An experimental setup for the polymerization of liquid propylene was used to carry out main polymerizations with and without a prepolymerization step. Two types of prepolymerization are introduced: at a constant temperature and at rapidly increasing reactor temperatures. With the present catalyst system, at high polymerization temperatures, a prepolymerization step will increase polymerization rate. Hydrogen was used to control the molecular weight of the product. The polymerization reactor was used in combination with a gas chromatograph to determine the vapor-liquid equilibrium (VLE) of the propylene-hydrogen system. Measurements are compared to predictions of different equations of state. The Peng-Robinson equation of state is best able to describe the VLE for this binary system. A temperature-dependent interaction parameter was derived from the fits to the measured data.