[+]Enlarge Credit: Courtesy of Bernard Lotz

Isotactic polypropylene (iPP), the number-two commercial polymer, was known for decades to adopt three crystalline forms, α, β, and γ, and in the past decade a fourth form, δ, was identified. Now, a researcher in France has found a fifth crystal form, ε. If ε-iPP can be better characterized and its synthesis scaled up, it could have industrial applications—for lightweight iPP formulations, for example.

In iPP, all of the polymer’s methyl groups, one per propylene unit, reside on the same side of the main chain. The polymer was discovered in the 1950s. Since then, it has seen wide commercial adoption, by itself and combined with other plastics, to create products ranging from chairs to carpets to diapers.

The α, β, and γ forms have varied properties and are used industrially in different ways. Scientists are still looking for commercial applications for the δ form. Now, polymer chemist Bernard Lotz of the National Center for Scientific Research (CNRS) and the University of Strasbourg, in France, reports a fifth form, ε-iPP, which he found and characterized using electron microscopy and electron diffraction (Macromolecules 2014, DOI: 10.1021/ma5009868).

Lotz found the ε crystals in a stereodefective sample, in which some methyl groups are on the wrong side of the chain. The new phase was a minor component—probably less than 1%—of a very small sample. And the sample was lost when Lotz’s lab was moved to a new location. “These circumstances are quite unfortunate, but they do not weaken the major message conveyed by this work—observation of a new crystal modification,” Lotz writes.

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“The new polymer has not yet been characterized as much as would be desirable,” says polymer chemist Stefano Valdo Meille of Polytechnic University of Milan, in Italy. Still, he concurs with Lotz that substantial stereodefectivity is most likely a key factor in its formation. Lotz’s work, Meille says, will stimulate scientists “to reproduce the data and provide complementary results that will allow them to gauge the value of the proposed structure and its relevance.”