Based on the precedent of decarboxylative photoredox reactions we propose an oxetane/azetidine radical intermediate formed by oxidation of the carboxylate anion with the excited form of the photocatalyst followed by decarboxylation (see Figure for the full mechanistic picture). Reacting oxetane 1 with ethyl acrylate in the presence of TEMPO did not form the usual product 2a. Oxetane-TEMPO adduct 17 was instead isolated in 21% yield (after chromatography) and its structure confirmed by X-ray crystallography, supporting the proposed oxetane radical (Scheme 3). Intermolecular reactions that involve radicals at benzylic positions often lead to low yields of the desired product and increased side reactions such as homo-coupling or reduction of the benzylic radical. The reasons behind this, and the effect of additional substituents at the benzylic position on reactivity, are not well understood. To provide insight into the key radical-addition step, we compared the behavior of a series of aryl acetic acids, with different substitution at the benzylic center, in the reaction with ethyl acrylate (Scheme 4). Only oxetane and azetidine undergo the desired Giese reaction pathway efficiently (>50% 2a = III-A, and 13a = IV-A). Very little or no dimer formation was observed for the 3-and 4-membered rings, in contrast to methylene, gem-dimethyl and tetrahydropyran linkers. [Ir] = [Ir{dF(CF3)ppy}2(dtbbpy)]PF6 (see Scheme 2). The significant differences in reactivity of aryl acetic acids with different benzylic linkers was further investigated computationally to provide insights into the underlying features that lead to these disparities. Since maximizing the yield of Giese product requires minimization of undesired dimerization, we sought to identify the origins of dimer formation. The relative quantities of dimer observed vary significantly depending on the identity of the benzylic substituents (Scheme 4). We isolated two factors that minimize dimer formation: (1) decreasing the stability of the benzylic radical, and (2) enhancing 𝜋-delocalization of the benzylic radical into the aromatic system. First, computational studies showed the Giese addition of benzylic radicals into methyl acrylate to range from moderately exergonic to slightly endergonic (DG = -11.7 to +0.6 kcal mol -1 ), indicating the possibility of reversible radical additions for some substrates (Figure ; see Figure and Supporting Table for the computed DG data of the Giese equilibrium). The relative stability of the benzylic radicals was then calculated (stabilities determined through H-atom exchange equilibria with cyclopropyl radical II; Figure and Supporting Figure ) and found to directly influence the equilibrium position of the Giese addition, with a linear relationship (R 2 = 0.87, Figure ). This Giese equilibrium determines the extent of dimerization: as the radical becomes less stable, the driving force for the Giese addition increases and dimerization is disfavored (Figure ). For example, the relative instability of cyclopropyl radical II causes an exergonic addition to the acrylate (DG = -11.7 kcal mol -1 ), resulting in rapid Giese quenching of this radical and therefore dimer suppression. The cyclopropane radical however, is known to be very unstable and more prone to ring opening than its larger-ring counterparts, presumably leading to its increased degradation and consequently, a low yield of Giese product II-A (19%) and no observable dimer II-B. Conversely, the gem-dimethyl benzylic radical VII is stabilized to the extent that the addition to the acrylate becomes endergonic (DG = +0.6 kcal mol -1 ) and is therefore reversible, allowing a build-up of this radical in the reaction and increasing the likelihood of dimerization (Figure ). The energy barriers for the Giese addition were calculated to be relatively low, ∆G ‡ = 15.3-19.5 kcal mol -1 . For substrates with only a small, or non-existent, driving force (e.g. cyclobutyl, THP and gem-dimethyl), the reverse process will have similar forward and backward reaction barriers, leading to balanced concentrations of benzylicand Giese adduct radicals. However, for systems with a larger driving force (e.g. oxetane, methylene and cyclopropyl), the Giese process will be irreversible resulting in low concentrations of benzylic radicals.