A composition for use in forming a multi-block copolymer from a single polymerizable monomer, said copolymer containing therein two or more segments or blocks differing in branching index, a polymerization process using the same, and the resulting polymers, wherein the composition comprises the admixture or reaction product resulting from combining: (A) a first olefin polymerization catalyst, (B) a second olefin polymerization catalyst capable of preparing polymers differing in chemical or physical properties from the polymer prepared by catalyst (A) under equivalent polymerization conditions, at least one of catalyst (A) or catalyst (B) being capable of forming a branched polymer by means of chain walking or reincorporation of in situ formed olefinic polymer chains, and (C) a chain shuttling agent.