Block copolymerization process and product

A block copolymerization process for the production of impact resistant ethylene-propylene block polymers at high productivity rates, which comprises

• I. in a first stage producing a propylene prepolymer in the presence of no more than about 5 vol % of inert diluents by polymerizing propylene at pressures sufficient to maintain propylene in liquid phase, at a residence time of about 30 minutes or below and at temperatures between about 115.degree. F. and about 165.degree. F. in the presence of a catalyst composition containing the components
• (a) an aluminum trialkyl or an aluminum trialkyl at least partially complexed with an electron donor compound, and
• (b) titanium tri- or tetrahalide supported on magnesium dihalide, or a complex of a titanium tri- or tetrahalide with an electron donor compound supported on magnesium dihalide, the components (a) and (b) being provided in a molar ratio of Al/Ti between about 10 and about 400;
• II. in a second stage contacting the prepolymer of the first stage with a monomer mixture of ethylene and propylene at a pressure sufficient to maintain propylene in liquid phase, at a temperature between about 115.degree. F. and about 165.degree. F., a residence time of between about 30 and about 450 minutes in the substantial absence of inert diluents, and
• III. separating the product of Step II from the unreacted monomer.