The thermal expansion of crystalline syndiotactic polypropylene is simulated to elucidate the origins of the transverse thermal expansion anisotropy observed in these materials. The calculations are carried out using a lattice dynamics method and empirical force fields. For the transverse thermal expansion coefficients, the results obtained for different crystal structures and force fields bracket the experimental results. Thermal expansion is calculated to be greater along the b axis than along the a axis, in agreement with experiment. This transverse thermal expansion anisotropy is found to arise from larger thermal stresses, lower stiffness, and less Poisson coupling along the b axis.