Carbocationic polymerization, “cation

Carbocationic Polymerization Using a Cation Pool" as an Initiator... 

Matyjaszewski, K. and M. Sawamoto, Controlled/Living Carbocationic Polymerization, Chap. 4 in Cationic Polymerizations Mechanisms, Synthesis, and Applications, K. Matyjaszerski, ed., Marcel Dekker, New York, 1996. 

Most carbocationic and cationic ring-opening polymerizations are chain processes proceeding with carbocations and/or onium ions as the active species. Nonchain processes which occur via cationic and electrophilic intermediates will be discussed in Chapter 7. 

Until recently, knowledge about absolute and relative rates of reaction of alkenes with carbocations was very limited and came almost exclusively from studies of carbocationic polymerizations [119-125]. The situation changed, when it became obvious that reactions of carbocations with alkenes do not necessarily yield polymers, but terminate at the 1 1 product stage under appropriately selected conditions (see Section III.A). Three main sources for kinetic data are now available Relative alkene and carbo-cation reactivities from competition experiments, absolute rates for reactions of stable carbocation salts with alkenes, and absolute rates for the reactions of Laser-photolytically generated carbocations with alkenes. All three sets of data are in perfect mutual agreement, i.e., each of these sets of data is supported by two independent data sets. 

In addition to protonic acids, Lewis acids are the most common initiators of carbocationic polymerizations. Two mechanisms are possible. Direct initiation is rare and usually slow. The more prevalent mechanism is by cocatalysis in binary systems, with the Lewis acid acting as a coinitiator or catalyst rather than as initiator. Cationating or protonating species are the true initiators, which are therefore the species incorporated at the polymer s end group. The most common initiator is adventitious water in insufficiently dried systems. Thus, mechanistic studies should be performed under stringently dry conditions or in the presence of proton traps such as hindered pyridines. In addition to water, the protonating reagent may be an alcohol, carboxylic acid, amine, or amide [Eq. (28)]. 

Carbenium ions apparently are formed =100 times less efficiently than radicals, with both radical and carbocationic polymerization operating simultaneously when kinetically possible. Although isobutene does not polymerize radically, styrene readily polymerizes by a radical mechanism. Thus, radical polymerization of styrene dominates in wet systems where the cations are trapped by water to form inactive oxonium ions. Polymerization is =100 times faster in super-dry systems, demonstrating that cationic polymerization must dominate, and that the rate constant of cationic polymerization is approximately 104 times higher (=100 x =100) than that of the radical polymerization ( radical) = 80 mol- -L-sec-1 at 25° C). 

More reliable estimates of monomer reactivity are available from reactions of model compounds (Chapter 2). For example, the rate constants of addition of the same standard benzhydryl carbenium ion to various substituted styrenes correlate very well to Hammett s + = 4.9 [193]. Addition of various p-substituted benzhydryl cations to the same standard alkene yielded p(Acr = — 5.1 [193]. These results demonstrate that carbocationic polymerizations are extremely sensitive to even small changes in the monomer structure. They also demonstrate that the reactivity of carbenium ions scales nearly perfectly to the... 

Transfer reactions should be absent in living polymerizations but are usually significant in classic carbocationic polymerizations. However, they appear to be suppressed in new living cationic polymerizations. This... 

It is possible to work at either lower cation concentrations—but this could lead to stronger effects of impurities (adventitious moisture)—or to use more specialized flow reactors. The dynamic equilibration between active and dormant species offers another solution to this problem. In this case, the sensitivity to impurities is low due to the high total number of chains, but the momentary concentration of propagating carbocations is tremendously reduced. This approach is always used in new controlled/ living carbocationic polymerizations, as we will discuss in detail in this chapter. 

Early Developments till the Discovery of Controlled Initiation. Under suitable conditions any electrophilic species may induce cationic polymerizations (1). As a practical matter, the most convenient cationogens are Bronsted acids alone or in conjunction with Frie-del-Crafts acids (1). Systematic research on the initiation of carbocationic polymerization became possible by the discovery of coinitiation by British investigators (2-4). These workers found that the strong Lewis acid BF3 alone is unable to initiate isobutylene polymerization but in the presence of suitable cationogens, i.e., H2O, immediate and vigorous polymerization ensues. Their formalism ... 

Recent review articles on the following topics were published the controversy concerning the cationic ring-opening polymerization of cyclic acetals (213), photoinitiators for cationic polymerization (21A), living polymerization and selective dimerization (215). raacroraonomers (216), and functional polymers and sequential copolymers by carbocationic polymerization (217). 

Since the appearance of the major review on the living carbocationic polymerization of olefins [1], a large body of pertinent additional data have been generated relative to this subject [2-17]. The significance of these data prompts us to combine this recently-acquired information with earlier data and to integrate all kinds of cationic olefin polymerizations into a comprehensive mechanism, be these induced by means of a purposely-added initiator or by an impurity, both of which can lead to conventional (presence of chain transfer and/or termination) or living (absence of chain transfer and irreversible termination) polymerizations. 

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