Precision functionality is the key feature of next-generation radical polymerization enabling the implementation of applications such as controlled drug delivery, self-healing material design, and optoelectronic materials. The incorporation of functionality is however diametrically opposed to diffusion-controlled growth-inhibiting termination reactions. The fundamental bottleneck remains the identification of a generic and flexible protocol to accurately map the shortlong termination reactivity. Herein, we introduce a unique framework based on the reversible additionfragmentation chain transferchain length dependenttermination (RAFT-CLD-T) method that encompasses an extension of state-of-the-art fundamental theories and a correction for possible polymer matrix effects. Applied to methyl methacrylate (MMA) polymerization, the shortlong termination reactivity is accurately quantified for the first time. Data analysis reveals the deficiency of currently used simplified models to describe the true shortlong termination reactivity and the dominance of short-chain diffusivity. The proposed framework and insights are a turnkey prerequisite for the fundamental understanding of radical polymerization processes and to complete current macromolecular diffusion theories.