Cocatalysts maximum rate

Although neutral nucleophiles (e.g., DMAP and Cy3P) have been shown to be effective as cocatalysts for these processes, in these instances there are initiation periods prior to achieving the maximum rate of copolymer production. This is best illustrated in Figure 8.8 for the formation of the copolymer from CHO and C02, as revealed by IR traces during in-situ IR monitoring. 

The decisive contributions made by Plesch to the understanding of the chemistry of cationic polymerisation also includes a pioneering piece of research in this specific area. In 1950, he published a classical paper on the role of trichloroacetic acid as cocatalyst in the polymerisation of isobutene by titanium tetrachloride in hexane at low temperature An analysis of the dependence of the maximum polymerisation rate upon the concentration of monomer was carried out by us frcm the plots published by Plesch and it can be concluded that a second power law was approximately followed. The presence of an excess of TiCl4 over the cocatalyst gave a maximum rate proportional to the concentration of the latter. We assume that in these crmditions the following mechanism accounts for the experimental observations ... 

Cocatalysts enhance overall average activity primarily by accelerating the development of polymerization rate and to a lesser degree by raising the maximum rate attained [52,684,686-688]. The induction time is shortened (and usually eliminated), and the polymerization rate then rises to its maximum more quickly. The more rapid development of activity in the presence of cocatalyst suggests that active sites are reduced, and even initiated, more quickly. The higher rate suggests that some sites are created that would not have become active otherwise. 

Hydroesterification with Co catalysts most commonly utilize pyridine or alkyl pyr-idines such as -y-picoline as cocatalysts. In a massive screening study, Co with pyridine promotors, unsubstituted in the ortho positions, gave the highest proportion of linear esters with either 1-octene oi a mixture of isomeric internal n-dodecenes. The promoter comparison was conducted at 160 C and 16.0 MPa for octene and 170°C and 18.0 MPa for the internal dodecenes, in both cases in an excess of CH3OH. The maximum rate for... 

When the relative speeds of anion transfer and intrinsic reaction are equal then the PTC system is at its maximum rate. However, if the transfer step is slower then the system is limited by the rate of transfer. If the reaction rate is the slower of the two, then the system is reaction rate limited. In either case, there are many parameters that can be manipulated to increase the overall rate of reaction catalyst structure [10], catalyst concentration [11], agitation [12], temperature [13], water content [14], solvent choice [15], inorganic anion [3] and cation [16], and even cocatalysts [17] can be used. The parameters that are important in traditional PTC systems may also be important in SCF/PTC systems. 

Ethylene Homo- and Co- polymerization. Ethylene homopolymerization and copolymerization with 1-hexene as a comonomer were carried out at 70 "C using AlEt, as a cocatalyst. The rate profiles of homo- and co-polymerization were changed drastically as the Hg/Ti ratio of the catalyst was changed. In homopolymer-ization the time to reach maximum rates becomes short as Hg/Ti ratio of the catalyst increases (Fig. 6). At the low Hg/Ti ratios to less than 2.6 the polymerization rate increases slowly to reach a steady-state value, which remains unchanged for an experimental period. At high Mg/Ti ratios, ca., Mg/Ti 16.5, poly-... 

For the two catalyst systems NdV/DIBAH/EASC and NdV/TIBA/EASC the ratios of Ai/ Nd were varied over a broad range from 5 to 50. It was shown that kp depends on the nature of the cocatalyst (TIBA and DIBAH) and on the respective concentrations of these cocatalysts. As kp is not constant the authors suggest that it is better considered as an apparent rate constant ka. For the cocatalyst TIBA the dependence of ka on nA /nN< results in a S-shaped curve with a maximum in ka = 311 L mol-1 min 1 at TiBA/ Ndv = 50 while activation with DIBAH leads to an inverse u-shaped curve with a maximum ka 220 L -mol 1 min 1 at Dll Aii/ NdV = 35 (Fig. 11 in Sect. 2.2.8) [179]. 

Any interpretation of these polymerisations based on this type of interaction as the basic initiation step (i.e., without the participation of the mtaiomer) is therefore incorrect. On the other hand, our mechanism involving the intervention of a monomer-catalyst complex justifies the necessity of the presence of the mcaiomer when catalyst and cocatalyst are mixed, if initiation rates passing throu a maximum are to be obtained. 

The polymerisation of benzene through repeated nucleophilic substitutions on the rings was studied by Kovacic et al. using ferric chloride as catalyst and water as cocatalyst. This system is of course outeide the realm of cationic polymerisation throu the double bcmd of an olefin, but illustrates well the role of water in Friedel-Crafts polycondensations. The authors showed that the rate of this reaction went throu a maximum at a catalyst/cocatalyst ratio of one and attributed this observation to the high activity of ferric chloride monohydrate ... 

It can be assumed that since [CClsCOOHJo < [TiC jo, all the carboxylic acid is com-plexed with the catalyst and consequently the rate of polymerisation is directly proportional to the concentration of the cocatalyst, i.e. the first equilibrium in the above scheme is very strongly shifted to the right-hand side. An inspection of Plesch s plots shows moreover that at relatively low titanium tetrachloride concentrations, and constant trichloroacetic acid concentration, the maximum polymerisation rate depended on [TiCl4], in agreement rvith our proposed Adg3 mechanism. However, when the... 

A dual role is played by methanol, which is both a solvent for reagents and products and a cocatalyst. Studies on tiie epoxidation mechanism, on the formation of TS-1 peroxi s and on acid properties of TS-l/HjO system (72-75), suggest that methanol takes part in the reaction mechanism by promoting the formation of the active species (Figure 1). Accordingly, reaction kinetics reaches maximum efficiency in the presence of methanol (72, 13). Initial turnover frequencies of 1-2 s have been observed at 40°C, in 92wt% methanol. Other polar solvents, even pure water, are usable provided that a decrease in the rate of reaction and in the yields is tolerated (72). 

Reference Year Powder material/ cocatalyst Reduction by Oxidation by Temperature Maximum H CO during oxidation step (% conversion of H2O/CO2, rate per g solid) No. of cydes/ reactor type/ g solid Characterization... 

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