Isoselective 4-methylpentene polymerization by pyridylamido hafnium catalysts

Abstract

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 (T_m = 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.