Typically, thermoset foams are produced by chemical foaming with simultaneous control of curing reaction and foaming gas generation rates in a heating process. In this study, to analyze the effect of the curing degree on bubble nucleation separately from gas generation rate, the thermoset polymers with different degrees of crosslinking were prepared by the polymerization before the foaming. Mixtures of styrene and divinylbenzene with different ratios were polymerized to prepare the model thermoset polymers of different degrees of crosslinking and then foamed by a pressure quench method with supercritical carbon dioxide. Surface tension and rheological properties were measured. The molecular weight between the crosslinking points was estimated from the rheological measurement. The foaming experiments elucidated that the distance between crosslinking points was the most important factor in determining the bubble nucleation of foaming. Namely, when the distance between the crosslinking points was shorter than the critical bubble diameter, bubbles could not be nucleated. In other words, the distance between the crosslinking points could determine the foamability of the crosslinking polymers.