  We study the unfolding of a single polymer chain due to an external force. We
use a simplified model which allows to perform all calculations in closed form
without assuming a Boltzmann-Gibbs form for the equilibrium distribution.
Temperature is then defined by calculating the Legendre transform of the
entropy under certain constraints. The application of the model is limited to
flexible polymers. It exhibits a gradual transition from compact globule to
rod. The boundary line between these two phases shows reentrant behavior. This
behavior is explained by the presence of residual entropy.
