## 3 Results and Discussion
### Effect of Crosslinker Content on Polymerization Rate and Evolution of Molecular Weight Averages  
The NMC of STY/DVB with a [TEMPO]/[BPO] ratio of 1.1 was taken as a base formulation to study the effect of DVB content on the polymerization rate (Figure 3), molecular weight development (Figures 4 and 5), gel fraction versus conversion, and the SI versus conversion. It is observed in Figure 3 that the polymerization proceeds faster as the amount of DVB crosslinker in the formulation is increased.  
Figure 2: Conversion versus time profiles for the NMC of STY/DVB at [DVB] = 1 wt.% and [TEMPO]/[BPO] = 1.1 (triangles) against reference cases of FRP of STY (Case 1, circles), NMP of STY (Case 2, squares), and FRC of STY/DVB (Case 3, diamonds).  
Figure 3: Effect of DVB content on conversion versus time for the NMC of STY/DVB in scCO\({}_{2}\) at [TEMPO]/[BPO] = 1.1, T = 120 \({}^{\circ}\)C, and P = 207 bar.  
Although not all the samples were characterized for molecular weight development, a linear relationship between number average molecular weight (M\({}_{\text{n}}\)) and conversion is observed in Figure 4 (see trending line) for Case 5 of Table 1. The data point at 20% monomer conversion seems to deviate from the linear trend. As observed in Figure 5, which shows a plot of weight average molecular weight (M\({}_{\text{w}}\)) versus conversion, the gelation point occurs precisely between 20% and 25% monomer conversion, a situation that may explain the deviation from the linear trend. It is clearly observed in Figure 5 that M\({}_{\text{w}}\) increases sharply during the pre-gelation period until the gelation point is reached, and then decreases for the sol fraction during the post-gelation period (see the trending lines), behavior typical for a system with polymer network formation.  
Experimental profiles of gel percentage versus monomer conversion at three levels of crosslinker are shown in Figure 6. The exact occurrence of the gelation points is not captured with precision, but the qualitative trend, illustrated by the trending lines, is clear: the gelation point occurs sooner when the concentration of DVB is increased. The gelation point for the system with 3 wt.% DVB content (Case 6) occurs between 5 and 10% monomer conversion; it occurs between 15 and 20% monomer conversion for the system with 1.5 wt.% DVB content (Case 5) and it takes place between 20 and 30% conversion for the system with 1 wt.% DVB content (Case 4). According to Figure 5, the gelation point for Case 4 takes place at about 25% conversion, which agrees with the gelation point observed in Figure 6.  
As shown in Figure 7, the swelling index (SI) is very low before the gelation point. As shown by the trending lines, a sudden increase in the SI around the gel point vicinity and then a gradual decrease after the gelation period is observed for both situations (Cases 4 and 5). Open markers in Figure 7 are repeats of polymerization. Although it is not observed clearly, most of the solid markers are repeats of the measurement for a single sample. These results show that the repeatability of the technique is good, and the reproducibility, understood as replication of the experiments in the same laboratory, is rather large. The gelation points inferred from Figure 7 are slightly higher than those obtained from Figures 5 and 6, but the qualitative trend is correct, since the gelation point at the lower level of crosslinker ([DVB] = 1 wt.%) occurs later than the corresponding one for the higher level ([DVB] = 1.5 wt.%).  
Figure 6: Effect of crosslinker content on gel development for the NMC of STY/DVB in scCO\({}_{2}\) at [TEMPO]/[BPO] = 1.1, T = 120 \({}^{\circ}\)C, and P = 207 bar.