Synthesis of a Rigid Cyclic Polymer from Cycloamylose and Its Solution Properties

Small-angle X-ray scattering measurements were made for nine cyclic amylose tris(n-butylcarbamate) (cATBC) samples ranging in the weight-average molar mass M_w from 1.6 �~ 10⁴ to 1.1 �~ 10⁵ to determine the z-average mean-square radius of gyration <S²>_z and the particle scattering function in two good solvents, tetrahydrofuran (THF) and methanol (MeOH) at 25 deg;C and in a θ solvent, 2-propanol (2PrOH) at the θ temperature (35 °C). Static light scattering measurements were carried out for cATBC in 2PrOH to determine M_w and the second virial coefficient A₂. The dimensional properties are consistently explained by the current theories for wormlike ring having substantially the same model parameters obtained for linear analogues, that is, the Kuhn segment length &lamda;⁻¹ of THF, 2PrOH, and MeOH are 75, 20, and 11 nm, respectively. Furthermore, number of hydrogen bonds decreases with the order of THF, 2PrOH, and MeOH, as is the case with linear ATBC. These results indicate that cATBC has rigid helical backbone stabilized by intramolecular hydrogen bonds in THF and it loosens with increasing solvent polarity. Indeed, lyotropic liquid crystallinity was found for cATBC in THF. In the θ condition, 2PrOH at 35 °C, cATBC has large positive A₂ values, (1.3-1.4) �~ 10⁻⁴ mol g⁻² cm³. It is successfully explained by the simulation results considering intermolecular topological interaction.

(by K. Terao and T. Sato)

Fig. 1. Chemical structure of cyclic amylose tris(n-butylcarbamate).

Fig. 2. Molar mass dependence of z-average radius of gyration <S²>_z^1/2 for cATBC (unfilled circles) and linear ATBC (filled circles) in THF at 25 °C. Solid black curves, theoretical values for cylindrical wormlike ring. Dashed and dot-dashed lines, theoretical values for rigid ( λ⁻¹ = ∞) and rather flexible ( λ⁻¹ = 4 nm) cyclic cylinders. (Color online)

Fig. 3. Polarized light micrograph of concentrated THF solutions for (a) cATBC45K (c = 0.69 g cm⁻³) and (b) cATBC75K (c = 0.45 g cm⁻³), where c denotes polymer mass concentration. (Color online)