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 quasihexagonal packing. The hexagonality index H, defined as the rati o of the largest to the smallest distance between the axis of the refe rence helix and its six nearest neighbors, appears to be a reliable in dicator of the presence of helices of a single handedness or, respecti vely, of both in a given crystal structure. A detailed analysis of the general energetic and entropic factors favouring chiral crystallizati on of helical polymers is carried out. It is shown that a hexagonal or pseudohexagonal arrangement (i.e. the six-fold coordination) in eithe r the crystalline or in a precrystalline state, promotes chiral crysta llization and is in its own turn favoured by clustering of isochiral h elices.