Various types of aggregates in mono- or bifunctional polyisoprenyllithium in hydrocarbons

The addition of tert-butyllithium on 1,2-bis(isopropenyl-4 phenyl)ethane in pure hexane leads to the quantitative synthesis of a dilithium compound as shown previously by analysis of the reactional mixture. Its characteristics of difunctional initiator for the synthesis of polydienes in pure hydrocarbon solvents was proved by indirect methods. In this study it is shown by mass spectrometry that difunctional oligopolyisoprene is solely obtained at the first stages of the initiation reaction. Viscosimetric measurements show that, for low degree of polymerization and low concentration in Li, cyclic aggregates are preferentially formed by intramolecular association. A Li-7 n.m.r. study in hexane of various mono- and difunctional organolithium compounds confirms their trend to associate in hydrocarbon solvents and, for alpha,omega-dilithiopolyisoprene, the influence of the chain length on the formation of aggregated species involving more than four lithium atoms. A change in the concentration in Li has little influence on the distribution of the various aggregates in the 4 x 10(-4) x 10(-3) mol l(-1) Li concentration range. The addition of LiCl reduces the amount of the most associated species, while the presence of 15 vol% THF leads to their disappearance. Copyright (C) 1996 Elsevier Science Ltd.