Deactivation and activation

Spontaneous polymerizations of monomers without the addition of initiator or catalyst are relatively rare. Examples are the thermal polymerization of styrene (Section 20.1.3) and the charge-transfer-initiated polymerization of monomers with opposite polarities (Section 18.1.2.2). It is often difficult to distinguish these genuine spontaneous reactions from nongenuine ones where polymerization is induced by some unrecognized admixture to the system. 

As a rule, monomers can only undergo polymerizations if they are suitably activated. Thus, strongly polarized double bonds or rings with heteroatoms can easily be induced to polymerize. 

Because of their free electron pairs or electron shell vacancies heteroatoms are particularly susceptible to attack by catalyst. Since heteroatom-containing groups react both by polycondensation and polyaddition mechanisms, they are especially readily catalyzed. The polymerization of rings with heteroatoms (lactams, lactones, trioxane, etc.) can also be easily initiated. On the same basis, however, deactivation of the chain can also occur frequently in these substances therefore, only low degrees of polymerization can be obtained. On the other hand, the activation of cycloalkanes in polymerization reactions proceeds with difficulty. The following discussion will therefore concentrate on the polymerization of rings and monomers with multiple bonds. 

Practice shows that different monomers respond differently to the various classes of initiators (Table 16-15). Styrene, for example, is polymerized by free radicals produced by the decomposition of dibenzoyl peroxide, by cations formed from BF3 + H2O H [Bp30H] , by anions from UC4H9, or by Ziegler catalysts [e.g., TiCU plus A1(C2H5)3]. Vinyl esters, on the other hand, can only be polymerized free radically in the condensed (fluid) phase, formaldehyde only cationically and anionically, acetaldehyde only cationically, etc. 

Whether a monomer can be activated to polymerize by a given class of initiator can be deduced from the polarization of the bonds and the steric effects. Formaldehyde has a negative partial charge on the oxygen atom and a positive partial charge on the carbon atom ( CH2=0 ). A cation R (e.g., a proton H ) can therefore attack at the oxygen atom, thus inducing polymerization  

On the first page of this chapter, we saw that benzene is unreactive toward bromine  

In order to force a reaction to occur, we introduced a Lewis acid into the reaction mixture, which generated a better electrophile (Br is a better electrophile than Br2). In fact, all of the reactions we have explored thus far have been examples of benzene reacting with powerful electrophiles (Cl, N02, alkyl , acyl , and SO3). Now, we will turn our attention to the nucleophile—how can we modify the reactivity of the aromatic ring  

To answer this question, we wiU explore substituted benzene rings, and we wiU consider the effect that a substituent wiU have on the reactivity of the ring. Benzene itself (CeHe) has no substituents. But consider the structure of phenol  

In this compound, the aromatic ring has one substituent an OH group. What effect does this substituent have on the nucleophilicity of the aromatic ring Is this compound a better nucleophile than benzene  

The oxygen atom is withdrawing electron density from the ring. Remember that the aromatic ring is only a nucleophile in the first place because it is electron-rich (from all of those tt electrons), so withdrawing electron density from the ring (via induction) should render the ring less nucleophilic. But we re not done yet. We need to consider one other factor resonance. 

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In addition to [A ] being qiiasi-stationary the quasi-equilibrium, approximation assumes a virtually unperturbed equilibrium between activation and deactivation (equation (A3.4.125)) ... 

An important example for the application of general first-order kinetics in gas-phase reactions is the master equation treatment of the fall-off range of themial unimolecular reactions to describe non-equilibrium effects in the weak collision limit when activation and deactivation cross sections (equation (A3.4.125)) are to be retained in detail [ ]. 

A. Electrophiles capable of substituting both activated and deactivated aromatic rings ... 

E. Activation and Deactivation by Other Groups i. General Effects and Summary... 

The diEFerent activation and deactivation influences are seen in 4,5-bromo-2-phenylpyridazin-3-one (209) which reacts with methoxide, hydrazine, or secondary amines at the 6-position. Related N—H and iV -alkyl halopyridazinones behave similarly. 

Relative reactivity of ring-positions based on positional selectivity of polychloro-azines must be regarded with caution because of the unequal activating effects of the chlorine substituents on each other. Also, it should be emphasized that one cannot use the positional selectivity in di- and tri-substitutions to assess relative reactivity of different positions. In such substitutions, the reactivity is determined by a complex combination of activating and deactivating effects which are unequal at the ring-positions (cf. Sections II, E, 1, II, E, 2,c, and II,E,2,e). 

No differences in operability and catalyst behavior (activity and deactivation) in the two plants were discernible. The expected catalyst lifetime in a commercial plant, calculated from the movement of the temperature profile down the catalyst bed with time, in both cases will be more than 16,000 hrs under the design conditions. 

Polymer formation during the Kharasch reaction or ATRA can occur if trapping of the radical (123), by halocarbon or metal complex respectively, is sufficiently slow such that multiple monomer additions can occur. Efficient polymer synthesis additionally requires that the trapping reaction is reversible and that both the activation and deactivation steps are facile. 

Emulsion polymerization has proved more difficult. N " Many of the issues discussed under NMP (Section 9.3.6.6) also apply to ATRP in emulsion. The system is made more complex by both activation and deactivation steps being bimolecular. There is both an activator (Mtn) and a deactivator (ML 1) that may partition into the aqueous phase, although the deactivator is generally more water-soluble than the activator because of its higher oxidation state. Like NMP, successful emulsion ATRP requires conditions where there is no discrete monomer droplet phase and a mechanism to remove excess deactivator built up in the particle phase as a consequence of the persistent radical effect.210 214 Reverse ATRP (Section 9.4,1,2) with water soluble dialky 1 diazcncs is the preferred initiation method/87,28 ... 

The chemical mechanisms of transition metal catalyses are complex. The dominant kinetic steps are propagation and chain transfer. There is no termination step for the polymer chains, but the catalytic sites can be activated and deactivated. The expected form for the propagation rate is... 

Several researchers (1,5,13-14) have shown evidence of classes of sites having different activities. Accordingly, blmodal distributions with distinctly different activity and deactivation characteristics were studied. As before the most active sites were assumed the least stable, as has been suggested by others (1-3). One major class of site was a high activity fast deactivating (HAFD) site and the other a low activity slow deactivating (LASD) site. The ratio of the mean HAFD... 

Minchin, R.F. and Boyd, M.R. (1983). Localization of metabolic activation and deactivation systems in the lung. Significance to the pulmonary toxicity of xenobiotics. Ann. Rev. Pharmacol. Toxicol. 23, 217-238. 

Mantovani A, Allavena P, Vecchi A, Sozzani S. Chemokines and chemokine receptors during activation and deactivation of monocytes and dendritic cells and in amplification of Thl versus Th2 responses. Int J Clin Lab Res 1998 28 77-82. 

Bismuth tra-tri lluoromcthancsulfonate, Bi(OTf)3, and BiCh were found to be effective catalysts for the Friedel-Crafts acylation of both activated and deactivated benzene derivatives such as fluorobenzene.19 Ga(III) triflate is also effective for Friedel-Crafts alkylation and acylation in alcohols and can tolerate water.20 This catalyst is water-stable... 

Since many metallic catalysts have high adsorption affinities, we often find that certain poison molecules are adsorbed in an immobile form after only a very few collisions with the catalyst surface. In this situation, the outer periphery of the catalyst particle will be completely poisoned while the inner shell will be completely free of poison. The thickness of the poisoned shell grows with prolonged exposure to poison molecules until the pellet is completely deactivated. During the poisoning process, the boundary between active and deactivated regions is relatively sharp. 

Adolfson, R., and Moudrianokis, E.N. (1976) Molecular polymorphism and mechanisms of activation and deactivation of the hydrolytic function of the coupling factor of oxidative phosphorylation. Biochemistry 15, 4164—4170. 

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