Initiators carbonium ion

Very powerful initiators of carbonium-ion polymerization were recently reported by Plesch.32b They belong to the salt-like class of organic compounds and dissociate readily into C104 ions and carboxonium positive ions. The latter are sufficiently reactive to initiate carbonium-ion polymerization of styrene. 

The most widely accepted mechanism (Whitmore, 13) for the polymerization of olefins involves the so-called carbonium ions. In accordance with this mechanism a carbonium ion (usually a tertiary ion) adds to the olefin to form a higher molecular weight carbonium ion which then yields the olefin polymer by elimination of, usually, a proton. With acid catalysts (e.g., sulfuric acid) the initial carbonium ion is formed by addition of the hydrogen ion from the acid to the extra electron pair in the double bond (the pi electrons) ... 

Evidently, however, the use of strong acids or stable dual-acid anions is only one of the necessary requirements to initiate carbonium ion reactions in general and cationic polymerizations in particular. A specific substrate (Lewis base) which is able to accept the proton (or carbonium ion) and can be converted into a new conjugated Lewis acid is equally important. This newly formed electrophile in conjunction with the deprotonated dual-acid will be the reactive species. 

In the case of alkyl vinyl ethers [27, 30] reaction rates were again high and the same experimental technique was used. However, the initiation reaction did not appear to be as fast as that in the polymerization of Al-vinylcarbazole, and the conversion/time data showed evidence of an initial acceleration to a maximum rate of polymerization, particularly in runs carried out at —25°C. The mechanism of initiation was assumed to involve direct addition of the initiating carbonium ion to the double bond of the monomer, in the light of related evidence from similar reaction in the presence of strong nucleophiles [80, 81]. At 0°C there was also an indication of a contribution from a termination reaction. Polymer yields were always in excess of 75%, however, and the termination process was neglected in the kinetic analysis. The simple scheme envisaged is ki... 

This is the first step, i.e. formation of the initial carbonium ion. These ions may be formed in many different ways protonation of an alcohol loss of nitrogen from a diazonium ion addition of a proton to an alkene or the interaction of a Lewis acid with a halide. 

Bronsted sites, that initiate carbonium ion reactions, and Lewis sites, giving ton radical reactions, coexist, althouth it appears that in practical usage the Bronsted acidity predominates. When using y-Al O as a suppwn, undesirable side reactions such as cracking, isomerization and coke formation always exist. These give unwanted products and lead to catalyst deactivation. 

Polystyrene (S) was grafted on the butyl backbone by converting the ally lie chloride sites to initiating carbonium ions (R AlR Cl ). The product has thermoplastic elastomer properties. Most recently Kennedy and coworkers have developed techniques for grafting... 

It is not apparent whether the initial abstractions in oxidation involve hydrogen atoms or hydride ions. Simplicity favors a radical mechanism, with similar abstractions for the initial and final hydrogen removals. The good correlation with reactivities in known radical reactions is also suggestive. However, initial carbonium ion formation would also involve the same qualitative dependence on structure, although structural effects are usually more pronounced for carbonium ions than for radicals. Qualitatively, the oxidation results correlate well with carbonium ion reactivities found in the solvolysis of substituted aUyl halides (142), again with internal olefins appearing slow in oxidation. 

Since the metal sulfate catalyst has both Bronsted and Lewis acid sites, it is expected that many n bases with nonbonding electrons such as —O— and —Cl and tt bases hke olefin will undergo acid-base equilibria, thus initiating carbonium ion or carbonium ion-like reactions. Table II summarizes the acid- catalyzed reactions on metal sulfate catalysts and shows the versatility of these systems. Although this table includes some examples of industrial work, our results and others clearly show the general trend in the strength of acid sites required for each specific reaction. But detailed discussion of correlation between the catalytic activity and the acidic property is reserved for the next section. 

As can be seen, there are two possible ways in which the chain-initiating carbonium ion is formed. If the first way, involving the formation of the catalyst-carbon linkage by loss of hydride ion from a paraffin is very rapid, then we should expect that paraffins should crack as readily as olefins, since the same intermediate structure is formed. Yet we do know that the catalytic cracking of olefins occurs at considerably lower temperatures than that of paraffins. Thus, it appears that the explanation given by Thomas (4) involving the formation of a small amount of olefin to serve as the proton acceptor is the more likely one. 

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