A new view of the anionic diene polymerization mechanism

We investigated the anionic polymerization of butadiene in d-heptane solvent using tert-butyl lithium as initiator. Two complementary techniques were used to follow the polymerization processes: H-1 NMR and small angle neutron scattering (SANS). The time resolved H-1 NMR measurements allowed us to evaluate quantitatively the kinetics of the processes involved. The initiation event commences slowly and then progressively accelerates. This indicates an autocatalytic mechanism. The microstructure of the first monomer units attached is to a high extent 1,2. The disappearance of initiator---at about 10% monomer conversion---signals the onset of the normal similar to6% vinyl content of the chain. Small angle neutron scattering was used to study the aggregation behavior of the carbon lithium head groups. It is well known that the polar head groups aggregate and form micellar structures. For dienes in nonpolar solvents the textbook mechanism assumes the formation of only tetramers during the propagation reaction. By combining H-1 NMR and SANS results we were able to determine quantitatively the aggregation number during all stages of the polymerization. Our measurements show the existence of large-scale structures during the initiation period. The initial degree of aggregation of more than 100 living polymer chains diminished as the polymerization progressed. In addition, even larger, giant structures with N-agg >> 1000 >> 1000 and R-g approximate to 1000A were found.