Unusual crystallization mode of tubular helical 2,3- exo-disyndiotactic polynorbornene

The present contribution reports the study of the peculiar crystalline structure of the 2,3-exo-disyndiotactic polynorbornene (dsPNB) obtained by polymerization with a TiCl4 based catalysts. Using X-ray diffraction measurements (Fig. 1a) combined with molecular modelling approaches (including molecular mechanics and dynamics together with quantum mechanical methods), the stereochemistry was proved with stereoregular chains adopting an unusual tubular helical conformation in the crystalline state [1]. Interestingly, the estimated coherence length along the backbone axis is much higher than the value predicted by the average degree of polymerisation suggesting that what is modelled as a single chain is plausibly a sequence of oligomers. Moreover, the helices packing generates an empty accessible core, in which guest molecules (e.g. toluene) and chain terminals can be easily hosted (Fig. 1b). A guest like I2 influences very substantially the relative intensities of the diffraction patterns while leaving peak positions unchanged as expected for a unique polymer conformation in which I2 replaces toluene or other hydrocarbon guests inside dsPNB channels [2]. Molecular modelling confirms the stability of empty and host-guest crystalline structures. In particular, MD simulations rationalises both the guest exchange mechanism and the role of chain tails in the crystalline structure.