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 from a high degree of covalence to that of a pair of free, solvated ions [86]:

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. Investigation of polymerizations of alkyl vinyl ethers, for instance, with BF₃ •O(C₂H₅)₂ in toluene or in methylene chloride at 79C, showed that the rate of monomer consumption falls in the following order [87]:

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

Table 4.1 Relative rates of reactions of alkenes in 1,2-dichloroethane at 24C with PhCH₂ L and (Ph)₂CL cation^a

Another example is a study of the differences in the rates of reactions of various alkenes with two cations, PhCH₂ and (Ph)₂CH, generated by electron pulses [88]. Carbon cations, free from complexities, such as ion pairing and cation aggregation that my be encountered in typical polymerizations, were used. Table 4.1 shows some of the data that was reported [88]. 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 40C.Thepolymerizations were carried out in a 50/50 mixture of CH₂Cl₂ 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. 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, results in formations of new carbon cations. They, upon formation, pair off immediately 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 [90]:

The degree of association of the ion pairs depends also upon the nature of the counterion and on the temperature of the reaction medium. Completely dissociated ion pairs allow chain growth to take place free from the influence of counterions. The carbon cations simply add directly to the double bonds of the incoming monomers. Propagation rates for such reactions are greater than for those with tight ion pairs [91]. The efficiency of the counterion is related to its acid strength.

In isobutylene polymerization at 78C the following Lewis acids were rated in the order of their efficiencies [92]:

BF₃ >AlBr₃ >TiBr₄ >BBr₃ >SnCl₄

The polymerization reactions can be complicated, sometimes, by two different types of propagation paths. Some chains may grow without a terminal counter-ion as free propagating species. Other polymeric chains, however, may be paired off with counterions. It should be noted that when references are made to free propagating ions, the ions are free from electrostatic influences of the anions. They are, however, still associated with, and interact with polar or polarizable solvent molecules or monomers.

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مواضيع ذات صلة

Living Cationic Polymerizations

Pseudo-Cationic Polymerization

Steric Control in Cationic Polymerization

Electro Initiation of Polymerization

Charge Transfer Complexes in Ionic Initiations

One Electron Transposition Initiation Reactions

Metal Alkyls in Initiations of Cationic Polymerizations

Lewis Acids in Cationic Initiations

Initiation by Protonic Acids

Cationic Polymerization

The Chemistry of Ionic Chain-Growth Polymerization

Polymer Preparation Techniques