This reduced monomer consumption is presumably due to the steric demand of the prodrugmonomer, preventing high molar mass polymers. Looking at the drug load of P2 -P6, HPLC analysis delivers better monomer conversion and higher particle yields; however, at the cost of much reduced starting concentrations of prodrug (37.5 and 25 mol%) (cf. Table ). The larger fraction of co-monomers seems to alleviate and compensate for the steric constraints of 2, improving the particle yield but not the final DLm inside the particles. The high DLm of P1 and P2 particles could therefore be explained by short oligomers and monomers that could become entrapped during particle synthesis. Such entrapped units could impair the stability and performance especially of the P1 nanoparticles. We therefore investigate the leaching of such low molecular weight oligomers and prodrug monomer from the nanoparticles. When we stir P1 nanoparticles in water for 48 h, and analyze the supernatant by HPLC, we are unable to detect any leaching from the particles (see Figure ). We therefore infer that the proportion of entrapped prodrug that leaches prematurely from the particles is low and/or the nanoparticles do not swell strongly in water, which prevents early release of the prodrug. This is a good characteristic for the intended theranostic application. Considering the desired application, we continue with our highly loaded P1 nanoparticles that exhibit also a desirable diameter d ≈ 120 nm for uptake by cells.