Polypropylene Piece by Piece

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Science  21 Apr 2006:
Vol. 312, Issue 5772, pp. 338-339
DOI: 10.1126/science.312.5772.338d

The properties of polypropylene plastic depend strongly on the relative stereochemistry of the methyl groups appended to every second carbon in the polymer chain. Elasticity, for example, arises from a structure with interspersed segments of random methyl configuration (termed atactic) and strictly parallel configuration (isotactic). Harney et al. present a catalytic system to prepare bulk quantities of polypropylene in which isotactic or atactic segments of any specified length can be incorporated in any desired order (demonstrated up to a total polymer molecular weight of ∼170,000). The technique is especially promising for studying relations between molecular structure and bulk properties.

The catalyst precursor is a Zr(CH3)2 complex with amidinate and pentamethylcyclopentadienyl ligands. Scission of a Zr-CH3 bond by an anilinium borate salt yields a catalyst that selectively produces isotactic polypropylene. The authors previously showed (by adding half an equivalent of borate) that if only half of the precursors lose CH3, ligand migrations between precursor and active catalysts result in atactic polymer. They now find that the CH3 groups can be restored to the catalyst complexes, regenerating the precursor, by adding another Zr(CH3) compound bearing a bulky neopentyl substituent. They can thereby tune polymer stereochemistry by successively adding either borate (to prepare an isotactic segment) or the bulky Zr methyl transfer agent (for an atactic segment). — JSY

Angew. Chem. Int. Ed. 45, 2400 (2006).

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