Horizontally and Vertically Concerted Steric Strategy in α-Diimine Nickel Promoted Ethylene (Co)Polymerization†

Main observation and conclusion Steric bulk plays a significantly paramount role in late transition metal promoted olefin (co)polymerization in terms of polymer molecular weight and catalyst thermal stability. Numerous sterically encumbered nickel and palladium catalysts have been developed, but they are usually uni-directional. In this contribution, from a bi-directional side the distinctive horizontally and vertically concerted steric strategy was well-developed and applied to alpha-diimine nickel catalysts. In ethylene polymerization, the increase of vertically steric bulk (H, Ph, Biph) led to an enhanced polymer molecular weight, a slightly ascended branching density; likewise increasing horizontally steric bulk (Ph, Nap, Ant) further resulted in the increase of polymer molecular weight and branching density. As a result, in sharp contrast to Ni1(H) without steric bulk, Ni5(Ant) with horizontally and vertically concerted steric bulk was thermally stable, showed very high activities at a level of 10(7) g center dot mol(-1)center dot h(-1), and produced polyethylenes with the highest molecular weight of 518.8 x 10(4) g center dot mol(-1) and the broader range of branching density from 2.4/1000C to 27.8/1000C. In the copolymerization of ethylene with methyl 10-undecenoate, Ni5(Ant) also provided the highest copolymer molecular weight but gave the lowest incorporation of co-monomer. This work sheds light on the steric effect on olefin (co)polymerization.