Termination in cationic polymerization

Kennedy and co-workers10 studied the kinetics of the reaction between Me3Al and t-butyl halides using methyl halide solvents as a model for initiation and termination in cationic polymerization. Neopentane was generated rapidly, without side reactions and rates were determined by NMR spectroscopy. The major conclusions were ... 

Propagation involves the consecutive additions of monomer molecules to the carbonium ion at the growing chain end. Termination in cationic polymerization usually involves rearrangement to produce a polymer with an unsaturated terminal unit and the original complex or chain transfer to a monomer and possibly to the polymer or solvent molecule. Unlike free-radical polymerization, termination by combination of two cationic polymer growing chains does not occur. 

Low-molecular weight azo compounds have frequently been used in cationic polymerizations producing azo-containing polymers. Thus, the combination of ionically and radically polymerizable monomers into block copolymers has been achieved. Azo compounds were used in all steps of cationic polymerization without any loss of azo function as initiators, as monomers and, finally, as terminating agents. 

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. 

Once a compound has been shown to polymerise, the most interesting question for me is What is stopping the chains from growing When that question has been answered we must know much about the kinetics of the system and at least a little about its chemistry. Before entering into an account of the reactions which stop chains from growing, it is important to make once again a clear distinction between termination and transfer reactions. There is no reason for not adhering to the radical chemist s definition of termination a reaction in which the chain-carrier is destroyed. In cationic polymerizations there are two main types of termination reaction ... 

In the present context the word termination is applied not to the breaking-off of a physical chain, i.e., the cessation of growth of a particular molecule, but to the complete destruction of a kinetic unit, which means the irreversible annihilation of one ion pair. This kinetic termination, which is a well-understood feature of radical polymerizations, is a comparatively rare event in cationic polymerizations it may occur in several different ways and in some systems not at all. 

The foregoing discussion has shown that termination reactions in cationic polymerization may be of many different kinds, that they may differ for apparently closely related systems, and that they may even be entirely absent. However, the polymers produced in many of these reactions are of low molecular weight and this means that transfer reactions are dominant. They may take on an even greater variety of forms than the termination reactions and their classification and discussion are still in an early stage of development. 

This reaction may account in part for the oligomers obtained in the polymerization of pro-pene, 1-butene, and other 1-alkenes where the propagation reaction is not highly favorable (due to the low stability of the propagating carbocation). Unreactive 1-alkenes and 2-alkenes have been used to control polymer molecular weight in cationic polymerization of reactive monomers, presumably by hydride transfer to the unreactive monomer. The importance of hydride ion transfer from monomer is not established for the more reactive monomers. For example, hydride transfer by monomer is less likely a mode of chain termination compared to proton transfer to monomer for isobutylene polymerization since the tertiary carbocation formed by proton transfer is more stable than the allyl carbocation formed by hydride transfer. Similar considerations apply to the polymerizations of other reactive monomers. Hydride transfer is not a possibility for those monomers without easily transferable hydrogens, such as A-vinylcarbazole, styrene, vinyl ethers, and coumarone. 

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 addition of the anion takes place at the unsubstituted carbon atom, which, in this case, carries a partial positive charge. Since the growing chain end is a genuine anion, chain termination can occur by addition of a reactive cation. As in cationic polymerization, combination of two growing ends is not possible. Chain transfer with electrophiles can also occur. 

Since carbocations are involved in cationic polymerization, a possible side reaction is their isomerization through hydride (alkyde) migration to more stable (less reactive) carbocations. This can lead to a polymer of broad molecular weight distribution or, if the isomerization is irreversible, to termination. 

A hypothesis which may explain the experimental observations can be developed as follows Transfer has been assumed to occur by proton transfer to monomer. Previous studies (18,19) indicate that propagation and transfer have similar transition states in cationic polymerizations. For this reason it is possible that these two processes may both occur within the ion-counterion-monomer complex. Termination has been assumed to occur by ion-counterion collapse (20), for example, for EtAlCl2 ... 

Whether all chains bear a terminal vinylic double bond has not been clearly established, and it would be somewhat astonishing if vinylic double bonds did not undergo side reactions since their reactivity in cationic polymerization is quite high. However, the occurrence of terminal p-vinyl benzyl groups is confirmed by the fact that the formed macromonomer readily copolymerizes with butyl acrylate. 

The reaction of an unsaturated compound with an antagonist function located at the end of a polymer chain is still the most commonly used method to synthesize macromonomers. We have already mentioned some processes that can be used to introduce into the chain end of a macromolecule a functional group, e.g. by deactivation of living carbanionic sites and transfer reactions of various kinds in cationic polymerization. We have also described some methods used to link an active terminal double bond to the chain end originally bearing hydroxy groups. 

Chain transfer is the most common chain terminating reaction in cationic polymerization and can include transfer to monomer, solvent, and polymer. Termination by combination with the counterion can also occur in some systems. In some cases, cationic polymerization may be used to prepare stereoregular polymers. Although the exact mechanism is unclear, it is known that stereoregularity varies with initiator and solvent.18 Lower temperatures also tend to favor more stereoregular polymers. 

In cationic polymerizations, the occurrence of living systems is limited. Therefore the search for a medium where only cationic initiation would take place is even more difficult. It is usually possible to exclude, or at least limit, propagation transfer and termination can rarely be excluded. Even so, some simplification can be achieved, and it has been exploited. 

A special kind of termination in ionic polymerizations is the mutual combination of anionic and cationic living chains (see Chap. 5, Sect. 5.8). When the two polymers consist of different monomers, block copolymers are formed. The two macroions can also consist of the same monomer. 

The functional terminator method in cationic polymerization calls for nucleophiles ( Nu-T) with functional groups (T). As required, the nucleophilic part of the terminator, but not the function Y, should combine with the living end (e.g., 11, Scheme 4) and thereby attach the function Y to the last repeat unit (the co-end or tail group) of polymer 12. The terminators should meet the following criteria ... 

When compared with the multifunctional initiators, the corresponding terminators are less available in cationic polymerization [202]. The situation is in sharp contrast to anionic living polymerization, where a variety of multifunctional terminators are developed (e.g., Cl2MeSiCH2CH2Si-MeCl2) [203,204]. However, a series of multifunctional silyl enol ethers were recently found to be effective in multiple termination of living cationic polymers of vinyl ethers [142,147,205,206] and a-methylstyrene [159,207] (Scheme 10). 

Termination by Irreversible Recombination with Counterion Irreversible collapse of ionic species via recombination with counterion is the most trivia] potential source of termination in cationic ring-opening polymerization. The relations discussed in Section II.B.6.C may also be applied to the present discussion. The termination will occur if counterion is nucleophilic enough (as compared with monomer) to compete successfully with monomer in Sn2 nucleophilic substitution and monomer is not nucleophilic enough to displace the counterion from the covalent termini ... 

The first part of this paper is a critical review of model studies in cationic polymerization. In the second part we describe and discuss our investigations exploring the effect of a variety of alkylaluminum/alkyl halide initiating systems under a variety of conditions on the competitive reactions of the Keimedy-Gillham scheme (9). This scheme represents a comprehensive set of model reactions developed for the study of competitive reactions in cationic olefin polymerizatioa It involves the cationation of a nonpolymerizable (steric-hindrance) olefin under simulated polymerization conditions and the complete analysis of reaction products which in turn reflect initiation, propagation, termination and transfer. 

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