Isoprene Polymerization by Organometallic Compounds. II

From the above results it would certainly appear that the ionic character of the growing ion pair (polymer−M+) is a very important factor in governing the structure of isoprene homopolymers. Another factor which may be sufficient or necessary for the formation of the unique cis-1,4-eonfiguration is the presence of an orienting surface or complex. It is well known that organolithium compounds are associated in hydrocarbon media, whereas they are probably monomolecularly dispersed in ethers and amines. It is apparent, then, that in hydrocarbon solutions the carbon-lithium chain end may be associated with other carbon-lithium pairs from unreacted initiator molecules or even with other chain ends or, in some cases, it may be associated with the lithium atoms on the surface of lithium particles. The fact that greater than 90 per cent cis-1,4-structure is obtained in hydrocarbons by use of either an apparently clear solution of filtered n-butyllithium or a heterogeneous system of lithium dispersion suggests that the massive surface of the lithium particle is not a necessary factor for the stereospecificity of the reaction. Whether we should consider the butyllithium or the lithium-initiated polymerizations as homogeneous solution reactions or heterogeneous surface reactions depends on the definition of a surface, since an associated complex or micelle may also conceivably serve as an orienting specie in the same way as a surface does. We emphasize this since so many stereospecific polymerizations such as the Ziegler system and the Phillips system appear to occur under heterogeneous conditions, presumably on an orienting surface of rather gross dimensions.