Carbonium ion polymerizations

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. 

One might anticipate that this type of termination in carbonium ion polymerization might be minimized or even completely avoided by complexing the gegen ion with a suitable electron-seeking molecule. Experiments designed with this purpose in mind are presently underway in our laboratories. If the termination could be prevented we would be able to synthesize living carbonium ion polymers. 

The third mode of termination which occurs in some carbonium ion polymerizations involves rearrangement of the active carbonium ion into an inactive one which cannot continue the propagation. These reactions can be avoided to a great extent by working at sufficiently low temperatures, and on the whole, they only contribute significantly to the termination reaction in a few systems. 

In anionic polymerization, as in carbonium ion polymerization, termination does not involve bimolecular reaction between two growing chains. Neither can recombination of ions lead to termination, since a carbon-metal bond is highly polar, in the case of alkali metals frequently completely ionized, and in every case very reactive. The termination step leading to the formation of a terminal C=C double bond is not too probable. This reaction involves the formation of a metal hydride, and this does not contribute greatly to the driving force. Consequently, such a termination is observed at higher temperatures only and it is probably more common in coordination polymerization where the metals involved are less electropositive. 

The charged end of a polymer and its counter-ion may recombine and form a stable covalent bond thus terminating the propagation of polymerization. Such a termination is frequently observed in carbonium ion polymerizations. For example, polymerization of a vinyl monomer if initiated by hydrochloric acid produces a carbonium ion and a chlorine-counter-ion. These two ions recombine readily forming a stable covalent C-Cl bond which does not propagate further the polymerization and forms, therefore, the dead end of a polymeric molecule. Actually, the recombination of carbonium ion with Cl- ion is such a rapid reaction that usually it follows immediately the formation of the relevant carbonium ion. This prevents the formation of a polymeric molecule and gives instead an addition product of HC1 to the reactive C=C double bond. A polymeric product can be obtained if the ions recombination is slowed down by sufficiently powerful solvation. For example, a solution of styrene in nitromethane, but not in a hydrocarbon, can be polymerized by HC1 (2), since the recombination of the solvated ions is sufficiently slow to permit the formation of a polymer. 

In ionic polymerization a hydride (H-) transfer or a proton transfer are the analogues of the hydrogen atom transfer in radical polymerization. A hydride (H-) ion transfer is observed in many isomerizations and dimerizations of hydrocarbons which proceed via carbonium-ion mechanism. A similar process is responsible for chain transfer ip some carbonium-ion polymerizations. The transfer of negative ions like Cl- is also common, e.g. triphenyl methyl chloride is an efficient transfer agent in such a polymerization. Transfer of a proton is, on the other hand, a very common mode of termination of anionic polymerization. Indeed, this mode of termination was discussed previously in connection with branching reactions, and it was postulated in the earliest studies of anionic poly-... 

Evidence for carbonium ion polymerization is inhibition by amines, bases, or water, and the increase in yield and rate by high dipole moment solvents at low temperatures. 

III. Carbonium Ion Polymerizations with Trialkylaluminums and the Characterization of AlRa-Lewis Base Systems by NMR Spectroscopy... 

A Theory of Initiation and Propagation of Carbonium Ion Polymerizations with Trialkylaluminum Catalysts. Trialkylaluminums or dialkylaluminum halides in conjunction with suitable cocatalysts in polar solvent are active polymerization catalysts. For example, when cocatalytic amounts of tert-butyl chloride are added to a quiescent mixture of trialkylaluminums or dialkylaluminum halides in methyl chloride solvent in the temperature range —30° to —100°C., immediate polymerization commences (2, 3, 4, 5, 6). 

Equation (26) is the ideal copolymer composition equation suggested [203] early in the development of copolymerization theory but which had to be abandoned in favour of eqn. (23) as a general description of radical copolymerization. Only in this particular case are the rates of incorporation of each monomer proportional to their homopolymerization rates. It was shown that the reactivity of a series of monomers in stannic chloride initiated copolymerization followed the same order as their homopolymerization rates [202] and so eqn. (26) could be at least qualitatively correct for carbonium-ion polymerizations and possibly for reactions carried by carbanions. This, in fact, does not seem to be correct for anionic polymerizations since the reactivities of the ion-paired species at least, differ greatly. The methylmethacrylate ion-pair will, for instance, not add to styrene monomer, whereas the polystyryl ion-pair adds rapidly to methylmethacrylate [204]. This is a general phenomenon no reaction will occur if the ion-pair is on a monomer unit which has an appreciably higher electron affinity than that of the reacting monomer. The additions are thus extremely selective, more so than in radical copolymerization. There is no evidence that eqn. (26) holds and the approximate agreement with eqn. (25) results from other causes indicated below. 

Carbonium-ion polymerization n. Cationic -C" " initiated polymerization reaction. 

Cationic polymerization n. Process in which the active end of the growing polymer molecule is a positive ion. If the ion is a carbo-nium ion, it is referred to as carbonium ion polymerization, a polymerization reaction with a positive or cationic initiator, i.e., a cationic initiator. 

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