I made four different samples of poly acrylic nitrile (PAN) by going through a precipitation polymerization. The system was initiiated by the redox system $\ce{K2S2O8}$, $\ce{FeSO4}$ and $\ce{Na2S2O5}$.

The difference between the four samples was the used amount of $\ce{Na2S2O5}$. So my understanding was, that the $\ce{Na2S2O5}$ recovers the oxidized $\ce{Fe^{3+}}$ back to $\ce{Fe^{2+}}$. Therefore more initiations would take place. I noted the time each sample needed until they start to blur. To my surprise at a certain point, the more $\ce{Na2S2O5}$ was added the longer took the sample to precipitate. Now I wonder why this is the case. I'm thankful for every answer!


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