Real time SANS study on head group self-assembly for lithium based anionic polymerizations

Small angle neutron scattering was used to in situ study the aggregated structures formed in the course of the polymerization of butadiene and isoprene in deuterated n-heptane. The samples were designed to have equal degrees of polymerization. These measurements showed, at low Q, that the start of the butadiene propagation event was accompanied by the presence of highly extended large-scale structures. As propagation progressed these initial structures diminished in size and were replaced, at least in part, by star-like aggregates. At the cessation of the polymerization reaction the star micelles, mid-Q regime, exhibited a mean aggregation state of 8.4. At lower conversions (and thus lower chain molecular weights) the presence of large three-dimensional aggregates was indicated. Conversely, the isoprene system in its initial moments of propagation did not show the same extent of large-scale structures although the low Q data did indicate the formation of architectures larger than the star-like aggregates. The star shaped micelles exhibited the mean degree of aggregation of 4. These results demonstrate that the association behavior of these polar dienyllithium headgroups is more varied than permitted by the current ‘textbook’ mechanism where the solitary permissible aggregation state is four. These findings concur with those suggested from a recent semi-empirical and ab initio quantum chemistry based series of calculations.