Isoselective 4-Methylpentene Polymerization by Pyridylamido Hafnium Catalysts

The stereospecific polymerization of 1-alkenes is a continuing issue for metal catalysis. We have initially reported isoselective 4-methylpentene polymerizations by nonconventional pyridylamido hafnium catalysts with different bridgehead substituents (L(R1R2)Hf(Me)2, L = pyridylamido; 1: R1 = 2-iPr–Ph, R2 = H; 2: R1 = tBu, R2 = H; 3: R1 = R2 = Me). The effects of polymerization temperature, monomer concentration, and bridgehead substituents were systematically investigated. Regardless of the bridgehead substituent and polymerization temperature, highly isotactic poly(4-methylpentene)s (PMPs) ([mmmm] > 99%) with high melting temperatures (Tm = 229–240 °C) were produced. The bridgehead substituents of pyridylamido hafnium catalysts remarkably affected the polymerization activity and molecular weight distribution (MWD) of the resultant PMPs. Catalyst 1 showed much higher activity (14 500 kg PMP (mol Hf h)−1) than catalysts 2 and 3. PMPs produced by catalyst 2 displayed unimodal distributions (MWD = 3.0–4.7), but polymers produced by catalysts 1 and 3 had clear bimodal distributions (MWD > 10). Besides, X-ray powder diffraction clearly showed that crystalline forms of the resultant PMPs were dependent on the polymerization temperature and catalyst structure.