Helical Conformations,Internal Motion and Helix Sense Reversal in Polyisocyanates, and the Preferred Helix Sense of an Optically Active Polyisocyanate

Empirical force field studies of poly(alkyl isocyanates) are presented. A barrier of 12.5 kcal/mol for cis-trans isomerization of the polymer-backbone′s conjugated partial double bonds makes the calculated energy of helix sense reversal agree best with corresponding experimental results. The alternating cis and trans torsion angles are found to be distorted, to about 170° and -55° for right-handed helices, by strong repulsions between the-carbon of each alkyl side chain and the neighboring backbone atoms of four consecutive monomer units. Polyisocyanates are predicted to possess a soft collective internal motion, in which rotation per monomer around the polymer′s helix axis varies widely, accompanied by large variations of the backbone′s torsional angles, at low energy cost. When the β carbon of a normal alkyl side chain acquires an absolute configuration (R) by methyl substitution, the left-handed helical conformation is predicted to be more stable than the right-handed one by about 0.5 kcal/mol per monomer unit. Reversal of the helical sense involves conformational change of several consecutive monomers, and the angle between the helices of opposite senses is about 130°.