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

Abstract

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, 2ΔG h, and the helical reversal free energy, ΔG r. 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 ΔG r were nearly the same but those of |2ΔG h| 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.

Double screw-sense inversions of helical chiral-achiral random copolymers of fluorene derivatives in phase separating solutions

“…The enantiomer excess 2f P À1 of the chiral-achiral random copolymer chain can be calculated by the matrix method for the Ising model. 28,30,31 The matrix includes the statistical weights of the P-and M-states and the helix reversal, which are respectively written as…”

The Macromolecular Route to Chiral Amplification

“…One approach to evaluating the Ising model for the quenched random field is a numerical transfer-matrix calculation, [42,43] where one specifies specific arrangements of the groups along the backbone within the random constraint. Each specific realization can then be theoretically evaluated since the detailed characteristics of the chiral field are specified.…”

“…[43] The theoretical work leaves little question of the accuracy of the conformational model proposed for this polymer.…”

Chiralitätsverstärkung in dynamischen supramolekularen Aggregaten

“…[24] Bei der Copolymerisation zu Poly(n-hexylisocyanat) unter Zusatz eines chiralen Monomers genügten schon kleine Mengen der chiralen Verbindung, um ein enantiomerenreines Polymer zu erhalten. [25,26] Mischungen chiraler Monomere lieferten gleichfalls Polyisocyanate, deren optische Aktivität eine nichtlineare Abhängigkeit vom Enantiomerenüber-schuss zeigte. [27] Zwei Effekte beeinflussen die Chiralitäts-verstärkung: das Befehlsprinzip ("sergeants-and-soldiers") und das Mehrheitsprinzip ("majority-rules").…”