## 4 Conclusions  
To summarise, thermosensitive, injectable smart hydrogel LDDSs were developed and thoroughly investigated. The hydrogels were produced from PCEC triblock copolymers, which were successfully synthesized via ROP with CL and PEG in the presence of Zr(acac)\({}_{4}\) as a catalyst. It was discovered that the Zr(acac)\({}_{4}\) catalyst exhibits outstanding activity in the polymerization of CL and, due to its improved biosafety profile, may be an interesting alternative to tin-based catalysts, which are typically used in these types of reactions.  
The method of PCEC triblock copolymers synthesis with used of Zr-catalyst was repeatable. Given the low M\({}_{n}\) values, it is reasonable to conclude that the obtained copolymers have an acceptable \(D\) index. The optimal polymerization conditions for ROP of CL in the presence of PEG were determined to be 130 \({}^{\circ}\)C and 6 h; the CL/Zr(acac)\({}_{4}\) molar ratio was 1000:1. The high percentage of monomer conversion and adequate yield values demonstrated that Zr(acac)\({}_{4}\) catalyst is sufficient in the ROP process.  
In the case of thermosensitive hydrogel formulation, the most favourable PCEC copolymer properties were observed for macromolecules containing PEG 1000 as a hydrophilic block with a CL/PEG ratio close to 2.0 [m/m]. For these copolymers, the sharp sol-gel transition was clearly observed at the temperature slightly lower than physiological, ensuring a rapid (<5 min) formation of stable gel with good mechanical properties (G\({}^{\prime}\)\(\approx\) 90,000 Pa for 20 wt% PCEC-A2.0 hydrogel) at 37 \({}^{\circ}\)C. The optimal PCEC concentration was determined to be 20 wt% to 30 wt%. These findings appear to be consistent with previous research on PCEC injectable hydrogels [3; 17].  
The analysis of the 5-FU release profiles reveals that the release follows first order kinetics, with a release time of 12 h. Fitting the data to the Higuchi and Korsmeyer-Peppas mathematical models indicates that the diffusion mechanisms of the antitumor drug are clearly predominant. The hydrophilic nature of the 5-FU and the matrix clearly explains this phenomenon.  
Finally, injectable smart hydrogels capable of releasing a hydrophilic antitumor drug were developed. The obtained hydrogels appear promising as short term LDDSs, potentially useful in antitumor therapy, due to the repeatable and well-controlled release rate, as well as the biodegradability and biosafety of PCEC.  
The following are available online at [[https://www.mdpi.com/article/10.3390/ijms22158330/s1](https://www.mdpi.com/article/10.3390/ijms22158330/s1)]([https://www.mdpi.com/article/10.3390/ijms22158330/s1](https://www.mdpi.com/article/10.3390/ijms22158330/s1)).  
A.K.: conceptualization, chemical, structural, analytical and pharmaceutical research, methodology, investigation, formal analysis, and writing--original draft; M.Z.-P.: analytical research, methodology, writing--original draft; A.Z.: biological research, methodology, investigation; S.K., A.P.: analytical research; E.O.: investigation, formal analysis, and writing--original draft and editing M.S.: conceptualization, chemical, structural, analytical and pharmaceutical research, investigation, writing--original draft, editing, and supervision. All authors have read and agreed to the published version of the manuscript.  
This work was founded by the FW23/N/21:MAT Statutory Project of Medical University of Warsaw, 1 Banacha St., Warsaw 02-097, Poland.  
Not applicable.  
The data presented in this study are available on request from the corresponding author.  
We would like to express our very great appreciation to [PERSON] for her valuable help with the copolymer synthesis and hydrogel properties analysis. Moreover, we would like to express our special thanks of gratitude to [PERSON] for her valuable help with rheological properties analysis.  
The authors declare no conflict of interest.