Bimolecular activation

The polymerizations (a) and (b) owe their success to what has become known as the persistent radical effect."1 Simply stated when a transient radical and a persistent radical are simultaneously generated, the cross reaction between the transient and persistent radicals will be favored over self-reaction of the transient radical. Self-reaction of the transient radicals leads to a build up in the concentration of the persistent species w hich favors cross termination with the persistent radical over homotermination. The hoinolermination reaction is thus self-suppressing. The effect can be generalized to a persistent species effect to embrace ATRP and other mechanisms mentioned in Sections 9.3 and 9.4. Many aspects of the kinetics of the processes discussed under (a) and (b) are similar,21 the difference being that (b) involves a bimolecular activation process. 

Atom transfer radical copolymerization can be described by a scheme similar to that shown in Scheme 9.48 except that bimolecular activation steps must be added ( Section 9.4). Copolymerization by ATRP through 2001 has been reviewed by Kelly and Matyjaszewski.554 A summary of ATRP copolymerizalions appears in Table 9.21. 

As products accumulate, however, or if inert gas is added to the system, the activated molecules of product, which would normally activate fresh molecules of the reactant, might to a greater and greater extent become deactivated by collisions with inert molecules, and the reaction would then depend more and more upon the ordinary kind of bimolecular activation. 

The data on the third-order recombinations of radicals and atoms present us with the possil)ility of calculating the rates of the inverse process, namely, the rates of bimolecular activation of molecules. To preserve generality, let us consider the association of two active species A and B to form the stable product AB. If the reaction is sufficiently exothermic or the product AB has few internal degrees of freedom, the mechanism of the association is complex and must involve the agency of a third body M. The mechanism may involve either or both of the following paths ... 

At low pressures the rate of deactivating collisions will be very small, almost all of the activated molecules formed will dissociate and hence the rate determining step will be the bimolecular activation process. Now equation (2.18) reduces to... 

In addition, all bimolecular activation-controlled reactions are independent of the degree of polymerization [6]. Simple 8 2 reactions between reactive groups attached to chain ends of mono-disperse macromolecules in a wide range of molecular weights are independent of the DP [7, 8] in the range of 20-2,000 [7]. This was shown on three different reactions. In the first one, the reactivities of chlorine-terminated low and high molecular weight polystyrenes towards polystyryllithium are equal in benzene and cyclohexane solvents ... 

Formally, this process may be thought of as a superposition of bimolecular activation and unimolecular decay. The bimolecular step occurs in the collisional activation of an AB+ ion above one (or more) of its dissociation thresholds A+ fragment ions appear as products of the unimolecular decay of the activated ion. In the process noted above, Rg is often a rare gas atom. 

Equation 15.1 describes a sigmoidal activation curve as long as ki and k2 exist. However, no sigmoidal curve was observed at any of the pH values examined for the Trx-PG s. In addition, the fusion protein exhibited no difference (within error) in activation in the absence or presence of a 1 1 molar ratio of pepsin molecules (Fig. 15.16), whereas r-PG activation was accelerated in the presence of exogenous pepsin (Fig. 15.1a). Again, if activation of Trx-PG by pepsin is much faster than self-activation, as was observed in r-PG, faster activation would be expected with exogenous pepsin, but no such effect was observed. These results do not discount the existence of bimolecular activation of Trx-PG, but would suggest that bimolecular activation was extremely slow in comparison to unimolecular activation. Based on the results above, the activation of Trx-PG followed... 

The ability of this host to encapsulate more than one guest suggests its use as a reaction chamber for bimolecular activities such as the Diels-Alder reaction. Indeed, when both p-benzoquinone and cyclohexadiene were present in solution, an initial 200-fold rate acceleration was observed, but the optimism faded upon discovery of a product inhibition effect. Since the product is a better fit for the cavity, no turnover could be observed. However, the catalytic turnover was observed for a pair of p-benzoquinone and thiophene dioxide derivatives since the adduct is ejected from the capsule in favor of two reactant molecules. 

Sepharose-bound pepsinogen was employed by Bustin and Conway-Jacobs (7) to show that unimolecular pepsinogen activation is possible. In their experiments, activation of immobilized pepsinogen by exposure to acid was confirmed by the generation of isoleucine NH2-terminus in the bound pepsinogen as well as the production of proteolytic capability in the enzyme, which was attached to the column. Subsequently, McPhie (8) used the appearance of a spectral change to kinetically study pepsinogen activation above pH 3.75. These data were interpreted as consistent with mixed activation, i.e., the existence of both unimolecular and bimolecular activation processes. 

Those giving deactivation by reversible atom or group transfer and involving a bimolecular activation process (Scheme 63). For the systems described, the deactivator (X-Y) is a transition metal complex where Y is the metal in a higher oxidation state. Y is then the metal in a lower oxidation state. Y is inert with respect to monomer. Y can be considered as a catalyst for the process shown in Scheme 62 and many aspects of the kinetics are similar. The best-known example is ATRP (Section 3.04.6.5) where the deactivator X-Y is, for example, a copper(II) halide. 

Recently, Cui and coworkers [101] reported that Co(Salen) (L25 in Scheme 10.6) incorporated in chiral MOFs also could go through bimolecular reaction pathways for the HKR of racemic epoxides with up to 99.5% ee. Crystal structure analysis suggests that the MOP structure brought Co(Salen) units into a highly dense arrangement and close proximity which could enhance the bimetallic cooperative interactions. The same bimolecular activation process in Co(Salen)-based MOP has also been found by lin and coworkers [102]. 

Mechanistic study shows that the high activity of the catalyst is mainly derived from the enhanced cooperative activation effect and the enrichment of reactants in the nanoreactor (Scheme 10.20b). This result strongly confirms the cooperative activation effect in the asymmetric reaction and is corroborated by a DFT calculation that the activation energy could be greatly reduced when the reaction goes through a bimolecular activation pathway (Scheme 10.22) [104]. 

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