TABLE OF CONTENTS An anionic mechanism is proposed for those polymerizations initiated by alkali metal organometallic species, where there is good reason to assume that the metal is strongly electropositive relative to the carbon (or other) atom at the tip of the growing chain [21, 143 151]. However, analogous to the discussion of the active species in cationic polymerization, a multiplicity of active species may be involved as propagating species in anionic polymerization as shown below [150]. In contrast to cationic polymerization, however, there is experimental evidence for the involvement of many of these species under certain experimental conditions [145, 147, 148].
Although the ability of alkali metals, such as sodium, to initiate polymerization of unsaturated organic molecules has long been known (the earliest record dating back to the work of Matthews and Strange [8] and of Harries [7], around 1910, on polymerization of dienes), the mechanism had remained largely obscure due to the heterogeneous character of this type of catalysis. The pioneering work of Higginson and Wooding [152] on the homogeneous polymerization of styrene by potassium amide in liquid ammonia, and that of Robertson and Marion [153] on butadiene polymerization by sodium in toluene, merely showed the important role of the solvent in participating in chain transfer reactions.
The true nature of homogeneous anionic polymerization only became apparent through studies of the soluble aromatic complexes of alkali metals, such as sodium naphthalene. These species are known to be radical anions [154 158], with one...
