In situ studies have been performed on the gas phase homogeneous free radical polymerization of vinyl acetate. These studies retained the polymeric radicals in the gas phase by growing so few of them simultaneously (<10(2)-10(3) molecules cm(-3)) that they could not encounter one another. Experiments were conducted in the uniform environment of an expansion cloud chamber using nucleation of drops for detection. However, the experiment was designed so that the analysis of the data would not have to rely on an unreliable theory of nucleation. Results of these studies indicate fairly conclusively that (1) small individual polymer molecules can induce nucleation in supersaturated vapors, and (2) the rate constant for polymer propagation is considerably smaller in the vapor than the corresponding value in the liquid. In fact, a quantitative upper bound can be placed on the rate constant. The knowledge gained in these experiments makes it possible to prescribe the design of future investigations that will be capable of yielding quantitative values (rather than bounds) for the rate constants as well as for the sizes of the polymers able to induce nucleation.