Optical Rotation of Random Copolyisocyanates of Chiral and Achiral Monomers:  Sergeant and Soldier Copolymers

Optical rotation (OR) of random copolymers of chiral 2,6-dimethylheptyl isocyanate (NIC) and achiral hexyl isocyanate (HIC) was measured as a function of mole fraction x of the chiral monomer, temperature, and molecular weight, with hexane, 1-chlorobutane, and dichloromethane as the solvents. The data as a function of molecular weight were analyzed by the statistical mechanical theory of copolymers developed (Gu, H.; et al. Polym. J. 1997, 29, 77−84), in which a polyisocyanate chain is modeled by an alternating sequence of left-handed and right-handed helices occasionally interrupted by helical reversals. The theory involves two parameters, the left-handed−right-handed free energy bias, 2ΔGh, and the helical reversal free energy, ΔGr. With appropriate values for these parameters, the experimental OR values were well described by the theory. When compared with poly((R)-i-deuterio-n-hexyl isocyanate (i = 1, 2), the values of ΔGr were nearly the same but those of |2ΔGh| were much larger: 71 ± 14 cal/mol for the terpene derived chiral unit vs 1∼2 cal/mol for the deuterated chiral monomer units. These free energy values are reasonable considering the chemical structures of the respective chiral monomer units.