Stereospecificity in the propagation step

The diene monomers give predominantly 1,4-polymers in hydrocarbon solvents if polymerized using lithium-based initiation. Isoprene, under these conditions, gives a predominantly cis-1,4 polymer but with butadiene the proportions of cis- and frans-1,4 are fairly evenly distributed. Once ain this phenomenon is characteristic of lithium compounds sodium- and potassium-based initiation gives mixed structures even in hydrocarbon solvents. Polymerization in polar solvents such as tetrahydrofuran leads to largely 3,4-polyisoprene or 1,2-butadiene with 

Styrene only forms an isotactic polymer under heterogeneous reaction conditions. This is the case with sodium or potassium alkyls [185,186]. It has been reported that the lithium alkyls produce isotactic polystyrene under homogeneous reaction conditions, but later investigation showed that the presence of lithium hydroxide or oxide is necessary [187]. 

In the polymerization of acrylates, interaction of lithium with carboxyl groups of incoming monomer and the penultimate unit of the polymer chain is the dominant feature of theories of stereoregulation [168,191]. The stable form of the chain end is proposed to involve secondary bonding of the lithium to the penultimate carboxyl group thus holding it in a fixed configuration, viz. 

The incoming monomer unit would then be forced, either because of steric interactions, or by the interaction of its carboxyl group with lithium at the chain-end, to add in a specific manner to re-form the same loose ring structure present initially. One variant of this mechanism [192] involves a covalently bonded six membered ring formed by enolization of the active chain end followed by alkoxide ion attack on the penultimate carboxyl group. In polar solvents, or in the presence of moderate amounts of them, competition for solvation of the counter-ion would be produced and the intramolecular solvation producing the stereospecificity would be reduced in effectiveness as the ether concentration is increased. Replacing the lithium counter-ion with sodium or other alkali metal would be 

The above mechanisms for cis-1,4 polymerization of isoprene or isotactic polymerization of acrylates assume that the configuration of each unit is fixed at the moment of reaction and that no racemization occurs between additions of monomer molecules. Little evidence for the validity of the mechanisms was available when suggested. Recently it has been possible to obtain information, from NMR studies, on the reaction path. This evidence is of two types and depends on the polymerization of stereospecifically deuterated monomers to determine the mode of approach of monomer molecules and on direct observations of NMR spectra of the terminal monomer unit in the polymer. 

---