CHIRAL CRYSTALLIZATION OF HELICAL POLYMERS

Crystallization of achiral or racemic helical polymers in chiral cryst als is quite frequent and while in some cases it may relate to the the rmodynamic stability of the chiral polymorph, in others it must be ass ociated with kinetic factors. Analysis of the available literature dat a suggests that in the second instance a key role must be played by th e nucleation step, i.e. specifically by the formation of precrystallin e entities. Furthermore a survey of the chiral crystal structures for helical polymers evidences that they are frequently characterized by a quasi-hexagonal packing. The hexagonality index H, defined as the rat io of the largest to the smallest distance between the axis of the ref erence helix and its six nearest neighbors, appears to be a reliable i ndicator of the presence of helices of a single handedness or, respect ively, of both in a given crystal structure. A detailed analysis of th e general energetic and entropic factors favoring chiral crystallizati on of helical polymers is carried out. It is shown that a hexagonal or pseudohexagonal arrangement (i.e. the 6-fold coordination) in either the crystalline or precrystalline state, promotes chiral crystallizati on and is in its own turn favored by clustering of isochiral helices.