Building on a recently introduced technique for continuous online monitorin
g of absolute weight-average molecular weight M-w of polymers during polyme
rization reactions, a method is introduced here for finding a useful approx
imation to the polymer mass distribution and moments at each instant during
the reaction. The method is based on finding the instantaneous weight-aver
age mass at each instant, M-w,M-inst, from an expression involving the deri
vative of the cumulative M-w, which is directly monitored online. The metho
d is restricted to chain growth polymerization where individual chains are
produced and terminate rapidly compared to the overall time it takes for co
mplete monomer conversion to polymer. Data from two different polymerizatio
n experiments that were monitored online were then analyzed. One of these e
xperiments is from a reaction for poly(vinylpyrrolidone), which produced a
bimodal population, and the other from a reaction that produced a broad, un
imodal distribution. Finally, other possibilities for online polydispersity
measurements are briefly discussed.