Covalent ester

For continuing polymerization to occur, the ion pair must display reasonable stabiUty. Strongly nucleophilic anions, such as C/ , are not suitable, because the ion pair is unstable with respect to THE and the alkyl haUde. A counterion of relatively low nucleophilicity is required to achieve a controlled and continuing polymerization. Examples of anions of suitably low nucleophilicity are complex ions such as SbE , AsF , PF , SbCf, BE 4, or other anions that can reversibly coUapse to a covalent ester species CF SO, FSO, and CIO . In order to achieve reproducible and predictable results in the cationic polymerization of THE, it is necessary to use pure, dry reagents and dry conditions. High vacuum techniques are required for theoretical studies. Careful work in an inert atmosphere, such as dry nitrogen, is satisfactory for many purposes, including commercial synthesis. 

The reverse reaction (ion formation) can occur in two ways internally, by attack of the penultimate polymer oxygen atom, or externally, by attack of a monomer oxygen atom (chain growth). The external process is about 10 times slower than the internal process in bulk THF (1). Since ion formation is a slow process compared to ion chain growth, chain growth by external attack of monomer on covalent ester makes a negligible contribution to the polymerization process. 

Studies have shown that, in marked contrast to carbanionic polymerisation, the reactivity of the free oxonium ion is of the same order of magnitude as that of its ion pair with the counterion (6). On the other hand, in the case of those counterions that can undergo an equiUbrium with the corresponding covalent ester species, the reactivity of the ionic species is so much greater than that of the ester that chain growth by external attack of monomer on covalent ester makes a negligible contribution to the polymerisation process. The relative concentration of the two species depends on the dielectric constant of the polymerisation medium, ie, on the choice of solvent. 

Silver(I) triflate is widely applied to the preparation of various derivatives of triflic acid, both covalent esters [66] and ionic salts For example, it can be used for the in situ generation of iodine([) triflate, a very effective lodinatmg reagent for aromatic and heteroaromatic compounds [130] (equations 65 and 66)... 

The hemicelluloses are soluble in alkali, and can therefore be readily separated from the cellulose component by alkali extraction. However, this can only be done when the wood has first been delignified. This is because they are probably linked to lignin via covalent ester linkages (see Chapter 3) which need to be cleaved... 

The Commentator s arguments can be answered summarily by scrutinising their basis, which is, in his own words, minute amounts of reactive carbenium ions being in equilibrium with covalent esters . The Commentator has produced no evidence of any kind for the existence of an equilibrium between ions and esters in the systems of interest here. 

The major approach to extending the lifetime of propagating species involves reversible conversion of the active centers to dormant species such as covalent esters or halides by using initiation systems with Lewis acids that supply an appropriate nucleophilic counterion. The equilibrium betweem dormant covalent species and active ion pairs and free ions is driven further toward the dormant species by the common ion effect—by adding a salt that supplies the same counterion as supplied by the Lewis acid. Free ions are absent in most systems most of the species present are dormant covalent species with much smaller amounts of active ion pairs. Further, the components of the reaction system are chosen so that there is a dynamic fast equilibrium between active and dormant species, as the rates of deactivation and activation are faster than the propagation and transfer rates. The overall result is a slower but more controlled reaction with the important features of living polymerization (Sec. 3-15). 

The species present in cationic ring-opening polymerizations are covalent ester (IX), ion pair (X), and free ion (XI) in equilibrium. The relative amounts of the different species depend on the monomer, solvent, temperature, and other reaction conditions, similar to the situation described for ionic polymerization of C=C monomers (Chap. 5). 

ACCA-3 ) that will form a covalent (ester) bond with the appropriate amino acid and deliver it to the site of translation. The sequence of the acceptor stem, which is made up of the 5 and 3 ends of the tRNA, is important in the recognition of a given tRNA by the correct charging enzyme. ... 

For example, styryl cations react with perchlorate to form covalent esters that are relatively stable at low temperatures [81], whereas the more basic triflate anion tends to abstract j3-protons from carbenium ions in a transfer reaction [56]. The basicity of the counteranion determines the contribution of /3-proton elimination relative to propagation, and therefore the limiting molecular weight in a polymerization. 

Acid adds slowly to styrene to form covalent esters, which have been detected directly by, 9F NMR [101]. Carbenium ions capable of propagation are then generated by ionization of the covalent esters with excess acid. However, the concentration of carbenium ions is too low to detect directly by spectroscopy, and propagation was initially proposed to occur by a nonionic mechanism. 

Even very small amounts of anhydrous perchloric and triflic acids (<10-3 M) polymerize styrene rapidly and quantitatively [123,126-130], Carbenium ions were initially not detected in these systems, and they were therefore proposed to proceed by a pseudocationic mechanism in which covalent esters react directly with styrene in a concerted muticenter rearrangement [128], However, short-lived carbenium ions have since been detected directly by stopped-flow UV [17-19,131]. The mechanism of the propagation step in these systems is discussed in more detail in Section lV.D.2.a. 

Dormant Species and Pseudocationic Propagation The majority of propagating chain ends in most cationic polymerizations initiated by protonic acids and/or cocatalyzed by Lewis acids do not exist as carbenium ions, but are instead dormant species. The two major types of dormant species are onium ions and covalent esters or halides. The covalent species are formed by reversible reaction of carbenium ions with nucleophilic anions onium ions are generated by reaction of carbenium ions with noncharged nucleophiles such as ethers, sulfides, and amines. Because the majority of propagating chain ends exist as dormant species, they are often the only species that can be detected spectroscopically ... 

The initiating systems based on Lewis acids with covalent esters and halides offer some advantages. First, the number of chains can be easily controlled by the concentration of the ester or halide used as an initiator. Second, the polymerization rate and the proportion of carbocationic species may be easily adjusted by the strength and concentration of the Lewis... 

Trapping agents, such as malonate anions, naphthoxides, and phosphines have been used to determine the concentration of chain carriers in controlled/living and other carbocationic systems [85,249,250]. These strong nucleophiles react with all sufficiently electrophilic species, including not only carbocations but also onium ions and covalent esters. Thus, the discussed measurements can provide only the total concentration of active and dormant end groups. In principle, the kinetics of formation of the product in the trapping experiments could resolve more and less active species but only if they are present at comparable concentrations. As discussed before, carbocations are present in ppm quantities in comparison with dormant species. 

Figure 26 presents typical H NMR spectra of covalent esters under various conditions. 

Controlled/living systems can be usually obtained when the polymerization is sufficiently slow and when either nucleophilic anions or additives are present (Sections IV and V). This means that the proportion of carbenium ions should be low and conversion to dormant species, fast. Nevertheless, under such conditions cationic species can be detected by dynamic NMR, by ligand exchange, salt, and solvent effects, and by other methods discussed in Chapters 2, 3, and in this section. Under typical controlled/living conditions, dormant species such as onium ions and covalent esters predominate. It is possible that the active species are strongly solvated by monomer and by some additives. These interactions may lead to a stabilization of the carbocations. However, in the most general case, this stabilization has a dynamic sense and can be described by the reversible exchange between carbocations and dormant species. 

Before going into details, let us make a brief statement that propagation in new controlled/living carbocationic systems has nearly the same mechanism as in the conventional systems discussed in Chapter 3, which consists of the electrophilic addition of carbenium ions to alkenes. The main difference is that carbenium ions are in dynamic equilibria with dormant species (covalent esters and onium ions). The correct choice of structures and concentrations of activators and nucleophilic additives as well as those of initiator allows for the preparation of polymers with predetermined molecular weights, low polydispersities, and controlled end functionality, including block copolymers (see Chapter 5). 

Propagation proceeds by the electrophilic addition of carbenium ions to double bonds with the regeneration of carbocations. The transition state is relatively late, and it was estimated that approximately half of the charge is transferred into the developing carbocation (Chapter 2). This may explain the fact that dormant species (covalent esters and onium ions) do not react directly with alkenes. The charge on the a-C atoms in the dormant species is not sufficient for the formation of the transition state. A multicenter rearrangement process is additionally disfavored by entropy. In contrast, a two-step process in which carbocations are formed and then very rapidly add to alkenes is free of this difficulty. 

At the time of this writing, this area remains controversial. On the one hand, we have and NMR spectra, along with conductance studies, in support of the presence of free silylenium ions in solution, whereas Si and Cl NMR data are used to support the claim that PhaSiCl04 is simply a covalent ester in solution, as it has long been known to be in the solid state. 

Since initiation is rapid the rate coefficient, fe, can be regarded as fep directly. Unfortunately, however, a complex situation has arisen because of additional uncertainty as to the nature of active centres, over and above the question of ion pairing. There appears to be agreement among the various groups of workers that a covalent ester species participates in an equilibrium also involving the propagating species, i.e.. 

Both the carbamates and the phosphorus derivatives form a covalent (ester) bond with the serine OH of the enzyme in essentially the same manner as does acetylcholine. Taylor... 

Proteinase inactivation occurs by a stoichiometric reaction between proteinase and inhibitor that results in the formation of a covalent ester bond between the reactive site residue of the inhibitor (Arg in antithrombin) and the active site residue (Ser in the proteinase). The proteinases thrombin, factor Xa, factor IXa, and, less effectively, factor Vila and factor XIa are all inactivated by antithrombin (Figure 36-16). Other SERPINS can inactivate procoagulant proteinases, heparin cofactor II can inactivate thrombin, and O I-proteinase inhibitor can inactivate factor Xa. An altered a i -proteinase inhibitor (a i -proteinase inhibitor... 

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