Ethylene was copolymerized with 1,9-decadiene using methylaluminoxane-activated rac-Et(Ind)(2)ZrCl(2) at 120 degrees C to produce macromonomers with pendant and terminal vinyl groups with a maximum of 6.5 vinyl groups per polymer chain. The macromonomers were copolymerized with ethylene and 1-butene or 1-octene using a constrained geometry catalyst (dimethylsilyl(N-tert-butylamido)(tetramethylcyclopentadienyl)titanium dichloride) at 120 degrees C in toluene. The resulting branched polymers showed three distinct populations during crystallization analysis fractionation, indicating the formation of a new fraction produced by the incorporation of macromonomer chains into ethylene/1-butene or ethylene/1-octene copolymer chains. These novel polyolefins have complex branched structures consisting of three main components: a high-crystallinity fraction (macromonomers), a low-crystallinity (or amorphous) fraction (a-olefin copolymer), and a third component resulting from the cross-linking of the two previous components (cross-product). These branch block copolymers have microstructures that can be tightly controlled by varying the conditions during the two stages of polymerization required for their synthesis and are promising new materials for specialty polyolefin applications.