The ultraviolet absorption spectra of living anionic polymerizing systems have been measured with sytrene, butadiene, and isoprene as the Inonolners; with lithium, sodium, arid potassiunl as the alkali metals; and with tetrahydrofuran, cyclohexane, and benzene as the solvents. For any rnononler the similarity of the spectra obtained with the various metals and solvents leads t o the conclusion that the alkali metal -carbon band has the same structure under the various conditions. From other evidence the bond is considered to be ionic in all cases. The diene nlonomer anions in tetrahydrofuran solution were found t o undergo a series of isornerizations, some of them co~nparable in speed t o the co~lsumption of monorner.Many polymerizations of olefins by anionic reagents occur by the addition of the metalorganic con~pound to the monomer ~nolecule, M, as the chain initiation step. Chain propagation then proceeds by the addition of further inonolner units to the carbon-metal bond a t the end of the chain, re-forming the bond a t each step.There is disagreement about the relative ainount of ionic or covalent character of this bond when different allrali metals are used and when the polarity of the solvent is changed. With lithium in particular (1, 2), it has been suggested that in nonpolar solvents the carbon-lithium bond is largely covalent in character and that with dienes this covalency is the cause of the stereoregulating action found in nonpolar solvents. Addition of polar solvents such as tetrahydrofuran (THF) was supposed to proinote ionization of the carbonlithium bond, the ion-pair reacting to form a inore random n~icrostructure.If the carbon-metal bond is essentially ionic, charge delocalization may be expected to occur with conjugated monomers, and the resultant polynler anions may be expected to show optical absorption in the near ultraviolet region a t roughly the same wavelength as observed for the corresponding cations (3). The use of such an absorption band to ineasure the concentration of active centers in the butyllithium-catalyzed poly~nerization of styrene in benzene has been described previously (4). The effect of addition of tetrahydrofuran to such a system has been briefly inentioned ( 5 ) . T o learn inore about the carbon-metal bond, a more general study of the changes in absorption spectra observed in the poly~nerization of styrene, butadiene, and isoprene has now been carried out in hydrocarbon solvents, in tetrahydrofuran, and in their mixtures with organo-lithium, -sodium, and -potassium reagents.