The kinetics of ethylene oxide polymerization in THF, with Na, K and Cs-naphthalene complexes as initiators, have been studied in detail.
Polymerization is first order with respect to monomer without chain termination or transfer. The complicated dependence of the polymerization rate on active centre concentration is due to strong association. Rate constants of the propagation and thermodynamic parameters of association have been determined from this dependence.
Contact type ion pairs are shown to be the active propagating species. Their activity increases and association ability decreases from Na + to Cs+. Solvation of the active species plays the decisive part in this process.
PUBLISHED data concerning anionic polymerization of epoxides indicate that the reaction mechanism is not so simple as sometimes suggested. The nature of the active centres, the mechanism of their interaction with the monomer and environment, the role of solvation and of dielectric constant are not yet clear.
Alkali metal alkoxides have been frequently used as initiators. Β’~-7) The low solubility of these initiators in common solvents requires addition of alcohol, so making the investigation extremely complicated. <3-7> Complexes such as sodiumnaphthalene are more convenient in view of their solubility and stability in some ethers. These initiators were used in ethylene oxide polymerization first by Szwarc and Richards es) and then by Dudek Β’9) in a study which is not widely known.
This paper deals with the kinetics of ethylene oxide polymerization in tetrahydrofuran (THF). Sodium, potassium and caesium-naphthalene complexes were used as initiators.
EXPERIMENTAL
All chemicals were thoroughly purified before use. THF was treated finally with Na-naphthalene and was stored in vacuum. Ethylene oxide was treated in the same way hut partial polymerization was caused. The monomer was redistilled and the procedure was repeated.
The usual high-vacuum technique was used for all manipulations. The flask wall was thoroughly heated, then treated with the initiator solution and afterwards with pure solvent. The initiator concentrations were determined either by titration or spectrophotometrically. The extinction coefficients for complexes were taken as 4750 (Na +, 364 nm), 6430 (K +, 365 nm) and 8500 M -a cm -a (Cs +, 367 nm). Cs--naphthalene was always used only freshly prepared in view of its instability.
The kinetics of polymerization were studied by dilatometry. Polymerization proceeded to completion and conversion was calculated in the usual way. For bulk polymerization, thermometric and gravimetric methods were used also.