Hi,
i am starting my master thesis And I am doing polymer synthesis following by precipitation method (dissolution in solvent, non solvent precipitation, centrifugation) repeated 2-3 times. I have some impurities in my NMR analysis.
So I want to know if precipitation method in cold ethyl ether is enough to purify a polymer chain ? if yes, it the best procedure to do a good purification through This method?
Example: addition drop by drop is important ? Stirring fast also ?
Is it better than soxhlet extraction ?
Another suestion regarding the purity of final product. Is GPC can confirm the formation of product ? How can I confirm clearly that the reaction work and the product is formed (in complement with NMR) ?
thanks
Hello Saya Bird,
the purification depends on the polymer and the side products/impurities, which you want to remove.
Additional purification methods might be following:
Dialysis; I used this to remove some salts, which could leave the polymer solution through the membrane of the dialysis tube, whereas the polymer remained in the tube.
Fractionated precipitation: I used this to separate blockcopolymers from homopolymers or different moecular weight fractions of the same polymer from each other. You add a non solvent to a polymer solution and the less soluble polymer (depending on their chemistry in the case of block copolymer) or the polymer with the higher molecular weight (in the case of different molecular weights) precipitates first as a polymer gel.
There are also methods, that require more elaborate equipment, like preparative GPC or separation by LCCC.
If your separation does not remove the impurities, maybe add the solvent slower (drop by drop, as you mentioned) and use another solvent (or non-solvent). Maybe you polymer precipitates so easily, that the impurities are trapped in your solid polymer. If the polymer precipitates slower the impurities may stay dissolved (similar to the case of fractionated precipitation).
The confirmation of your product also depends on the polymer. Maybe some groups on your monomer were changed by the polymerization, which could be detected by IR-spectroscopy.
For the chracterization of polyurethanes (as an example), remaining isocyanate groups can be titrated.
If you use a new method to obtain a known polymer, you can also compare it's properties with literature values (i.e. the glass transition temperature via DSC) to get a hint if the reaction might have worked.
There are plenty of complex and simple methods to characterize a polymer, but it depends on your polymer if you can use them.
Indeed, high stirring rate and slow polymer feeding rate benefits purification because it doesn't allow precipitated polymer particles to coagulate, trapping impurities within them. But if your initial polymer solution is too concentrated and viscous, it doesn't help, because polymer drop outer layer precipitate and becomes impassable faster then non-solvent get in inner layer and dissolve impurities. In that case you can try to dilute initial polymer solution or use for precipitation solvent/non-solvent mixture, varying its composition.
You can try to match the polarity of the impurity with the non-solvent as best you can. For example if you know that the impurity is highly polar and that the polymer is not soluble in alcohol, try alcohol. I agree with Mikhail that high stirring/slow polymer addition is very important, but I would always get low yield from too dilute of polymer solutions and difficulty collecting it. 2-3 repetitions is typical.
Dear Saya,
I think of somethink like this:
https://www.sigmaaldrich.com/US/en/products/filtration/concentration-filters/centrifugal-filters
(or similar brands).
There are filters with different high-MW cutoffs and solvent compatibilities available. Handling is rather simple! You just centrifuge your liquid (ideally maybe to nearly dry, you may want to repeat a 2nd step for higher purification). The high-MW material is after centrifugation on top and the low-MW impurities should run through....
Alfred
What polymer and it's approximate Molecular weight. Based on these we have to select the solvent. Once we have correct solvent then dissolving in it and precipitating by adding partially dissolvingl solvent, the impurities can be separated. Once the impurities are separated twice or thrice polymer can be dried by evaporating the solvent to get the pure polymer.
You can do IR and DSC for MP or Tg.
is your polymer crystalline or amorphous; from that you may improve impurities elimination
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