Solution properties of polybutadiene and <i>trans</i>‐polyisoprene fractions

Abstract

Abstract Linear polybutadienes, prepared with butyllithium as catalyst, and polybutadienes branched by exposure to γ‐radiation have been fractionated and the fractions examined by osmometry and light scattering. Turbidimetric second virial coefficients ( A 2 τ ) of mixed polymer fractions are virtually the same as those of the higher molecular weight components of the mixtures for most compositions. This is true both for mixtures of linear with linear and linear with branched polymer. The higher the molecular weight of the fraction, the greater the effect; the addition of 1% microgel to a linear polymer reduced A 2 τ by a factor of three. The presence of microgel or high molecular weight branched polymer has been shown to be responsible for the very high molecular weights previously reported for polybutadienes from light scattering measurements. It is conveniently removed by shaking the solutions with calcium sulfate. Second virial coefficients obtained either by light scattering or osmometry are, within the limits of experimental error, uninfluenced by branching in the polymer. In general those factors which lead to increased branching also result in increased polydispersity, and it is the latter which results in the decrease in A 2 τ . The fall of the osmotic second virial coefficient ( A 2 τ ) with molecular weight is much smaller than would be calculated theoretically, and the fall in A 2 τ is greater than would be expected, notwithstanding the fact that for some of the fractions M̄ u /M̄ n &lt; 1.1. This indicates that A 2 π is sensitive to the low molecular weight species presented in the fractions, whereas the reverse must apply to A 2 τ . Natural of synthetic trans ‐polyisoprene showed analogous behavior to polybutadiene, although, owing to experimental difficulties, sharp branched fractions could not be obtained. The following viscosity–molecular weight relationships were obtained in benzene solution: [η] = 1.45 × 10 −4 M 0.76 for butyllithium‐catalyzed polybutadienes, and [η] = 4.37 × 10 −4 M 0.65 for natural and synthetic trans ‐polyisoprenes.