Effects of steric repulsion on helical conformation of poly(N-propargylamides) with phenyl groups

N-Propargylamides with one, two, and three phenyl groups at the cc-position of the carboxyl group [HCequivalent toCCH(2)NHCOR; 1, R = C(CH3)(2)C6H5; 2, R = CCH3(C6H5)2; 3, R = C(C6H5)(3)] were polymerized with a rhodium catalyst, (nbd)Rh+B-(C6H5)(4) (nbd = 2,5-norbornadiene), to obtain the corresponding polymers in 85-91% yields. Poly(1) possessed a moderate molecular weight (Mn = 6300) and was thoroughly soluble in chloroform and dichloromethane. On the other hand, poly(2) and poly(3) were not totally soluble in the solvents. The M-n's of chloroform-soluble parts were less than 3000. The secondary structure of these three polymers in chloroform was examined by UV-vis spectroscopy with varying temperature. It was found that only poly(1) could adopt helical conformation even at 60 degreesC. By the copolymerization of either monomer 2 or 3 with HCequivalent toCNH(2)CO(CH2)(4)H (4), the solubility of the polymers was effectively improved, and the M.'s were remarkably increased. When the content of unit 4 in poly(2-co-4)s was 25% and above, the copolymers could form helical conformation with different degrees, among which poly(2(0.40)-co-4(0.60)) showed the largest helicity. When the content of unit 4 of poly(3-co-4)s exceeded 95%, the copolymer took helical structure partly.