*3.1. Materials*

D,L-lactide (3,6-dimethyl-1,4-dioxane-2,5-dione, 99%) was purchased from Corbion (Netherlands) and recrystallized in butyl acetate before use. Poly(ethylene glycol) methyl ether (mPEG) with a molecular weight of 5000 Da, stannous (II) 2-ethylhexanoate (SnOct2) were purchased from Sigma-Aldrich and used as received. All organic solvents were of analytical grade and used without further purification. Double distilled water was used for all experiments. Oxaliplatin (trans-R,R-cyclohexane-1,2-diamine)oxalatoplatinum(II)) was synthesized using a procedure described in literature [56].

## *3.2. Synthesis of Block Copolymers*

mPEG113-*b*-P(D,L)LA*n* diblock copolymers were synthesized by ring-opening polymerization of D,L-lactide in the presence of mPEG. Stannous (II) 2-ethylhexanoate (0.14% wt./wt. with respect to the amount of lactide) was used as a catalyst. By varying the ratio of lactide to mPEG in the reaction, it was possible to control the polymerization degree of PLA block. Before polymerization, the reactants were dried under vacuum for 30 min at room temperature.

For example, for synthesis of mPEG113-*b*-P(D,L)LA62 block-copolymer, D,L-lactide (5.04 g, 35 mmol), mPEG113 (5 g, 1 mmol) were placed into a dried polymerization flask equipped with a magnetic stirrer. Then, 0.017 mL of 1M hexane solution of stannous (II) 2-ethylhexanoate was poured into the flask. Hexane was removed by evaporation in vacuum. The reaction flask was closed with a glass stopper and immersed into an oil bath preheated to 140 ◦C. The polymerization was carried out for 24 h under argon atmosphere with constant stirring. The reaction product was cooled to room temperature and dissolved in tetrahydrofuran (10 mL) and precipitated twice, first using excess of cold (+5 ◦C) diethyl ether and then cold hexane (200 mL). The synthesis of block copolymers with different composition was carried out analogously.

## *3.3. Characterization of mPEG-b-P(D,L)LA Copolymers*

The degree of lactide conversion, PLA block length and number-average molecular weight of the synthesized polymers were determined by 1H NMR. Spectra were recorded on a 300 MHz Bruker WP-250 SY spectrometer in 5 mm o.d. sample tubes. For measurements 30 mg of block copolymer was dissolved in 1 mL of CDCl3. The integrals of the peaks corresponding to the PLA methine protons (-CH, 5.15 ppm) and PEG methylene protons (-CH2-, 3.65 ppm) were used to calculate PLA block length and number average molecular weight ( *Mn*) of the synthesized block copolymers (Figure S5 of the Supplementary Materials). The degree of conversion was found to be 90–95% for all the synthesized polymers. It was calculated using the integrals of the peaks corresponding to the unreacted lactide and PLA methine protons. The residual monomer was successfully removed by precipitation, as was confirmed by the absence of the corresponding signal (around 4.97–5.05 ppm) on the 1H NMR spectrum (Figure S5 of the Supplementary Materials). Thus, the purity of synthesized polymers is not less than 99%.

Gel permeation chromatography (GPC) was performed to determine molecular weight and polydispersity index of the synthesized block copolymers (Figure S6 of the Supplementary Materials). Chromatograms were recorded on a Knauer system consisting of a pump, a refractometric detector, and Phenogel 5 μm 103 Å column. The sample concentration

was 5 g/L, THF was used as the mobile phase (40 ◦C and 1 mL/min), and the column calibration was performed with polystyrene standards (Polymer Laboratories).

## *3.4. Preparation of mPEG-b-P(D,L)LA Nanoparticles*

Drug-free and drug-loaded nanoparticles based on mPEG-*b*-P(D,L)LA copolymers were prepared by solvent displacement (nanoprecipitation) method with acetone as organic solvent [57]. Briefly, mPEG-*b*-P(D,L)LA (50 mg) was dissolved in acetone (10 mL). Double distilled water (10 mL) was added dropwise into the solution under stirring. The organic solvent was removed through evaporation for 4 h at room temperature.

To prepare oxaliplatin-loaded mPEG-*b*-P(D,L)LA nanoparticles, 2.5 mg of oxaliplatin (5% wt./wt. with respect to the amount of block copolymer) was preliminarily dissolved in water. Then, the nanoparticles were prepared similarly to the drug-free mPEG-*b*-P(D,L)LA nanoparticles. Finally, the aqueous suspensions were centrifuged (40000 g, 30 min) to remove the residues of the organic solvent and free drug, and the precipitated nanoparticles were dispersed in water and freeze-dried.

## *3.5. Characterization of mPEG-b-P(D,L)LA Nanoparticles*

Measurements of size distribution and zeta-potential (ζ-potential) were performed by dynamic light scattering (DLS) on a Zetasizer Nano ZS instrument (Malvern Ltd.), equipped with a He-Ne laser with a wavelength of 633 nm at a scattering angle of 173◦. All of the experiments were carried out three times. The data are presented as mean ± standard deviation.

The morphology of nanoparticles was characterized by transmission electron microscopy (TEM) using a Titan 80–300 TEM/STEM (FEI) microscope at accelerating voltage of 300 kV with a BM-Ultrascan (Gatan) camera operating in the bright field mode. Thincarbon-film-coated copper TEM grids were glow-discharged for 10 s in the Pelco easiGlow system. A 3 μL droplet of the aqueous suspensions with concentration of 0.5 g/L was deposited on the carbon side of the grid and incubated for 1 min. Then, the carbon side of the grid was rinsed with 10 μL of distilled water, and right after that, 10 μL of uranyl acetate solution with a concentration of 1 wt.% was applied to the grid and incubated for 30 s. The excess of the solution was removed after each step by touching the grid edge with filter paper.

Synchrotron SAXS measurements of the aqueous suspensions of nanoparticles were performed at the European Molecular Biology Laboratory (EMBL) on the storage ring PETRA III (DESY, Hamburg) on the EMBL-P12 beamline equipped with a 2D photon counting pixel X-ray detector Pilatus 2 M (Dectris). The scattering intensity, *I(s)*, was recorded in the range of the momentum transfer 0.02 < *s* < 4.5 nm<sup>−</sup>1, where

$$\mathbf{s} = (4\pi \sin \theta) / \lambda \tag{1}$$

*2θ* is the scattering angle, and λ = 0.124 nm is the X-ray wavelength. The measurements were carried out at 23 ◦C using continuous flow operation over a total exposure time of 1 s divided into 20 × 50 ms individual frames to monitor for potential radiation damage (no radiation effects were detected). For each sample, 20 scattering curves were captured to improve the quality of the obtained data. The data were corrected for the solvent scattering and processed using standard procedures with the program PRIMUS [58]. Data analysis was performed using the software suite ATSAS [59]. Pair distance distribution and volume size distribution functions were calculated using the program GNOM [60]. Before SAXS measurements, all suspensions were centrifuged (10000 rpm, 10 min).

## *3.6. Evaluation of Drug Loading*

The content of oxaliplatin (weight ratio of the drug to the block copolymer) in the freeze-dried nanoparticles was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). An axial ICP-AES 720-ES spectrometer (Agilent Technologies, USA) was used for measurements with a low flow axial quartz torch with 2.4 mm inner

diameter injector tube (Glass Expansion, Australia), a double-pass glass cyclonic spray chamber (Agilent Technologies), a OneNeb nebulizer (Agilent Technologies, USA), and a Trident Internal Standard Kit (Glass Expansion). Samples were introduced manually to reduce washing volume, without preliminary digestion or dilution. A detailed description of the measurement process can be found in the Supplementary Materials. The drug loading content DLC and encapsulation efficacy EE of oxaliplatin-loaded mPEG-*b*-P(D,L)LA nanoparticles were calculated according to the following equations:

$$DLC = \frac{m\_1^{OxPt}}{m\_{NP}} \times 100\% \tag{2}$$

$$EE = \frac{m\_1^{OxPt}}{m\_o^{OxPt}} \times 100\% \tag{3}$$

where *mOxPt* 1 is amount of incorporated oxaliplatin in nanoparticles, *mNP* is amount of nanoparticles, and *mOxPt* 0is initial amount of oxaliplatin.
