The distinction between the condition Xn trends to infinite and Xw trends to infinite in fact illustrates well the phenomenon of gelation. Xn trends to infinite means that the average DP per molecule is infinite, because the number of molecules reduces to zero. On the other hand, Xw trends to infinite means that the average DP per unit is infinite. That is, some units belong to infinite species . We think when Xn tends to infinity, which means that the mol number of molecules is approaching zero but not quite there. The mol number of molecules is assumed to be approaching zero, when there is only one molecule left, which means that the mol number of molecules is 1/ 6.02×10²³.