Titanium catalytic systems, study

A highly obscure feature of cationic polymerization is the great phenomenological difference between aliphatic and aromatic monomers. The survey by Brown and Mathieson [84] of the behaviour of a very wide range of monomers towards trichloroacetic acid is particularly illuminating in this respect. Unfortunately, there are so few studies with aliphatic olefins that detailed comparisons must be confined to isobutene. It is well known that isobutene cannot be polymerised by conventional acids, such as sulphuric, perchloric, hydrochloric, or by salt-like catalysts such as benzoyl perchlorate, whereas all these catalysts readily give at least oligomers from aromatic olefins. Even when the same catalytic system, (e.g., titanium... 

Research by Ziegler and Natta showed that organoaluminum compounds in combination with titanium halogenides are active catalysts of the polymerisation of olefines at low pressure. This inspired the study of other organometal compounds (e.g., organotin compounds like tetraal-kyltin) in similar catalytic systems. It has been proved experimentally. 

A recent systematic study of the role played by titanium alkoxide, 2-propanol, and molecular sieves (MS) has permitted the development of an efficient catalytic system furnishing chiral sulfoxides with high ee.54 This catalyst has a new composition Ti(0-i-Pr)4/(/ ,/ )-DET/i-PrOH (1 / 4 / 4), in the presence of 4A MS, which is a combination of the Modena47 and Sharpless systems. Using this new system, the Orsay group achieved the highest enantioselectivity in catalytic asymmetric oxidation of sulfides by a nonenzymatic method (Table 8). 

Thus, the experimental data show that the composition of catalytic systems does not influence the stereoregularity of the corresponding polymer fractions but only their relative content. Hence, the stereospecificity of the active centers of these catalysts is the same including the one-component catalyst TiCl2. This confirms the concepts on the monometallic character of AQ in the Ziegler-Natta catalysts. The possible existence of chiral titanium atoms on the titanium chloride surface was studied by Cossee and Arlman ... 

A catalytic system which has been greatly studied is based on titanium cyclopenta-dienyl derivatives. Belov et al. studying ethylene polymerization with the... 

The synthesis of the Ind-amido titanium complexes [(C9H5R)SiMe2NBut]TiX2 (Scheme 304) with alkoxo and amido substituents at 2- and 3-indenyl position has been reported and the molecular structures of the derivatives for R = NMe2 and N(CH2)4 have been determined by X-ray diffraction. The methyl derivatives are activated with B(C6F5)3 and studied as catalytic systems for the ethylene/l-octene co-polymerization. A dramatic effect of the indenyl substituent nature on catalyst efficiency and polymer properties is observed.740... 

Atactic polypropene has been synthesized with homogeneous catalytic systems based on mono-Cp trialkoxo titanium complexes activated by MAO.951 Syndiotactic polystyrene has been synthesized with different mono-Gp trialkoxo titanium derivatives activated by MAO and AlMe3, and the catalytic efficiency has been compared with bis-Cp titanium catalysts.952 The titanium ligands affect both catalytic activity and stereoregularity of the polypropylene obtained. For the CpTi(OPrn)3/MAO system, factors influencing the propylene polymerization, such as temperature, Al/Ti molar ratio, and monomer pressure, have been studied. 

Studies on dichloro ansa-Cp titanium compounds as catalytic systems, in the presence of MAO for the polymerization of ethylene and propylene, have been described.1684... 

TiCl4 immobilized on surface of PE modified by graft polymerization of vinylpyridine, acrylonitrile, methyl vinyl ketone, allyl alcohol, allylamine, allyl sulfide or acrylic acid, has been studied with IR spectroscopy and XPS I166J. Cluster aggregates of Ti(IV) fixed on the surface, were discovered. Their formation did not cause the reduction of Ti(IV). In binary catalytic systems of Ti(IV) on an organo-aluminum polymeric support, Ti(IV) was reduced to Ti(III), and Ti(II) appeared at 343 K. The high and stable activity of immobilized titanium compounds during ethylene polymerization was attributed to deceleration of Ti(IV) reduction processes. 

Okamoto et reported a catalytic system based on an aminetris(aiyl-oxide) titanium complex, which gives a low-valent titanium species in the presence of trimethylsilyl chloride and magnesium powder. This system was studied in radieal ring opening of epoxides and oxetanes to generate the corresponding aleohols. 

Lentz and coworkers reported that air-stable titanocene difluoride can act as a catalyst for hydrodefluorination reactions in the presence of silane derivatives, giving excellent yields even at low temperature. The authors expanded the scope of the reaction to other substrates such as vinylic and allylic fluorides and a range of catalysts.Mechanistic studies indicated that a titanium(m) hydride is the active species, and the silane derivative is the hydride source. This catalytic system is not only interesting form the environmental point of view, but also for the synthesis of new organic molecules as it represents a new synthetic route to obtain inaccessible aminopyridine derivatives." ... 

The study of ethylene and propylene copolymerisation, on vanadium and titanium catalysts of various compositions [70], led to the conclusion that studied catalytic systems contain two or three types of AC. This conclusion has been made as a result of the analysis of the MWD curves, carbon nuclear magnetic resonance spectroscopy analysis, and copolymers composition fractionation data. The analysis of a large number of copolymer fractions, produced by their dissolution in several solvents at various temperatures, has indicated the existence of several types of AC different both in stereospecificity and in reactivity. According to the authors of [70], a combination of copolymer fractionation results with gel chromatography data indicates the presence of two or three types of AC. 

Changes in the kinetic activity distribution of the macromolecule growth centres can be observed in the butadiene polymerisation process when a titanium catalyst is prepared in situ. The function F(/w of the studied catalytic system has several maximums, indicating different types of AC in the butadiene polymerisation process and caxaXytk. complex prepared in situ, which is directly in the monomer solution. The number of maximums depends on the conversion (Figure 3.48). Five different types of AC have been identified for the studied system each of them is responsible for the synthesis of the polymer fraction with a specific MW Type -lnM = 7.1-7.8 Type II - / M = 9.4-9.9 Type III - / M = 11.0-12.0 Type IV - / M = 12.7-13.2 and Type V-lnM = 14.6-14.8. 

A variety of Group 4 metal complexes, in combination with common olefin polymerization activators, have been evaluated as potential catalysts for syn-diospecific polymerization of styrene (for reviews, see Refs. 114, 115, 123, and 426). Monocyclopentadienyl and monoindenyl titanocenes generally exhibit the highest activities (eq. 5) (112-127). Curiously, half-sandwich titanium-trifluoride-based catalysts are more active than their trichloride analogues (124,427,428). The polymerization mechanism for sPS formation is under debate. Kinetic studies and spectroscopic investigations of the catalytic systems suggest a cationic Ti(III) complex as the active species (123). 

From both the experimental and theoretical points of view, the most thoroughly studied catalytic systems are undoubtedly allylnickel(II) systems and monocyclopentadienyl titanium complexes. In the case of the nickel systems, chain growth proceeding by BD insertion into the allyl-transition metal bond was proven directly by NMR spectroscopy for both 1,4-trans- and 1,4-cis-regulating catalysts. In this case, the proposed mechanism for stereoregulation suggests that the cis-trans... 

The accessible product range is also broadened thanks to the exceptional intrinsic stereospecificity and molecular veight control of the catalytic system. High quality and consistency are favoured by easy and smooth control of the polymerization conditions due to the catalyst properties. In addition, future progress can be foreseen as a result of specialized development studies of this type of titanium trichloride catalyst currently under way. 

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