STEREOCHEMICAL CONTROL IN ANIONIC-POLYMERIZATION OF 2-VINYLPYRIDINE AND RELATED VINYL MONOMERS

The alkylation, silylation and other electrophilic reactions of 1-lith io-1,3-bis(2-pyridyl)butane (2a) in THF at -78 degrees C leads to meso -like products that may be accounted for by intramolecular coordinatio n of Li ion with the penultimate 2-pyridyl group, This is also demonst rated by proton-transfer equilibria between 1-lithio-2-ethylpyridine a nd 2a, allowing the determination of the Delta H and Delta S Of intram olecular chelation in 2a, The Delta S of chelation is nearly zero, pro bably as a result of chelation-induced desolvation of Li ion, Surprisi ngly, however, the corresponding addition of 2-vinylpyridine to 2a and higher P2VPLi homologs in THF is not stereoselective. In toluene at - 78 degrees C the t-BuLi-initiated polymerization of 2-VP is actually s omewhat syndiotactic (rr=0.44). The presence in this polymerization of at least one equivalent of t-BuOLi or other tertiary ROLi surprisingl y leads to isotactic P2VP, SEC viscometric and other studies indicate that both the P2VPLi- and P2VPLi-ROLi complexes are monomeric in tolue ne, The stereochemistry is apparently due to a 1:1 P2VPLi-ROLi complex that discriminates between the two pro chiral faces of the anion by p referential coordination of the (ROLi(2)) moiety to the meso-prochiral face, Isotactic-like addition now occurs by electrophilic attack of 2 -VP that is syn with respect to the (ROLi(2)) 'counter ion.'