Transfers in cationic polymerizations

Possibilities for chain transfer in cationic polymerization are abundant.119,138 Proton transfer to the monomer is a rather general transfer reaction leading to two isomeric unsaturated end structures ... 

A connection between eqn. (56) and transfer in cationic polymerization has not yet been observed. It is assumed that the great intensity of proton transfer is caused by the existence of the equilibrium (57), comprising a non-classical ion with a loose, more mobile proton... 

Chain growth occurs through a nucleophilic attack of the carbanion on the monomer. As in cationic polymerizations, lower temperatures favor anionic polymerizations by minimizing branching due to chain transfer reactions. 

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 ... 

The DPs obtained in cationic polymerizations are affected not only by the direct effect of the polarity of the solvent on the rate constants, but also by its effect on the degree of dissociation of the ion-pairs and, hence, on the relative abundance of free ions and ion-pairs, and thus the relative importance of unimolecular and bimolecular chain-breaking reactions between ions of opposite charge (see Section 6). Furthermore, in addition to polarity effects the chain-transfer activity of alkyl halide and aromatic solvents has a quite distinct effect on the DP. The smaller the propagation rate constant, the more important will these effects be. 

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. 

Several chain transfer to polymer reactions are possible in cationic polymerization. Transfer of the cationic propagating center can occur either by electrophilic aromatic substituation or hydride transfer. Intramolecular electrophilic aromatic substituation (or backbiting) occurs in the polymerization of styrene as well as other aromatic monomers with the formation of... 

For polymerizations carried out to high conversions where the concentrations of propagating centers, monomer, and transfer agent as well as rate constants change, the polydispersity index increases considerably. Relatively broad molecular-weight distributions are generally encountered in cationic polymerizations. 

TABLE 5-5 Monomer Transfer Constants in Cationic Polymerization of Isobutylene in CH2CI2... 

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. 

Various Brpnsted and Lewis acids can also be used in cationic polymerization of styrene.118 138 159 The molecular weight, however, is difficult to control. Of the usual transfer processes, transfer to the monomer is the most significant reaction. An additional difficulty, the occurrence of Friedel-Crafts reactions, arises if polymerization is carried out in aromatic solvents. As a result, cationic polymerization of styrene usually leads to ill-defined products and is mainly of academic interest.159... 

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 ... 

Solvent polarity and temperature also influence ihe results. The dielectric constant and polarizability, however, are of little predictive value for the selection of solvents relative to polymerization rates and behavior. Evidently evety system has to he examined independently. In cationic polymerization of vinyl monomers, chain transfer is the most significant chain-breaking process. The activation energy of chain transfer is higher than that of propagation consequently, the molecular weight of the polymer increases with decreasing temperature. 

We then have a strong (M — 1)+ peak and weaker (M + 29) and (M + 41)+ peaks. The larger cations probably are similar to those formed in cationic polymerization (Section 10-8B), whereas formation of the (M — 1)+ cation corresponds to the hydrogen-transfer reaction discussed in Section 10-9. 

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. 

These are much slower than to the preceding group of monomers, evidently because of the lower reactivity of oxonium, sulphonium, ammonium, phos-phonium and siloxonium, ions. Moreover, monomers with these heteroatoms are strongly basic, and therefore cations are preferentially solvated by the monomers. This reduces the probability of other kinds of transfer to solvent, impurities, etc. Many heterocycles, e. g. A-substituted aziridines, thiethanes [62], tetrahydrofuran [63], under suitable conditions polymerize by a living mechanism, i. e. without transfer. In situations where transfer does occur, it is assumed to proceed by the mechanism disscussed previously, for example by transfer to the counter-ion. With regard to transfer intensity, vinyl ethers can be ordered between the hydrocarbon monomers and the heterocycles. The mechanism of transfer in their polymerization has yet to be studied. 

Table 17 Transfer Coefficients to Various Aromatic Compounds in Cationic Polymerization of Styrene"...

Cyclic phosphates polymerize by both cationic and anionic mechanisms. Anionic polymerization is the method of choice for preparation of high molecular weight polymers because in cationic polymerization the reaction of exocyclic ester group leads to chain transfer ... 

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