Topochemical reactions have been extensively studied in organic and polymer chemistry, and are conventionally triggered by heat and/or light. Pressure, as one of the fundamental thermodynamic parameters, has recently been applied to study phase transitions of inorganic and hybrid nanomaterials on both microscopic and macroscopic scales. Relatively fewer studies have been focused on high-pressure behaviors, especially pressure-induced reactions, on organic and polymer molecules and assemblies. Polytriacetylenes (PTAs) are unique conjugated polymers with all-carbon main-chains consisted of alternating double bonds and diacetylene units. Although diacetylene units are prime examples capable of light-induced topochemical polymerization into polydiacetylene, PTAs are found to be very stable under light irradiation. Here we report our observation that in a diamond anvil cell (DAC), applying low pressure leads to crystallization and ordering of PTAs bearing linear alkyl side-chains. Further increasing pressure leads to irreversible cross-linking reactions, resulting in materials of graphyne-like structures, appearances, optical properties, and solubilities. Findings in this paper thus reveal the potential of using pressure to guide conjugated polymer self-assembly and to induce chemical reactions toward new materials discoveries.