Activation-deactivation processes

We can distinguish several sub-classes of activation-deactivation processes according to their mechanism. These are shown in Scheme 9.1 -Scheme 9.3. 

Equation (89) shows that the allowance for the variation of the charge of the adsorbed atom in the activation-deactivation process in the Anderson model leads to the appearance of a new parameter 2EJ U in the theory. If U — 2Er, the dependence of amn on AFnm becomes very weak as compared to that for the basic model [see Eq. (79)]. In the first papers on chemisorption theory, a U value of 13eV was usually accepted for the process of hydrogen adsorption on tungsten. However, a more refined theory gave values of 6 eV.57 For the adsorption of hydrogen from solution we may expect even smaller values for this quantity due to screening by the dielectric medium. 

In the case of hydrogenase, the substrate hydrous are always present and so at reducing potentials the enzyme will generate hydrogen. In solutions of H2, the hydrogen-oxidizing activity can also be observed. Because measurements can be made over timescales of milliseconds to hours, it was possible to observe both the extremely rapid reaction of the enzyme with hydrogen, and the slow activation/deactivation processes at more positive potentials. 

This reversible activation/deactivation process can be summarized as follows H + PM R— H-R— H-R-Ga-GDP-G(3-Gy interaction— H-R + Ga-GTP + G 3 Gy complex— active Ga-GTP activates effector proteins —> downstream effects deactivation occurs via the GTPase activity of Ga so that Ga-GTP — Ga-GDP + P — Ga GDP binds G(3-Gy — the inactive GDP-Ga-Gfi Gy complex is re-formed. 

The model developed has been used to study the effect of catalyst pretreatment and operation temperatures upon the activation/deactivation processes during isopropyl alcohol (IPA) dehydrogenation on a Cu/Si02 catalyst. 

E.L. Agorreta, J.A. Pefia, J. Santamarfa and M. Monzdn, A kinetic model for activation-deactivation processes in solid catalysts,"Ind. Eng. Chem. Res. In press". 

There is an urgent need for an analysis of bottleneck properties in collisional activation-deactivation processes, which is more general than equation (8.4), although (8.4) will probably be useful initially in characterising the position and the severity of the bottleneck. Whether or not there are other patterns of transition probabilities, not exhibiting bottle-... 

The basic mechanism of SFRP is the alternating activation-deactivation process between large amounts of dormant species and small amounts of propagating radicals ... 

In the quasi-equilibrium for the catalytic activation-deactivation process (Scheme 2), Eq. 1 takes the form... 

Mikami et al. [101] devised catalytic system in which two KRs were acting simultaneously on a racemic catalyst, leading to an activation/deactivation process. One enantiopure activator selected one enantiomer of the catalyst, leading to activation, whereas another enantiopure compound selected preferentially the other enantiomer to give a deactivated catalyst. 

The penultimate effect on the kinetics of the activation/deactivation processes may be quite significant. For example, model studies of activation of various dimers in ATRP copolymerization of methyl acrylate (MA) and methyl methacrylate (MMA) showed the following relative values of act for dimeric species H-MA-MA-Br, H-MMA-MA-Br, H-MA-MMA-Br, and H-MMA-MMA-Br to be 1, 4.6, 19, and 96, respectively. The back-strain effect resulting from the presence of a MMA penultimate unit, and formation of a thermodynamically more stable radical from a MMA-Br terminal unit, increased the values of act by 5 and 20 times, respectively, in comparison to MA penultimate/terminal imits. The combined effects resulted in a 100-fold increase of k ct for H-MMA-MMA-Br relative to H-MA-MA-Br (116). 

Marosi, L., Cox, G., Tenten, A., et al. (2000). In Situ XRD Investigations of Heteropolyacid Catalysts in the Methacroleinto Methacrylic Acid Oxidation Reaction Structural Changes during the Activation/Deactivation Process, J. Catal., 194, pp. I40-I45. 

Over the past two decades, LRP has attracted great attention as a useful tool for preparing well-defined, low-polydispersity polymers [8-11]. The basic concept of LRP is a reversible activation-deactivation process (Scheme 11.1), where P-X is a dormant (end-capped) chain and P is a polymer radical. Due to this process, LRP allows fine control of polymer architectures retaining the advantages of conventional free radical polymerization such as simplicity, robustness, and versatility. For further details, see Reference [Hh]. Here, we briefly describe the mechanism of LRP. 

For MMA polymerization with X=TeMe, Yamago a al. synthesized low-polydispersity polymers (Mw/M 1.15) by the addition of a small amount of dimethyl ditelluride (MeTe)2, without which Mw/Mn exceeded 1.35 due to a small Cex (e.g., 3.6 at 60° C ). This suggests an inaease of k a in the presence of (MeTe)2. A kinetic study on the role of (MeTe)2 demonstrated that (MeTe)2 worked as an efficient deactivator of P to in situ generate MeTe (and P-TeMe), and MeTe" then worked as a highly reactive activator of the dormant species P-TeMe. Namely, there is a rapid reversible activation-deactivation process mediated by (MeTe)2, that is, P-TeMe+MeTe" P" + (MeTe)2 as another activation mechanism besides E)T, accounting for the observed dramatic improvement of the polydispersity controllability. 

Low-mass compounds such as shown in Figure 4 have been studied with their activation/deactivation processes as models for polymer adducts, and some of them are actually used as efficient initiating adducts of LRP. Examples are shown below. 

It is useful to interpret Eqs. (2.6)-(2.9). The dissociation rate at low pressure is equal to /ci[M], the rate of collisional activation [Eq. (2.6)]. At high pressures, the collisional activation/deactivation processes establish an equilibrium ratio of AB and AB, described by the rate-coefficient ratio ki/k-i, and the unimolecular dissociation process of AB, reaction (2), becomes rate determining [Eq. (2.8)]. In recombination at low pressures, association and redissociation of AB are much more frequent than collisional stabilization, such that an equilibrium between A, B, and AB is established, as described by the rate-coefficient ratio fc 2//c2 Collisional stabilization of AB, reaction (-1), is then rate determining [Eq. (2.7)]. At high pressures, collisional stabilization is so frequent that the rate of association of A and B, reaction (2), determines the recombination rate [Eq. (2.9)]. 

---