I need to get itp file for 300 repeat unit of a polymer using OPLS-AA/Gromacs. I have used this web ( From PDB to GROMACS via OPLS — Group of Characterization of Materials. GCM — UPC. Universitat Politècnica de Catalunya) to get itp for small molecules, but they have limited size.

I have tried gmx pdb2gmx but that did not work for my pdb file.