  A statistical mechanical description of flexible and semi-flexible polymer
chains in a poor solvent is developed in the constant force and constant
distance ensembles. We predict the existence of many intermediate states at low
temperatures stabilized by the force. A unified response to pulling and
compressing forces has been obtained in the constant distance ensemble. We show
the signature of a cross-over length which increases linearly with the chain
length. Below this cross-over length, the critical force of unfolding decreases
with temperature, while above, it increases with temperature. For stiff chains,
we report for the first time "saw-tooth" like behavior in the force-extension
curves which has been seen earlier in the case of protein unfolding.
