Initiators, cationic, from olefins

Similarly, cationic initiators such as protic or Lewis acids jne also not very effective as polymerization catalysts for these systems. The basicity of the skeletal nitrogen atoms of the cyclophosphazene rings overwhelms the reactivity of the cationic initiators towards olefin polymerization. Thus, the cyclophosphazene rings are readily protonated or metalated and thus prevent both protic and Lewis acids from initiation of polymerization. 

In the polystyrenes produced by cationic initiators most of the chain-ends are terminal indanyl groups, and olefinic groups are rare. As this terminal indanyl group cannot be aluminated like a double bond, the amount of tritium incorporated comes only from the initial AlBr2CH2CHPh-groups and the few residual terminal double bonds and it, therefore, represents (approximately) the total number of initiated chains. 

Photo-NOCAS reactions of p-dicyanobenzene with 2-methylpropene in acetonitrile afforded novel 3,4-dihydroisoquinoline derivatives, as shown in Scheme 132 [482], This photoreaction is initiated by a single electron transfer from olefin to p-dicyanobenzene. Acetonitrile as a nucleophile combined with the alkene radical cation and the resulting radical cation adds to the radical anion of 1,4-di-cyanobenzene. Cyclization to the ortho position of phenyl group followed by loss... 

The use of ethers as cocatalysts for the cationic polymerisation of alkenyl monomers induced by Lewis acids has received little systematic attention and the mechanism through which these compounds operate is not well understood. The complex diethyl-ether-boron fluoride has been extensively used as a very convenient cationic initiator, but mostly for preparative purposes. As in the case of alcohols and water, ethers are known to act as inhibitors or retarders in the cationic polymerisation of olefins, if used obove cocatalytic levels, because they are more nucleophilic than most rr-donor monomers. Imoto and Aoki showed that diethyl ether, tetrahydrofuran, -chloro-diethyl ether and diethyl thioether are inhibitors for the polymerisation of styrene-by the complex BF3 EtjO in benzene at 30 °C, at a concentration lower than that of the catalyst, but high enough (0.5 x 10 M) to quench the active species formation for a time. Their action was temporary in that the quenching reaction consumed them, and therefore induction periods were observed, but the DP s of the polystyrenes were independent of the presence of such compounds, as expected from a classical temporary inhibition. 

Initiation (or olefin protonation) In this step, a f-butyl cation is formed from isobutene. A sec-butyl cation is formed from 1-C4= or 2-C4=. The sec-butyl cation can form a f-butyl cation by methyl shift, or it can undergo hydride transfer from isobutane, forming n-C and a t-butyl cation. 

Studies on the dimerization and hydrogenation of olefins with transition metal catalysts in acidic chloroaluminate (iii) ionic liquids report the formation of higher molecular weight fractions consistent with cationic initiation [3-6]. These studies ascribed the occurrence of the undesired side reaction to both Lewis acid and proton-catalyzed polymerization routes. Studies have shown that when protons, from HCl as the source, are dissolved at ordinary temperatures and pressures in the... 

When, however, initiations take place by one-electron transposition, they occur as a direct result of oxidation of free radicals. They can also take place through electron transfer interactions involving electron donor monomers. The carbon cations can form from olefins in a variety of ways. One way is through direct additions of cations, like protons, or other positively chaiged species to the olefins. The products are secondary or tertiary carbon cations ... 

The ability of a proton from a protonic acid to initiate a polymerization depends upon the nucleophilicity of the conjugate base,A . If it is low, protonation or cationization of the olefin is a step of initiation ... 

The initiation mechanism of olefins by AIB3 was explained similarly. The intermediate compound of aluminum bromide and the olefin is expected to lose HBr. Although initially the reaction mixture is free from protonic acid, it could form under conditions where initiation takes place by a conjugate acid catalyzed system. In addition, all cationic polymerizations of olefins should be considered as typical examples of general carbocadonic reactivity in electrophilic reactions. The s arate mechanisms are to be looked upon as various examples that differ only in the nature of the initial electrophile. They always lead to the related trivalent alkyl cation when polymeric chain growth is initiated ... 

These initiations were originally reported by Bawn and co-workers. The cations must be used in low concentrations to insure complete dissociation from their respective counterions. Stable organic cations can be formed from olefins, aromatic structures, or compounds with heteroatoms possessing unshared electrons, like oxygen, nitrogen, or sulfur. Some examples of stable cations that can initiate cationic polymerizations are ... 

Pseudocationic polymerizations can be distinguished from true ones by the temperature dependence of the rate of polymerization and the effect of added water. That is, pseudocationic reactions proceed slowly at low temperatures —90 C), whereas cationic polymerizations are still vary rapid. Furthermore, the rate of a pseudocationic polymerization is practically unaffected by the addition of water (up to [H2O]/[initiator] = 10 1). By contrast, in true cationic polymerizations, even at very low concentrations, water strongly affects the polymerization. Carbocations from olefins, in fact, are instantly destroyed by added water (see Chapter 18). Metal halides form hydrates with water. The concentration of these hydrates, and therefore the water concentration, then affects the polymerization rate and the degree of polymerization. 

Use of carbocations to initiate the Schmidt reaction has been the subject of several investigations during the last two decades. The cationic intermediate can be generated from secondary or tertiary alcohols or from olefins. The first example was described by the Pearson group for the synthesis of a bicyclic enamine. However, various drawbacks still remain when carbocations are involved limited stability of the intermediates, low selectivity, and the need... 

Vinylic copolymers derived both from cycloolfins and from a cycloolefin and an acyclic olefin are known. These are obtained by using cationic initiators or anio-nic-coordinative catalysts of the Ziegler-Natta type. 

The first report of NHC-containing osmium compounds acting as catalysts came from Esteruelas and co-workers in 2005. Thus, cationic benzyli-dene complexes 44 were prepared by reaction of the corresponding 16-electron precursors [(NHC)OsCl(p-cymene)][OTf] (NHC = IMes or IPr) with phe-nyldiazomethane, and their potential as initiators for olefin metathesis was probed in the RCM of diethyl diallylmalonate, the ROMP of cyclooctene, and a variety of self- and cross-metathesis reactions (Equation (7.10)). Although they were not as efficient as standard ruthenium-benzylidene metathesis initiators, compounds 44 displayed, nevertheless, a fairly decent activity. More importantly, in addition to being the first NHC-Os catalytic application, this study constituted a rare example of osmium catalysed C-C bond formation. 

The photo-NOCAS reaction was first described by McMahon and Arnold and is a photonucleophilic Sfj2Ar aromatic substitution between dicyanobenzene and an olefin in the presence of electron donor photosensitizers (phenanthrene or biphenyl) in acetonitrile-methanol solutions. This reaction system has been researched extensively in recent times. As shown in Scheme 6, the single electron transfer from olefin to photo-excited electron-deficient dicyanobenzene forms the cation radical of the olefin, which initiates a quenching reaction with nucleophile solvent methanol molecules and forms the methoxyalkyl radical. Addition of an electron transfer photosensitizer (phenanthrene or biphenyl) to the reaction mixture increases the efficiency of the reaction simply by absorbing more Hght. The excited state of the photosensitizer donates an electron to dicyanobenzene to give the photosensitizer radical cation and dicyanobenzene radical anion. The photosensitizer radical cation then oxidizes the olefin. 

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