How can you simulate the effect of a solvent on a polymer in an MD simulation?
You have generally two choices a) implicit and b) explicit solvent. Presumably, you don't have good interaction parameters for explicit solvent, since you are likely doing a generic polymer melt.

There are a variety of solvation models; LAMMPS, for instance, implements a dielectric pair style for which you can attenuate the dielectric constant of your solvent (and a few other things). Other popular approximations are generalized Born models. 

This paper uses GROMACS to simulate poly(styrene sulfonate) and poly(allylamine hydrochloride) in explicit and implicit solvent, as an example from the literature.