Ethylene polymer and process for preparing same

Abstract

Ethylene is polymerized, optionally with a small amount of an .alpha.-olefin, in the presence of a Ziegler catalyst in multiple stages by using a polymerization apparatus including at least three polymerization reactors. In the polymerization process, the following three ethylene polymer components U, H and L are prepared sequentially in an optional order; a component U having an .alpha.-olefin content .alpha..sub.U of 0.1-10 wt. % and an intrinsic viscosity [.eta.].sub.U satisfying 2.times.[.eta.].sub.W .ltoreq.[.eta.].sub.U .ltoreq.5.times.[.eta.].sub.W in which [.eta.].sub.W is the intrinsic viscosity of the entire ethylene polymer, the ratio R.sub.U of component U in the entire ethylene polymer satisfying 0.3.times.[.eta.].sub.W .ltoreq.[.eta.].sub.U .times.R.sub.U .ltoreq.0.8.times.[.eta.].sub.W ; a component H having an .alpha.-olefin content .alpha..sub.H of up to 20 wt. % and an intrinsic viscosity [.eta.].sub.H satisfying ([.eta.].sub.W).sup.1/2 .ltoreq.[.eta.].sub.H .ltoreq.1.5.times. [.eta.].sub.W, the ratio R.sub.H of the component H satisfying 0.25.times.[.eta.].sub.W .ltoreq.[.eta.].sub.H .times.R.sub.H .ltoreq.0.7.times.[.eta.].sub.W ; and an ethylene polymer component L having an .alpha.-olefin content .alpha..sub.L of up to 10 wt. % and an intrinsic viscosity [.eta.].sub.L satisfying 0.2.ltoreq.[.eta.].sub.L .ltoreq.2.0, the ratio R.sub.L of the component L satisfying [.eta.].sub.L .times.R.sub.L .ltoreq.0.05.times.[.eta.].sub.W. The ethylene polymer satisfies the requirement of .alpha..sub.U .times.R.sub.U .ltoreq.0.2.times.(.alpha..sub.U .times.R.sub.U +.alpha..sub.H .times.R.sub.H +.alpha..sub.L .times.R.sub.L), and has [.eta.].sub.W of 1.5 to 4.0. The catalyst used is such that, when the component H is prepared independently, the molecular weight distribution Mw/Mn of the obtained ethylene polymer is in the range of from 7 to 17 and the rate constant of deactivation Kd at said polymerization is larger than 0.3 hr.sup. -1.