Cationic Polymerization of styrene

To further demonstrate the stability and unreactivity of the isopropenylferrocene carbenium ion, a stable derivative of this ion was prepared and evaluated as an initiator for the cationic polymerization of styrene. The derivative prepared for this purpose was the tetrafluoroborate salt of the isopropenylferrocene carbenium ion. Even at 20°C a 26% yield (Mjj = 11,315, = 18,815) of... 

TABLE 5-9 Activation Energy for Rate of Cationic Polymerization of Styrene... 

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... 

The cationic polymerization was further evidenced by studying the effect of added proton scavengers, such as ammonia and trimethylamine, and the copolymerization with a-methylstyrene and isobutylvinylether. It is now believed that both radical and cationic polymerization of styrene are able to proceed by ionizing radiations even in bulk at room temperature, and the latter polymerization is much more predominant in the absence of cation scavengers, though it is effectively suppressed in their presence. 

Overberger, C. G., G. F. Endres and A. Monaci Ionic polymerization. VII. Relative reactivities of mono- and />-dialkylbenzenes as molecular terminating agents in the cationic polymerization of styrene. J. Amer. chem. Soc. 78, 1969 (1956). 

The cocatalytic activity of alkyl halides in the cationic polymerization of styrene in the presence of stannic chloride (17), in the polymerization of butadiene with Et2AlCl-cobalt compound (10) and R3Al-cobalt compound (23) catalyst systems and in the cationic polymerization of isobutylene (12) and styrene (13) in the presence of Et2AlCl is well documented. It is reasonable to propose that a reaction between Et2AlCl and a labile chlorine atom on PVC results in the generation of a carbonium ion on the polymer backbone. 

The cationic polymerization of styrene sulfide has been reinvestigated by Van Craeynest (15). With triethyloxonium tetrafluoroborate as initiator, a rapid and quantitative polymerization was observed, followed by a slow degradation of die polymer to a mixture of cis and tram 2,5-diphenyl-l, 4-dithiane and as and tram 2,6-diphenyl-1,4-dithiane. Since the BF4 counter ion is not capable of forming a covalent bond, a back-biting reaction via sulfonium ions seems the plausible mechanism for the dimer formation. The polymerization initiated with dimethyl sulfate showed the same characteristics a fast polymerization is followed by degradation to the same mixture of isomeric diphenyl- 1,4-dithianes. However, the mwts-2,5-diphenyl derivative was the only isomer that crystallized from the solution. It is therefore reasonable to accept that with dimethyl sulfate also, the cyclic dimers of styrene sulfide are formed by a back-biting type of degradation of the polymer and not by the mechanism shown above. 

Mah et al. demonstrated the effect of counterions on the cationic polymerization of styrene [35-37]. The radiation-induced polymerization is much more sensitive to impurities than the catalytic polymerization, as the former involves the cationic species in a free ion state. Thus, one can expect, in the presence of stable anions, the promotion of the cationic polymerization because of the ion-pair formation between the dimer cation and the counterion. The effect was... 

Despite of this inherent limitation, several spectacular results have been obtained. It should be noted that the initiation mechanism of the cationic polymerization of styrene described above was also deduced from the results of pulse radiolysis experiments. The pulse radiolysis combined with other techniques, such as the matrix isolation technique, the electron spin resonance technique and usual polymerization techniques, definitely provides a powerful means for investigating fundamentals of polymerization. 

Recently Russian workers claimed that the presence of cocatalysts was not a necessary requirement for cationic polymerization of styrene and isoprene with stannic chloride catalyst provided the temperature and/or the dielectric constant of the medium was high enough (53—55). Similar ideas have also been expressed by Japanese investigators (56). 

Chain branching occurs in cationic polymerization much as it does in free-radical polymerization. Propose a mechanism to show how branching occurs in the cationic polymerization of styrene. Suggest why isobutylene might be a better monomer for cationic polymerization than styrene. 

Not many initiators belong to this class even though the halogenoacetic, fluorosulphonic, and other acids are included. A detailed analysis of their polymerization mechanism is obscured by complex formation with monomer and with solvent, by the occurrence of aggregates, and by anion reactions in acids with an unstable anion. In spite of its apparent simplicity, initiation by Bronsted acids has not yet been investigated in detail. The pseudo-cationic polymerization of styrene is an instructive example. 

Williams et al. studied the radiation-initiated cationic polymerization of styrene. By conductivity measurements they were able to determine very small amounts of ions and thus directly determine the propagation rate constant [381] under otherwise comparable conditions, k+ is about 30times larger than k. ... 

Cationic polymerizations of styrene, 2-methylpropene, vinylnaphthalene, indene, etc. at temperatures about 273 K yield only low polymers. Polymerization of styrene with HC104 in chlorinated solvents at room tern-... 

The dissociation constants of trityl and benzhydryl salts are KD 10 4 mol/L in CH2C12 at 20° C, which corresponds to 50% dissociation at 2-10-4 mol/L total concentration of carbocationic species (cf. Table 7) [34]. The dissociation constants are several orders of magnitude higher than those in analogous anionic systems, which are typically KD 10-7 mol/L [12]. As discussed in Section IV.C.l, this may be ascribed to the large size of counterions in cationic systems (e.g., ionic radius of SbCL- = 3.0 A) compared with those in anionic systems (e.g., ionic radius of Li+ 0.68 A), and to the stronger solvation of cations versus anions. However, the dissociation constants estimated by the common ion effect in cationic polymerizations of styrene with perchlorate and triflate anions are similar to those in anionic systems (Kd 10-7 mol/L) [16,17]. This may be because styryl cations are secondary rather than tertiary ions. For example, the dissociation constants of secondary ammonium ions are 100 times smaller than those of quaternary ammonium ions [39]. 

There is also only limited information available on the dissociation of ion pairs to free carbenium ions, especially from macromolecular systems (cf., Sections II.D and IV.B). The dissociation constants in cationic polymerizations of styrene are approximately KD = 10-6il moFL in CH2CI2, depending on the temperature and structure of the counteranions... 

Table 16 Dissociation Constants of Growing Ion Pairs in Cationic Polymerizations of Styrenes...

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

In contrast to p-alkoxystyrenes, styrene lacks an electron-donating, car-bocation-stabilizing substituent, and thus it is much less reactive and forms a much less stable growing carbocation. It has therefore been believed that controlled/living cationic polymerization of styrene would be very difficult. 

The active site in chain-growth polymerizations can be an ion instead of a free-radical. Ionic reactions are much more sensitive than free-radical processes to the effects of solvent, temperature, and adventitious impurities. Successful ionic polymerizations must be carried out much more carefully than normal free-radical syntheses. Consequently, a given polymeric structure will ordinarily not be produced by ionic initiation if a satisfactory product can be made by less expensive free-radical processes. Styrene polymerization can be initiated with free radicals or appropriate anions or cations. Commercial atactic styrene polymers are, however, all almost free-radical products. Particular anionic processes are used to make research-grade polystyrenes with exceptionally narrow molecular weight distributions and the syndiotactic polymer is produced by metallocene catalysis. Cationic polymerization of styrene is not a commercial process. 

Considerable advances have taken place in the 1990s with regard to cationic polymerization of styrene. Its uses to make block copolymers and even living cationic polymerization have been reported (171). 

Proton transfer to monomer is the best known and most widely studied transfer reaction in the cationic polymerization of vinyl monomers. Cationic polymerization of styrene and isobutylene belong to the comprehensively studied examples More recently, it has been shown by H-NMR that there are two kinds of double bonds formed as result of transfer in the polymerization of isobutylene ... 

Ionic chain polymerizations are especially sensitive to traces of moisture and other impurities which can terminate ions. Water and other similar compounds terminate ions by transferring a proton or negative fragment (see Chapter 8) For example, in the cationic polymerization of styrene, the cationic chains are effectively terminated by proton transfer to water molecule ... 

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