Recent transition metal catalysts for syndiotactic polystyrene

Syndiotactic polystyrene has attracted much interest in scientific and industrial research after its first synthesis in 1985 and has led to a fast commercialization of this polymer. The catalyst systems used for this coordination polymerization of styrene are a key point in this development to provide high polymerization activities and syndiotacticities of the polymers obtained. This literature review gives a comprehensive overview on the recent transition metal catalysts comprising the literature since about 2000 and especially on the transition metal complexes investigated in the syndiospecific homopolymerization of styrene. It includes the polymerization activity of the catalysts, the syndiotacticity of the polymers received as well as the discussion of the relationships between catalyst structure and polymerization activity. The complex-coordination mechanism of the syndiospecific polymerization of styrene is summarized in general at the beginning. The review of the recent transition metal catalysts for the syndiospecific styrene polymerization includes transition metal complexes, cocatalysts (methylaluminoxanes and boron compounds), activators and chain transfer agents, and supported and heterogenized catalysts. Transition metal complexes contain group 4 transition metal complexes (mono- and bis-cyclopentadienyl complexes, metal complexes of other ring systems such as indenyl, fluorenyl and other complexes, di- and multi-nuclear complexes, and non-metallocene complexes), and metal complexes of other transition metals (groups 8–10, rare earth metals, and others). The chapter on mono-cyclopentadienyl complexes demonstrates an overview on the investigations using unsubstituted cyclopentadienyl and pentamethylcyclopentadienyl complexes, on the influence of the variation of the structure of the cyclopentadienyl ligand, and on the effect of the variation of ancillary complex ligands besides cyclopentadienyl. This summary also considers recent developments in the preparation of new transition metal complexes based on the synthesis of completely novel π-ligands of the half-metallocenes, the success in attaining high syndiospecificities with transition metal complexes based on rare earth metals, the coordination polymerization in aqueous systems, the syndiospecific living polymerization, and new activators for the catalysts, with regard to syndiotactic polystyrenes.