Cocatalysts mechanism

Friedel-Crafts (Lewis) acids have been shown to be much more effective in the initiation of cationic polymerization when in the presence of a cocatalyst such as water, alkyl haUdes, and protic acids. Virtually all feedstocks used in the synthesis of hydrocarbon resins contain at least traces of water, which serves as a cocatalyst. The accepted mechanism for the activation of boron trifluoride in the presence of water is shown in equation 1 (10). Other Lewis acids are activated by similar mechanisms. In a more general sense, water may be replaced by any appropriate electron-donating species (eg, ether, alcohol, alkyl haUde) to generate a cationic intermediate and a Lewis acid complex counterion. 

The catalytic cycle (Fig. 5) (20) is well estabUshed, although the details of the conversion of the intermediate CH COI and methanol into the product are not well understood the mechanism is not shown for this part of the cycle, but it probably involves rhodium in a catalytic role. The CH I works as a cocatalyst or promoter because it undergoes an oxidative addition with [Rh(CO)2l2]% and the resulting product has the CO ligand bonded cis to the CH ligand these two ligands are then poised for an insertion reaction. 

Figure 33 The catalytic mechanism for the production of borane-terminated isotactic polypropylene (z-PPs) via in situ chain-transfer reaction by a styrene/hydrogen consecutive chain-transfer reagent allowing the utilization of MAO cocatalyst (50). (Adapted from ref. 74.)...

Abstract Zirconocenes have been used for a long time in the field of olefin polymerization using MAO as cocatalyst. The equivalent hafnocenes were seldom used due to a lack of productivity while using MAO activation. In the last few years borane and borate activation has come into the focus of research for olefin polymerization. A variety of different hafnocenes were used to investigate the polymerization mechanism and the different cocatalysts. 

Independent of the ligand system, two different activation methods have been used in performing the propylene polymerization experiments. In both cases, the catalytic activities and molecular weights of the polymers are a sensitive function of the aluminum content provided by the activators. This dependence suggested an additional reversible chain transfer to aluminum when activating with MAO. As lower contents of A1 are provided in the polymerization system in the case of in situ activation with TIBA/borate, the only mechanism occurring is the chain back-skip. Furthermore, the differences in the polymer microstructures prepared with MAO and borate as cocatalysts are reflected. They sustain the proposed reversible chain transfer. 

Natta s bimetallic mechanism stipulates that when the catalyst and cocatalyst components are mixed, the chemisorption of the aluminium alkyl (electropositive in nature) occurs on the titanium chloride solid surface which results in the formation of an electron-deficient bridge complex of the structure shown... 

Many other compounds have been shown to act as co-catalysts in various systems, and their activity is interpreted by analogous reactions [30-33]. However, the confidence with which one previously generalised this simple picture has been shaken by some extremely important papers from Eastham s group [34], These authors have studied the isomerization of cis- and Zraws-but-2-ene and of but-l-ene and the polymerization of propene and of the butenes by boron fluoride with either methanol or acetic acid as cocatalyst. Their complicated kinetic results indicate that more than one complex may be involved in the reaction mechanism, and the authors have discussed the implications of their findings in some detail. 

Thus, our experiments with isobutene show that for this monomer the Gantmakher and Medvedev theory is not applicable they also show that for isobutene CH2C12 is not a cocatalyst to TiCl4. However, it was still possible that the polymerisation of styrene at the lowest water concentration was due not to residual water, but either to co-catalysis by the solvent or to direct initiation by the Gantmakher and Medvedev mechanism. However, since we found the molecular weight to be independent of both the water and the TiCl4 concentration and the rate at low water concentration to be independent of the TiCl4 concentration, these alternatives appeared unlikely. 

The three-component synthesis of benzo and naphthofuran-2(3H)-ones from the corresponding aromatic alcohol (phenols or naphthols) with aldehydes and CO (5 bar) can be performed under palladium catalysis (Scheme 16) [59,60]. The mechanism involves consecutive Friedel-Crafts-type aromatic alkylation and carbonylation of an intermediate benzylpalla-dium species. The presence of acidic cocatalysts such as TFA and electron-donating substituents in ortho-position (no reaction with benzyl alcohol ) proved beneficial for both reaction steps. 

These experimental results suggested a hydrogen-bonding mediated cooperative Bronsted acid catalysis mechanism (Scheme 6.28). Thiourea cocatalyst 9 is viewed to coordinate to mandelic acid 20 through double hydrogen-bonding, stabilizes the acid in the chelate-hke cis-hydroxy conformation, and acidifies the a-OH proton via an... 

Jun s proposed mechanism was probed by a deuterium labeling experiment using N-methyl co-catalyst 66 (Equation 9.8). Hydridoimidoyl complexes (e.g., 64), which are implicated by Jun in C—C bond formation, cannot form when secondary amine 66 is used as a cocatalyst. Instead, C—H reductive elimination from a complex... 

In both the mechanisms the polymer chain grows from the catalyst surface by successive insertion reactions of the complexed monomer, the R group originally present in the cocatalyst ending up as the terminal group of the chain. 

These cocatalyst effects observed in the stereospecific polymerization of aliphatic monoaldehyde by the organoaluminum catalyst are similar to those reported by Letort for free cationic polymerization. We prefer the coordinated cationic mechanism to the coordinated anionic one proposed by several workers. 

In order to establish this supposition of a restricted rotation around the coordination bond, it should be assured that the contribution of the dissociation mechanism is negligibly small in the hydrogen-exchange reaction. For this purpose, a 1 1 mixture of (ZJ-propene-l-rf, (composition d0, 3.6% Z-l-dlt 86.1 % E-l-, 7.5% Z-l, 2-d2,1.4% E-l,2-d2,0% and 1, l-d2,1.4%) and propene-d6 (purity, propene-powder catalyst at room temperature, and HD gas (98 %) was added as a cocatalyst. In this experiment, the amounts of Mo—H and Mo—D sites are approximately equal during reaction. The primarily exchanged products formed by associative and dissociative mechanisms are described respectively in Scheme 11. 

Two parallel routes for the elimination of glycol formate are suggested, involving either reaction with H2 or with cocatalyst water. The detection of formic acid in the reaction products suggests another mechanism, with initial production of formic acid from H2 and C02, followed by reaction with the oxirane. This mechanism is not favored however since the yields of glycol formates varied substantially when various substituted oxiranes were reacted. This would not have been expected in a mechanism with formic acid as an intermediate. A third mechanism, not considered by the authors, could proceed through initial production of propylene carbonate, followed by reduction to the mono- or di-formate. 

---