**4. Conclusions**

Drug-free and oxaliplatin-loaded biodegradable mPEG113-*b*-P(D,L)LA*n* nanoparticles were prepared by a simple nanoprecipitation technique. The influence of hydrophobic block length on the structure, size, morphology, and drug loading content of mPEG113- *b*-P(D,L)LA*n* nanoparticles was investigated. It was observed that in aqueous solution mPEG113-*b*-P(D,L)LA*n* copolymers, where *n* = 62–173 monomer units, form spherical nanoparticles with hydrodynamic diameters ranging from 32 to 56 nm. The "core-corona" structure of the block copolymer nanoparticles was confirmed by SAXS. Tailoring of P(D,L)LA block length results in variation in both core-corona interface area and tethering density of hydrophilic PEG chains on the surface of P(D,L)LA core of the mPEG113- *b*-P(D,L)LA*n* nanoparticles, which affects oxaliplatin loading content. An increase in P(D,L)LA block length from 62 to 173 monomer units results in a decrease in core-corona interface area from 2.7 × 10<sup>20</sup> to 1.5 × 10<sup>20</sup> nm2/g and tethering density of PEG chains from 1.6 to 1.0 nm<sup>−</sup><sup>2</sup> and a reduction in the oxaliplatin loading content from 3.8 to 1.5% wt./wt. Thus, we suppose that oxaliplatin is adsorbed on the core-corona interface of the mPEG113-*b*-P(D,L)LA*n* nanoparticles. SAXS measurements revealed that oxaliplatin loading does not affect the size and structure of the block copolymer nanoparticles.

The size and structure of polymeric nanoparticles are crucial characteristics that should be considered in the design of targeted nanoformulations of anticancer agents. The developed oxaliplatin formulation based on 32 nm mPEG113-*b*-P(D,L)LA*62* nanoparticles loaded with 3.8 wt.% of drug with 76% encapsulation efficiency can be considered as a promising candidate for treatment of various types of cancer. In vitro and in vivo tests will be performed in order to compare its efficacy and toxicological profile with pure oxaliplatin.

**Supplementary Materials:** The following are available online, Figure S1: Guinier plots for the mPEG113-*b*-P(D,L)LA*n* nanoparticles, Figure S2: DLS intensity size distribution curves for oxaliplatinloaded mPEG113-*b*-P(D,L)LA*n* nanoparticles, Figure S3: Guinier plots for oxaliplatin-loaded mPEG113- *b*-P(D,L)LA*n* nanoparticles, Figure S4: Pair distance distribution functions *P(R)*–*R* for oxaliplatinloaded mPEG113-*b*-P(D,L)LA*n* nanoparticles, Figure S5: Typical 1H NMR spectrum of mPEG113- *b*-P(D,L)LA*n* (300 MHz, CDCl3), Figure S6: GPC curves for synthesized mPEG113-*b*-P(D,L)LA*n* copolymers, Table S1: Conditions of ICP-AEC measurements.

**Author Contributions:** Conceptualization, N.G.S. and S.N.C.; validation, Y.A.K., A.A.P., and D.S.V.; formal analysis, E.V.R., E.V.S., and A.A.N.; investigation, Y.A.K., A.A.P., E.V.R., and D.S.V.; writing— original draft preparation, E.V.R. and Y.A.K.; writing—review and editing, D.R.S., N.G.S., and A.I.K.; visualization, E.V.R. and Y.A.K.; supervision, N.G.S. and S.N.C.; project administration, N.G.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Russian Science Foundation, gran<sup>t</sup> number 18-73-10079. **Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** This work was carried out using the equipment of the resource centers of the National Research Center "Kurchatov Institute". Authors are grateful to European Molecular Biology Laboratory (EMBL) for SAXS experiments on the storage ring PETRA III (DESY, Hamburg).

**Conflicts of Interest:** The authors declare no conflict of interest.

**Sample Availability:** Samples of the compounds are not available from the authors.
