Efficient Synthesis of Hydroxylated Polyethylene via Copolymerization of Ethylene with 5-Norbornene-2-methanol using Bis(β-enaminoketonato)titanium Catalysts

The copolymerizations of ethylene with 5-norbornene-2-methanol (NBM) using bis(β-enaminoketonato)titanium complexes [RNC(CH3)CHCO(CF3)]2TiCl2 [1a: R = Ph, 1b: R = C6H4Me(p), 1c: R = C6H4tBu(p), 1d: R = C6H4F(p), 1e: R = C6H4Me(o)] and [R2NC(CF3)CHCO(R1)]2TiCl2 [2a: R1 = Ph, R2 = Ph; 2b: R1 = Ph, R2 = C6H4Me(p); 2c: R1 = Ph, R2 = C6H4tBu(p); 2d: R1 = Ph, R2 = C6H4F(p); 2e: R1 = C6H4Me(p), R2 = Ph] have been investigated. By optimizing the catalyst structures and polymerization conditions, outstanding catalytic activity and efficient polar comonomer incorporation have been achieved. Catalysts 1b,c and 2b,c with electron-donating groups (Me and tBu) display improved tolerance toward polar group, exhibiting higher catalytic activities. Among these catalysts, 1b produced the copolymer with the highest NBM incorporation. Using Et2AlCl as the pretreated reagent, the high NBM incorporation up to 22.0 mol % with a catalytic activity of 700 kg/molTi·h has been achieved by catalyst 1b. DFT calculations have been utilized to explain the high catalytic activity and efficient comonomer incorporation in the present catalyst system. The alternating ethylene–NBM sequence can be detected at high NBM incorporation, as demonstrated by 13C NMR (DEPT) spectra.