We report a catalytic system that produces olefin block copolymers with alternating semicrystalline and amorphous segments, achieved by varying the ratio of alpha-olefin to ethylene in the two types of blocks. The system uses a chain shuttling agent to transfer growing chains between two distinct catalysts with different monomer selectivities in a single polymerization reactor. The block copolymers simultaneously have high melting temperatures and low glass transition temperatures, and therefore they maintain excellent elastomeric properties at high temperatures. Furthermore, the materials are effectively produced in economically favorable, continuous polymerization processes.

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A kinetic model was derived to investigate the effects of reversibility in chain transfer to metal in olefin polymerization. The model predicts both number- and weight-average molecular weights, M n and M w, as a function of several reactor input variables including the constants for chain transfer and reversible transfer. A number of interesting and nonobvious insights into molecular weight distributions are gained from these simulations. The most revealing result is the variation of the molecular weight distribution (M w/M n) with conversion. In the absence of reversible transfer, the M w/M n is always greater than or equal to 2. Regardless of the magnitude of the reversible transfer constant, M w/M n approaches 2 as M n approaches its maximum value. More dramatic deviations from M w/M n = 2 are observed for higher chain transfer constants. Polymerization data from two bis(phenoxyimine)zirconium catalyst systems are presented to demonstrate these effects. These results indicate that singular polymerizations should not be used to explore for reversible transfer characteristics, but rather a series of polymerizations should be conducted over a range of polymer conversions.