Understanding the molecular origin for better control of polymorphism is critical for the development of high performing, large area production of organic electronics. We explore a series of organic semiconductor systems with the same conjugated core and various bulky side chains to understand the polymorphic phase space. We demonstrate that substituting a bulkier silicon atom for a carbon atom in the side-chains compromises the packing environment such that they inhibit side chain rotation. We prove that either allowing, or preventing, the rotation of bulky sidechains triggers cooperative transition or nucleation and growth, respectively. We investigate the impact of both types of polymorphic transitions on electronic performance. By inducing nucleation and growth, we can access two kinetically stable polymorphs and study their electronic performance. From triggering cooperative transition, we have in situaccess to different polymorphs with rapid reversible polymorphic transition for applications in next-generation smart multifunctional materials. This work offers a simple molecular design tool to access both polymorphic transition pathways and incorporate their advantages to organic semiconductors.