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2,3-exo-Diheterotactic Dicyclopentadiene Oligomers: An X‑ray Powder Diffraction Study of a Challenging Multiphase Case

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posted on 2019-03-31, 00:00 authored by Arnaldo Rapallo, William Porzio, Giorgia Zanchin, Giovanni Ricci, Giuseppe Leone
Cycloaliphatic polyolefins have attracted attention for use as functional materials, but they are rarely observed in the crystalline state. New semicrystalline dicyclopentadiene oligomers were obtained with an iminopyridine chromium complex in combination with methylaluminoxane. Fourier transform infrared and nuclear magnetic resonance measurements did not allow us to establish the chain stereochemistry, and size exclusion chromatography measurements merely revealed that the obtained sample contained oligomers of different molecular masses. The wide-angle X-ray diffraction (WAXS) powder pattern showed a remarkable crystalline character of the sample, and its study was considered the main route toward the comprehension of the obtained material. By analogy with the norbornene olefin, there was conjecture that the obtained dicyclopentadiene oligomers featured a 2,3-exo-diheterotactic stereoregularity. Then, different ad hoc computational techniques were implemented to describe the WAXS powder pattern in terms of a model mixture of perfect crystals of oligomers with different numbers of monomer units and with the mentioned stereoregularity. This modeling allowed us to shed light on the structures of crystals constituted by oligomers of different masses, to estimate their abundance in the sample, and to confirm the suggested chain stereochemistry, which has never been observed before in dicyclopentadiene macromolecules.

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