In my M.Sc research long time ago, I polymerized acrylic acid (AA) several times but I never polymerized vinyl pyridine (VP). My answer is a general rule that applies to vinyl monomers (including AA & VP).

When you get a bottle of liquid AA, you will see a statement indicating the presence of inhibitor (such as HQ or MEHQ). An inhibitor is a material which prevents undesirable free radical polymerization of the monomer during transportation & storage.

To carry out successful polymerization, the inhibitor must be removed and this is done by several ways but the easiest one is solvent extraction. You just add a suitable amount of dilute aqueous NaOH solution & then shake the separatory funnel. Two layers form (organic which contains AA & aqueous which contains the inhibitor) . Discard the aq. layer & collect the organic layer. You will have wet AA so you add a drying agent after which you just decant the dry liquid AA carefully into the flask in which polymerization is to be done.

If you use a larger amount of initiator than usual, then you will get a polymer with a low average molecular mass. With respect to the authors, I disagree with them emphasizing that polymerization will be affected negatively since more initiator means more radicals (from the initiation step) going to the propagation step. This rate-determining step will involve formation of smaller polymeric chains & this leads to lower average molecular mass of the resulting polymer.

Dear all, the authors of the paper claim using higher initiator loading for the sake to compensate the part that will react with the inhibitor (free radicals scavenging). However this stragy also may favor the birth of other problems such as the 'cage' and 'gel' effects, branching, low conversion, low molecular weight, and so on.

It is advised to take off the inhibitor prior to polymerization. My Regards

Thanks a lot for both your response. I understand the presence of inhibitor is undesirable. However, my main aim is to obtain high affinity imprints. If you check the article i have mentioned, they elute oligomers, unsaved monomers and low affinity imprints already. My main doubt is what is should i eclectic in my high affinity imprints if i remove the inhibitor since several works of molecular imprinting do not remove the inhibitor and follow a similar synthesis strategy

Hi,

The presence of an inhibitor at the beginning of the reaction leads to an induction period during which there is no polymerization. After this period, i.e. when the inhibitor has been completely consumed, polymerization occurs at a rate identical to that of polymerization in the absence of inhibitor. However, as mentioned by Dr. Abdelkader BOUAZIZ it is possible to have other problems such as the rapid increase in the viscosity of the medium. Under these conditions, the diffusion rate of the radicals decreases and therefore the concentration of the radicals and the heat increase because the reaction is exothermic (Tromsdorff effet or gel effect).

So, I advise you to eliminate the inhibitors before polymerization and for that there are several ways:

1- It is possible, for example, to add a large amount of initiator to the reactor to combat the effect of the inhibitor, however, this technique is not suitable in all cases,

2- It is possible to raise the temperature to cause thermolysis of the inhibitor, but the monomer must have a high thermal stability,

3- It is possible to distill the monomer and inhibitor charge, but the monomer must have a good thermal stability, moreover such an operation is difficult to achieve and the boiling point of the inhibitor is, in many cases, higher than that of the monomer,

4- It is possible to remove the inhibitor by adding a dilute solution, e.g. soda ash, and then washing the monomer charge with water to remove all traces of caustic compounds.

5- It is possible to adsorb the inhibitor with a compound such as alumina or silica gel.

With my best regards

Excess initiator, and a reduced molecular weight of the product polymer, should be less of a problem if your desired polymer will be cross-linked, because the cross-links vastly increase the product's molecular weight.