We consider a collapsed polymer chain in a poor solvent sufficiently below the O point under the action of a force to unwind it. Using a simple phenomenological mean-field-model, we show the polymer coil changes from the collapsed state to the stretched state via a first-order transition as the stretching force is increased. The phase diagram is calculated which consists of a first-order line with a tricritical end point. The lifetimes of the metastable states are also calculated and the hysteresis effects are discussed. Possible connections to molecular gateway and memory devices in chemical engineering are also explored.
