This 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 data (Fig. 1a) combined with molecular modelling studies (including the use of 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 [1,2]. Interestingly, the estimated coherence length along the backbone axis is much higher than the value predicted by the average degree of polymerisation thus suggesting that a sequence of oligomers can packs along the chain axis. The resulting helices three dimensional assembly generates an empty accessible core, in which guest molecules (e.g. toluene, see Fig. 1b) or chain terminals can be easily hosted. A guest like I2 influences substantially the relative intensities of the diffraction patterns while leaving peak positions unchanged as expected for a unique polymer conformation in which I2 replaces guest molecules inside dsPNB channels [2]. Molecular modelling confirms the stability of empty and host-guest crystalline structures. MD simulations rationalises the guest exchange mechanism and the role of chain tails in the crystalline structure. In particular, it has been demonstrated that chain ends of dsPNB has a sufficient mobility to “fill” the empty cavities of the structure even at room temperature [3].
Unusual crystallization mode of tubular helical 2,3-exo-disyndiotactic polynorbornene: a combined X ray diffraction / molecular modelling study
FAMULARI, ANTONINO;BAGGIOLI, ALBERTO;MEILLE, STEFANO VALDO
2014-01-01
Abstract
This 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 data (Fig. 1a) combined with molecular modelling studies (including the use of 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 [1,2]. Interestingly, the estimated coherence length along the backbone axis is much higher than the value predicted by the average degree of polymerisation thus suggesting that a sequence of oligomers can packs along the chain axis. The resulting helices three dimensional assembly generates an empty accessible core, in which guest molecules (e.g. toluene, see Fig. 1b) or chain terminals can be easily hosted. A guest like I2 influences substantially the relative intensities of the diffraction patterns while leaving peak positions unchanged as expected for a unique polymer conformation in which I2 replaces guest molecules inside dsPNB channels [2]. Molecular modelling confirms the stability of empty and host-guest crystalline structures. MD simulations rationalises the guest exchange mechanism and the role of chain tails in the crystalline structure. In particular, it has been demonstrated that chain ends of dsPNB has a sufficient mobility to “fill” the empty cavities of the structure even at room temperature [3].File | Dimensione | Formato | |
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