The solubility parameter and glass transition temperature of l,2-polybutadiene with different 1,a-unit content have been measured. T, increases with increasing 1,Zunit content, whereas no essential change of solubility parameter has been observed. So, it may be concluded that chain flexibility arising from internal rotation about single bond is the sole factor which determines glass transition. Chain flexibility was then studied by computation of energy of rotational isomerization E and the steric factor E characterizing hindrance to internal rotation. Finally, the potential barrier to internal rotation U was obtained by correlating 6 and c via Tg. The results are shown in Table . v , c and U all increase with increasing 1,2-unit content indicating rising of Tg is a result of increasing chain stiffness. Determination of the solubility parameter of 1,2polybutadiene by viscometry with toluene-cyclohexane (similar molar volume) as mixed solvent was examined and proved to be reliable. The exponent a in the Mark-Houwink equation for 1,2-polybutadiene-toluene system was estimated from the solubility parameter of polymer and solvent according to the method of van Krevelen and Hoftzd6 and found to be 0.725. This value of a was used as a first approximation for the calculation of molecular weights from GPC data. The Mark-Houwink equations finally established for the system, 1,2-polybutadiene-toluene (30Β°C) with different 1,2-unit contents are given in eqs. (8)-(10).