## 2 Results and Discussion
** **b (\%)**} & \multirow{2}{*}{**\(M_{\rm e}\) **(Da)**} & \multirow{2}{*}{**\(PD\)** * [FOOTNOTE:*]Footnote *: Calculated by the weight method: \({}^{\rm B}\) calculated from \({}^{\rm B}\)H-NMR analysis (spectra of a crude reaction mixture; the conversion has been calculated by the integration of the characteristic signal of the monomer (\(\delta=5.03\) ppm) and the polymer chain (ranged from \(\delta=5.13\) to \(5.18\) ppm): \({}^{\rm B}\) determined by GPC; \(M_{\rm e}\) corrected by a factor of \(\alpha=0.58\)[41]: \({}^{\rm B}\) MC (macrocylic content) determined by MALDI TOF MS; \(As shown in Tables 1-4 the yield of the ROP process was dependent on the _rac_-LA/catalytic system's molar ratio, reaction medium, temperature and reaction time. The ROP yield of the _rac_-LA process ranged from 16% to 58% for ZnEt\({}_{2}\)/GAc (Tables 1 and 2) and from 15% to 83% for ZnEt\({}_{2}\)/PGAc catalytic systems (Tables 3 and 4). Only in one case (**PLA 20**, Table 2) the polymeric product was obtained in a trace amount. It was found that this type of solvent had an essential influence on the process' yield, that is, toluene was found to be an optimum polymerization medium. For ZnEt\({}_{2}\)/PGAc catalytic system, the yields of PLA were in the range of 35%-83% (medium--toluene, Table 3), 29%-47% (medium--THF, Table 4) and 15%-36% (medium--CH\({}_{2}\)Cl\({}_{2}\), Table 4). In comparison, the yields of the ROP products of _rac_-LA catalyzed by ZnEt\({}_{2}\)/GAc ranged from 28% to 58% (medium--toluene, Table 1), 23%-43% (medium--THF, Table 2) and 0%-21% (medium--CH\({}_{2}\)Cl\({}_{2}\), Table 2). Moreover, the ROP yields increased when the reaction temperature was raised from 40 to 80 \({}^{\circ}\)C. For example, PLAs were obtained with a high yield: 61% (**PLA 24**), 68% (**PLA 27**) and 83% yields (**PLA 28**), respectively (Table 3). The PLA yield tended to decrease with increasing of _rac_-LA/catalytic system molar ratio. For **PLA 27**, **PLA 28**, **PLA 33** and **PLA 37**, the corresponding yield values were 68%, 83%, 63% and 75%, respectively (Table 3). The process yield also increased with the reaction time increasing. For example, PLAs were obtained with 53% (**PLA 25**, reaction time 16 h), 59% (**PLA 26**, reaction time 24 h) and 68% yields (**PLA 27**, reaction time 48 h), respectively (Table 3). The molecular weight of PLAs was also dependent on the _rac_-LA/catalytic system molar ratio, reaction medium, temperature and reaction time (Tables 1-4). The average molecular mass (\(M_{n}\)) values of PLA increased when the reaction time, reaction temperature and _rac_-LA/catalytic system molar ratio were increased. The \(M_{n}\) values of PLA determined by the GPC were in the range of 1200-9900 Da (ZnEt\({}_{2}\)/PGAc catalytic system, Tables 3 and 4) and 1300-6800 Da (ZnEt\({}_{2}\)/GAc catalytic system, Tables 1 and 2). When the process was carried out in the presence of a ZnEt\({}_{2}\)/PGAc catalytic system (where the molar ratio of catalyst to monomer was 1:100, reaction temp. 80 \({}^{\circ}\)C), the \(M_{n}\) results were: 9900 Da for **PCL 37** (reaction time 48 h), 9300 Da for **PLA 36** (reaction time 24 h) and 8600 Da for **PLA 35** (reaction time 16 h) (Table 3). In comparison, when the ZnEt\({}_{2}\)/GAc catalytic system was used, \(M_{n}\) results were 6800 Da for **PLA 14** (reaction time 48 h), 6600 Da for **PLA 13** (reaction time 24 h) and 5900 Da for **PLA 12** (reaction time 16 h), respectively (Table 1). As was shown, the PLAs obtained in the presence of ZnEt\({}_{2}\)/PGAc catalytic system were generally characterized by a higher \(M_{n}\) when compared to the PLAs synthesized in the presence of ZnEt\({}_{2}\)/GAc. Moreover, when ROP was carried out in toluene, the synthesized PLAs were characterized by a higher \(M_{n}\) than that of PLAs synthesized in THF or CH\({}_{2}\)Cl\({}_{2}\). The \(M_{n}\) values determined from GPC were comparable to the viscosity analysis results (\(M_{\rm{c}}\)), as well as those of \(M_{n}\) calculated from \({}^{1}\)H-NMR.