The photostability of polymeric materials is crucial for their applicability, especially under potentially harsh environmental conditions. In the current study, the influence of methyl-substitution on the photochemical stability of photoinitiator-derived benzoyl end groups is systematically investigated by a combination of pulsed-laser polymerization and subsequent size exclusion chromatography coupled with electrospray ionization mass spectrometry (PLP-SEC-ESI-MS), chemically induced dynamic nuclear polarization nuclear magnetic resonance spectroscopy (CIDNP-NMR), and density functional theory (DFT) calculations. Poly(methyl methacrylate)s (pMMA) were synthesized employing benzointype photoinitiators with systematically substituted benzoyl moieties (i.e., 2-methylbenzoin, 3-methylbenzoin, 4-methylbenzoin, 2,4-dimethylbenzoin, 2,6-dimethylbenzoin, 2,4,6-trimethylbenzoin, 2,3,5,6-tetramethylbenzoin, and 2,3,4,5,6-pentamethylbenzoin). Photoinduced cleavage of the photoinitiator-based end group (irradiation at 351 and 355 nm) occurs solely for polymeric species with benzoyl end groups carrying no or only one ortho-methyl substituent/s, whereas all of the other substitution patterns lead to stable chain termini. The theoretical calculations suggest that the different reactivity can be traced back to shifts of the n-pi* transitions by approximately +0.25 eV. The current investigation unambiguously evidences that methylation in both orthopositions of the benzoin-type photoinitiator critically enhances the photostability of the resulting polymer chain termini providing a clear instruction for photoinitiator design leading to polymers with stable chain termini.