Asymmetric Polymerization of Triphenylmethyl Methacrylate Using 9-Alkyl-9-fluorenyllithium–(–)-Sparteine Complexes. Influence of Organolithium Structure on the Propagation Stereochemistry

Abstract

Asymmetric anionic polymerization and oligomerization of triphenylmethyl methacrylate (TrMA) were carried out with the complexes of (–)-sparteine (Sp) with 9-methyl-9-fluorenyllithium (MeFlLi) and 9-ethyl-9-fluorenyllithium in toluene at −78°C in conjunction with the polymerization of TrMA with Sp–9-fluorenyllithium (FlLi) complex. As well as Sp–FlLi complex, Sp–MeFlLi complex gave an optically active ([α]D+360°), highly isotactic polymer with one-handed helical conformation; the polymer had a higher molecular weight than that obtained by Sp–FlLi under the same reaction conditions. The oligomers obtained at the feed ratio of monomer to initiator of two, three, and five were converted to oligo(methyl methacrylate) [oligo(MMA)]; the oligo(MMA)s were first separated in terms of degree of polymerization and then into diastereomers. The dimer meso (m) and racemo (r) and the trimer mm and mr were resolved into enantiomers to determine the ratio of isomers for each oligomer. On the basis of the change in isomer content of the dimer with the change in the feed ratio of monomer to initiator, it was concluded that the dimer anion having S absolute configuration is more active toward TrMA monomer than that having R configuration in both the systems with MeFlLi and EtFlLi in contrast to the reaction with Sp–FlLi where the R-dimer anion is more active. The stereochemistry in the propagation of a trimer anion to a tetramer anion and thereafter appeared similar to the reaction system with Sp–FlLi where ---RRR--- isomers predominantly propagate. The predominant propagation of the S-dimer anion results in a larger amount of the isomeric anions which do not propagate to the optically active polymer.