Control of polymorphic behavior is crucial for designing functional organic semiconductor devices as even a slight structural difference may translate to dramatically different electronic properties. One route to controlling structure is through stimulus-induced polymorph transitions, which allows for switching those electronic properties. However, despite advances in predicting crystal structures, the molecular design characteristics governing the polymorphic transition mechanism remains unknown. Here, we systematically investigate a series of n-type organic semiconductor molecules based on 2-dimensional quinoidal terthiophene with varying alkyl side chain lengths to modulate two distinct polymorph transitions, one cooperative martensitic transition and the other non-cooperative nucleation and growth transition. In the three molecular systems, we observe that shortening the alkyl chain past a critical length suppresses the cooperative polymorph transition by limiting the alkyl chain conformation change. On the other hand, the nucleation and growth transition temperature increases as the side chain length decreases, possibly driven by the increase in the melting point of the alkyl chains. We also found that tuning the alkyl chain length modulates the associated quinoidal to aromatic biradical switching that drives the nucleation and growth transition, suggesting a synergy between the crystal structure and electronic structure. Ultimately depending on the exact mechanism of the polymorph transition, adjusting the alkyl chain length may lead to tuning of the polymorph transition temperature or suppression of the transition altogether. This offers a potential molecular design rule to target a particular transition mechanism based on the desired behavior for the system.