Blocks on the Move

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Science  21 Sep 2007:
Vol. 317, Issue 5845, pp. 1653
DOI: 10.1126/science.317.5845.1653b

Living polymerization is a double-edged sword. On the one hand, this class of techniques—all of which feature a tight association between catalyst and growing chain that inhibits termination—affords exquisite control over the molecular structure of the polymer and the uniformity of the bulk product distribution. On the other hand, such control comes at the expense of cost and throughput efficiency, as each chain requires its own appended catalyst and monomers must be added in batches. Hustad et al. present an alternative approach to the preparation of diblock copolymers, in which, rather than adding consecutive batches of different monomers to a living system, they set up successive continuous reactors and limit polydispersity by using a chain shuttling agent. Rapid chain transfers between a hafnium catalyst and zinc shuttling agent allow each catalyst molecule to extend >300 chains while keeping the chain lengths roughly uniform across the sample. By feeding pure ethylene into the first reactor and adding a longer α-olefin to the second, the authors achieve a product distribution with two distinct segments (high- and very-low-density polyethylene) in each chain and a polydispersity of only 1.67. — JSY

Macromolecules 40, 10.1021/ma0717791 (2007).

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