Cationic polymerization Isomeric species

Whereas the cationic polymerization of furfurylidene acetone 3a engenders crosslinked structures (25), the use of anionic initiators results in linear structures (26). However, the propagation is preceded by an isomerization of the active species which eliminates the steric hindrance to propagation arising from the 1,2-disubstitution in the monomer structure. A proton shift from the 4- to the 2-position places the negative charge at the extremity of the monomer unit and the incoming monomer can add onto this anion without major restrictions. The polymer structure thus obtained is ... 

If concentrations of carbenium ions are too low to be observed directly, they must be detected indirectly in kinetic studies of the racemiza-tion of optically active dormant species, ligand exchange and/or detailed studies of the effect of substituents, solvent and salts. Some of the most convincing and elegant work in this area was presented in Chapter 2 using primarily benzhydryl derivatives. As discussed in the next section, correlations between ionization rates and equilibrium constants, rates of solvolysis and rate constants of electrophilic addition can be interpolated and in some cases extrapolated to cationic polymerizations of alkenes to evaluate the reactivities of various active species and the dynamics of their isomerization. 

Because the chain transfer to polymer is fast as compared with reformation of active species of propagation [Eq. (128)] and there is a reaction pathway, which due to the formation of isomerized products is irreversible [reaction (129)], continuous degradation of the already formed polythiirane chains occurs if the reaction system is kept unterminated [159]. Also isolated polymers, treated with cationic initiators degrade to low molecular weight, predominantly cyclic oligomers. Consequently, cationic polymerization of thiiranes is very strongly affected by chain transfer to polymer processes. 

The chemistry of cationic polymerizations is usually significantly more complicated than that of anionic polymerizations. In many cases, the initiator added to start the polymerization does not, itself, start the polymerization, but first forms the actual active species in the polymerization mixture, itself, by reacting with itself, a cocatalyst, or the monomer. In addition, the initiating and growing species can participate in a whole series of side reactions such as transfer, isomerization, or termination reactions. 

In the field of cationic polymerization, on the other hand, a number of examples are known in which a propagating carbocation isomerizes (rearranges) into an energitically more stable carbocation that in turn propagates This isomerization polymerization should be clearly distinguished from the monomer-isomerization polymerization that involves isomerization of the starting monomer, not the propagating species. 

Vinyl cyclohexane and other similar monomers give analogous phantom polymers under similar conditions, and the reaction has been explored in great detail by Kennedy and his collaborators [9f, 74]. The occurrence of such relatively fast isomerizations makes it appear very likely that in these polymerizations the propagating species is a cation. 

Bestian and Clauss proposed that the polymerization occured with isomerization on a cationic alkyltitanium species or one of its associated forms. Propagation by anionic and cationic species accounts for their results more easily. Most of the oligomer low molecular weight product was from anionic type propagation (Equation 8). However, the 7.8% of the dimer and the 30% of the trimer fractions were produced by cationic propagation of the n-butyl group (Equation 9). 

The cationic nature of Ziegler catalysts have been proposed by Sinn, Winter and Tirpitz (90) who found that the polymerization of styrene, of vinylethers, of butadiene and of isoprene by Ziegler catalysts required the presence of trace amounts of proton-active substances. These same cationic catalyst species isomerized heptene and isoheptene. No definite results could be obtained for propylene and it... 

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. 

As indicated by the direction of the arrows the isomeric 7-membered oxonium ion dominates in the polymerization of the 5-membered 1,3-dioxolane whereas in the polymerization of the 7-membered 1,3-dioxepane cationated monomer dominates. This is apparently due to the differences in strain of the involved rings. Kinetic analysis of the polymerization of these two monomers has shown that the isomeric (enlarged) oxonium ions can be treated as the kinetically dormant species propagation and depropagation on these species proceed with almost identical rates. This explains why for the same starting concentration of initiator, as observed by Plesch (19), 1,3-dioxepane polymerizes over 100 times faster than T,3-dioxolane. This is because the proportion of the productively active species is higher for the former than for the latter monomer. 

The anodic oxidation of alkanes in anhydrous hydrogen fluoride has been studied at various acidity levels from basic medium (KF) to acidic medium (SbFs) to establish optimum conditions for the formation of carbenium ions . The oxidation potential depends on the structure of the hydrocarbon methane is oxidized at 2.0 V, isopentane at 1.25 V vs Ag/Ag. Three cases of oxidation can be distinguished. In basic medium, direct oxidation of the alkane to its radical cation occurs. In a slightly acidic medium, the first-formed radical cation disproportionates to cation, proton and alkane. The oxidation is, however, complicated by simultaneous isomerization and condensation reactions of the alkane. In strongly acidic medium, protonation of the alkane and its dissociation into a carbenium ion and molecular hydrogen occurs. In acidic medium n-pentane behaves like a tertiary alkane, which is attributed to its isomerization to isopentane. The controlled potential electrolysis in basic medium yields polymeric species. 

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