initial chain (𝑆𝚤𝐶 """"" ) with reaction time (shown up to 10 days; panel a), and an exponential decay of the number average molecular weight (𝑀𝑛 """" ) with reaction time (panel b), calculated for 𝑀𝑛 """" (𝑡 " ) of 12 kDa (blue trace), 6 kDa (purple traces), and 3 kDa (green traces) at three environmental •OH steady-state concentrations of [•OH]SS = 10 -15 M, 10 -16 M and 10 -17 M (shades of the corresponding colour). c, d) A reaction network plot illustrating the results of Monte Carlo simulations with 20 initial PEG chains and simulation termination criteria of 𝑆𝚤𝐶 """"" = 1 and 8 in panels c and d, respectively, with the resulting nr% (i.e., the number of PEG molecules with a given MW over the total number of chains obtained) contributions of individual PEG molecules of given MW plotted below, with the initial MW distribution shown as a dashed line. The PEGs are colour-coded according to MW from high (blues) over medium (purples) to low (greens) MWs. e, f) Averaged outputs of 50 Monte Carlo simulations on 5000 initial PEG molecules with an initial MW distribution of 6380 ± 400 Da. Simulation results are depicted as weight percent (wt%) distributions of the MWs of PEG molecules after 𝑆𝚤𝐶 """"" of 1, 2 and 8 (relative to the initial distribution (i.e. 𝑆𝚤𝐶 """"" = 0)). The changes in the wt% contribution of PEG molecules with selected specific MWs (i.e., MW = 6380 Da (blue); MW = 4400 Da (purple); and MW= 440 Da (green) with increasing simulated 𝑆𝚤𝐶 """"" is sg in panel f. The 95% confidence intervals of the simulated outputs are shown as shaded areas around the averages in panels e and f.