The chemical structure of the two polymers is reported in Figure together with the corresponding glass transition temperatures. Infiltration processes were performed in a conventional ALD reactor, operating in quasi-static mode at 70°C, using TMA and H2O as metal and oxygen precursor, respectively, as shown in Figure . Information about solubility and reactivity of TMA molecules in the two polymer matrices was obtained by monitoring polymer swelling by operando spectroscopic ellipsometry (SE) and analysing chemical composition of the infiltrated polymers by ex-situ x-ray photoelectron spectroscopy (XPS). Experimental results were combined with DFT simulations of explicit TMA infiltration and polymer swelling to provide a consistent picture of TMA interaction with the different polymer matrices, identifying the energetically favourable binding configurations of TMA with PMMA and PLA polymer chains and elucidating the origin of the differences between the two polymers. In addition, we demonstrate that polymer swelling can be well described, and hence predicted, within DFT simulations which opens up new avenues to predicting novel precursorpolymer systems for SIS applications.