To understand the thermal self-initiated polymerization of vinyl monomers during the divinylbenzene refining process, the differences in reaction behavior among these monomers were studied by density functional theory calculations. The calculations reveal that para-divinylbenzene (para-DVB) exhibits higher reactivity than other monomers in polymerization. Specifically, among the first step of initiation which is a Diels-Alder reaction, the reaction between para-DVB molecules shows the lowest activation energy (Ea) of 134.6 kJ/mol. Subsequently, para-DVB preferentially undergoes molecule-assisted homolysis with the products of the aforementioned Diels-Alder reaction and formatting free radicals with an Ea of 74.8 kJ/mol and a reaction rate constant (kTST) of 3.48 × 10−3 L/mol∙s. In the propagation stage, para-DVB also exhibits the highest monomer reactivity, with a kTST approximately 1.4–2.25 times greater than other monomers. The polymerization preformed under refining conditions confirms that the consumption of para-DVB is faster than that of other monomers, consistent with the DFT calculations.