Cation-initiated reaction

Due to its high speed, the cationic initiation reaction contributes to the over-all lactam consumption, in contrast to the anionic polymerization in which the disproportionation is usually very slow as compared with the fast propagation reactions involving acyllactam growth centres. Neutral and protonated polymer amide groups can also take part in the dispro-... 

The molecular mass of PVFA-co-PVAm can be adjusted by the production of PVFA. High-molecular-mass polymers are yielded by radical polymerization of VFA [58, 59], while rather low-molecular-mass products (oligomers) can be obtained by specific cationic initiation reactions [60-62]. The differ-... 

The cationic initiation reaction involving a monomer and a protonic acid HA can be written as ... 

In the presence of cationic initiators, the possibiUty for loss of pendant ether groups to form free alcohol is another side reaction that usually results in color formation because of the highly conjugated products formed. 

The most important reaction with Lewis acids such as boron trifluoride etherate is polymerization (Scheme 30) (72MI50601). Other Lewis acids have been used SnCL, Bu 2A1C1, Bu sAl, Et2Zn, SO3, PFs, TiCU, AICI3, Pd(II) and Pt(II) salts. Trialkylaluminum, dialkylzinc and other alkyl metal initiators may partially hydrolyze to catalyze the polymerization by an anionic mechanism rather than the cationic one illustrated in Scheme 30. Cyclic dimers and trimers are often products of cationic polymerization reactions, and desulfurization of the monomer may occur. Polymerization of optically active thiiranes yields optically active polymers (75MI50600). 

HC1, HBr, and HI add to alkenes by a two-step electrophilic addition mechanism. Initial reaction of the nucleophilic double bond with H+ gives a carbo-cation intermediate, which then reacts with halide ion. Bromine and chlorine add to alkenes via three-membered-ring bromonium ion or chloronium ion intermediates to give addition products having anti stereochemistry. If water is present during the halogen addition reaction, a halohydrin is formed. 

Synthetic polymers can be classified as either chain-growth polymen or step-growth polymers. Chain-growth polymers are prepared by chain-reaction polymerization of vinyl monomers in the presence of a radical, an anion, or a cation initiator. Radical polymerization is sometimes used, but alkenes such as 2-methylpropene that have electron-donating substituents on the double bond polymerize easily by a cationic route through carbocation intermediates. Similarly, monomers such as methyl -cyanoacrylate that have electron-withdrawing substituents on the double bond polymerize by an anionic, conjugate addition pathway. 

The Tafel slopes obtained under concentrations of the chemical components that we suspect act on the initiation reaction (monomer, electrolyte, water contaminant, temperature, etc.) and that correspond to the direct discharge of the monomer on the clean electrode, allow us to obtain knowledge of the empirical kinetics of initiation and nucleation.22-36 These empirical kinetics of initiation were usually interpreted as polymerization kinetics. Monomeric oxidation generates radical cations, which by a polycondensation mechanism give the ideal linear chains ... 

As shown in Fig. 6, when the salt [28 2 ] is dissolved in a variety of solvents, the anion [2 ] is rapidly consumed by reaction with the cation [28 ] and reaches an apparent equilibrium, the position of which depends on the solvent polarity. Clearly, the initial rate of the cation-anion reaction increases as the solvent polarity decreases. 

Polyene Cyclization. Perhaps the most synthetically useful of the carbo-cation alkylation reactions is the cyclization of polyenes having two or more double bonds positioned in such a way that successive bond-forming steps can occur. This process, called polyene cyclization, has proven to be an effective way of making polycyclic compounds containing six-membered and, in some cases, five-membered rings. The reaction proceeds through an electrophilic attack and requires that the double bonds that participate in the cyclization be properly positioned. For example, compound 1 is converted quantitatively to 2 on treatment with formic acid. The reaction is initiated by protonation and ionization of the allylic alcohol and is terminated by nucleophilic capture of the cyclized secondary carbocation. 

Generally speaking, a monomer with electron-releasing groups will be more rapidly polymerized by cationic initiators. Anionic initiators polymerize olefins with electron-withdrawing groups more rapidly. A more sensitive test of the nature of the reaction is the behavior of a mixture of two such monomers in copolymerization in which they compete for the intermediate. This will be discussed in more detail in Chapter XII on polar versus radical mechanisms. 

Because visible light is not energetic enough to break chemical bonds, direct production of free radicals by the photoinitiator does not occur. Instead when cationic initiation is needed, as for reaction with epoxies, DIBF is used in conjunction with an iodonium compound such as 4-octyloxyphenyl-phenyliodonium hexaf luoroantimonate (OPPI). It has been proposed that when irradiated, DIBF and OPPI interact to form a cationic species. 

The hydride route involves the initial reaction with hydrogen followed by coordination of the substrate the well-known Wilkinson catalyst [RhCl(PPh3)3] is a representative example. A second possible route is the alkene (or unsaturated) route which involves an initial coordination of the substrate followed by reaction with hydrogen. The cationic catalyst derived from [Rh(NBD)(DIPHOS)]+ (NBD = 2,5-norbornadiene DIPHOS = l,2-bis(diphenyl)phosphinoethane) is a well-known example. The above-mentioned rhodium catalysts will be discussed, in the detail, in the following sections. 

A remarkable feature of iridium enantioselective hydrogenation is the promotion of the reaction by large non-coordinating anions [73]. This has been the subject of considerable activity (anticipated in an earlier study by Osborn and coworkers) on the effects of the counterion in Rh enantioselective hydrogenation [74]. The iridium chemistry was motivated by initial synthetic limitations. With PFg as counterion to the ligated Ir cation, the reaction ceases after a limited number of turnovers because of catalyst deactivation. The mechanism of... 

The sections are divided by the coordination number of the reacting ion defined as the number of donor atoms that interact with the metal. The nomenclature used for the ligands is L for neutral molecules that act as ligands and X for anions that act as ligands. Most of the examples in this section will involve cations [ML ]+ or [MX ]+, but there will be a short section on bare metal anions, M . The anions of more complexity than M will be discussed in Section IV on clusters. Many reactions produce an initial product that continues to react resulting in further coordi-native changes and possibly redox changes. Tables I and II will indicate the initial reaction product and other major reaction products. 

It is unfortunate that many workers have not appreciated how essential a clue to the kinetics can be provided by the kinetic order of the whole reaction curve. The use of initial rates was carried over from the practice of radical polymerisation, and it can be very misleading. This was in fact shown by Gwyn Williams in the first kinetic study of a cationic polymerization, in which he found the reaction orders deduced from initial rates and from analysis of the whole reaction curves to be signfficantly different [111]. Since then several other instances have been recorded. The reason for such discrepancies may be that the initiation is neither much faster, nor much slower than the propagation, but of such a rate that it is virtually complete by the time that a small, but appreciable fraction of the monomer, say 5 to 20%, has been consumed. Under such conditions the overall order of the reaction will fall from the initial value determined by the consumption of monomer by simultaneous initiation and propagation, and of catalyst by initiation, to a lower value characteristic of the reaction when the initiation reaction has ceased. 

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