Again, please keep the list in the loop.
Others may wish to comment.
As I said before, if you have a linear chain that spans the box
and then connect the 2 atoms at the ends, across the box,
you are forming a ring, not an infinite linear chain. If the
box is big, you can think of it an in infinite chain if you like,
but it is really a ring.
You can also fill your box with N monomers and bond them as
one long chain, so long as every (N,N+1) pair is "close" to each
other in a periodic sense, so that the long chain crosses the box
many times. But if you also bond 1 to N, then you have a ring.