Chain-Walking Olefin Polymerizations with Donor-Substituted Half-Sandwich Chromium Complexes: Ethylene/Propylene Copolymer Look-Alikes by Polymerization of Propylene

Six half-sandwich chromium complexes, in which the carbocyle is substituted by a nitrogen containing heterocycle as additional ligand were investigated regarding their polymerization behavior toward propylene. For each catalyst precursor, polymerization series in which the influence of the monomer concentration and of the polymerization temperature were analyzed, were conducted, using methylaluminoxane (MAO) as cocatalyst. Besides, several polymerizations with 1-butene were made, which brought further aspects into the interpretation of the for the polymerization of propylene complex NMR spectra. Because of the chain-walking tendency of all investigated systems, by sole polymerization of propylene, polymers are obtained, which have a structure which resembles those of ethylene/propylene copolymers with regioinverted propylene units. The ethylene share of the polymers with a value up to 30 mol % is astonishingly high. The different tendencies toward 2,1-insertions with succeeding chain-straightening of the chromium complexes and of each single catalyst by variation of the polymerization conditions is expressed in a broad range of the glass transition temperatures of the synthesized polymers, which vary between −14 and −38 °C.