Self-Poisoning in Crystallization of Helices

Crystallization of complex molecules such as polymers sometimes shows peculiarities absent in atomic or small-molecule systems. They are due to tortuous kinetic pathways that involves complex conformational ordering additional to the simple adoption of correct position and orientation. One such anomaly is the growth rate minimum as a function of crystallization temperature or solution concentration, or multiple minima, exhibited by monodisperse long-chain alkanes.¹ These occur at the transition from extended to folded-chain growth, or once-folded to twice-folded etc. The slow-down and even complete cessation of growth are due to the numerous but not quite stable folded-chain depositions blocking the productive extended-chain ones at the growth surface.
Such stark anomalies tell us much about the self-poisoning that takes place at the growth surface of all polymers. However, growth rate minima and self-poisoning should not be restricted to chain folding. Here we consider the possibility of self-poisoning in polymers due to incorrect helical order blocking the growth of crystals with ordered helical chains. We show, using two different kinetic models, that growth rate minima due to “helical self-poisoning” are possible.

1. G. Ungar et al., PRL 2000, 85, 4397.