Hi all,
I am carrying out a catalytic reaction to break down a polymer (PCL-PEG-PCL, 2k-2k-2k). I am unsure how to calculate the number of moles of polymer. Say that I want 3.0 mg of substrate. If I take the molecular weight of the polymer as it is (6000 g/mol), that would work out to be 0.5umol (u = micro).
If I want to add 5mol% of the catalyst (MW = 518 g/mol), that would work out to be 0.013 mg, which is far too small a value to weigh.
A colleague of mine suggested that after obtaining the number of moles of polymer, multiply that value with the total number of repeating units of monomers, i.e. 18+45+18, I would get 40.5umol, and the amount of catalyst I would need is 1 mg (doable). In this case, am I adding 5mol% based on the no. of moles of monomers?
I thought of calculating the amount of polymer I need based on the amount of catalyst I want to put in as well. So for example, if I want to put 1mg of catalyst to make up 5mol%, I would need 38.6umol of polymer, which works out to be 0.23g.
Pretty confused as to how I should go about this. Does anyone have a suggestion or a different way to calculate?
Many thanks!
Dear Ht Liang, I see that your problem is with amount of the catalyst. Simply prepare a given concentration of it and take the volume that corresponds to the mass you want to use. Since it is a low mass, the solution will be better if it is a low concentration one, say 1g/l.
For the polymer, if only the end groups are of interest, then better to get their equivalent by titration. My Regards
If you are following a paper, it should indicate what the "mol%" refers to. There is no wrong answer, but obviously if it is "mol% with respect to the polymer" or "mol% with respect to the repeating unit" the difference (as you have already calculated) is substantial. Again, if you are following a paper, check also for weights other than percentage amount fraction (i.e. mol%).
If possible I would increase the amount of substrate I am working with, 3.0 mg is a low amount if it is necessary to add catalyst gravimetrically.
Otherwise, again if possible, I would work by diluting a catalyst solution.
Best regards
You can measure the molecular weight of the copolymer by size exclusion chromatography and from the results, you will be able to calculate the degradation (%).
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