Counterions, tetrahydrofuran polymerization

The oxonium ions could, in principle, be in equilibrium with minute quantities of carbon cations, CH2 , that are more active. All evidence to date, however, shows that in tetrahydrofuran polymerizations the presence of carbon cations is negligible in the propagation process. Also, the rate constant for propagation of free macroions with the counterions is equal, within experimental error, to the rate constant for macroion-counterion pairs. This does not appear to depend upon the structure of the anion studied." Such information, however, was obtained on large anions. > th smaller anions, differences in the rates of propagation of macrocations and those of macroion-counterion pairs has not been ruled out. 

Terminations in tetrahydrofuran polymerizations can depend upon the choice of the counterion, particularly if the reaction is conducted at room temperature. In many reactions the chain continues to grow without any considerable chain termination or transfer. This produced the term living polytetrahydrofuran. Thus, in polymerizations of tetrahydrofuran withPFe or SbFe counterions, the molecular weights of the products can be calculated directly from the ratios of the initiators to the monomers. The molecular weight distributions of the polymers from such polymerization reactions with PFe and SbF6 , however, start out as narrow, but then broaden. This is believed ... 

Cationic ring-opening polymerization is the only polymerization mechanism available to tetrahydrofuran (5,6,8). The propagating species is a tertiary oxonium ion associated with a negatively charged counterion ... 

The propagation rate constant and the polymerization rate for anionic polymerization are dramatically affected by the nature of both the solvent and the counterion. Thus the data in Table 5-10 show the pronounced effect of solvent in the polymerization of styrene by sodium naphthalene (3 x 1CT3 M) at 25°C. The apparent propagation rate constant is increased by 2 and 3 orders of magnitude in tetrahydrofuran and 1,2-dimethoxyethane, respectively, compared to the rate constants in benzene and dioxane. The polymerization is much faster in the more polar solvents. That the dielectric constant is not a quantitative measure of solvating power is shown by the higher rate in 1,2-dimethoxyethane (DME) compared to tetrahydrofuran (THF). The faster rate in DME may be due to a specific solvation effect arising from the presence of two ether functions in the same molecule. 

A review is given on the kinetics of the anionic polymerization of methyl methacrylate and tert.-butyl methacrylate in tetrahydrofuran and 1,2-dimethoxy-ethane, including major results of the author s laboratory. The Arrhenius plots for the propagation reaction+are linear and independent of the counterion (i.e. Na, Cs). The results are discussed assuming the active centre to be a contact ion pair with an enolate-like anion the counterion thus exhibiting little influence on the reactivity of the carbanion. 

A most thorough investigation of the tetrahydrofuran system was recently reported by Dreyfuss and Dreyfuss (18). They initiated the polymerization bv the decomposition of benzenediazonium hexafluorophos-phate (PhNj, PFg), which provides a Ph+ ion and an extremely stable PFg counterion. It seems that the stability of the counterion is the reason for the simplicity of the system which is... 

Polymerization of 5-membered cyclic ether, tetrahydrofuran with SbF6 counterion, is the best example of such a system. 

Electron-withdrawing substituents in anionic polymerizations enhance electron density at the double bonds or stabilize the carbanions by resonance. Anionic copolymerizations in many respects behave similarly to the cationic ones. For some comonomer pairs steric effects give rise to a tendency to altemate. The reactivities of the monomers in copolymerizations and the compositions of the resultant copolymers are subject to solvent polarity and to the effects of the counterions. The two, just as in cationic polymerizations, cannot be considered independently from each other. This, again, is due to the tightness of the ion pairs and to the amount of solvation. Furthermore, only monomers that possess similar polarity can be copolymerized by an anionic mechanism. Thus, for instance, styrene derivatives copolymerize with each other. Styrene, however, is unable to add to a methyl methacrylate anion, though it copolymerizes with butadiene and isoprene. In copolymerizations initiated by w-butyllithium in toluene and in tetrahydrofuran at-78 °C, the following order of reactivity with methyl methacrylate anions was observed. In toluene the order is diphenylmethyl methacrylate > benzyl methacrylate > methyl methacrylate > ethyl methacrylate > a-methylbenzyl methacrylate > isopropyl methacrylate > t-butyl methacrylate > trityl methacrylate > a,a -dimethyl-benzyl methacrylate. In tetrahydrofuran the order changes to trityl methacrylate > benzyl methacrylate > methyl methacrylate > diphenylmethyl methacrylate > ethyl methacrylate > a-methylbenzyl methacrylate > isopropyl methacrylate > a,a -dimethylbenzyl methacrylate > t-butyl methacrylate. 

The propagation rates for methyl methacrylate polymerization in polar solvents like tetrahydrofu-ran or dimethylformamide are lower than the rates of initiation [203]. There is no evidence, however, that more than one kind of ion pairs exist [204-206]. The ion pairs that form are apparently craitact-ion pairs [203]. Furthermore, based on the evidence, the counterions are more coordinated with the enolate oxygen atoms of the carbonyl groups than with the a-carbons. As a result, they exert less influence on the reactivity of the carbanions [203]. The amount of solvation by the solvents affects the reaction rates. In addition, intramolecular solvation from neighboring ester groups on the polymer chains also affects the rates. In solvents like dimethylformamide, tetrahydrofuran, or similar ones [203], the propagating chain ends-ion pairs are picmred as hybrid intermediates between two extreme structures. This depends upon the counterion, the solvent, and the temperature [203] ... 

Heterocyclic Monomers.—Reviews of the polymerization of tetrahydrofuran (THF) were published. Rate constants of propagation of THF on macroesters and macroions were measured. In the polar solvent nitromethane, where macroesters are not important, it was shown that k and k t are identical within experimental error, and are not influenced by the nature of the counterion. It was postulated that the active centres are so highly solvated by monomer that free ions and ion-pairs are indistinguishable in terms of reactivity. 

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