Cationic active center, structure

As shown earlier in this chapter, there are several systems in cationic ring-opening polymerization, in which quantitative initiation may be achieved and there is essentially no transfer and/or termination. Thus, all the chains have the initiator moiety as the head group and active centers at the growing chain end. These cationic active centers may be deactivated with suitable nucleophiles, leading to specific structure of the terminal end group. 

Cationic active center will be stabilized by electron-donating substituents that delocalize positive charge, while anionic active center is favored with electron-withdrawing or electronegative substituents which delocalize negative charge as shown in the following (a) carbocationic and (b) carbanionic structures ... 

The initial solution of the crystal structure of the Torpedo enzyme [28], followed by the mammalian AChE structure [29], revealed that the active center serine lies at the base of a rather narrow gorge that is lined heavily with aromatic residues (Fig. 11-6). The enzyme carries a net negative charge, and an electrostatic dipole is oriented on the enzyme to facilitate diffusional entry of cationic ligands. Crystal structures of several inhibitors in a complex with AChE also have been elucidated [25]. 

In summary, we have shown that stable cationic charge centers can significantly enhance the reactivities of adjacent electrophilic centers. Most of the studied systems involve reactive dicationic electrophiles. A number of the reactive dications have been directly observed by low temperature NMR. Along with their clear structural similarities to superelectrophiles, these dicationic systems are likewise capable of reacting with very weak nucleophiles. Utilization of these reactive intermediates has led to the development of several new synthetic methodologies, while studies of their reactivities have revealed interesting structure-activity relationships. Based on the results from our work and that of others, it seems likely that similar modes of activation will be discovered in biochemical systems (perhaps in biocatalytic roles) in the years to come. 

Coordination by oxygen atoms is not the only mechanism with which cations can be bound in the cavity of a natural or non-natural receptor, however. The crystal structure of acetylcholinesterase, an enzyme that catalyzes the hydrolysis of the neurotransmitter acetylcholine into choline and acetate, with the inhibitor deca-methonium (Me3N+(CH2)ioNMe3+) included inside the active center showed an... 

Although heterobimetallic complexes with alkylated rare-earth metal centers were proposed to promote 1,3-diene polymerization via an allyl insertion mechanism, details of the polymerization mechanism and of the structure of the catalytically active center(s) are rare [58,83,118-125]. Moreover, until now, the interaction of the cationizing chloride-donating reagent with alkylated rare-earth metal centers is not well-understood. Lanthanide carboxylate complexes, which are used in the industrial-scale polymerization of butadiene and isoprene, are generally derived from octanoic, versatic, and... 

Dilute solutions (x <0.05) are particularly interesting since, in these solids, M cations can be considered as virtually isolated in the AO matrix, and mutual M M interactions are absent. Therefore, the intrinsic catalytic behavior of isolated centers M can be signled out (25). The catalytic behavior can be compared for M in matrices of the same symmetry but different ionicity (e.g., AO = MgO, CaO, NiO, or SrO) or those also differing in symmetry (e.g., AO = NiO or ZnO). For example, it has been shown that the activity for N2O decomposition of both Ni2+ and Co2+ ions is lower when the ions are hosted in ZnO than in MgO (330). A progressive increase of x allows one to follow the insulator to semiconductor transition of the MxAi xO system in a controlled way without changing the local structure of M as surface active centers. 

The structure of the cationic chain ends is not clear in the polymerization of cyclic formals. Two different kinds of active centers and hence two types of propagation reactions have been proposed (9) ... 

Mechanistic Aspects of Cationic Copolymerizations The relative reactivities of monomers can be estimated from copolymerization reactivity ratios using the same reference active center. However, because the position of the equilibria between active and dormant species depends on solvent, temperature, activator, and structure of the active species, the reactivity ratios obtained from carbocationic copolymerizations are not very reproducible [280]. In general, it is much more difficult to randomly copolymerize a variety of monomers by an ionic mechanism than by a radical. This is because of the very strong substituent effects on the stability of carbanions and carbenium ions, and therefore on the reactivities of monomers substituents have little effect on the reactivities of relatively nonpolar propagating radicals and their corresponding monomers. The theoretical fundamentals of random carbocationic copolymerizations are discussed in detail and the available data are critically evaluated in Ref. 280. This review and additional references [281,282] indicate that only a few of the over 600 reactivity ratios reported are reliable. 

Cation exchanged zeolites are successfully applied as catalysts or selective sorbents in separation technologies. " For both catalytic and sorption processes a concerted action of polarizing cations and basic oxygen atoms is important. In addition, transition metal cation embedded in zeolites exhibit peculiar redox properties because of the lower coordination in zeolite cavities compared to other supports." " Therefore, it is important to establish the strength and properties of active centers and their positions in the zeolite structure. Various experimental methods and simulation techniques have been applied to study the positions of cations in the zeolite framework and the interaction of the cations with guest molecules.Here, some of the most recent theoretical studies of cation exchanged zeolites are summarized. 

Correlations of structures and reactivities for anionic and cationic ring-opening polymerization are reviewed. The following topics are discussed chemical structure of active species and their isomerism, determination of active centers concentration, covalent vs ionic growth and correlations between structures of active centers or monomers and their reactivities. 

Many papers have been published concerning the structure of the active centers in anionic and cationic ring-opening polymerization reactions of oxacyclic monomers. Recently, attention has been paid in our laboratory to the influence of the structure of complex carbonium salt initiators, especially of the dioxolanyllum salts used for initiating the cationic polymerization reactions of trioxane, tetrahydrofuran and dioxolane, on the course of the polymerization ( ). 

Polymerization by Transition-Metal Complex Catalysts. Mlly M12, and M13 have been polymerized by Et3Al/TiCl4 catalysts between 50° and 80 °C in n-hexane, the reaction times ranging from a few hours to several days. The polymers obtained have the same structure as those obtained by cationic polymerization. By analogy with mechanisms proposed in the literature (38, 39), the structure shown in Equation 24 may be proposed for the active center. 

Up till now, the predominant and, it should be mentioned, successfully solved problems have been related to the determination of the nature (cationic, free-radical or anionic) and the structure of the active center of the growing polymer chain represented by an asterisk in Scheme 1. However, the investigation of the process of the direct insertion of the monomer in the polymer chain, i.e. everything represented in Scheme 1 by an arrow - was considered to be of secondary importance, with the exception of anionic coordination polymerization. It is usually a priori assumed that this is an elementary single-stage activation transition in the literal sense without any peculiar features, and if these features even exist, they are completely predetermined by (Fig. 1). 

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