Preparation of syndiotactic polystyrene using the doubly bridged dinuclear titanocenes

As a new kind of dinuclear metallocene four DBDM (doubly bridged dinuclear metallocene) that hold two different bridging units linking two metallocenes have been prepared and their polymerization properties have been pursued. The selected bridging ligands for DBDM were polymethylene and dialkoxy terminated derivatives to bond two cyclopentadienyls and two titanium centers, respectively. The syntheses of new dinuclear metallocenes 5–8 were able to be achieved by the reaction between dinuclear half-titanocenes 1 and 2 and ditrimethylsiloxy terminated derivatives 3 and 4 at −78 °C. The EI mass spectra and 1H and 13C NMR spectral data were very informative to identify their formulations as well as structural features. The polymerization of styrene was conducted by using DBDM 5–8. From the polymerization studies it was found that (i) DBDM 7 and 8 holding 2,2-diethyl-1,3-dipropanoxy (DEP) bridge not only show greater activity but also produce higher syndiotactic polystyrene than DBDM 5 and 6 holding 1,1,4,4-tetramethyl-1,4-dibutanoxy (TMB) bridge between two titanium centers, (ii) activities increase with increase in the polymerization temperature, the amount of cocatalyst and monomer concentration, (iii) the most crucial factor to control the stereoregularity of the resulting polystyrene is the styrene concentration in reaction system, (iv) surprisingly the catalyst structure does not have much effect on molecular weight of the obtained polystyrenes. The most significant feature from this study is that the second bridging dialkoxy terminated ligands connecting two titanium centers likely exert more pronounced influence than the polymethylene bridges between two Cp groups on the activity of the catalyst as well as the stereochemistry of the generated polymers.