Polymeric carbonium ion

Of particular interest are certain ionic graft copolymerizations in which the polymerization reaction is initiated only on the macromolecular framework and no homopolymer is formed. An example is provided by the formation of polymeric carbonium ions from chloride-containing polymers, such as poly(vi-nylchloride), in the presence of diethylaluminum chloride ... 

An alternative mechanism involves the addition of the polymeric carbonium ion to a double bond in cts-1,4-polybutadiene, the latter formed in situ by the polymerization of butadiene by the Et2AlCl-cobalt compound catalyst system. 

The mechanism proposed in Reaction 3—i.e., the generation of a polymeric carbonium ion by the reaction of Et2AlCl with PVC and the addition of the carbonium ion to a double bond in cts-1,4-polybutadiene —would appear to be applicable to the polymer-polymer grafting reaction. The monomer-polymer grafting reaction may involve polymerization of butadiene on the polymeric carbonium ion site or the reaction between polybutadiene generated in situ and the polymeric carbonium ion. 

These findings are inconsistent with the essentially random nature of the final copolymer. Weissermel et al[5] proposed a rearrangement mechanism which accounts for the redistribution of the comonomer units. Transacetalization is the term given to this rearrangement. In 1972, Cherdron[8) described this redistribution as the result of "the reaction of the polymeric carbonium ions with oxygen atoms of the same or of another polymer molecule (intramolecular and intermolecular transacetalization)". He depicted the mechanism as follows ... 

Among the aryl olefins, 1,1-diphenylethylene is half-protonated in 71% H2SO4 (Deno ct al., 1959). Although ultraviolet spectra (Deno et al., 1960 Grace and Symons, 1959) and half-protonation points (Deno et al., 1960) have been reported for a-methylstyrene and other monoaryl olefins, we now know that some of these observations were made on rearranged and polymerized carbonium ions. The following two equations represent valid cases of protonation of monoaryl olefins. 

One of the side reactions that can complicate cationic polymerization is the possibility of the ionic repeat unit undergoing the well-known carbonium ion rearrangement during the polymerization. The following example illustrates this situation. 

Through a study of the influence of thiophene and other aromatic compounds on the retardation and chain transfer on the polymerization of styrene by stannic chloride, the relative rates of attack of a carbonium-ion pair could be obtained. It was found that thiophene in this reaction was about 100 times more reactive than p-xylene and somewhat less reactive than anisole. ... 

The cationic polymerization of cardanol under acidic conditions has been referred to earlier [170,171], NMR studies [16] indicated a carbonium ion initiated mechanism for oligomerization. PCP was found to be highly reactive with aldehydes, amines, and isocyates. Highly insoluble and infusible thermoset products could be obtained. Hexamine-cured PCP showed much superior thermal stability (Fig. 12) at temperatures above 500°C to that of the unmodified cardanol-formaldehyde resins. However, it was definitely inferior to phenolic resins at all temperatures. The difference in thermal stability between phenolic and PCP resins could be understood from the presence of the libile hydrocarbon segment in PCP. 

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. 

While a planar configuration characterizes the last monomeric unit of a polymeric chain growing by a radical or carbonium ion mechanism, a tetrahedral configuration should be attributed to the end of a growing polymeric carbanion. Hence an isotactic or a... 

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. 

Redox initiation is commonly employed in aqueous emulsion polymerization. Initiator efficiencies obtained with redox initiation systems in aqueous media are generally low. One of the reasons for this is the susceptibility of the initially formed radicals to undergo further redox chemistry. For example, potential propagating radicals may be oxidized to carbonium ions (Scheme 3.44). The problem is aggravated by the low solubility of the monomers (e.g. M VIA. S) in the aqueous phase. 

Two pieces of direct evidence support the manifestly plausible view that these polymerizations are propagated through the action of car-bonium ion centers. Eley and Richards have shown that triphenyl-methyl chloride is a catalyst for the polymerization of vinyl ethers in m-cresol, in which the catalyst ionizes to yield the triphenylcarbonium ion (C6H5)3C+. Secondly, A. G. Evans and Hamann showed that l,l -diphenylethylene develops an absorption band at 4340 A in the presence of boron trifluoride (and adventitious moisture) or of stannic chloride and hydrogen chloride. This band is characteristic of both the triphenylcarbonium ion and the diphenylmethylcarbonium ion. While similar observations on polymerizable monomers are precluded by intervention of polymerization before a sufficient concentration may be reached, similar ions should certainly be expected to form under the same conditions in styrene, and in certain other monomers also. In analogy with free radical polymerizations, the essential chain-propagating step may therefore be assumed to consist in the addition of monomer to a carbonium ion... 

The addition of a cation to an olefin to produce a carbonium ion or ion pair need not end there but may go through many cycles of olefin addition before the chain is eventually terminated by neutralization of the end carbonium ion. Simple addition to the double bond is essentially the same reaction stopped at the end of the first cycle. The addition of mineral acids to produce alkyl halides or sulfates, for example, may be prolonged into a polymerization reaction. However, simple addition or dimerization is the usual result with olefins and hydrogen acids. The polymerization which occurs with a-methyl-styrene and sulfuric acid or styrene and hydrochloric acid at low temperatures in polar solvents is exceptional.291 Polymerization may also be initiated by a carbonium ion formed by the dissociation of an alkyl halide as in the reaction of octyl vinyl ether with trityl chloride in ionizing solvents.292... 

Another probable example of initiation by carbonium ions is the polymerization in alkyl bromides as solvents, in which the alkyl group is found at the end of the polymer chain.291... 

Another differential reaction is copolymerization. An equi-molar mixture of styrene and methyl methacrylate gives copolymers of different composition depending on the initiator. The radical chains started by benzoyl peroxide are 51 % polystyrene, the cationic chains from stannic chloride or boron trifluoride etherate are 100% polystyrene, and the anionic chains from sodium or potassium are more than 99 % polymethyl methacrylate.444 The radicals attack either monomer indiscriminately, the carbanions prefer methyl methacrylate and the carbonium ions prefer styrene. As can be seen from the data of Table XIV, the reactivity of a radical varies considerably with its structure, and it is worth considering whether this variability would be enough to make a radical derived from sodium or potassium give 99 % polymethyl methacrylate.446 If so, the alkali metal intitiated polymerization would not need to be a carbanionic chain reaction. However, the polymer initiated by triphenylmethyl sodium is also about 99% polymethyl methacrylate, whereas tert-butyl peroxide and >-chlorobenzoyl peroxide give 49 to 51 % styrene in the initial polymer.445... 

It needs to be said at the outset that my attempts at clarification have not been made easier by the discovery [4] of the pseudocationic polymerizations early in 1964. Since exploration and revaluation of these reactions are still only in their early stages, there are inevitably many loose ends and open questions and probably also some inconsistencies in the present work. Some aspects of pseudocationic polymerization have been reviewed [5-7]. It should be noted that this discovery makes many of the theoretical discussions in Reference 1 of purely historical interest. Since the publication of Reference 1 several reviews on, and relevant to, cationic polymerization [8] and on carbonium ions [9] have appeared. 

The polymerizations of olefins catalysed by metal halides have been interpreted in terms of carbonium ions as the reactive species since the work of Hunter and Yohe (see p.109). Although the discovery of co-catalysis subsequently showed that their theory is not valid for many systems,... 

However, a search of the literature reveals that in fact there was until 1964 very little evidence concerning the presence of carbonium ions during polymerizations. At this point it is necessary to distinguish carefully between a demonstration that an olefin can form carbonium ions in the presence of a catalyst, (e.g., H2S04) or a syncatalyst, (e.g., SnCLpHjO), and a demonstration that these ions are present during the polymerization reaction, and necessarily connected with it. (See also reference 1, Chapter 1.)... 

Relatively few attempts have been made to demonstrate the presence of ions in the polymerization of styrene, the most extensively studied of all monomers. Pepper [11] made conductivity studies on stannic chloride solutions in various solvents with and without monomer and added water, using open systems. He concluded that his results shed little light on the question of whether chain-carrying cations were present (which, indeed, he presumed) or on their concentration. Brown and Mathieson [12] found that for the polymerization of styrene by chloroacetic acids in nitromethane, the conductivity was indistinguishable from zero when no water was added, although the reaction rate was appreciable, and with increasing amounts of added water the conductivity increased, but the polymerization rate decreased. Therefore their results gave no useful information on the question of the participation of carbonium ions. 

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