Glassy state and secondary structures of chiral macromolecules: Polyisocyanates and polyketones

The temperature dependence of the optical activity in the amorphous and therefore nonbirefringent solid state is used here to study the conformational properties of polyisocyanates and isotactic polyketones. Optically active polyisocyanates, which are not entirely amorphous in the solid state, were rendered amorphous by blending, while the glass transition temperature of the inherently amorphous polyketones was varied from 145 to 1 °C by terpolymerization of alternating styrenic/CO units with alternating 1-alkene/CO units. In both polymer systems, the optical activity changed significantly at temperatures only above T_g. This result, which was controlled for the contribution of degradation and epimerization of stereocenters in the polyketones, requires that the chiral optical properties of these polymers arise from long-range conformational properties of the chain that are impeded from changing with temperature only in the glassy state. This is consistent with the known character of the polyisocyanates but acts to reveal the previously uncertain basis of the optical activity in the isotactic polyketones studied here.