Conformational study of helical poly(propiolic esters) in solution

Chiral and achiral propiolic esters [HC=CCO2R. where R = n-hexyl (HexPr), (CH2)(4)Cl (CBPr), (R)- and (S)-CH2CHMeEt (MBPr), (1S,2S,SS)-myrtanyl (MyrtPr), and (1R,2S,SR)-menthyl (MentPr)] were copolymerized in the presence of [(nbd)RhCl](2) in order to study the conformational properties of poly(propiolic esters). A clear cooperative effect on helical conformation was obtained in the copolymerization of CBPr with MyrtPr. A similar positive nonlinear relationship between the enantiomeric excess of MBPr and the observed chiroptical properties of copolymers was also recognized, which means a relatively long persistence length of the helix of poly(propiolic esters). In the copolymerization of HexPr with MentPr, on the other hand, randomly coiled (co)polymers were obtained when the MentPr content was around 60%. H-1 NMR spectra of poly(propiolic esters) gave well-resolved two diastereotopic signals, attributed to the a-methylene protons in the side groups, owing to the slow helix-helix transformation on the NMR time scale. The energy barrier for the helix-helix transformation of poly(HexPr) was determined to be more than 18.5 kcal/mol by the variable-temperature NMR technique. The NMR study of the (co)polymers also enabled us to estimate the free energy difference between the helical and randomly oiled states (Delta G(r) for poly(HexPr) = 1.59 +/- 0.16 kcal/mol at 22 degrees C).