## 2 Results and Discussion
### Synthesis and Structural Analysis of Copolymers
Figure 3 describes the relationship between the percent agreement of the estimated M\({}_{\rm n}\) and the theoretically calculated M\({}_{\rm n}\). The dependence of M\({}_{\rm n}\) of the obtained copolymer was determined to be directly proportional to M\({}_{\rm n}\) calculated from the feed ratio; the relationship appears to be linear (Figure 4), indicating that the polymerization was carried out in a well-controlled manner. Furthermore, a comparison of samples synthesized under identical conditions, namely PCEC-7 and PCEC-9, revealed that the process was repeatable. Figure 2: \({}^{13}\)C NMR spectrum of PCEC triblock copolymer. The M\({}_{\text{n}}\) and dispersity (\(D\)) of the obtained PCEC triblock copolymers were determined by gel permeation chromatography (GPC) (Table 1). The M\({}_{\text{n}}\) values were in satisfactory agreement with theoretical and estimated by \({}^{1}\)H NMR values, especially for copolymers with a M\({}_{\text{n}}\) above 2000 g/mol. Moreover, the synthesized copolymers were characterized by relatively narrow \(D\) index (ranged from 1.45 to 1.60). A series of PCEC copolymers were synthesized to determine the optimal CL/catalyst ratio. According to results mentioned above and preliminary tests of hydrogels gel-forming properties (Table 1 and Figure 3), the M\({}_{\text{n}}\) of PEG was set at 1000 g/mol, and the CL/PEG ratio was 1.3, 1.7, or 2.0 [m/m]; the reaction time and temperature were 6 h and 130 \({}^{\circ}\)C, respectively. Under these conditions, the obtained copolymers exhibit gel-forming properties with LCST close to 37 \({}^{\circ}\)C. Furthermore, the percentage agreement between theoretical and experimental M\({}_{\text{n}}\) calculations was close to 100%. The CL/catalyst molar ratio of the prepared samples ranged from 100:1 to 1250:1. Table 2 displays the results of the synthesis. Figure 4: Estimated M\({}_{\text{n}}\) of PCEC versus theoretical M\({}_{\text{n}}\) values calculated from feed ratio. Figure 3: The relationship between the percent agreement of the estimated M\({}_{\text{n}}\) and the theoretical M\({}_{\text{n}}\). When the agreement of \(\mathrm{M_{n}}\) of copolymers with the theoretical values calculated from feed ratio was compared, a decrease in percent agreement was observed with a decrease in catalyst amount from approximately 130% for 100:1 [mol/mol] to approximately 90% for 1250:1 [mol/mol]. The \(\mathrm{M_{n}}\) agreement increased to the limit value (at 1000:1) as the catalyst amount was decreased; further reduction of the catalyst amount worsened the agreement, and the obtained \(\mathrm{M_{n}}\) values were below theoretical assumptions (Figure 5). Furthermore, no significant relationship was perceived between the aforementioned agreement and the CL/PEG ratio. Based on these findings, the optimal CL/Zr(acac)\({}_{4}\) molar ratio was determined to be 1000:1; the percentage agreement of \(\mathrm{M_{n}}\) of copolymers between theoretical and experimental calculations appears to be the closest to 100% (Table 2 and Figure 5). To evaluate the polymerization reaction kinetics, a series of PCEC copolymers were synthesized in variable time periods using PEG 1000 at 130 \({}^{\circ}\)C; the monomer/catalyst molar ratio was 1000:1 and the CL/PEG ratio was 2:1 [m/m] (Table 3). It was discovered that the conversion, products yield and M\({}_{\text{n}}\) as well \(D\) increased concurrently with the reaction time; the relationship between M\({}_{\text{n}}\) and the reaction time is presented in Figure 6. Zr(acac)\({}_{4}\) appears to be an effective catalyst for ROP of CL in the presence of PEG as a co-initiator; the conversion value was high even though the reaction time was very short (89% of conversion after 0.5 h). The results of the reaction yield interpretation are fairly convergent, but the observed products yield values are significantly lower when compared to conversion.