Synthesis of new Cr(II) complexes with bidentate phosphine ligands and their behavior in the polymerization of butadiene - Influence of the phosphine bite angle on catalyst activity and stereoselectivity

Some new chromium(II) complexes were synthesized by reacting CrCl2(thf) with various bidentate phosphines [bis(diphenylphosphino)methane (dppm); 1,2-bis(diphenylphosphino)ethane (dppe); 1,3-bis(diphenylphosphino)propane (dppp); bis(diphenylphosphino)amine (dppa)] and a new family of catalysts for the polymerization of butadiene was prepared by reacting the above chromium complexes with methylaluminoxane (MAO). These systems were found to be in some cases extremely active and they gave predominantly 1,2-polybutadienes (1,2 content >85%) having different tacticity (syndiotactic or predominantly isotactic) depending on the phosphine ligand bonded to the chromium atom. The influence of the type of ligand on catalyst activity and selectivity is discussed.