Syndiospecific Polymerization of Styrene Catalyzed by Half‐titanocenes Containing Monodentate Anionic Nitrogen Ligands

Abstract

Main observation and conclusionStyrene polymerization catalyzed by the half‐titanocenes CpTiCl2[1,3‐R2(CH2N)2C=N] (6b: R= 2,6‐Me2C6H3, T4: R = 2,4,6‐Me3C6H2; T5: R = 2,6‐iPr2C6H3) was carried out in the presence of methylaluminoxane (MAO). Compared to the styrene conversion (31%) and syndiospecific index (45%) using reported 6b as precatalyst, T5 bearing ligand with isopropyl substitutes on the N‐aryl‐rings exhibits much higher styrene conversion (61%) and syndiospecific index (99%), indicating that the catalytic behavior could be improved obviously by the introduction of electronic donating and steric bulky substituents. One N atom in imidazolin‐2‐iminato ligand was replaced by O atom, affording half‐titanocenes CpTiCl2[3‐C6H5(CH2N)(CH2O)C=N (T1) and CpTiCl2[2,6‐Me2(C6H3O)(NiPr2)C=N] (T2). Compared to 6b, both higher styrene conversion and syndiospecific index are afforded by using half‐titanocene T1 containing 2‐imino‐3‐phenyloxazolidine ligand. All the results illustrate that both the chemical structure and the nature of substituents of the ligand have obvious influence on the styrene conversion and syndiospecific index in the polymerization of styrene. All the resulting syndiotactic polystyrenes (sPSs) are highly syndiospecific (rrr > 99%). Correspondingly, the sPS prepared using T5/MAO catalytic system exhibits high melting point and narrow molecular weight distribution. The results might show new light on designing more efficient half‐titanocenes for styrene polymerization with both high styrene conversion and high syndiospecific selectivity.