OPTICAL-ROTATION OF RANDOM COPOLYISOCYANATES OF CHIRAL AND ACHIRAL MONOMERS - SERGEANT AND SOLDIER COPOLYMERS

Optical rotation (OR) of random copolymers of chiral 2,6-dimethylhepty l 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 he lices occasionally interrupted by helical reversals. The theory involv es two parameters, the left-handed-right-handed free energy bias, 2 De lta G(h), and the helical reversal free energy, Delta G(r). With appro priate values for these parameters, the experimental OR values were we ll described by the theory. When compared with poly((R)-i-deuterio-n-h exyl isocyanate (i = 1, 2), the values of Delta G(r) were nearly the s ame but those of \2 Delta G(h)\ were much larger: 71 +/- 14 cal/mol fo r the terpene derived chiral unit vs 1 similar to 2 cal/mol for the de uterated chiral monomer units. These free energy values are reasonable considering the chemical structures of the respective chiral monomer units.