Generating a polymer

Hi,

I have a doubt regarding the generation of a polymer like PVC in lammps.
If i want to do the polymer generation using an all atom approach and not a united atom one,
then how do i specify in the def.chain file that i have a monomer consisting of two atom types( -CH2- and -CHCl )??

or do i HAVE to use it by taking 1 monomer itself as a bead ? (united atom)

thanks in advance,
prateek