We report the synthesis of one new boronate ester-based covalent organic framework (COF) and two new covalent organic polymers (COPs) made with fluoranthene-containing monomers and hexahydroxytriphenylene. The structure of the monomer heavily influences whether this material forms a highly ordered mesoporous material (COF) or an amorphous, microporous material (COP). The synthesis of the fluoranthene monomers was carried out using a divergent strategy that allows for systematic structural variation and the ability to conduct a careful structure-function study. We found that small structural variations in the monomers dramatically affected the crystallinity, surface area, pore structure, and luminescence properties of the polymers. While each of the monomers contains the same fluoranthene core, the resultant pore sizes range from microporous (10 Å) to mesoporous (37 Å), with surface areas ranging from ∼500 to 1200 m2/g. To help explain how these small structural differences can have such a large effect, we carried out a series of molecular dynamics simulations on the polymers to obtain information with atomic-scale resolution on how the monomer structure affects non-covalent COF layer stacking.