  Polymorphism rationalizes how processing can control the final structure of a
material. The rugged free-energy landscape and exceedingly slow kinetics in the
solid state have so far hampered computational investigations. We report for
the first time the free-energy landscape of a polymorphic crystalline polymer,
syndiotactic polystyrene. Coarse-grained metadynamics simulations allow us to
efficiently sample the landscape at large. The free-energy difference between
the two main polymorphs, $\alpha$ and $\beta$, is further investigated by
quantum-chemical calculations. The two methods are in line with experimental
observations: they predict $\beta$ as the more stable polymorph at standard
conditions. Critically, the free-energy landscape suggests how the $\alpha$
polymorph may lead to experimentally observed kinetic traps. The combination of
multiscale modeling, enhanced sampling, and quantum-chemical calculations
offers an appealing strategy to uncover complex free-energy landscapes with
polymorphic behavior.
