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Long chain branched (LCB) polyethylene block copolymers having thermoplastic elastomer character were made using mixed metallocene catalysts. Conceptually, the synthesis can be divided into two steps. Step 1 involves the generation of vinyl-terminated, crystallizable macromonomers, and step 2 involves the incorporation of these macromonomers into an amorphous or plastomeric copolymer backbone. In practice, the two steps may be conducted sequentially or simultaneously. The polymer properties depend on the catalyst pair and the process conditions selected, which determine the populations of reactive macromonomer and the probability of incorporating them into the backbone. One such useful pair is the mixture of Cp2ZrCl2 and (C5Me4SiMe2NC12H23)TiCl2, activated with MAO. In the presence of a mixed ethylene/butene feed the Cp2ZrCl2 catalyst, by virtue of its low comonomer incorporating capability, will produce primarily crystalline polyethylene macromonomers. The titanium catalyst, on the other hand, has a higher affinity for comonomers and will consume comonomer and macromonomers during the polymerization which produces plastomeric backbone containing, in one case, over 20 mol % butene. Microtensile test on the polymers showed good elastic recovery and good high-temperature tensile strength. The properties of the resultant comblike polymers will be governed by the topological details of the branched polymer as well as the LCB distribution. To study the latter distribution, branched model polymers having dissimilar LCB and backbone compositions were synthesized. GPC-FTIR analysis provided the LCB distribution pattern, revealing a progression of statistically branched polymers with the highest molecular weight molecules containing the highest levels of branching. Upon cooling from the melt, the crystalline segments (primarily in the LCB portions) would cocrystallize to form crystalline domains embedded in an amorphous matrix, as confirmed by transmission electron microscopy.

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