We show that the mechanism for the negative axial thermal expansion is fundamentally different in polyethylene (PE) and isotactic polypropylene (iPP) crystals. For PE, axial contraction increases the entropy, thereby leading to negative axial thermal expansion. In contrast, for iPP, axial expansion increases the entropy, which alone would lead to positive axial thermal expansion. The negative axial thermal expansion in iPP occurs as an elastic response to the positive transverse thermal expansion, and is based on potential energy effects.