Study On Synthesis Of Multiblock Polyethylene Via Chain Shuttling Polymerization

Brookhart research group found polyethylene with high molecular weight and high branching density can be made by(a-diimine)nickel catalyst with stronger axial steric hindrance through coordination polymerization.However,there was little research about binuclear(a-diimine)nickel catalyst.Two binuclear(a-diimine)nickel catalysts with conjugated backbone(cat.l and cat.2)were used for ethylene polymerization in this paper.Electrons on those two Ni2+could transfer through the conjugated a-diimine backbone in these two catalysts.The influence of the catalyst structure,polymerization temperature,polymerization pressure and the kind of cocatalyst on the polymerization activity,molecular weight,melting point,crystallinity and branching structure of the polymers in ethylene polymerization has been studied.In ethylene polymerization catalyzed by cat.1 and cat.2,organochloroaluminates were more active than aluminoxanes and produced higher amout of polymer in combination with nickel catalysts.This behavior was mainly related to Lewis acidity and size of the cocatalysts.An obviously increasing of branching density and remarkably decline of polymerization activity,molecular weight and crystallinity of polyethylene happened along with the rising of polymerization temperature.There was no obvious melting peak on the DSC curses of the polyethylene obtained at 60 ?.Some short branches(such as methyl,ethyl,propyl et al.)and long branches were detected on molecular chain.The content of methyl is the most.The branched structure became more complicated when temperature increased,for characteristic peak of isobutyl became more and more obvious.We found the catalytic behavior of the two Ni2+ in cat.l and cat.2 was not the same in ethylene polymerization.One of the Ni2+ can produce polyethylene with high branching density and the other Ni2+ can produce polyethylene with low branching density.This was because electrons on those two Ni2+ could transfer through the conjugated a-diimine backbone.Thus,these two metals could influence to each other.Then we prepared multiblock polyethylene by chain shuttling polymerization catalyzed by cat.l.The chain shuttling agent is ZnEt2 and the cocatalyst is AlEtCl2.The multiblock polyethylene was made of polyethylene with high branching density and polyethylene with low branching density.In the same time,we investigated the influence of Zn/Ni,polymerization temperature and polymerization pressure on the polymerization behavior.The ratio of polyethylene with low branching density in multiblock polyethylene increased with the rising of polymerization pressure,leading to the increase of crystallinity and tensile strength of multiblock polyethylene.Compared with three OBCs(made by Dow),the multiblock polyethylene which was prepared under 25 ?and 1.2MPa had close tensile strength and lower elongation at break.This means the multiblock polyethylene had smaller distortion than OBCs under the same force.An obviously increasing of branching density and remarkably decline of polymerization activity,molecular weight and crystallinity of the multiblock polyethylene happened along with the rising of temperature.The crystallinity and tensile strength of the multiblock polyethylene is very small at 40 ?.When polymerization temperature is increased to 60 ?,there was no obvious melting peak on the DSC curves of the resultant polyethylene.Therefore,in order to make sure of enough tensile strength,polymerization at 25 ? is more suitable.The molecular weight distribution of polymer prepared by cat.2 at 25 ? and 1.2MPa is very wide.This is because the chain-walking ability of Ni2+ in cat.2 is worse than Ni2+ in cat.1.The crystallinity of polymer prepared by cat.2 is too high and the ratio of polyethylene with low branching density is not suitable for chain shuttling polymerization.