Propagation species in cationic polymerization

Aldehyde polymers were probably known well over 100 years ago [322-324]. In spite of that, polyoxymethylene is the only product from aldehyde polymerization that is produced in large commercial quantities. Formaldehyde polymerizes by both cationic and anionic mechanisms. An oxonium ion acts as the propagating species in cationic polymerizations [322, 323]. In the anionic ones, the propagation is via an alkoxide ion. 

Acid-catalyzed polymerizations of vinyl compounds have been extensively investigated in the past and several publications summarize the results of this research Nearly half a century has passed since the propagating species in cationic polymerization were claimed to be carbocations In spite of this historical background, many critical problems on the nature of the propagating species still remain unsolved, partly because its concentration and rate constants have not yet been determined in most cationic polymerization systems. 

Studies on the propagating species in cationic polymerization may be important for two reasons First, they can contribute toward developing new methods for synthesis of new polymers and oligomers second, they yield an insight into the nature of unstable carbocations in organic media, which play a vital role in many organic reactions. Synthetic research and kinetic/analytical studies should thus be able to influence and inspire each other. 

This review, mainly based on our recent investigations, has discussed the nature of the propagating species in cationic polymerization of vinyl monomers, classified cationic initiators into metal halides (MXJ and nonmetal halides (non-MX ), and pointed out clear differences between the propagating species derived from these two classes of cationic initiators. The article also emphasizes the unique characteristics of the non-MX initiators that have eluded systematic studies for a long time. In particular, it should be noted that these initiators specifically form two independent propagating species (dissociated and non-dissociated species). 

The expressions (Eqs. 5-34 and 5-42) for Rp in cationic polymerization point out one very significant difference between cationic and radical polymerizations. Radical polymerizations show a -order dependence of Rp on while cationic polymerizations show a first-order depenence of Rp on R,. The difference is a consequence of their different modes of termination. Termination is second-order in the propagating species in radical polymerization but only first-order in cationic polymerization. The one exception to this generalization is certain cationic polymerizations initiated by ionizing radiation (Secs. 5-2a-6, 3-4d). Initiation consists of the formation of radical-cations from monomer followed by dimerization to dicarbo-cations (Eq. 5-11). An alternate proposal is reaction of the radical-cation with monomer to form a monocarbocation species (Eq. 5-12). In either case, the carbocation centers propagate by successive additions of monomer with radical propagation not favored at low temperatures in superpure and dry sytems. 

The polymerizability of monomers having a 1,5-pentanedioxy central group were lower than monomers containing a diphenyl sulfone group. This was due to a difference in the reactivity and concentration of the cation-radical propagating species in the polymerization [214]. 

Termination reactions cannot be eliminated in radical polymerizations because termination reactions involve the same active radical species as propagation therefore, eliminating the species that participates in termination would also result in no polymerization. Termination between active propagating species in cationic or anionic processes does not occur to the same extent because of electrostatic repulsions. Equation (1) represents the rate of polymerization, Rp, which is first order with respect to the concentration of monomer, M, and radicals, P, while Eq. (2) defines the rate of termination, Rt, which is second order with respect to the concentration of radicals. To grow polymer chains with a degree of polymerization of 1000, the rate of propagation must be at least 1000 times faster than the rate of termination (which under steady state condition is equal to the rate of initiation). This requires a very low concentration of radicals to minimize the influence of termination. However, termination eventually prevails and all the polymer chains produced in a conventional free radical process will be dead chains. Therefore they cannot be used in further reactions unless they contain some functional unit from the initiator or a chain transfer agent. 

As we have seen earlier, the propagating species in cationic chain polymerization is a positively charged carbon species. The older term for this trivalent, trigonal, positively charged carbon ion is carbonium ion which we have used up to this point. Olah [10] proposed that the term carbenium ion be used instead of carbonium ion, the latter being reserved for pentavalent, charged carbon ions, and the term carbocation for both carbonium and carbenium ions. Since the term carbenium ion is not universally followed, to avoid the controversy we will henceforth refer to the propagating species as carbocations. Most text and journal references use the term carbocation and the term carbocation polymerization is used synonymously with cationic polymerization in the literature. 

Thus cationic polymerizations show a first order dependence of Rp on Ri or initiator concentration in contrast to radical polymerizations which show a one-half-order dependence of Rp on Ri- The difference is a consequence of their basically different modes of termination. Termination is second order in the propagating species in radical polymerization but only first order in cationic polymerization. 

In general, these anions are associated with a counterion, typically an alkali metal cation. The exact nature of the anion can be quite varied depending on the structure of the anion, counterion, solvent, and temperature [3-5]. The range of possible propagating species in anionic polymerization is depicted in terms of a Winstein spectrum of structures as shown in Equation 7.2 for a carbanionic chain end (R ) [3, 6]. In addition to the aggregated (associated) (I) and unaggregated (unassociated) (2) species, it is necessary to consider the intervention of free ions (5), contact... 

An anionic mechanism is proposed for those polymerizations initiated by alkali metal organometallic species, where there is good reason to assume that the metal is strongly electropositive relative to the carbon (or other) atom at the tip of the growing chain [21,143-151]. However, analogous to the discussion of the active species in cationic polymerization, a multiplicity of active species may be involved as propagating species in anionic polymerization as shown below [150]. In contrast to cationic polymerization, however, there is experimental evidence for the involvement of many of these species under certain experimental conditions [145,147,148]. 

The cationic polymerizations of cyclic acetals are different from the polymerizations of the rest of the cyclic ethers. The differences arise from great nucleophUicity of the cyclic ethers as compared to that of the acetals. In addition, cyclic ether monomers, like epirane, tetrahydrofuran, and oxepane, are stronger bases than their corresponding polymers. The opposite is tree of the acetals. As a result, in acetal polymerizations, active species like those of 1,3-dioxolane, may exist in equilibrium with the macroalkoxy carbon cations and tertiary oxonium ions [69]. By comparison, the active propagating species in the polymerizations of cyclic ethers, like tetrahydrofuran, are only tertiary oxonium ions. The properties of the equilibrium of the active species in acetal polymerizations depend very much upon polymerization conditions and upon the structures of the individual monomers. 

Table 9. Reported evidence for the formation of living/long-lived propagating species in cationic vinyl polymerization...

The kinetic expressions which describe the rate and degree of polymerization in cationic polymerizations are derived in a manner analogous to that for radical polymerization. The results are similar with the main difference being that the direct and inverse dependencies of the rate and degree of polymerization, respectively, on the initiator concentration or initiation rate are both first-order, not half-order as in radical polymerization. The difference arises from cationic termination being mono-molecular in the propagating species instead of bimolecular as in radical polymerization. 

Such a behavior is expected in cationic polymerization where, because carbocations are not reacting among themselves, only one propagating species is involved in the termination reaction. 

It is generally accepted that there is little effect of counterion on reactivity of ion pairs since the ion pairs in cationic polymerization are loose ion pairs. However, there is essentially no experimental data to unequivocally prove this point. There is no study where polymerizations of a monomer using different counterions have been performed under reaction conditions in which the identities and concentrations of propagating species are well established. (Contrary to the situation in cationic polymerization, such experiments have been performed in anionic polymerization and an effect of counterion on propagation is observed see Sec. 5-3e-2.)... 

Cationic polymerization of alkenes involves the formation of a reactive carbo-cationic species capable of inducing chain growth (propagation). The idea of the involvement of carbocations as intermediates in cationic polymerization was developed by Whitmore.5 Mechanistically, acid-catalyzed polymerization of alkenes can be considered in the context of electrophilic addition to the carbon-carbon double bond. Sufficient nucleophilicity and polarity of the alkene is necessary in its interaction with the initiating cationic species. The reactivity of alkenes in acid-catalyzed polymerization corresponds to the relative stability of the intermediate carbocations (tertiary > secondary > primary). Ethylene and propylene, consequently, are difficult to polymerize under acidic conditions. 

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