Hello Subhankar Mandal, there are two fundamental pre-processing tools that you require for implementing a polymer simulation in LAMMPS. They are Avogadro2 (https://www.openchemistry.org/projects/avogadro2/) and moltemplate (https://github.com/jewettaij/moltemplate). Here are the steps that you will need to follow.

Avogadro2:

1. Draw the required monomer, end group and solvent molecules in Avogadro2.

2. Use the in-built OpenBabel tool to optimize their geometries and export them as cartesian XYZ files.

3. Enter the co-ordinates from the XYZ files in a spreadsheet (Excel), and zero out the co-ordinates of the relevant central atoms (chiral-centers, polymerizing linkages, etc.), and rewrite your xyz files.

Moltemplate: This is a slightly tedious job but this is one of the best ways to build polymer. Please consult the provided examples. They are very helpful.

1. Build your .lt files for your monomers, end-groups, and solvent molecules, manually by entering the co-ordinates from the corresponding xyz files and atom types from the oplsaa.lt file provided in the forcefields folder of moltemplate.

2. Build you .lt file for the polymer by connecting the monomers, and end groups.

3. Build your system.lt file for the initial LAMMPS simulation cell containing the polymer and the solvent.

4. Run moltemplate.sh and cleanup_moltemplate.sh to generate the required LAMMPS datafile and the initialization, and OPLS/AA forcefield parameters for your system.

Additionally, you can also use PACKMOL (https://m3g.github.io/packmol/) together with moltemplate if required. Consult the examples in moltemplate to see which method suits you.