Initiators carbenium salts

Velichkova, R. S., Toncheva, V. D., and Panayotov, I. M., Macromonomers fi om vinyl and cyclic monomers prepared by initiation with stable carbenium salts, J. Polym. Sci., Part A Polym. Chem.. 25. 3283-3292, 1987. 

The foregoing analysis shows how a new potential carbenium salt initiator can be rapidly assessed in terms of its relative solubility, the relative position of its iommolecule... 

Because Reaction (24) is reversible, eventually all the carbenium salt is consumed in irreversible Reaction (25). Similar behavior was observed for polymerization of tetrahydrofuran initiated with trityl salts. In this case, however, hydride transfer from tetrahydrofuran molecule is followed by proton expulsion to form 2,3-dihydrofuran, which complicates the initiation mechanism [27,28] ... 

The studies of initiation of tetrahydrofuran polymerization with differently substituted carbenium salts shed some light on the reasons of low... 

Unravelling the mechanism(s) of initiation, whereby chain carriers are formed in a system. This topic includes such studies as the mode of proton transfer frran a Br nsted acid to a monomer, the steps involrred in the interaction of a Lewis acid with an olefin (in the absence and/or in the presence of a cocatalyst), the reaction of stable carbenium salts with variais vinylic moieties, etc. 

The second category of carbenium salts includes much less stable entities which cannot be easily isolated. t-Butyl-, benzyl- and 1-phenylethyl carbenium salts with such anions as BF , C10 , etc., are typical examples of such highly reactive initi-... 

The above observations are obviously not intended as a rejection of this very useful technique. We shall have numerous opportunities to underline the important contributions made by its judicious exploitation. Perhaps the most interesting application of conductivity in cationic polymerisation relates to measurements in model systems such as the study of the self-ionisation of initiators, the interactions of Lewis acids with cocatalysts, and of course the extent of dissociation of stable carbenium salts. 

Ledwith and Sherrington have already discussed some of the basic aspects of polymerisation processes involving stable carbenium salts as initiators and reviewed the literature in this field up to 1974. Since then, some interesting and pertinent material has been published which makes it necessary to reassess the interpretation of the older work. We therefore cover this whole area of cationic polymerisation from its beginning. 

The first detailed study of a cationic polymerisation of vinyl ethers induced by stable carbenium salts was reported in 1971 by Bawn et Isobutyl vinyl ether was polymerised with trityl and trc ylium hexadiloroantimonates and trityl fluoroborate. From calorimetric measurements of the rate of polymerisation, it was concluded that all the initiator used was consumed roon after mixii and the assumption was made that an equal number of active species was formed in this fast initiation reaction. Propagation rate constants were thus obtained and attributed to the action of free ions. It was als) claimed that no significant termination took place during the polymerisation ce successive monomer additions produced polymerisations having the same propagation rate constant. Later work performed in the same laboratory on other vinyl ethers... 

The study of the polymerisation of cyclopentadiene by stable carbenium salts has been the exclusive domain of S walt and his group. In 1967, they reported the first observations on a system involving trityl hexachloroantimonate in methylene chloride between — 70 and 20°C Because of the interesting features of this polymerisation, the work was pursued by kinetic and mechanistic investigations. In I969 it was shown that initiation took place by direct addition and that transfer and termination were imimpor-tant particularly in the first stipes of the process. The kinetics of initiation were followed by visible spectroscopy and provided furthw evidence for a one-to-one reaction. Thus,... 

Isrprene has been polymerised by both tropylium and trityl salts in nitrobenzene by Gaylord and Svestka who also found that less polar media were ineffective for this reaction. The complex behaviour of these systems did not allow any fundamental conclusion to be reached on the medianism of initiation except that some incorporation of trityl groups (but not of tropylium groups) was detected in the polymers. Interestingly, isoprene seems to display a limiting nucleophilicity and sit at the boundary between monomers polymerisable and non-polymerisable by these stable carbenium salts. 

The extremly high susceptibility of N-vinylcarbazole towards cationic polymerisation is weU illustrated by the ease and rapidity with which this monomer is converted into polymer even at low temperatures under the action of very small concentrations of stable carbenium salts. We have already mentioned that oxonium salts are inactive for the polymerisation of olefins. Turchi et al. tried triethyloxonium hexafluorophos-phate as a possible initiator for the polymerisation of this monomer and proved in a series of elegant experiments that this salt is not an initiator. Polymerisation was observed but caused by traces of PFs present in the system. The importance of this paper stems from its conclusive evidence showing that even the most basic among the vinylic monomers is insensitive to oxonium salts. 

The work of Aso et al. on this monomer has been discussed at lei th by Ledwith and Sherrington in their review This investigation does not contribute substantially to the basic understanding of the mechanism of initiation by stable carbenium salts and no further comments are therefore necessary in the present context. 

WhUe this nomomer cannot be polymerised either by trityl or by tropylium salts, Plesdi and Pask were able to promote its polymerisation by using diphenylethylium hexafluoroantimonate. This recent discovery opens up new interesting potentials in the field of carbenium-salt initiated polymerisatkm of aliphatic olefins. 

The major drawback concerning the use of these stable carbenium salts as initiators for the polymerisation of vinylic monomers, is of course their inabflity to attack certain olefins such as isobutene. The search for alternative catalysts within this famfly has resulted in the remarkable success obtained by Plesch already quoted. Hopefully more such new initiators will become avaflable after this breakthrough. 

In 1960, Olah and coworkers reported a study on the polymerisation of olefins by various carbenium salts prepared at low temperature just before the addition of the monomer. AUcyl tetrafluoroborates in particular were drown to be fairly active initiators, but the molecular weight of the products were low. The above salts are not stable under normal conditions, but can be hailed in an inert atomsphere at low temperature and their reactivity is such that they can attack alij atic olefins such as butene and isdrutene. However, the very instability of these initiators is an obvious stumbling block inst fundamental studies. 

Kennedy and collaborators have studied the polymerisation of olefinic monomers induced by catalytic combinations consistir of mixtures of organic chlorides and aluminium alkyls. Although the authors claimed that these mixtures gave rise to carbenium salts capable of initiating viiq l polymermtion, we believe that the process leading to active species does not necessarily imply the formation of transient carbenium ions derived from the catalyst pair. For this reason, these systems are not discussed in this section but rather in Chapter IV together with other aluminium halide initiated polymerisations. 

This crystalline complex was found to initiate the polymerisation of isobutene at —78°C more effectively than an equivalent amount of AIQ3 alone or with f-butyl chloride added to the mixture. This was taken as a proof that the activity of the complex pointed to its structure approaching that of a carbenium salt. This work was never continued with other monomers to demonstrate that the f-butyl cation was the initiating species. 

Within the context of stable carbenium salts initiatron, we already examined a very interesting and successful study on the block copolymeriation of a-methylstyrene with cyclopentadiene performed by Vairon and Villesange (see Sect. V-A-4-b). The preparation of the product required three basic operations (i) the living anionic polymerisation of a-methylstyrene to give monodisperse macromolecules, (ii) transformation of their end groups into stable carbocationic moieties, and (iii) initiation of the polymerisation of cyclopentadiene from these active ends under conditions of minimal transfer and termination reactions. Thus, the macroinitiators in the second polymerisation were generated by a controlled anionic polymerisation and allowed tiie synthesis of a triblock near-isomolecular copolymer. 

In the second group of systems diarylhalonium salts, failing to initiate polymerization generate the carbenium salts by decomposition induced by free radicals formed photochemk Benzophenone, benzil or maleic anhydride were used as sources of free radicals ... 

Some other salts, e.g. (CfiH5)3C SbF (formed in situ), resulted predominantly in H transfer or proton elimination 19). These facts have been rationalized (Adv. Polymer Sci. 37, 1980 Sect. 3.2.1) when we discussed factors influencing the relative proportions of initiation by direct addition. These factors include first of all the ability of the second THF molecule to add to the oxonium ion formed by reaction between THF and the initiating carbenium ion (preformed or transient). Steric hindrance, like that observed in oxonium ions formed by triphenylcarbenium ions and THF, is... 

Carbenium salts [e.g. (C6H5)3C A ] initiate the polymerization of DXL by hydride-transfer which introduces an additional complication 28,29-33,34). 

Cationic polymerization of lactones can be initiated by a wide variety of protonic and Lewis acids, and by oxonium and carbenium salts. 

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