The phase behavior of many conjugated polymers is rich with both crystalline and liquid crystalline phases. Recent computational efforts have identified the isotropic-tonematic transition temperature for polymers such as poly(3hexylthiophene- 2,5-diyl) (P3HT). Herein, model predictions are combined with experimentally determined values of the equilibrium melting temperature as a function of chain length to provide the complete phase behavior for P3HT. Additionally, because a full description of the phase behavior requires proper accounting for the regioregularity of the chain, a thermodynamic relationship is derived to predict this behavior as a function of both chain length and regioregularity and the impact of regioregularity on the expected phase diagram is discussed. (C) 2016 Wiley Periodicals, Inc.