Research on controlling cationic polymerization through a degenerative chain-transfer (DCT) process has primarily focused on the family of vinyl ether monomers. To expand the monomer scope, a better understanding on what properties of chain-transfer agents (CTA)s to achieve satisfactory cationic DCT is necessary. In this work, we focused on para-methoxystyrene (pMOS) as the model monomer for electron-rich styrenics and screened a library of CTAs varying in acidity and nucleophilicity. Our results showed that increasing the nucleophilicity of the CTAs significantly improved the control over the cationic DCT polymerization of pMOS. In contrast, acidic CTAs, which provide good control over the cationic DCT polymerization of vinyl ethers, do not exert appreciable control of the cationic DCT polymerization of pMOS. Furthermore, we discovered two new CTAs for controlling the cationic DCT polymerization of pMOS, 1-butaneselenol and benzeneselenol. Lastly, this systematic study allowed us to develop a hypothesis about how the electronic structure of the propagating carbocation dictates the characteristics of a CTA necessary to control the cationic DCT polymerization of a given monomer.