Formation pathways for high-molecular-mass compound growth are presented, showing why reactions between aromatic moieties are needed to explain recent experimental findings. These reactions are then analyzed by using quantum mechanical density functional methods. A sequence of chemical reactions between aromatic compounds (e.g., phenyl) and compounds containing conjugated double bonds (e.g., acenaphthylene) was studied in detail. The sequence begins with the H-abstraction from acenaphthylene to produce the corresponding radical, which then furnishes higher aromatics through either a two-step radical-molecule reaction or a direct radical-radical addition to another aromatic radical. Iteration of this mechanism followed by rearrangement of the carbon framework ultimately leads to high-molecular-mass compounds. This sequence can be repeated for the formation of high-molecular-mass compounds. The distinguishing features of the proposed model lie in the chemical specificity of the routes considered. The aromatic radical attacks the double bond of five-membered-ring polycyclic aromatic hydrocarbons. This involves specific compounds that are exceptional soot precursors as they form resonantly stabilized radical intermediates, relieving part of the large strain in the five-membered rings by formation of linear aggregates.