Abstract
The elementary reactions of isoprene polymerization initiated with n-butyllithium have been studied in n-heptane solution in order to obtain an access to a control of molecularweight distribution of the polyisoprene.
The absence of any termination and chain transfer reaction was confirmed by measuringthe concentration of Li attached to the polymer end (polymer-end-Li) and the molecularweights. The polymer-end-Li formation reaction was found to be first order with respect to the non-associated n-butyllithium, but not to the monomer concentration. It wasfound that the rate of polymerization increases with the polymer-end-Li concentration anddecreases with the initial n-butyllithium concentration, indicating reduction of the concentrationof active polymer end due to association with n-butyllithium. The rate of propagationin the polymerization where isolated low molecular weight polyisoprenyllithium is used asan initiator was found to be first order with respect to the monomer concentration, and asixfold association of the polymer-end-Li was inferred by a formula derived from therelation between the propagation rate and the polymer-end-Li concentration.
In conclusion, the elementary reactions of the n-butyllithium initiated polymerizationof isoprene are thought to consist of initiations through complex reactions between n-butyllithiumand isoprene, a propagation by non-associated polymer-end-Li, and three types ofassociation of n-butyllithium and polymer-end-Li, namely, two self-associations and across-association.
