Asymmetric polymerization of triphenylmethyl methacrylate leading to a one-handed helical polymer: Mechanism of polymerization

Article abstract of DOI:10.1021/ja00030a030

Asymmetric oligomerization of triphenylmethyl methacrylate (TrMA) was carried out with complexes of 9-fluorenyllithium and chiral ligands in toluene at -78°C, and the oligomers obtained were converted into methyl esters. The resulting oligo(methyl methacrylate)s were first fractionated by gel permeation chromatography in terms of degree of polymerization and further separated into diastereomers and optical isomers by high-performance liquid chromatography. The distribution of oligomers and the ratio of isomers in each oligomer gave important information on the mechanism of the asymmetric (helix sense selective) polymerization of TrMA. The reactivity of each oligomer anion depended greatly on its degree of polymerization and stereostructure. The oligomer anions whose asymmetric centers have R configuration in the system with the complex of 9-fluorenyllithium and (-)-sparteine and those of S configuration in the systems with (+)-(2S,3S)-2,3-dimethoxy-l,4-bis(dimethylamino)butane and (+)-(S)-1-(2-pyrrolidinylmethyl)pyrrolidine as chiral ligands predominantly propagated to a one-handed helical polymer. A stable helix starts at degree of polymerization 9 in the former system and at degree of polymerization 7 in the latter two systems. One helix turn seems to consist of three or four monomeric units. The main chain of the resulting polymer in the former system possessed RRR - absolute configuration and that in the latter systems SSS -, though both polymers are considered to be of the same helicity, P or M. These results indicate that the helicity of the polymer is not governed by the configuration of the main chain but by the chirality of the ligands.