Cocatalysts definition

It is not definitely determined by experiments whether or not the catalyst, R2A10A1R2, requires a cocatalyst for initiating the polymerization. This difficulty is due to the characteristic nature of the catalyst referred to above. 

Experiments described by Corey constitute a noteworthy example of double asymmetric induction where neither participant in the reaction is chiral [95] As illustrated in Figure 4.18 two different catalysts are necessary to achieve the best results. Control experiments indicated that the nucleophile is probably free cyanide, introduced by hydrolysis of the trimethylsilylcyanide by adventitious water, and continuously regenerated by silylation of the alkoxide product. Note that the 82.5% enantioselectivity in the presence of the magnesium complex shown in Figure 4.18a is improved to 97% upon addition of the bisoxazoline illustrated Figure 4.18b as a cocatalyst. Note also that the bisoxazoline 4.18b alone affords almost no enantioselectivity, and that the enantioselectivity is much less when the enantiomer of the bisoxazoline (Figure 4.18b) when used as the cocatalyst. Thus 4.18a and 4.18b constitute a matched pair of co-catalysts and 4.18a and ent-A. %h are a mismatched pair (see Chapter 1 for definitions). The proposed transition structure... 

The test was deliberately stopped after 5500 h. Compared to the Dimersol one-phase system, the Difasol nickel consumption was ten times lower and the cocatalyst consumption twice as low. No additional fresh IL was required. This definitely demonstrated the stability of the chloroaluminate IL under dimerization conditions. 

The cations formed in this way add on to the monomer and, so, start the polymerization. The cocatalyst is consequently improperly named and is definitely not cocatalytic. In actual fact, the initiating species is formed from it thus, it would be more proper to call it an initiator. The Lewis acid is also, correspondingly, not a catalyst, but a coinitiator. 

The type of monomers suitable for cationic polymerization are those containing an electron-donating substituent such as 1,1-dialkyl, alkene, alkoxy, and phenyl that stabilize the propagating cationic centers. Successftil industrial examples include polyisobutylene and its copolymers with dienes such as butyl mbber. In ionic polymerization, initiator is conventionally called a catalyst. However, by definition, catalyst and initiator are two different types of reagents. Catalyst takes part in reactions but can be removed from the final product if necessary. On the other side, initiator molecules or their fragments become a part of the produced chains after polymerization. In cationic polymerization, a single catalyst is not sufficient and a cocatalyst is required. Typical catalysts are Lewis acids such as BF3, AICI3, and TiCU that must be used with a protonic cocatalyst such as H2O and methanol ... 

Certain classes of enzymes require small, auxiliary, nonprotein molecules called cofactors, coenzymes, and prosthetic groups. Definitions for these three terms are somewhat arbitrary and, in fact, the term cofactor will be used in the following chapters to represent broadly the identity and functional roles of cocatalysts. The roles of cofactors are structural, functional, or both. They provide the enzyme with the chemical or photochemical capabilities lacking in the normal amino acid side chains. An enzyme devoid of a cofactor is called an apoenzyme. Apoenzymes are catalytically inactive. The active complex of the protein and the cofactor is termed a holoenzyme. The cocatalysts can be defined on the basis of the catalytic functions that are mediated (76). 

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