Applying the histogram Monte Carlo simulation method and the bond-fluctuation model, various phase transitions in single-polymer systems were investigated. The critical transition temperature (Theta point) in the coil-globule collapse transition of a macromolecular chain is accurately determined Finite-size scaling results near the transition point are verified. The first-order transition associated with the freezing/crystallization of a polymer at a temperature below the Theta point is also observed. The free energy profiles associated with these two transitions are explicitly computed. Furthermore, the unfolding phase transition associated with stretching a collapsed polymer chain is investigated. The free energy profile associated with the transition is explicitly computed. Results on the energy cumulants and free energy profiles provide direct evidences for the first-order nature of the unfolding phase transition.
