Variation of helical pitches driven by the composition of N-propargylamide copolymers

Five novel N-propargylacetamide and -propaneamide monomers [HCdropCCH(2)NHCOR; 4: R = CH2C6H5, 5: R = CH(C6H5)(2), 6: R = CH2CH2C6H5, 7: R = CH2CH(C6H5)(2), 8: R = CH2C(C6H5)(3)] were polymerized with a rhodium catalyst, (nbd)R+B-(C6H5)(4) (nbd = 2,5-norbornadiene), to provide polymers in 81-99% yields. Poly(4), poly(5), and poly(8) possessed low molecular weights (M = 31006100), presumably due to their low solubility in solvents, while poly(6) and poly(7) had moderate M-n's (15 000 and 16 700) and good solubility in several solvents including chloroform. The secondary structures of the five polymers in chloroform were investigated by UV-vis spectroscopy. Poly(4) adopted a helical structure to a certain extent, while poly(5)-poly(7) hardly did not. In contrast, poly(8) exhibited a UV-vis absorption peak around 365 nn, demonstrating that it invariably formed helices under the given conditions. Copolymers of monomer 8 with either chiral monomer 10 [HC=CCH2NHCOCH(CH3)CH2-CH3] or achiral monomers 1 [HC=CCH2NHCOC(CH3)(2)C6H5] and 9 [HC=CCH2NHCO(CH2)(5)CH3], i.e., poly(8-co-10)s, poly(8-co-1)s, and poly(8-co-9)s, showed good solubility and possessed higher M-n's than that of poly(8). Furthermore, with the increase of unit 8 in poly(8-co-10)s, poly(8-co-1)s, and poly(S-co-9)s, initial blue and then red shifts were observed in the lambda(max) of their UV-vis spectra and further in the CD signal of poly(8-co-10)s, demonstrating that all of these copolymers formed helices with different pitches, which depended on the compositions of the copolymers. Additionally, the sergeants and soldiers rule was observed in achiral/chiral poly(8-co-10)s.