Titanium-catalyzed polymerization

A similar dichotomy was observed in the titanium catalyzed polymerization of primary silanes coupled to the hydrogenation of norbornene (20). At low catalyst concentration (ca. 0.004H), essentially complete conversion of norbornene to an equimolar mixture of norbornane and bis-phenylsilyl- (and/or 1,2-diphenyl-disilyl)norbornane was observed. Under these conditions no evidence for reduction of titanium was obtained. At higher catalyst concentrations (> 0.02M) rapid reduction of the dimethyltitanocene to J, and 2 occurs and the catalytic reaction produces mainly polysilane (DPn ca. 10) and norbornane in ca. 80 per cent yields, and silylated norbornanes in about 20 per cent yield. 

Despite the early use of phosphonium salt melts as reaction media [12, 18, 25], the use of standard ionic liquids of type 1 and 2 as solvents for homogeneous transition metal catalysts was described for the first time in the case of chloroaluminate melts for the Ni-catalyzed dimerization of propene [5] and for the titanium-catalyzed polymerization of ethylene [6]. These inherently Lewis-acidic systems were also used for Friedel-Crafts chemistry with no added catalyst in homogeneous [7] as well as heterogeneous fashion [8], but ionic liquids which exhibit an enhanced stability toward hydrolysis, i. e., most non-chloroaluminate systems, have been shown to be of advantage in handling and for many homogeneously catalyzed reactions [la]. The Friedel-Crafts alkylation is possible in the latter media if Sc(OTf)3 is added as the catalyst [9]. 

Polybutadiene and polyisoprene are produced and used mainly as synthetic rubber on an industrial scale by using transition metal catalysts, especially titanium- and nickel-based ones. By contrast, only minor attention has been paid to the palladium-catalyzed polymerization of butadiene. A mixture of 1,2-polybutadiene and trans- and c/s-l, 4-polybutadiene was obtained by using PdCl2 as a catalyst (7, 2). 

After reaching the desired DP, 40-50 % of the oligomeric melt is transferred to the polymerization vessel. Titanium butoxide (50-150 ppm) or dibutyl tin oxide catalyst (100-250 ppm), or some combinations of the two catalysts, is added to catalyze polymerization at 260-275 °C. A vacuum of <0.15 kPa is applied to remove the condensed water so as to drive the reaction until the polymer reaches an intrinsic viscosity (IV) of 0.7-0.9 dL/g. 

The action of water in the titanium tetrachloride catalyzed polymerization is paradoxical, since water at —60 to —80° was present only in the solid phase its solubility in hexane at these temperatures is in the order of 10-10 moles per liter (Plesch et al., 83). It was found to be essential that the water be present as an extremely fine dispersion such as might result from the rapid bubbling of moist air through the liquid at the low temperature. Addition of liquid water which formed lumps of ice in the reaction mixture did not initiate polymerization. It may be concluded that a fine dispersion is necessary in order that reaction with titanium tetrachloride can occur and a chain reaction is initiated ... 

Dehydrogenative Coupling. Transition-metal catalyzed polymerization of silanes appears to hold promise as a viable route to polysilanes. A number of transition-metal complexes have been investigated, with titanium and zirconium complexes being the most promising (105—108). Only primary silanes are active toward polymerization, and molecular weights are rather low. The dehydrogenative polymerization is depicted in reaction 11, where Cp = cyclopentadienyl ... 

Living poly(n-hexylisocyanate) was previously prepared by titanium-catalyzed coordination polymerization as described by Rattan [1],... 

If the reaction between enol silyl ethers and a,/ -unsaturated ketones is attempted in the presence of a titanium Lewis acid, the mode of the reaction switches to 1,4-addition with reference to the unsaturated ketone [109-113]. The reaction of an enol silyl ether is shown in Eq. (30) [114]. Ketene silyl acetals react with a,j8-unsaturated ketones in similar 1,4-fashion, as exemplified in Eq. (31) [115]. Acrylic esters, which often tend to polymerize, are also acceptable substrates for a, -unsaturated carbonyl compounds [111]. A difluoroenol silyl ether participated in this cationic reaction (Eq. 32) [116], and an olefinic acetal can be used in place of the parent a-methylene ketone [111] to give the 1,5-diketone in good yield (Eq. 33) [117]. More results from titanium-catalyzed 1,4-addition of enol silyl ethers and silyl ketene acetals to a,f -unsaturated carbonyl compounds are summarized in Table 4. 

If the rate of anti-syn isomerization is relatively low, then the cis-trans selectivity can be determined by the formation of the anti- or the 5y/i-butenyl structure, for example from the t] -cis or the if-trans coordinated butadiene, in the catalyst complex. This is the mechanism of stereoregulation which was suggested in the mid-1960s by Cossee and Arlman [34, 35] for titanium-catalyzed butadiene polymerization, and which was reconsidered more recently for the allylne-odymium complex catalysts to explain their cis-trans selectivity [39], But it is also possible that the difference in reactivity between the anti and the syn structure of the catalytically active butenyl complex can determine the cis-trans selec-... 

F.J. Karol, Copolymerization and chain-branching studies with high-activity chromium and titanium catalysts, in R.P. Quirk (Ed.), Transition Metal Catalyzed Polymerization, Proceedings of the International Symposium, 2nd, Akron, OH, 1986, Cambridge University Press, Cambridge, 1988, pp. 702-718. 

With both sets of catalysts polymerization may be conducted at elevated temperature where the polymer dissolves in the diluent or at lower temperature where the polymer forms as insoluble granules (dispersion polymerization). Whether the polymer forms in solution or dispersion the vanadium catalyzed product is uniform and the titanium catalyzed product is heterogeneous. This provides further evidence that the structure is controlled by the catalyst and not by reaction conditions. 

The mechanism of the Ziegler-Natta-catalyzed polymerization of a substituted ethylene is shown in Figure 28.1. The monomer forms a tt complex (Section 6.5) with titanium at an open coordination site (i.e., a site available to accept electrons) and the coordinated alkene is inserted between the titanium and the growing polymer, thereby extending the polymer chain. Because a new coordination site opens up during insertion of the monomer, the process can be repeated over and over. 

It is relatively rare for the interactions with the polymeric support to be beneflcial to catalytic performance. However, one example of this was observed upon the deliberate positioning of a binol ligand in close proximity to polystyrene via amide linkages [15], which resulted in enhanced performance in the titanium-catalyzed addition of diethylzinc to aldehydes (Figure 5.2b). 

Schliiter et al. described the synthesis of soluble PPV derivatives from substituted aromatic dialdehydes via McMurry-type polycondensation reaction. With this low valent titanium catalyzed reaction (see Scheme 12), the obtained products are characterized by a double bond cisjtrans ratio of about 0.4 and an average degree of polymerization of about 30 [43]. 

To facilitate catalyst recovery, polymeric and dendrimeric TADDOL, and BINOL ligands have been used for the titanium-catalyzed diethylzinc addition reaction [49]. Moreover, ionic liquids and fluorous solvents have also been used as the reaction media to facilitate the separation of ligands [50]. The microporous metal-organic frameworks prepared from BINOL derivatives were applicable to heterogeneous diethylzinc addition to aldehydes in the presence of excess amount of Ti(O Pr)4 [51]. 

Addition polymerization can be accomplished not only through a free radical initiator as mentioned above, but also by some other means. The most important polymerization catalyst is of the type known as Ziegler-Natta catalyst. These two chemists discovered that a combination of chemicals titanium tetrachloride and triethyl aluminum is an excellent catalyst for polymerizing a number of olefins. They were awarded Nobel Prize in 1963 for this discovery. Subsequent research by others found that similar combinations of chemicals a transition element compound and triethyl aluminum or similar alkylating agent do catalyze polymerization of olefins. Specific combination of such chemicals allow formation of polymers of specific stereochemistry. 

An important metal-catalyzed polymerization is that initiated by Ziegler-Natta catalysts. They are typically made from titanium tetrachloride and a trialkylaluminum, such as trieth-ylaluminum, A1(CH2CH3)3. The system polymerizes alkenes, particularly ethene, at relatively low pressures with remarkable ease and efhciency. 

The polymerization of diphenyldiacetylene in the presence of vanadyl acetyl-acetonate and triethylaluminum gave an open-chain polymer 10) [13]. However, when tri(isobutyl)aluminum and titanium tetrachloride were used to catalyze polymerization of [11], the product was postulated to contain another type of repeat unit [14] 35). The soluble product from the latter polymerization possessed a cryoscopic molecular weight of 1270 and a softening point of 255°-260°C. It also had good heat resistance, losing only 3% of its weight after 6 hours at 250°C in argon. Definite catalytic activity toward the decomposition of nitrous oxide was exhibited by this product [12,14]. 

Polymerizations catalyzed with coordination compounds are becoming more important for obtaining polymers with special properties (linear and stereospecific). The first linear polyethylene polymer was prepared from a mixture of triethylaluminum and titanium tetrachloride (Ziegler catalyst) in the early 1950s. Later, Natta synthesized a stereoregular polypropylene with a Ziegler-type catalyst. These catalyst combinations are now called Zieglar-Natta catalysts. 

Amidinate complexes of copper(I) and copper(II) have been found to catalyze the polymerization of carbodiimides. The copper catalysts are highly active even under air and oxygen. It was shown that the catalytic activity of an air-stable copper(II) amidinato complex is almost equal to that of reported air-sensitive titanium(IV) amidinate initiators. Scheme 225 illustrates the proposed reaction mechanism. ... 

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