Metallocene-based branch-block thermoplastic elastomers

Long chain branched (LCB) polyethylene block copolymers having thermoplasti c elastomer character were made using mixed metallocene catalysts. Conceptu ally, the synthesis can be divided into two steps. Step 1 involves the gene ration of vinyl-terminated, crystallizable macromonomers, and. step 2 invol ves the incorporation of these macromonomers into an amorphous or plastomer ic copolymer backbone. In practice, the two steps may be conducted sequenti ally or simultaneously. The polymer properties depend on the catalyst pair and the process conditions selected, which determine the populations of rea ctive macromonomer and the probability of incorporating them into the backb one. One such useful pair is the mixture of Cp2ZrCl2 and (C5Me4SiMe2NC12H23 )TiCl2, activated with MAO. Tn the presence of a mixed ethylene/butene feed the Cp2ZrCl2 catalyst, by Virtue of its low comonomer incorporating capabi lity, will produce primarily crystalline polyethylene macromonomers. The ti tanium catalyst, on the other hand, has a higher affinity for comonomers an d 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 h igh-temperature tensile strength. The properties of the resultant comblike polymers will be governed by the topological details of the branched polyme r as well as the LCB distribution. To study the latter distribution, branch ed model polymers having dissimilar LCB and backbone compositions were synt hesized. GPC-FTIR analysis provided the LCB distribution pattern, revealing a progression of statistically branched polymers with the highest molecula r weight molecules containing the highest levels of branching. Upon cooling from the melt, the crystalline segments (primarily in the LCB portions) wo uld cocrystallize to form crystalline domains embedded in an amorphous matr ix, as confirmed by transmission electron microscopy.