r atom which reacts with the above aluminum compound; (iv) a halide of the formula R3X, wherein R3 is a C1-4 alkyl radical and x is a halide selected from the group consisting of chlorine and bromine; (v) titanium tetrachloride, to provide a molar ratio of Mg to Al1 from 4.0:1 to 6:1; Mg:Ti from 4 to 6; a molar ratio of Al1 to Ti from 0.5:1 to 3:1; a molar ratio of halide to Mg from 1.8:1 to 2.4:1