ESTIMATION OF THE CONFORMATIONAL CHARACTERISTICS OF POLYURETHANE CHAINS

Linear polyurethanes (PUR-D-k) were prepared by reacting 4,4'-methylen ediphenyl diisocyanate (MDI) with low-molecular-weight diols HO-(CH2)( k)-OH (k = 2, 3, 4, and 6) and fractionated by precipitation or extrac tion with mixtures of N,N-dimethylformamide (DMF) and dibutyl ether (D BE). The fractions were characterized by their weight-average molecula r weights <(M)over bar w> (light scattering) and intrinsic viscosities [eta] in DMF (good solvent) and mixtures of DMF with DBE (poor solven ts). The Kuhn statistical segment length l(K) was evaluated from the i ntrinsic viscosities in poor solvents. Low values of l(K), (12 to 14) x 10(-8) cm, indicate that benzene rings and virtual double bonds (ure thane groups) confine neighbour interactions to bonds within the repea ting units and assure independent rotations at the bonds adjoining the m.