Polymerization with titanium chloride

The catalytic activity of certain of the Friedel-Crafts catalysts was shown to decrease over a very wide range in the series boron fluoride, aluminum bromide, titanium tetrachloride, titanium tetrabromide, boron chloride, boron bromide and stannic chloride (Fairbrother and Seymour, mentioned in Plesch al., 83). When boron fluoride is added to isobutylene at dry ice temperatures, the olefin is converted to a solid polymer within a very few seconds. The time required for complete polymerization with aluminum bromide hardly extends to a few minutes while reaction times of hours are required with titanium chloride and periods of days with stannic chloride. 

Polymerization Catalysed by Acids and Bases. Carbonium ions and carbanions respectively are carriers of the chain transfer in cationic and anionic polymerizations respectively. Ionic polymerization mechanism was exploited for the synthesis of polymeric stabilizers in comparison with the free-radical polymerization only exceptionally. The cationic process was used for the synthesis of copolymers of 2,6-di-tert-butyl-4-vinylphenol with cyclopentadiene and/or for terpolymers with cyclopentadiene and isobutylene [109]. System SnCWEtsAlCla was used as an initiator. Poly(lO-vinylphenothiazin) was prepared by means of catalysis with titanium chlorides [110]. Polymers of 4-[a-(2-hydroxy-3,5-dimethylphenyl)ethyl]-vinylbenzene [111] and 3-allyl-2-hydroxyacetophenone [112] were also prepared under conditions of cationic polymerization. 

PREPARATIVE TECHNIQUES Ziegler-Natta polymerization with titanium halide/ aluminum cdkyl catalyst and, optionally, ether, ester, or silane activator. Catalyst may be deposited on a magnesium chloride support. Slurry and gas phase processes are used. Catcdyst systems based on metallocenes are under development. Typical comonomers are ethylene and 1-butene. 

Erom 1955—1975, the Ziegler-Natta catalyst (91), which is titanium trichloride used in combination with diethylaluminum chloride, was the catalyst system for propylene polymerization. However, its low activity, which is less than 1000 g polymer/g catalyst in most cases, and low selectivity (ca 90% to isotactic polymer) required polypropylene manufacturers to purify the reactor product by washing out spent catalyst residues and removing unwanted atactic polymer by solvent extraction. These operations added significantly to the cost of pre-1980 polypropylene. 

In order to obtain compounds with Ti-O-P and Zr-O-P units, the hexaethoxy-derivative, NsPaCOEOg, was treated with titanium and zirconium tetrachlorides. In each case, hygroscopic solids of the type NaPaCOEOiOaMCU (M = Ti or Zr) and ethyl chloride were obtained. The degree of polymerization of these solids was 1.6—1.8, and on the basis of their i.r. and n.m.r. spectra, two alternative structures, (46) and (47), were proposed. In an alternative route to the same type of compound, N3P3CI6 was treated with tetra-n-butoxytitanium in o-xylene. Butyl chloride was liberated and a solid was obtained which has been assigned the structure (48). Its thermal decomposition was studied by differential thermal analysis. 

Plesh (188) treated polyvinyl chloride and vinyl chloride-vinylidene chloride copolymers with aluminium chloride or titanium tetrachloride for initiating the carbonium polymerization of styrene. As expected (218)... 

The first high-activity catalyst for ethylene polymerization which avoided the necessity of washing was derived from titanium(III)alkoxy chloride and triisohexylaluminum (92) [for the patent literature, see (25)1. Another route starts with titanium trichloride. Thus, excellent results as regards activity are obtained with the so-called Stauffer TiCls which contains 30 mol% of aluminum, obtained according to the following reaction. 

This situation is somewhat reminiscent to that encountered in enzyme chemistry where the active biocatalyst is a combination of an apo-enzyme and a coenzyme, the components alone being complete inactive. Substrate specificity, which is so characteristic for enzymatic processes is also high in carbonium ion chemistry. For example styrene is polymerized by titanium tetrachloride—water, but not by titanium tetrachloride— alkyl chlorides 37) however, with stannic chloride catalyst alkyl chlorides are effective cocatalysts 88). In the same vein Plesch (93) showed that water is a better cocatalyst than acetic or chloroacetic acid in conjunction with titanium tetrachloride in isobutene polymerization, but Russel (94) found just the opposite with stannic chloride. 

In 1944 these experiments were reexamined by a team of Hungarian workers 139). They polymerized anethole in benzene with large amounts of titanium chloride and stannic chloride at ambient temperatures and obtained colorless powders of very low molecular weight (DP = 7—8). 

Active polymerization catalysts have been derived from organomagnesium compounds, for example by reaction with titanium (tv) chloride [7] the polymerization of various vinyl monomers has been initiated by organomagnesium compounds [8] and recently polymerization initiated by magnesium ate complexes has been described [9, 10]. 

The chemistry of Lewis acids is quite varied, and equilibria such as those shown in Eqs. (28) and (29) should often be supplemented with additional possibilities. Some Lewis acids form dimers that have very different reactivities than those of the monomeric acids. For example, the dimer of titanium chloride is much more reactive than monomeric TiCL (cf., Chapter 2). Alkyl aluminum halides also dimerize in solution, whereas boron and tin halides are monomeric. Tin tetrachloride can complex up to two chloride ligands to form SnCL2-. Therefore, SnCl5 can also act as a Lewis acid, although it is weaker than SnCl4 [148]. Transition metal halides based on tungsten, vanadium, iron, and titanium may coordinate alkenes, and therefore initiate polymerization by either a coordinative or cationic mechanism. Other Lewis acids add to alkenes this may be slow as in haloboration and iodine addition, or faster as with antimony penta-chloride. 

As outlined in Section III. A.3. a, the strength of the Lewis acid with mixed chloride and alkoxy derivatives decreases as the number of chloride ligands are replaced with alkoxy groups. Titanium chloride with one alkoxy group polymerizes styrene and a-methylstyrene Lewis acid with two alkoxy groups is too weak to initiate polymerization of styrene, but will initiate polymerization of a-methylstyrene and vinyl ethers. The Lewis acidity of titanium chloride derivatives with three alkoxy groups are so low that only vinyl ether polymerizations reach reasonable conversions. 

More universal is the method of Cp determination using selective tracers such as carbon monoxide and carbon dioxide which interact only with active metal = polymer bondsUsing tagged CO and CO2 as quenching agents systematic data have been accumulated so far on the influence of the composition of catalysts (titanium chlorides with various organometallic compounds) and polymerization conditions on Cp and kp values for the polymerization of ethylene and propylene IS)... 

Based on the number of radioactive tags in the polymer being at short ( 10 min) contact with CO the following values of for the polymerization of ethylene on titanium chloride catalysts were found (go °C, bulk and supported... 

On the basis of the reported data on the achieved maximum activity of a titanium chloride catalyst supported on MgCl2 ( 5x 10 g C2H4 per g Ti per h and per atm ) the minimum possible value of kp(kp ) can be evaluated if one assumes that the number of ACg is equal to the total content of titanium in the catalyst. In this case, kf" a 0.4 x 10 l/(mol x s). This value does not exceed and is close to the value of kp found with a radioactive quenching agent used in the ethylene polymerization on a catalyst of this type. 

The effect of polymerization conditions on Cp has been studied mainly for two-component systems based on titanium chloride and vanadium chloride . The number of propagation centers changes with the polymerization time proportionally to the reaction rate and is independent of the monomer concentration (0.2-2 mol/1 at 70 °C). Most interesting is the effect of the polymerization temperature on Cp. It has been found that with rising temperature Cp ingreases (Table 3). In the... 

An essential difference is observed for the chain transfer with hydrogen in the polymerization on bulk TiClj (the chain transfer is 0.5th order with respect to [Hj]) and on catalysts supported on MgClj (first — order chain transfer with respect to [Hj]). This difference leads to higher values of the melt index of polyethylene prepared on the TiClyMgClj catalyst in the presence of H in comparison with non-supported titanium chloride catalysts... 

The activity of the catalyst in the olefin polymerization changes a wide range mainly with the variation of the number of ACs. This number depends on the nature and crystalline structure of the transition metal compound, the presence of a support and catalyst modifier, the nature of a cocatalyst, and the polymerization conditions. The most pronounced increase of the number of active centers (up to nearly half of the total content of titanium in the catalyst) is achieved by supporting titanium chlorides on anhydrous highly dispersed magnesium chloride. Catalysts of this type show the highest activity amongst all known catalytic systems used for olefin polymerization. 

Homogeneous catalysts for the ethylene polymerization based on bis(cyclopenta-dienyl)titanium(IV) compounds [4], tetrabenzyltitanium [14], tetraallylzirconium and hafnium are formed with diethylaluminum chloride, dimethylaluminum chloride or triethylaluminum as co-catalysts. Their activities are poor (less than 200 kg PE/mol catalyst per h), so no industrial application resulted. 

Titanium tetrachloride is used as an intermediate in the production of titanium metal, titanium dioxide, and titanium chloride pigments, as a polymerization catalyst, in the manufacture of iridescent glass and faux pearls, and with ammonia to produce smoke screens. It is also used as a catalyst in many organic syntheses in the chemical industry. Titanium tetrachloride was formerly used with potassium bitartrate as a mordant in the textile industry, and with dye-woods in dyeing leather. 

An example of a macrocycle with oxygen donors to support a Ti catalyst for lactide polymerization has been reported by Frediani et a/. " The authors described several titanium chloride complexes bearing calix[4]arene ligands, which act as catalysts for solvent-free lactide polymerization (Figure 18). These complexes acted as dual-site catalysts with two polymer chains growing from one metal center. 

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