  We study the conformations of polymer chains in a poor solvent, with and
without bending rigidity, by means of a simple statistical mechanics model.
This model can be exactly solved for chains of length up to N=55 using exact
enumeration techniques. We analyze in details the differences between the
constant force and constant distance ensembles for large but finite N. At low
temperatures, and in the constant force ensemble, the force-extension curve
shows multiple plateaus (intermediate states), in contrast with the abrupt
transition to an extended state prevailing in the $N \to \infty$ limit. In the
constant distance ensemble, the same curve provides a unified response to
pulling and compressing forces, and agrees qualitatively with recent
experimental results. We identify a cross-over length, proportional to $N$,
below which the critical force of unfolding decreases with temperature, while
above, it increases wiyh temperature. Finally, the force-extension curve for
stiff chains exhibits "saw-tooth" like behavior, as observed in protein
unfolding experiments.
