The structural transitions of syndiotactic polystyrene in response to external stresses and temperature were investigated in detail by molecular dynamics simulations. Upon uniaxial compression along the b-axis, the nanoporous epsilon form was transformed into a lower-density porous "S-I" form, accompanied by marked elongation of the a-axis. The S-I form was transformed into the gamma form following stress reduction. The transition from the epsilon to the beta' form was also observed under tensile stress along the c-axis, which is associated with a conformational transition of the main chains. The stress responses and the reproducibility of the transitions were confirmed by extensive simulations. The thermal stability of the polymorphs was also examined via stepwise heating simulations. Experimentally observed thermal transitions of epsilon -> gamma and delta(e) -> gamma were reproduced. It was found that the S-I form is more stable than the epsilon and delta(e) forms under 0.27 GPa uniaxial stress along the b-axis. Considering the reproducibility of the stress-induced transitions and the thermal stability, the S-I form could be realized under an appropriate stress condition, though no experimental observation has been reported for now. (C) 2017 Elsevier Ltd. All rights reserved.