A process for preparing polymers, especially multi-block copolymer containing therein two or more segments or blocks differing in chemical or physical properties, by contacting propylene, 4-methyl-1-pentene, or other C4-8 α-olefins and one or more copolymerizable comonomers, especially ethylene in the presence of a composition comprising the admixture or reaction product resulting from combining: (A) a first metal complex olefin polymerization catalyst, (B) a second metal complex olefin polymerization catalyst capable of preparing polymers differing in chemical or physical properties from the polymer prepared by catalyst (A) under equivalent polymerization conditions, and (C) a chain shuttling agent.