Catalyst-cocatalyst combinations

The cationic polymerization of several para-substi-tuted a-methylstyrenes initiated by various Friedel-Crafts catalyst-cocatalyst combinations has been studied for the effects of catalyst type, monomer substituent and reaction solvent polarity on polymer structure and properties. By using solvent mixtures, the tacticity of the resulting polymers could be varied over a wide range, the syndiotactic form being favored in the more polar mixtures. 

Table II. Effect of Variations in Catalyst-Cocatalyst Combinations on MWD for the Polymerization of Monomers with Different p-Substitutes...

The first kinetic model for propagation in homogeneous systems was proposed by Ewen [47], assuming that the propagation took place as shown in Fig. 9.18. This scheme, shown for Cp2Ti(IV) polymerization of propylene, is representative of the kinetics for dl of the polymerizations with Group IVB metallocenes. In the scheme, species 1 and 4 represent coordinatively unsaturated Ti(IV) complexes that are-formally 16-electron pseudo-tetrahedral species, species 2 represents the interacting catalyst/cocatalyst combination, while intermediate 3 is shown with the monomer coordinated... 

Although all three catalyst-cocatalyst combinations were tested under the same conditions, note that the polymers with density 0.938 g mL-1 were made at a higher reactor temperature (about 10 °C higher). If corrected, the slopes of all three lines would be slightly lowered. 

Species (1) and (4) in the scheme represent coordinatively unsaturated Ti(IV) complexes that are formally dP 16felectron pseudotetrahedral species species (2) represents the interacting catalyst/cocatalyst combination, while intermediate species (3) is shown with the monomer coordinated at an a molecular orbital with the three non-Cp ligands and the transition metal occupying a common equatorial plane. The growing chain is held between two lateral coordination sites accommodating an unidentified non-Cp anion (R ) and the monomer. 

Under the best reported conditions (concerning activity) for the aforementioned catalytic systems (use of Rh(acac)(CO)2, molar ratios Rh/Phen = 1, PhNOa/Rh = 250, PhNH2/Rh = 125, 160 °C, 68 atm in methanol, total volume 75 ml) a total conversion was reached in 2 h (plus 1.5 h required to reach the final temperature), with a 80 % selectivity in methyl phenylcarbamate, 15 % in aniline, 6 % in N-methyleneaniline, and 1 % in N-methylaniline. Use of [Rh(CO)2Cl]2 as catalyst under the same conditions gave a slower, but more selective reaction (3 h at 160 °C to reach complete conversion, with a 89 % selectivity in carbamate) [164], An even higher selectivity (96 %) was reported for the pyridine-promoted reaction (Py = 25 ml), although a lower catalytic ratio (166.7) was used in a reaction run for 5.5 h at 130 °C [165] with [Rh(CO)2Cl]2 as catalyst. Unfortunately, the examples were chosen is such a way to prevent a complete comparison of the different catalyst-cocatalyst combinations imder exactly the same experimental conditions, so that it is impossible to say which is the best. 

A variety of initiators have been used for cationic polymerization. The most useful type of initiation involves the use of a Lewis acid in combination with small concentrations of water or some other proton source. The two components of the initiating system form a catalyst-cocatalyst complex which donates a proton to monomer... 

Description Polypropylene with a melt flowrate ranging from 0.1 to 1,200 can be produced with the Borstar PP process. Currently, Ziegler Natta catalysts are used, but there is a potential to use single-site catalysts latter. When producing homopolymers and random copolymers, the process consists of a loop reactor and a gas-phase reactor in series. One or two gas-phase reactors are combined with this arrangement when heterophasic copolymers are produced. Propylene, catalyst, cocatalyst, donor, hydrogen, and comonomer... 

The two-component catalytic systems used for olefin polymerization (Ziegler-Natta catalysts) are combinations of a compound of a IV-VIII group transition metal (catalyst) and an organometallic compound of a I-III group non-transition element (cocatalyst) An active center (AC) of polymerization in these systems is a compound (at the surface in the case of solid catalysts) which contains a transition metal-alkyl bond into which monomer insertion occurs during the propagation reaction. In the case of two-component catalysts an AC is formed by alkylation of a transition metal compound with the cocatalyst, for example ... 

In 1991 Farina carried out a detailed study on the effect of ligands on the reaction between vinyltributyltin and several halides and triflates [77], His conclusion was that the best ligand for this type of reaction was tris(2-furyl)phosphine. Halides (particularly iodides) and triflates are indeed the most frequently used leaving groups, although Roth and Sapino showed that fluorosulfonates can also be used [78] their catalyst was palladium(II) acetate. A look at the catalyst (or to be exact precatalyst) and cocatalyst combinations, together with solvent variations, which have been used in the papers cited above will make it clear that there is in fact no ideal system, but that each reaction will basically require optimization. 

Procatalyst and cocatalyst combine in solution to give the actual catalyst, a procedure most suitable for routine application. There are no synthetic steps necessary to prepare the catalyst prior to the catalytic reaction to be performed. 

Catalysts and Kinetics. Hundreds of variants and combinations of catalysts, cocatalysts, catalyst pretreatments, and reaction conditions have been discovered and described, mostly in the patent literature (28). It is now generally agreed that most coordination polymerizations are heterogeneous, but that some are clearly homogenous. The basic characteristic that distinguishes all Ziegler/Natta-type stereoregular polymerization catalysts is that... 

Sterically hindered Zr and Ti chelated phenoxide complexes represent a new class of homogeneous olefin oligomerization/polymerization catalysts when combined with cocatalysts such as MAO and FAB (eq 36).Spectroscopic investigations of the reaction between the Zr dibenzyl complex with FAB in toluene reveals the formation of the corresponding cationic complex associated with a benzylborate anion via Ph coordination (64 eq 36). Similar findings were obtained from bis(o-arylphenoxide)M(CH2Ph)2 complexes (M = Zr, Ti),2° while the corresponding dimethyl complexes yield unstable species after FAB activation. The products mediate the polymerization of ethylene and propylene. 

TEMPO and other organic nitroxyls have been used as catalysts in combination with numerous stoichiometric oxidants, such as sodium hypochlorite [24], PhI(OAc)2 [25], and sodium chlorite [26]. A number of recent studies have shown that NO -based redox cocatalysts enable these reactions to be conducted with O2 as the terminal oxidant [27]. The general catalytic cycle for these aerobic nitroxyl/NO -catalyzed alcohol oxidation reactions is depicted in Scheme 15.6a. A variation of this approach features halides as additives, in which the X2/HX redox couple is believed to mediate the NO2/NO and oxoammonium/hydroxylamine redox couples (Scheme 15.6b). 

Supported metallocene catalysts were combined with KIO montmorillonite by Weiss et al. During preparation of the clay, bentonite was treated with mineral acid, causing some of the octahedral alumina sheet to dissolve. This created dendritic silica, whose hydroxyl termination served to immobilize an alkylaluminum cocatalyst (either AlMes or Al Bus). The supported cocatalysts activated the simple metallocene dichlorides II and XI,... 

Gu and coworkers reported an improved protocol for the Miehael reaction between acetone and aromatic nitroalkenes by employing Ma s saccharide-derived catalyst 37 combined with acetic acid as cocatalyst (Scheme 19.40, left). The aeidie counterpart enhanced the performance of the catalytic system delivering the products in high yields (76-94%) and with high to excellent enantioselectivity (88-96% enantiomeric excess), whilst the catalyst... 

In very small concentrations, water acts as a co-catalyst, initiating polymerization in combination with a catalyst (e.g., SnCl4). However, in larger concentrations, it inactivates the catalyst (such as by hydrolysis of SnCU) or competes successfully with monomer for the catalyst-cocatalyst complex and inactivates the proton by forming hydronium ion [see Eqs. (P8.18.2) and (P8.18.4)] because the basicity of the carbon-carbon double bond is far less than that of water ... 

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