###### Abstract
Furthermore, block copolymers composed of PCL and poly(ethylene glycol) (PEG) exhibit LCST value close to physiological temperature, making them potentially useful as injectable hydrogel drug delivery systems (DDSs); the LCST value can be adjusted by varying the copolymer number average molecular weight (M\({}_{\text{n}}\)), CL/PEG molar ratio, and block copolymer topology [17]. In addition, these polymers are approved by the FDA [18]. Because PCL/PEG triblock copolymers are composed of hydrophilic PEG blocks (B) and hydrophobic PCL blocks (A), they can be divided into BAB (PEG-\(b\)-PCL-\(b\)-PEG, PECE) and ABA (PCL-\(b\)-PEG-\(b\)-PCL, PECE) types with slightly different properties [19]. The PCEC copolymer has a wider gel window, a longer in vivo persistence (about three weeks), and can be synthesized in a single step without the use of any coupling agents [3]. The PCEC sol-gel-sol transition mechanism can be explained by micelles formation and aggregation. Because of the amphiphilic nature of PCEC copolymer, the macromolecules self-assemble into micelles in aqueous media at ambient temperature. As the temperature rises, the diameter of the micelles rises, and the interactions between micelles become stronger; intermicellar bridges form as well. Furthermore, as the temperature approaches the LCST value, hydrophobic interactions between copolymer molecules are thermodynamically preferred over copolymer-water interactions, and the sol-gel transition occurs. When the temperature reaches the UCST value, the micelles disintegrate, the copolymer precipitates, and the hydrogel starts to flow [13; 20]. Ring-opening polymerization (ROP) is the most widely used method for producing PCL/PEG block copolymers. Due to its high reactivity, the vast majority of papers describe the procedure using stannous octaate (Sn(Oct)\({}_{2}\)) as a catalyst and PEG-diol as a co-initiator. However, there are some concerns about tin-based catalysts' biosafety; they can be neurotoxic and cause DNA damage [21]. Thus, in the biomedical field, a tin-free catalyst effective in ROP of cyclic esters is highly desirable [19; 22]. Zirconium(IV) acetylacetonate (Zr(acac)\({}_{4}\)) appears to be a promising alternative to Sn(Oct)\({}_{2}\) as a ROP catalyst. It has a low toxicity and excellent catalytic activity in the polymerization of \(\epsilon\)-caprolactone (CL), lactide (LA), and glycolide (GA). Furthermore, Zr(acac)\({}_{4}\) has been approved by the FDA as a drug and cosmetics additive, and it is less toxic than tin-based catalysts [21; 22; 23]. To date, various PCEC and PECE hydrogel formulations have been utilized as injectable DDSs [12; 24; 25; 26; 27; 28; 29; 30]. These poly(ether-ester)s show excellent biodegradability and biocompatibility. Moreover, their aqueous solutions exhibit rapid sol-gel transition upon heating at the temperature slightly below physiological and form stable gel with good mechanical properties [13; 19]. [PERSON] et al. [12] obtained a PCEC injectable hydrogel depot as growth hormone (hGH) delivery system. The in vitro drug release assay revealed the hGH was released in prolonged and controlled manner up to three weeks. The release kinetics was denoted as first order with reduced burst release observed at the initial stage of the experiment. Analysing the results of Korsmeyer-Peppas and Higuchi mathematical modelling, the mechanism of the release was denoted as non-Fickian transport, with a predominance of diffusional mechanisms. [PERSON] et al. [24] developed a blended injectable hydrogel DDS comprising PCEC and PECE copolymers. The copolymers were successfully synthesized via ROP using Sn(Oct)\({}_{2}\) as catalyst; the synthesis of PECE required the coupling reaction with hexamethylane diisocyanate at the second step of the reaction. In vitro drug release profile of diclofenac sodium indicated the prolonged and controlled manner of the release. The mathematical modelling revealed the diffusional mechanism of the release (\(n\) coefficient for Korsmeyer-Peppas model was 0.40).