Propagation in cationic polymerization

In the propagation step, the polarity of the medium affects strongly the reaction because the intimacy of the ion pair depends upon solvent polarity. The bond between the two ions can vary 

The chemical structures of the monomers also determine their reactivity toward cationic polymerizations. Electron-donating groups enhance the electron densities of the double bonds. Because the monomers must act as nucleophiles or as electron donors in the course of propagation, increased electron densities at the double bonds increase the reaction rates. It follows, therefore, that electron-withdrawing substituents on olefins will hinder cationic polymerizations. They will, instead, enhance the ability for anionic polymerization. The polarity of the substituents, however, is not the only determining factor in monomer reactivity. Steric effects can also exert considerable controls over the rates of propagation and the modes of addition to the active centers. Polymerizations 

Alkyl group = r-butyl i-propyl ethyl n-butyl methyl Another example is a study of the differences in the rates of reactions of various alkenes with two cations, PhCH2 and Ph2CH , generated by electron pulses. Carbon cations, free from complexities, such as ion pairing and cation aggregation that may be encountered in typical cationic polymerizations, were used. Table 3.1 shows some of the data that were reported.  

Chain-growth reactions with fairly tight ion pairs, that occur in medium of low polarity, require that the monomers be inserted repeatedly between the two ions. These consist of carbon cations on the terminal units paired with the counterions. The ion pairs are first loosened, or relaxed, complexations with monomers follow, and insertions complete the process. All insertions, of course, result in formations of new carbon cations. Upon formation, they immediately pair off with the counterions, and the process continues  

The mechanisms of such insertions consist of repeated push-pull attacks by the ion pairs on the double bonds of the incoming monomers  

Alkyl group = t - butyl - propyl ethyl n - butyl methyl 

Recently, Kolishetti and Faust [89] reported investigations of the polymerizations of p-methylstyrene in the presence of isobutylene, styrene, p-chlorostyrene, and 1,3 butadiene at —40°C. The polymerizations were carried out in a 50/50 mixture of CH2CI2 with methyl cyclohexane as the solvent and a weak Lewis acid, SnBr2Cl2. The reactions were conducted by mono additions of each monomer that was followed by instantaneous terminations. The results showed that p-methylstyrene is roughly 3.8 times more reactive than isobutylene, 4.8 times more reactive than styrene, 7.2 times more reactive than p-chlorostyrene, and 100 times more reactive than butadiene. 

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Figure 8.7 A simplified reaction scheme for initiation and propagation in cationic polymerization. (After Ref. 20.)...

S. Penczek, P. Kubisa, and R. Szymanski, Activated monomer propagation in cationic polymerizations. Makromol. Chem. Macromol. Symp. 1986, 3, 203-220. 

As already discussed, propagation in cationic polymerization of cyclic ethers by the ACE mechanism proceeds on tertiary 0x0-nium ion active species. Ionic species in general may exist in the form of ion-pairs (contact or solvent separated) and free ions. The fraction of each form is governed by a corresponding equilibrium constant that depends on the polarity of the medium. The knowledge of the fraction of different ionic forms, which is essential for the proper analysis of kinetics of anionic vinyl polymerization in which different forms show different reactivity, is less crucial in analyzing the kinetics of cationic polymerization of cyclic ethers because available data point out to equal reactivity of ion-pairs and free ions in propagation. 

The polarity of the solvent affects chain propagation in cationic polymerization. The higher the polarity, the stronger the equilibrium shift toward separated ion pairs and free ions and the faster the addition of monomers to these ions. For the polymerization of styrene under HCIO4 in CCI4—CICH2CH2CI mixtures with different... 

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then interact with excess monomer to start propagation. We studied in some detail the initiation of cationic polymerization under superacidic, stable ion conditions. Carbocations also play a key role, as I found not only in the acid-catalyzed polymerization of alkenes but also in the polycondensation of arenes as well as in the ring opening polymerization of cyclic ethers, sulfides, and nitrogen compounds. Superacidic oxidative condensation of alkanes can even be achieved, including that of methane, as can the co-condensation of alkanes and alkenes. 

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. 

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. 

Olefins can only be polymerized by metal halides if a third substance, the co-catalyst, is present. The function of this is to provide the cation which starts the carbonium ion chain reaction. In most systems the catalyst is not used up, but at any rate part of the cocatalyst molecule is necessarily incorporated in the polymer. Whereas the initiation and propagation of cationic polymerizations are now fairly well understood, termination and transfer reactions are still obscure. A distinction is made between true kinetic termination reactions in which the propagating ion is destroyed, and transfer reactions in which only the molecular chain is broken off. It is shown that the kinetic termination may take place by several different types of reaction, and that in some systems there is no termination at all. Since the molecular weight is generally quite low, transfer must be dominant. According to the circumstances many different types of transfer are possible, including proton transfer, hydride ion transfer, and transfer reactions involving monomer, catalyst, or solvent. 

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... 

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. 

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. 

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.)... 

Plesch, P H., Propagation Rate Constants in the Cationic Polymerization of Alkenes, pp. 1-16 in Cationic Polymerization and Related Processes, E. J. Goethals, ed., Academic Press, New York, 1984. Pregaglia, G. F., M. Minaghi, and M. Cambini, Makromol. Chem., 67, 10 (1963). 

Various side reactions greatly limit the conversions and polymer molecular weights that can be achieved in cationic polymerization of lactams. The highest molecular weights obtained in these polymerizations are 10,000-20,000. The most significant side reaction is amidine (XXXI) formation [Bertalan et al., 1984]. Propagation of the polymer chain... 

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. 

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. 

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