Possible participation of poly(butadienyllithium) nonassociated active centers in chain propagation: Ab initio calculations

Ab initio calculations of the thermodynamic parameters were carried out for the processes of dissociation of dimeric associates of crotyllithium and 1 -lithium-2,6-octadiene, as well as crotyllithium complexes with butadiene. It was shown that crotyllithium cannot be used as an adequate model for the active center of poly(butadienyllithium), because it does not take into ac count the interaction of lithium with the unsaturated penultimate chain uni t. In the case of 1-lithium-2,6,10-dodecatriene, it was demonstrated that t he coordination of lithium to the third unsaturated chain unit provides an appreciable contribution to the energy of the active center. It was found t hat the equilibrium constant for the dissociation of dimeric associates det ermined by HF/6-31G calculations, when 1-lithium-2,6-octadiene was employed to model the terminal unit, is close by an order of magnitude to estimates derived from kinetic measurements suggesting that chain propagation involv es, for the most part, the unassociated active centers. However, the monomo lecular dissociation of dimeric associates is hardly probable, because the activation energy is high (no less than 120 kJ/mol). The activation energy may be reduced by taking the contribution of the third chain unit into cons ideration as well as by assuming that dissociation proceeds through complex formation with the monomer. In this case, the intramolecular rearrangement of these complexes may be the limiting stage of chain propagation.