In this work, we report an intriguing coexistence of both cooperative and nucleation and growth type mechanisms during polymorphic transitions in single crystals of 2DQTT-o-B, one of the highest performing n-type organic semiconductors to date . The I-II phase transition, which exhibits cooperativity, is accompanied by a significant conformational change of the alkyl side chains, showing the importance of side chain flexibility in accessing cooperative phenomena. In stark contrast, the II-III transition shows a nucleation and growth mechanism driven by an increase in core interactions based on biradical formation in combination with melting of the alkyl side chains. This is the first time that both cooperative and noncooperative transitions are reported in a single system. Further, the phenomenon of biradical interactions directly inducing a polymorphic phase transition has not been reported before. Finally, we demonstrate the use of cooperative behavior through a thermally actuated switching device, a functionality unavailable to a nucleation and growth type mechanisms.