We determine the mechanism that governs polymerization during a process in which solid monomer is polymerized with a vapor-phase initiator. Gel permeation chromatography data show a bimodal molecular weight distribution at all processing conditions which can be attributed to two different polymerization mechanisms. Smaller chains form by polymerization at the vapor–solid interface, and larger chains form by polymerization within the solid. The monomer mobility and sublimation rate affect the polymerization rate and thereby affect the membrane structure. The molecular weight of the larger chains can be increased by increasing the polymerization temperature and the polymerization time. The ability to vary the polymerization time allows for tuning the solubility of the membranes. The process–structure–property relationships elucidated in this study can enable the fabrication of porous polymer membranes for applications in filtration, textiles, and sensors.