Active centre structure and the stereoregulation process in anionic polymerization

Abstract

The mechanism of stereoregulation in the anionic polymerization of butadiene and isoprene is discussed in terms of what is known about the structure of the active centres. This includes the effect of the presence of cis and trans isomers and the dynamics of their interconversion and in the isoprene case, the effect of two distinct types having 1.4 and 4.1 structures.The difflcult task of elucidation of the stereoregulation process is made much easier in anionic polymerization because of the good stability of the active centres under most conditions. Particularly important is the ability to characterize them by NMR spectroscopy in the form of oligomeric models but also by optical spectroscopy during actual polymerization processes. The first indication of interest mechanistically from N M R spectroscopy was the appearance of cis and trans forms of the model compounds t.bu. CH&H-CHz CH, Li+ (1) and t.bu. C--CHZC(CH,)~CH-~ L i ' (2) which mimic the active centres in butadiene and isoprene polymerizatioqrespectively. A cross-check with optical spectroscopy revealed that generally the two forms absorb at different wavelengths (trans at longer h) so that isomerization can be monitored during actual polymerization. The relative amounts of each depend on counter-ion and solvent but allylic lithium compounds generally, largely independent of the hydrocarbon chain attached, prefer the trans form in hydrocarbon solvents and cis form in polar solvents.This observation is of obvious interest in terms of the classic recipe for the preparation of highly cis-1.4 polyisoprene in non-polar solvents by lithiumbased initiation (3). One might expect that the predominant trans active centres would lead to a polymer rich in trans units. Since the exact opposite is observed then it is clear that the equilibrium state is not attained under most conditions. A study of the early stages of isoprene addition (4). together with measurements of the cis + trans isomerization rates, showed that the 0 1992 Huthig & Wepf Verlag, Basel