Isoprene polymerization by organometallic compounds. II

Abstract Dispersions of metallic lithium, sodium, and potassium and some of their organoderivatives were used to initiate the polymerization of isoprene in several solvents. The propagating species in all cases were ion pairs of the type . The structure of the resultant polyisoprene depends on the character of the propagating pair. This in turn depends mainly on the counterion M+ and the solvent type. One quality of this ion pair which is of utmost importance is its ionic character (or degree of charge separation). In solvents such as diethyl ether and tetrahydrofuran, where we might expect large charge separation in the ion pair, all initiatiors produce similar, though not necessarily identical, polymer structures comprising 1,2 ‐3,4‐, and trans ‐1,4‐adducts. With potassium‐dependent initiators, very nearly the same polymer structure is obtained in hydrocarbons as in the more basic ethers, attesting to the highly ionic character of the potassium‐carbon bond; this might perhaps be considered as the “limiting anionic behavior”. Sodium has smaller inherent ionic‐character and is more sensitive to changes in solvent basicity, but still it parallels the behavior of potassium. With lithium, the most covalent of the alkali metals, a dramatic change in behavior occurs when polymerization is conducted in hydrocarbon media; a 93% cis ‐1,4‐,7% 3, 4‐polyisoprene structure is obtained. This may be merely a limiting structural from at the low end of the scale of ionic character of the propagating ion‐pair. The steric factors arising from complexes formed between the propagating ion‐pair. The steric factors arising from complexes may also have an important influence on the polymer structures product in the various solvents. A continuous seriese of polyisoprene structures intermediate between those formed in hydrocarbons with Li and the limiting “anionic structure” can be obtained by use of such solvents as phenyl ether or anisole, or by judicious choice of appropriate hydrocarbon–solvent ratios with a number of suitable cosolvents.