Some properties of low molecular weight polybutadiene and polytetrahydrofuran

The proposition, that low molecular weight polymer fractions in good solvents behave as if they were under ⊖ conditions, has been examined experimentally. Series of monodisperse hydroxy‐terminated polytetrahydrofuran (PTHF), 82% 1,4‐polybutadiene (PBD), and 30% 1,4‐PBD were prepared, and values of M̄ n obtained by vapor‐pressure osmometry and endgroup analysis. The Mark–Houwink viscosity parameters K and ν were determined in a number of solvents. The general conclusion is that the proposition is invalid for these systems notwithstanding the fact that ν = 0.50 for one of them [82% 1,4‐PBD in methyl ethyl ketone (MEK) at 25°C]. For this particular case, the following evidence suggests that these are actually ⊖ conditions so that the apparent fulfilment of the proposition is fortuitous. ( 1 ) Cloud‐point precipitation yields ⊖ = 26 ± 3°C in MEK. ( 2 ) The value of K is close to that of K ⊖ found elsewhere for PBD in a different solvent at a similar temperature. ( 3 ) Application of the Kurata‐Stockmayer iterative procedure for estimating K ⊖ from data in good and bad solvents yields a reasonably small discrepancy (10%) between the K ⊖ values from data in toluene and MEK at 25°C for this polymer and only a 3% difference in the unperturbed dimensions (〈 r 0 2 〉/ M ) 1/2 derived from them. Measured melting points T m of PTHF ( M̄ n = 1000–13000), plotted as a function of chain length Z , viz., 1/ T m = 1/ T m 0 + 2 R / Z Δ H f , yield 43 ± 3°C and 1.6 kcal/submole, respectively, for the limiting melting point T m 0 and the heat of fusion Δ H f . The former is in good agreement with the value obtained dilatometrically for high molecular weight polymer, while the latter indicates a degree of crystallinity of ca. 54%.