Entries 1-3 in Table show the results of a first experiment using this recipe (and a higher and lower reducing agent concentration) performed under a coverslip. This successfully resulted in homogeneous brushes with the exception of a notable thickness gradient of a few mm wide around the edges, caused by oxygen diffusion from the surrounding air (see Figure ). This 'edge effect' has been noted by others performing airtolerant SI-ATRP in a liquid film sandwiched between the substrate and a cover . The reducing agent concentration was varied first, as that parameter is expected to be most critical when changing the setup (and thus, the amount of oxygen diffusion). Predicting the optimal amount of reducing agent is not trivial: too much will produce too much activator (Cu(I)) in the beginning of the polymerization reaction, leading to overly fast and poorly controlled ATRP. On the other hand, an insufficient amount of reducing agent results in poor oxygen tolerance, and thereby too slow polymerization (or none at all) . Both cases result in thin brushes. This experiment clearly shows that 10 mM ascorbic acid is excessive, since it results in thinner brushes than are obtained with lower concentrations. Next, a full factorial design was performed with three factors (monomer concentration, catalyst concentration, and reducing agent concentration) and two levels, i.e. a 2 3 design. From this, it was identified that the higher monomer concentration (1 M), higher catalyst concentration (0.2 mM), and lower reducing agent concentration (2 mM) produced the thickest brushes (# 4 in Table ). The full results are available in the SI.