where k1 (s -1 ) is the pseudo-first-order reaction rate constant, and kM (= 2.1 • 10 9 M -1 s - 1 ) is the previously-reported, experimentally-determined rate constant for reaction of •OH with a monomeric unit in dissolved PEG. The reactions following •OH attack (i.e., addition of dioxygen and intermolecular rearrangements leading to a chain scission) have reported rate constants that are higher than k1. The overall scission reaction rate, therefore, is close to being diffusion limited (since kdiff is usually around 10 -9 -10 -10 M -1 s -1 according to the Smoluchowsky equation). The average scissions per initial polymer (i.e., PEG) chains (hereafter abbreviated by the dimensionless (i.e., number of cuts per one chain) variable 𝑆𝚤𝐶 """"" (t)) were derived assuming that all monomeric units of a given PEG molecule have the same probability of reacting with •OH independently of the position of the monomeric units in the chain (Equation ): 30). The transition between the reaction-limited regime for small PEG molecules and the diffusion-limited regime for large PEG molecules is expected to occur around n = 30. In the reaction-limited regime, the reaction rate constant, kP, of a given PEG molecule is directly proportional to its number of repeating monomeric units n (Equation ):