2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/124173A 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.4 pages, 5 figures, ReVTeX4 style. Accepted in Phys. Rev. LettSoft Condensed MatterStatistical Mechanicstext