The second polymerization method involved circulating gaseous CPD through the molten sulfur using a pump. In this method, two flasks were connected by a tube and a reflux condenser was attached to the one serving as the CPD reservoir (Fig. ). Freshly distilled CPD was added to the flask with the condenser while sulfur was added to the other flask. Equal masses of CPD and sulfur were used (3.0 g of each). The inlet of a gas pump was inserted into the headspace of the flask with sulfur, while the outlet was submerged in the sulfur. Both the CPD and sulfur vessels were then heated to 140 °C and the CPD was pumped through the molten sulfur at an approximate rate of 500 mL/minute. After 60 minutes of reaction, the resulting viscous black liquid was poured into a silicone mold and cured for 24 h at 140 ºC in an oven to provide the final polymer for evaluation in thermal imaging. It is worth noting that any DCPD formed in the CPD reservoir (left flask, Fig. ) cannot enter the gas phase and react with the sulfur because the boiling point of DCPD is 170 ºC. And while ultra-thin sulfur polymer films have recently been made using gaseous monomers , the process in Figure constitutes the first bulk copolymerization of sulfur with a gaseous monomer-a process previously thought to be infeasible . The synthesized polymers were first characterized by differential scanning calorimetry (DSC) to determine the glass transition temperature. For the 50-poly(S-r-CPD) polymer prepared using the reflux method, a glass transition temperature of 41 °C was measured. This decreased to 3 °C when the sulfur content was increased to 67 wt%. The polymer prepared using gaseous CPD possessed the lowest glass transition temperature at -12 °C. These results are likely due to a number of factors. First, increasing sulfur content appears to result in a lower Tg for these specific polymers. Second, the reflux method results in the regeneration of some DCPD that can be incorporated into the polymer. DCPD is known to increase the Tg when used as a comonomer for this class of polymers . Indeed, in control experiments with the CPD system, adding exogenous DCPD as a co-monomer dramatically increased the glass transition temperature of the polymer.