isotactic polypropylene is polymerized from a hetereogenous organo-aluminium-titanium complex (Ziegler-Natta process) [110,1260], or less frequently from metallic oxides of chromium, vanadium or molybdenum bonded to an inert support (e.g. the Philips process) [447]. Transition metal ion contents vary in different commercial samples (Table 2.4). 

Cyclization of alkenes.1 This Ziegler-Natta polymerization catalyst can effect intramolecular cyclization of unactivated alkenes. Thus transmetallation of a Grignard reagent (1) with Cp2TiCl2 results in a titanium complex (2), which on... 

Any one of a group of addition polymerization catalysts involving titanium-aluminum complexes. Ziegler-Natta catalysts produce stereoregular (either isotactic or syndiotactic)... 

Since cationic titanium complexes have been proposed as intermediates in aUcene polymerization by soluble Cp2TiCl2/AlR3-based Ziegler-Natta catalysts (see Ziegler-Natta Catalysts), there has been considerable interest in the synthesis and reactivity of cationic titanium complexes (see Oligomerization Polymerization by Homogeneous Catalysis). 

Ziegler-Natta Polymerization. NVK and other monomers, such as vinyl ethers, 1,5-hexadiene, dihydrofuran, and dihydropyrans can be polymerized using a titanium, hafnium or zirconium pentamethylcyclopentadienyl complex as an initiator in the presence of a borane co-initiator, e.g., ... 

In 1958, Natta et al. published a study on Ziegler-Natta polymerizations of various vinyl monomers [362]. Natta and his co-workers showed that these polymerizations are very sensitive to steric hindrance at the double bound. In addition, Ziegler-Natta catalysts can generate a series of Lewis adds, which lead to complex side reactions. Therefore, only few attempts have been made to polymerize vinyl arenes by Ziegler-Natta catalysts. Heller and Miller obtained stereoregular polymers consisting of 1-vinylnaphthalene, 2-vinylnaphtha-lene and 4-vinylbiphenyl [363]. Polymerization by triethyl aluminum/titanium tetrachloride gave polymers in 75% to 90% conversion, which were characterized by IR and H-NMR spectroscopy and found to be at least 90% isotactic. Only 1-vinylnaphthalene produced a crystallizable polymer [362,363]. 

The regioselectivity of nucleophilic additions to the Co(CO)3BF4 complex has also been examined.i Ziegler-Natta polymerization of 2-trimethylsilylmethyl-1,3-butadiene catalyzed by triethylaluminum and titanium(IV) chloride gives predominantly cis-1,4-polymer. However, anionic polymerization yields a polymer whose microstmcture is conqtosed of cis-1,4-, trans-1,4-, and 3,4-units.i ... 

Ziegler-Natta catalysts are primarily complexes of a transition metal halide and an organometallic compound whose structure is not completely understood for all cases. Let us use as an example TiCU and R3AI. The mechanism of the polymerization catalysis is somewhat understood. This is shown in Fig. 14.6. The titanium salt and the organometallic compound react to give a pentacoordinated titanium complex with a sixth empty site of... 

The class of monocyclopentadienylamido (CpA) titanium complexes has attracted the interest for the polymerization of a-olefins with bulky side groups. This arises since conventional Ziegler-Natta catalysts are less effective in starting the copolymerization of ethene with 4-methyl-l-pentene. Homogeneous catalysts of the zirconium cyclopentadienyl type (Cp2M) with methylaluminoxane exhibit a low catalytic activity. 

The nature of the cocatalyst in the Ziegler-Natta systems exerts a significant effect on the polymerization rate and isotacticity of the polymer product Interesting data were obtained by Keii et al. when the cocatalyst was changed during polymerization. The pronounced effect of the cocatalyst is the basis of the concept on the bimetallic composition of ACg according to which the latter are represented, for example, as binuclear complexes including titanium and aluminium. 

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. 

Additionally, if the initiation reaction is more rapid an the chain propagation, a very narrow molecular weight distribution, MJM = 1 (Poisson distribution), is obtained. Typically living character is shown by the anionic polymerization of butadiene and isoprene with the lithium alkyls [77, 78], but it has been found also in butadiene polymerization with allylneodymium compounds [49] and Ziegler-Natta catalysts containing titanium iodide [77]. On the other hand, the chain growth can be terminated by a chain transfer reaction with the monomer via /0-hydride elimination, as has already been mentioned above for the allylcobalt complex-catalyzed 1,2-polymerization of butadiene. 

One of the most remarkable aspect on the bis-Cp titanium derivative chemistry has been the production of new and unprecedented variety of polyolefins. The use of this type of complexes as Ziegler-Natta pre-catalyts for the olefin polymerization has opened new possibilities to produce polyolefins with different properties, and significant effort has been devoted to the design of new bis-Cp catalyst structures. This section summarizes simple aspects related to the polymerization of ct-olefins catalyzed by bis-Cp titanium complexes containing a cr-Ti-C bond. A more comprehensive review of the catalytic applications of titanium complexes in the a-olefin polymerization processes is covered in Chapter 4.09. 

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