EFFECT OF MONOMER CONCENTRATION ON PROPENE POLYMERIZATION WITH THE RAC-[ETHYLENEBIS(1-INDENYL)]ZIRCONIUM DICHLORIDE METHYLALUMINOXANE CATALYST

The structural details of isotactic polypropene (iPP) produced with th e moderately isospecific racemic-ethylenebis(l-indenyl)zirconium dichl oride/methylaluminoxane (rac-(EBI)ZrCl2/MAO) catalyst are strongly dep endent on monomer concentration. Polymerizing propene at 50 degrees C in toluene, at propene concentrations in the experimentally measurable range 0.4-11 mol/L, rac-(EBI)ZrCl2/MAO shows activities in the range 2-350 kg(PP)/(mmol(Zr) . h) and yields polypropenes with M(v)'s from 8 800 to 36 600, percent mmmm pentads ranging from 54 to 86%, and corres ponding melting temperatures from 86 to 136 degrees C. Two types of re gioirregular (secondary) placements in an isotactic sequence of primar y propene insertions are observed in iPP synthesized in liquid monomer , erythro (E, ca. 0.4%) and three (T, ca. 0.2%). These secondary units are gradually converted into 1,3 propene units as the monomer concent ration is lowered. In ''starved catalyst'' conditions, that is, as [M] --> 0, rac-(EBI)ZrCl2/MAO produces atactic propene oligomers ((M) ove r bar(n) = 1080, mmmm = 9.1%). These effects are the most probable cau se for the discordance of literature data on metallocene-catalyzed pro pene polymerization. Three chain transfer mechanisms have been detecte d: beta-hydrogen transfer to the metal and beta-hydrogen transfer to t he monomer, both occurring after a primary insertion, and beta-hydroge n transfer to the monomer after a secondary insertion with the exclusi ve formation of a cis-2-butenyl end group.