## 2 Results and Discussion  
### Synthesis and Structural Analysis of Copolymers  
The primary goal of the study was to create a PCEC-based injectable and thermosensitive smart hydrogels and assess their suitability as a 5-FU delivery system. Furthermore, the suitability of Zr(acac)\({}_{4}\) as a catalyst for ROP of CL in the presence of PEG has been investigated. The optimal conditions for ROP (reaction time and CL/catalyst molar ratio) were specified in the first step of our research, for which the agreement between experimental and theoretically calculated M\({}_{\rm n}\) of copolymers was the closest to 100% and the product was obtained with high yield and high CL conversion. Simultaneously, the optimal M\({}_{\rm n}\) of PEG and CL/PEG ratio [m/m] were specified, providing the gel-forming properties of the obtained copolymer with LCST close to 37 \({}^{\circ}\)C.  
A series of PCEC triblock copolymers were synthesized via ROP process (Scheme 1). The copolymers were varied in CL/PEG (m/m) ratios and PEG molecular weight. Table 1 contains the data gathered. All of the samples were synthesized from 0.5 g of PEG characterized by various M\({}_{\rm n}\). The molar ratio of CL to catalyst was 100:1. The copolymerization process was carried out at a temperature of 130 \({}^{\circ}\)C.  
\begin{table}
\begin{tabular}{l c c c c c c c c c c c c c c c c} \hline  & \multicolumn{1}{c}{**Feed Ratio**} & \multicolumn{4}{c}{**NMR**} & \multicolumn{4}{c}{**GPC e**} \\ \hline
**Sample** & \multicolumn{2}{c}{**Time**} & \multicolumn{2}{c}{**PEG M\({}_{\rm o}\)**} & \multicolumn{2}{c}{**CL/PEG**} & \multicolumn{2}{c}{**M\({}_{\rm o}\)**} & \multicolumn{2}{c}{**C/PEG**} & \multicolumn{2}{c}{**M\({}_{\rm o}\)**} & \multicolumn{2}{c}{**M\({}_{\rm o}\)**} & \multicolumn{2}{c}{**M\({}_{\rm o}\)**} & \multicolumn{2}{c}{**M\({}_{\rm o}\)**} & \multicolumn{2}{c}{**M\({}_{\rm o}\)**} & \multicolumn{2}{c}{**M\({}_{\rm o}\)**} & \multicolumn{2}{c}{**M\({}_{\rm o}\)**} & \multicolumn{2}{c}{**M\({}_{\rm o}\)**} & \multicolumn{2}{c}{**M\({}_{\rm o}\)**} & \multicolumn{2}{c}{**M\({}_{\rm o}\)**} \\  & \multicolumn{2}{c}{**[**\%**]**} & \multicolumn{2}{c}{**[**\%**]**} & \multicolumn{2}{c}{**[**\%**]**} & \multicolumn{2}{c}{**[**\%**]**} & \multicolumn{2}{c}{**[**\%**]**} & \multicolumn{2}{c}{**[**\%**]**} & \multicolumn{2}{c}{**[**\%**]**} & \multicolumn{2}{c}{**[**\%**]**} & \multicolumn{2}{c}{**[**\%**]**} & \multicolumn{2}{c}{**[**\%**]**} & \multicolumn{2}{c}{**[**\%**]**} \\ \hline FCC-1 & 6 & 200 & 1.9 & 380 & 200 & 580 & 1000 & 200 & 8.6 & 4.2 & 1200 & 1700 & 1.53 & 59 & not soluble \\ FCC-2 & 6 & 200 & 1.5 & 300 & 20  
The molecular weight and PCL block size were found to be in good agreement with theoretical values, though the percentage of agreement varied depending on the M\({}_{\rm n}\) of the PEG and the theoretical M\({}_{\rm n}\) of the copolymers calculated from the feed ratio (Table 1). The agreement between measured and theoretical values of M\({}_{\rm n}\) of copolymers increased significantly as M\({}_{\rm n}\) of PEG increased from 200 g/mol to 1000 g/mol. When the M\({}_{\rm n}\) content of copolymers was increased to 2000 g/mol, similar results were obtained. Above this point, the percent of agreement appears to be independent of M\({}_{\rm n}\) of PEG, reaching a plateau around 125%. Consideration of the agreement of the PCL block size with theoretical values reveals converging conclusions. When the copolymer M\({}_{\rm n}\) is low, a slight difference between theoretical and experimental data results in a relatively high percent agreement value. This effect became less pronounced as M\({}_{\rm n}\) values increased. When the relatively high molecular weights of the synthesized copolymers are considered, it can be concluded that the M\({}_{\rm n}\) values (calculated from \({}^{1}\)H NMR spectra) were consistent with the theoretical values calculated from the CL and PEG feed ratios, and the control over the polymerization process was adequate.