Slow-chain-walking polymerization of ethylene and highly chain-straightening polymerization of 1-hexene to access semicrystalline polyolefins

Abstract

The production of semicrystalline polyolefins using α-diimine late transition metal catalysts is highly desired but challenging in both ethylene and α-olefin polymerization, which relates to fundamental steps such as slow-chain-walking and highly chain-straightening events. In this work, by the combination of 4-(diphenylamino)phenyl moiety with different bulky substituents including H, Me, CHPh2, and Ph groups, a new family of unsymmetrical α-diimine nickel catalysts Ni1-Ni4 has been prepared. Under the activation of co-catalyst, these catalysts are highly active (107 g mol-1h−1) for ethylene polymerization, giving polymers from branched ethylene oligomers to linear ultrahigh molecular weight polyethylenes (UHMWPEs) over a wide temperature range of 30–70 °C. Notably, the preferred Ni4 even enables a slow-chain-walking process and thus is capable of producing linear, semicrystalline UHMWPE (Mn = 1056 kg mol−1, brs = 9.1/1000C, Tm = 123.5 °C) at low ethylene pressure of 1 bar. Likewise, in 1-hexene polymerization Ni4 is highly selective (88.1%) for uncommon 2,1-insertion followed by complete chain-walking. Thus, the chain-straightening polymerization of 1-hexene efficiently yields semicrystalline poly(1-hexene) with a high Tm of 102.1 °C that resembles low-density polyethylene (LDPE).