Cationic polymerization energetics

In total, many possibilities of propagation result for the ions formed by the attack on the C = 0 double bond. According to the calculations, 4 of the structures which can be formed theoretically by interaction of an acrolein chain end with an acrolein monomer possess energetic preference. Two of them are the structures c and d. These results agree with the experimental cationic polymerizability ofacroleine(R = —CHO), as well as with the fact that in the cationically polymerized polyacroleine the following structures alternate with each other88) ... 

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then react with excess monomer to start propagation. The kinetics and mechanisms of cationic polymerization and polycondensation have been studied extensively.925-928 Kennedy and Marechal926 have pointed out that only cations of moderate reactivity are useful initiators, since stable ions such as arenium ions were found to be unreactive for olefin polymerization. On the other hand, energetic alkyl cations such as CH3CH2+ were too reactive and gave side products. 

The monomers most comprehensively studied in cationic polymerization are the non-planar tetrahydrofuran and 1,3-dioxolane. HowevCT, 5-membered rings do not assume such favorable conformations like the chair conformation of 6-membered rings. Consequently, the energetic barrier (pseudorotation barrier) between the con-... 

Its cationic polymerization yields a living polymer terminated by -0=CH2 group, whereas the energetically improbable -OCH2CH2+ group is never formed. Electrophilic attack of the -0=CH2 on oxygens of trioxepane yields two kinds of terminal sequences, either... 

The cationic polymerization of propylene, 1-butene, and higher 1-alkenes yields only very low molecular weight polymers DP < 10 - 20) with highly complicated strucmres that arise due to various combinations of 1,2-hydride and 1,2-methide shifts, proton transfer, and elimination, besides chain transfer during polymerization. In the polymerization of ethylene, initiation involving protonation and ethylation is quickly followed by energetically favorable isomerization ... 

In some cationic polymerizations the monomers may rearrange in the process of placements into the polymeric chains. They isomerize into energetically preferred configurations. The result is that the... 

Cationic polymerization of oxetanes has been more recently exploited to prepare a variety of functional polymer materials including functional polymer networks and energetic binders for solid rocket propellants. Polymerization of BCMO has been studied extensively in the past because of the industrial application of this process. After the production of Penton was discontinued the interest in polymers of BCMO ensued from the fact that they may serve as a precursor for various functional polyoxetanes. 

This parallelism is reflected in the proposed mechanism for the ionization of methane which shows that (a) the second step of the scheme invoives attack of an ethyl cation on methane, but the reaction cannot stop there, and goes on to (b), the third step, which involves attack of a secondary-isopropyl cation on methane. The primary and secondary alkyl cations are very strongly acidic species and are unstable under the reaction conditions. The condensation reaction essentially terminates with the much more weakly acidic tertiary-butyl ion. Alkane polycondensation and olefin polymerization side reactions producing stable, less acidic, tertiary ions obscured the simple alkylation reactions of the primary and secondary alkyl cations. Implicit in this mechanism, however, is that it is possible to react an acidic energetic primary cation (such as the ethyl cation) with molecules as weakly basic as methane and thus, the door was opened to new chemistry through activation of the heretofore passive, weakly basic, "paraffins" (20-24). 

Unequivocal demonstration of the formation of macrozwitterions is confined to anionic polymerization. Ironically zwitterions were first postulated as intermediates in cationic vinyl polymerizations initiated by Friedel-Crafts halides4. Friedel-Crafts halides, probably the most widely used cationic initiators, are molecules, not ions. However, formation of an anion with a metal-carbon bond seems to be energetically unfavourable and initiation is thought to occur by self ionization or involve a co-catalyst. 

In none of the C oFg salt syntheses had there been any indication of the existence of a polymeric-cation salt analogous to the (CioHg)2 PF6" salt described by Fritz and his co-workers. Face-to-face bonding of CioFj with CioFg appears not to be energetically favorable. This may be a consequence of the repulsive interaction of the electron-rich F ligands (particularly because of those electrons occupying the orbitals of carbon-... 

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