Dear Bahareh Razavi ,

I Don't know which type of polymer you are using either cross-linked resin or some other,

Anyway answer for your question is, Polar solvent means resin get swell more and also in some case non-polar also get swell more that based on the polymer functional group.,

Polymer have high capacity functional group that means high % of functional group means from this H+ or OH- loss will be more and the solvent functional group Pyrolidone Group from NMP, DMSO group effect will alter the molecular weight its quiet common with polar solvent,, so molecular weight difference will occur.,

Due to Weak  bond between functional group of the polymer or weak cross-linking some will be there due to polar solvent addition.

Due to more solvent absorption, some internal bonding and alteration you feel molecular weight affect., 

This are the reason for your problem but its minor,

Hope you got.,

Good Luck & Regards

R.Selva........

it's general by using polar solvents the molecular mass will alter because of its higher salvation that means removal of H+ and addition of solvent group., almost all polymer resin will have acid sensitive resin beads so polar solvent will affect molecular mass, the points explained by selva makes clear

I do not understand any questions or answers as they are not precise. What kind of polymer do you have? linear, branched or crosslinked. What do you mean "alter the molecular weight in solvent?" The chainsare chnging the entropy in the solvent not molecular weight. In case your solvent will react with the polymer-then you will change the molecular mass.

Helena

Let you know that the molecular weight of the polymer wont change with respect to the solvent, only the hydrodynamic volume of the polymer could be changed with respect to the solvent. If you take your polymer in two different solvents which have relatively different polarity, then your polymer should give more hydrodynamic volume in a solvent which has more polarity. This could be evidenced by size exclusion chromatography. 

In your case i hope that in DMSO you will get more hydrodynamic volume of the polymer.

solubility of polymer also affects its molecular weight during polymerization. Mostly, when high molecular weight is reached polymer becomes insoluble (depending on the solvent) and this in-solubility limits the further increase in weight.  This is the physical aspect however if solvent as some role in the mechanism of polymerization reaction, if might affect the molecular weight.

Dear all,

in polyaddition polymerization, the solvent may acts as a chain transfer agent limiting hence the molecular weight growth, and in sometimes the ultimate conversion may be also reduced. In step growth polymerization, the solvent mat also reacts with any of the end groups of a growing chain and blocking it from this site. This true if the solvent and the reaction conditions allow for such reaction.

The solvation power of a solvent is the key parameter behind the accurate and real measurement of the molecular dimensions. The concept of polar/apolar solvent should be used appropriately with respect to the nature of the polymer in question. Regards

As far my knowledge there is no effect of solvent on molecular weight of polymer except  the solvent itself participate in the polymerization reaction.