Organotitanium, polymerization

Although titanium does not form a stable bond with carbon, organotitanium polymers have been formed such as polyalkoxytitanoxanes and polymeric titanate esters (Figure 29.14). 

Gas-phase processes commercialized in the late 1960s offer much simpler operation. Since they eliminate solvent, solvent separation, recovery, and purification are unnecessary. Polymerization is carried out in stirred-vessel reactors or in fluidized beds. A very successful fluidized-bed process is Union Carbide s UNIPOL technology444 designed originally for producing HDPE and extended later to LLDPE. Ethylene is polymerized by injecting the fine catalyst powder (organotitanium or... 

Hexenylsilane was introduced as a co-monomer into the organotitanium-mediated polymerization of ethylene to produce silane-terminated ethylene-5-hexenylsilane copolymers. High activities and narrow polydispersities were observed in the polymerization-chain transfer process. Ethylene-5-hexenylsilane copolymer molecular weights... 

Organotitanium compounds have been intensively studied, initially mainly because of the discovery by Ziegler and Natta that ethylene and propylene can be polymerized by TiCl3-aluminum alkyl mixtures in hydrocarbons at 25°C and 1 atm pressure84 (Section 22-9). Organic compounds have been found to react with N2 and to act as catalysts in a number of other reactions. 

Considerable effort has been directed to the stereoselective synthesis of aimulated cyclopentadienyl complexes of the group 4 transition metals. Ci asymmetric and C2 synunetric aimulated cyclopentadienyl ligands have been used to prepare chiral organotitanium complexes, which are now being actively studied as catalysts for asymmetric synthesis and olefin polymerization see Asymmetric Synthesis by Homogeneous Catalysis). 

The catalytic polymerization of olefins using aluminum alkyls and titanium tetrachloride or organotitanium compounds 15, 16, 40, 185,... 

The only work on the catalysis of diene polymerization by structurally defined organotitanium compounds that can be mentioned is that of Dolgoplosk and coworkers [50]. As these authors have found, tetrabenzyltitanium and the tribenzyl-titanium halides can catalyze the stereospecific diene polymerization without any co-catalyst. 

Alkylaluminums have become very important as mixed catalysts for low-pressure polymerization of olefins (especially ethylene), a reaction discovered by Ziegler and his colleagues.263 Numerous investigations have been devoted to a study of the mechanism of this reaction e.g., it has been shown that an important role in the polymerization is played by the organotitanium compounds formed from mixtures of alkylaluminums and titanium tetrachloride.264... 

In our attempt towards such fibers we felt that if we utilized organic polymers (as a source of carbon) and suitably modified it with an organotitanium complex at the molecular level, we would have a system containing the right ingredients i.e. Ti and C. Such a polymeric system could conceivably be processed into a fiber [5]. 

Organometallic derivatives of titanium have been widely investigated, primarily as a result of their ability to act as excellent catalysts in polymerization reactions. The interest in organotitanium derivatives was largely initiated by the discovery of the Ziegler-Natta catalysts(1,2). 

With organotitanium compounds, especially carbometalation, substitution and insertion reactions tend to proceed. Carbometalation, as described in the reaction mechanisms of the Ziegler-Natta catalyst in Chapter 7, contains olefin polymerization with titanium compounds. Namely, titanium reacts with organoalumi-num compounds and the titanium atom forms a bridged structure (Ti—R—Al) with an alkyl group of organoaluminum compounds and this Ti—R adds to a carbon-carbon double bond. 

Patten, T.E. and Novak, B.M. (1996) Living organotitanium(IV)-catalyzed polymerizations of isocyanates. Journal of... 

Amin and Marks copolymerized alkenylsilanes and ethene with homogeneous mono- or bi-nuclear organotitanium complexes (Figure 12), ° which are known to be efficient a-olefm polymerization agents and in addition being able to effect silanolytic chain transfer. ... 

Figure 12 Organotitanium complexes for an efficient a-olefin polymerization with alkenylsilane incorporation and silanolytic chain transfer.

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