Macromolecular Stereochemistry: Effect of Pendant Group Structure on the Conformational Properties of Polyisocyanides^1,2

The hydroxyl function of the S enantiomer of the 1,2-acetone ketal of propylene glycol wasconverted, via the amine, to the isocyanide ([α]₆₄₆+32.9° (CHC1₃)), which was polymerized by using NiCl₂hexahydrate in the neat state to the derived poly((S)-2,2-dimethyl-l,3-dioxolane-4-methyl isocyanide) ([α]_S46+68.5° (p-dioxane); M_w= 165000). The ultraviolet spectrum of this brown solid in CHC1₃exhibited a band, as a shoulder on lower wavelength absorptions, with λ_max= 295 nm and 3.2 X 10² L mor¹ cm^-1, and a long low-intensity visible region tail. The circular dichroism (CD) spectrum was similar to published spectra for α-substituted asymmetricpolyisocyanides and showed three maxima (λ_max>nm; Δe, L mol^-1 cm^-1): 328, -0.082; 290, +0.015; 275, -0.047. High-field ¹³C NMR studies exhibited extensive chemical shiftdispersion for both the backbone carbon and the pendant carbons. Similar shift dispersion was encountered in poly(n-butyl isocyanide) and in poly(benzyl isocyanide). Poly((R)-α-phenylethyl isocyanide) also exhibited chemical shift dispersion superimposed on symmetrical but broadened lines in its ¹³C NMR spectrum. Zimm plots constructed fromwide-angle 633-nm light scattering data on poly((#)-a-phenylethyl isocyanide) (M_w= 107 000) and on poly((iS)-2,2-dimethyl-l,3-dioxolane-4-methyl isocyanide) (M_w= 165000) showed no angular dependence. The weight average hydrodynamic radii of thesepolymers in tetrahydrofuran were determined by photon correlation spectroscopy to be 51 and 24 Å, respectively. If one adopts a 4/1 helix with a pitch of 4 Å proposedin the literature for poly(a-phenylethyl isocyanide), our data yield a persistence length of 32 Å, which is substantially largerthan that for poly(S)-2,2-dimethyl-l,3-dioxolane-4-methyl isocyanide). The results show that some form of stereoirregularity, which could involve syn-antiisomerism about the carbon-nitrogen double bond, is seen in each of thepolymers studied. In addition, the chain dimension is strongly dependent on the pendant group structure.