Copolymers, especially multi-block copolymer containing therein two or more segments or blocks differing in tacticity, are prepared by polymerizing propylene, 4-methyl-1-pentene, or another C4-8 α-olefin 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 tacticity from the polymer prepared by catalyst (A) under equivalent polymerization conditions, and (C) a chain shuttling agent.