Soluble Catalytic Systems

The second reason is not so evident for example, the data were obtained for a soluble catalytic system based on biscyclopentandienyltitanium-dichloride (178, 179) for which case the propagation center formed contains a Ti(IV) ion. 

A detailed study of the oligomerization of ethylene in toluene with soluble catalytic systems of the type (C5H5)2TiRCl—AlC HjCl has been carried out by Fink et al. 

Furthermore, soluble catalytic systems containing homogeneous active centres yield polymers with very narrow MWD (Q 2) even when they are used in hetero-phasic processes (see Section 4.1.1). 

An excellent review on soluble systems, including also the more recent progresses in ethylene polymerization, is that of Sinn and Kaminsky who even developed soluble catalytic systems with extremely high activities by using alumoxane and biscyclopentadienyl-titanium or-zirconium compounds. Interesting results have also been obtained in ethylene oligomerization and in propylene dimerization... 

Doi et al. studied the effect of different aluminium alkyls on the polydispersity of syndiotactic polypropylene obtained with the V(acac)3-alkyl aluminium halide soluble catalytic system, at temperatures below —65 °C. These authors found that, by varying the type of aluminum alkyl not only the propagation and transfer rates are changed, but also the polymer polydispersity index decreases in the following order ... 

On the other hand, changes in MWD are expected when the aluminum alkyl undergoes a true transformation following a chemical reaction with a third additional component. Soluble catalytic systems based on titanocenes coupled with alumoxanes proved to be suitable for such study. Cihlaf et al. investigated the MWD... 

For polypropylene, it has been found that the MWD of both the crystalline and amorphous fractions, obtained with the a-TiClj—Al(C2Hs)2Cl catalytic system, are independent of the Al/Ti ratio. The same result was obtained for polypropylene with MgClj-supported high yield catalytic systems. Instead, with the ( 5115)2— TiCl2—-A1(CH3)2C1 soluble catalytic system, Chien observed a narrowing of polyethylene MWD with an increase in the Al,Ti ratio and explained it by an increase of chain transfer rate. 

In conclusion, electronic density of the transition metal may be influenced, case by case, by the effect of the reaction with aluminum alkyl and, as a result, the carbon-transition metal bond stability, olefin coordination and insertion capacity, stereochemical control of active centre and chain transfer and propagation processes, hence polymer MWD, may also be affected. This is particulary true for soluble catalytic systems for which the existence of active centres as bimetallic complexes is likely. 

MWD is usually broadened with a decrease of polymerization temperature. In the ethylene polymerization with the (C5H5)2TiClj—Al(CH3)2Cl soluble catalytic system, Chien attributed such behaviour to a greater decrease in termination rate in comparison to propagation rate as temperatures decreases. 

Recent research efforts concentrated on soluble catalytic systems, like di- y -cyclopentadienyl-diphenyltitanium and tetrabenzylzirconium complexed with methylaluminoxane, (CH3)2Al-[-0-Al(CH3)-]rt-Al(CH3)2. Such catalysts, however, yield products that contain only about 85% isotactic polypropylene, " and only if the reactions are conducted at low temperatures, -45 °C or lower. 

Apparently, soluble catalysts are obtained by reaction of Ti(OR)4 with AIR3 [144]. High-molecular-weight polyethene is obtained in variable amounts, with Al/Ti ratios ranging between 10 and 50. Similar results are attained by replacing titanium alkoxide by Ti(NR2)4 [145]. Soluble catalytic systems are also obtained by reaction of Ti(acac)3 [146] and Cr(acac)3 [147] with AlEts as well as by reaction of Cr(acac)3 and VO(acac)2 with AlEt2Cl in the presence of triethyl phosphite [121]. With vanadium catalysts the activity reaches its maximum at Al/V ratio = 50. Under these conditions up to 67% vanadium is in the bivalent oxidation state. Bivalent and trivalent compounds will be active. 

Even in experiments performed at strictly constant F, the copolymers obtaiied with heterogeneous catalysts sometimes have a significant compositional distribution (32,129,138,145,179,180). The possible explanation is that different active sites on the catalyst sur ce have different activities in respect to comonomers, i.e. different aixl rj values (66, 145, 169). On the other hand, when soluble catalytic systems are used, a relatively narrow compositional distribution is observed (32,179,180) showing ore type of active site. 

Ben2yloxy- and 2-tosyloxystyrene were hydroformylated under various reaction conditions to obtain the corresponding linear aldehydes. The best results (up to 70% linear aldehyde at 80 °C and low pressure) were obtained by using the catalyst precursor Pt(xantphos)Cl2 in toluene or the water-soluble catalytic system Rh (CO)2(acac)/2,7-bis(S03Na)2-xantphos in the biphasic medium water/toluene [72]. 

Amino acids and oligopeptides were used as ligands for Rh(CO)2(acac) in the aqueous biphasic hydroformylation of styrene. The water-soluble catalytic system maintained its activity practically unchanged during three recyded experiments [113]. 

Many other mono- and bidentate ligands have also been explored, and structures 8.18 and 8.19 are two examples. The ligand 8.18 in combination with PdCOAc) gives a water-soluble catalytic system for the selective oxidation of alcohols to the corresponding carbonyl derivatives. Most oxidation reactions are potentially hazardous. The use of water as the solvent makes oxidation reactions considerably safer. 

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