The disclosure concerns Ziegler-Natta catalysts. Specifically, magnesium and titanium containing catalysts are disclosed that do not exhibit a well known effect, the 'comonomer effect'. This effect manifests itself in decreasing activity with decreasing amounts of comonomer in a 1-olefin polymerization. Through the use of magnesium chloride, the effect is shown to be substantially reduced. Further, even where there are comonomers present, the catalysts of this invention show improved catalyst activity in the presence of magnesium chloride.