###### Abstract  
In this paper, injectable, thermosensitive smart hydrogel local drug delivery systems (LDDSs) releasing the model antitumour drug 5-fluorouracilacil (5-FU) were developed. The systems were based on biodegradable triblock copolymers synthesized via ring opening polymerization (ROP) of \(\varepsilon\)-caprolactone (CL) in the presence of poly(ethylene glycol) (PEG) and zirconium(IV) acetylacetonate (Zr(acac))\({}_{\lambda}\)), as co-initiator and catalyst, respectively. The structure, molecular weight (M\({}_{\text{n}}\)) and molecular weight distribution (_E_) of the synthesized materials was studied in detail using nuclear magnetic resonance (NMR) and gel permeation chromatography (GPC) techniques; the optimal synthesis conditions were determined. The structure corresponded well to the theoretical assumptions. The produced hydrogels demonstrated a sharp sol-gel transition at temperature close to physiological value, forming a stable gel with good mechanical properties at 37 \({}^{\circ}\)C. The kinetics and mechanism of in vitro 5-FU release were characterized by zero order, first order, Higuchi and Korsmeyer-Peppas mathematical models. The obtained results indicate good release control; the kinetics were generally defined as first order according to the predominant diffusion mechanism; and the total drug release time was approximately 12 h. The copolymers were considered to be biodegradable and non-toxic; the resulting hydrogels appear to be promising as short-term LDDSs, potentially useful in antitumor therapy. biomedical hydrogels; \(\varepsilon\)-caprolactone and poly(ethylene glycol) copolymers; antitumor drug delivery systems; local drug delivery systems; smart hydrogels +
Footnote †: journal: Applied Chemistry and Biomaterials Chemistry and Biom  
able to absorb large amounts of water or body fluids while maintaining integrity [9; 10]. In recent years, much attention has been paid to stimuli-responsive hydrogels, so-called smart hydrogels, as potential LDDSs. Temperature, pH, light, magnetic field, or biological factors such as enzymes or antibodies could cause these biomaterials' properties to change. The drug release from these hydrogels can be remotely controlled or triggered by the factors mentioned above. Whereas, injectable hydrogels are in situ gel-forming matrices that are administered as a free-flowing sol into the target tissue and form a stable gel at physiological conditions due to crosslinking processes. The most significant benefits of these systems are: ease of implantation, protection of the implantation site against infection, and increased patient comfort and compliance. Furthermore, because of local implantation, drug distribution is generally limited to the target tissue; therapy effectiveness is enhanced while side effects are minimised [9; 10; 11; 12; 13]. One of the strategy for developing injectable hydrogels is to obtain thermosensitive hydrogels undergoing sol-gel transition at physiological temperature. In particular, at room temperature, the polymer aqueous solution forms a free-flowing sol. As the temperature rises to the lower critical solution temperature (LCST), the viscosity of the hydrogel rapidly increases, forming a stable gel. As the temperature rises to the upper critical solution temperature (UCST), the gel-sol transition occurs and the hydrogel starts to flow [10; 14; 15]. Polyesters such as poly(\(\epsilon\)-caprolactone) (PCL), polylactide (PLA), and polyglycolide (PGA) have received a lot of attention in the field of thermosensitive hydrogels for biomedical applications. Their biocompatibility and biodegradability distinguish them as potential LDDSs; they are nontoxic and do not require surgical resection after use [16].