PE-EP and PE-EP-EPDM block polymers have been made in yields exceeding 70% with Ziegler-Natta catalysts by matching the polymerization time for each block to chain growth time and catalyst decay rates. A short residence time tubular flow reactor with multiple side stream feeds was used to accomplish this result. The amount of block polymer and chain fragments in the product was quantified by various techniques including DSC, solvent fractionation, and GPC/FTIR. PE-EP diblocks have low strength, as expected, and are not suitable for most TPE applications. However, PE-EP-EPDM triblocks with optimized EPDM block length could be coupled via olefinic unsaturation in the EPDM to form multiarm starlike structures consisting of a central elastomeric core with the PE blocks extending outward. The coupled triblocks gave processible polymers with tensile strengths of 12-18 MPa and tension sets less than 10%.