Ion‐pair structure in anionic polymerization. Effect of temperature and association on an allylic lithium compound in two polar solvents

Abstract

NMR studies are reported on enriched samples of a one-unit model of the active centres present in butadiene polymerization. Concentrations between 2 x M and 1M, and temperatures between -120" and 50" were investigated. The effect of ion-pair association on charge distributions in diethyl ether solutions is described. The effect of temperature on the overall structure in tetrahydrofuran solutions is discussed.The use of spectroscopic methods to study charge delocalized ion-pairs has contributed much to our knowledge of their structure in solution. Much of this refers to ion-pairs derived from aromatic compounds where optical spectroscopy has proved to be extremely valuable."' This technique is of less value in the case of allylic ion-pairs of the type present in anionic diene polymerization, although interesting information can still be ~btained.".~' I3C NMR studies on single (or two unit) monomer models, on the other hand, can give evidence on charge distribution and bond rotations in the active ~entre.'~"' The low natural abundance of the I3C nucleus however, means that concentrations in the range of 0.5 to 1.0 M have generally been used. In many cases (particularly with lithium compounds) the species observed is then in an associated state. The sensitivity of the NMR method can obviously be improved if I3C enriched samples are available, so that concentrations -lo3 M can be used. A comparison made with results obtained at higher concentrations may also be of value. This communication reports on I3C NMR spectra obtained on neopentylallyllithium (t.C,H,-CH,-CH=CH'CH,Li') prepared a) with 2-13C enriched butadiene or b) with a mixture of 55% 1-I3C enriched butadiene and 45% of 2-I3C enriched butadiene. The former samples were used generally in the high concentration experiments, where they are particularly valuable in the 0