## 2 Results and Discussion
As is known, in the MALDI-TOF MS spectra of PLA, two populations of chains can be observed (the even number and the odd number of lactyl units). An odd number of lactyl units shows that the PLA chain undergoes intra- and intermolecular transesterification. In our results, the MALDI-TOF MS spectra of the synthesized PLAs comprise two or three series of peaks (Figure 1). The primary series (**I**) corresponded to PLA macromolecule terminated with a hydroxyl group and a hydrogen atom (residual mass: _ca._ 41 Da, Na\({}^{+}\) adduct). The third series of peaks (**III**) also corresponded to PLA molecules terminated with a hydroxyl group and hydrogen atom (residual mass: _ca._ 57 Da, K\({}^{+}\) adduct). The second series of peaks, which had low intensity (almost unnoticeable) (**II**), corresponded to cyclic molecules (residual mass: _ca._ 23 Da, Na\({}^{+}\) adduct). The content of this population was determined on the basis of the intensity ratio of the peaks for linear and cyclic PLA. As was shown, the content of cyclic products generally increased with increasing of the temperature and polymerization time. In our previous paper, we reported that when ROP of CL was carried out in the presence of ZnEt\({}_{2}\)/PGAc catalytic system at 40-60 \({}^{\circ}\)C within 48 h or at 80 \({}^{\circ}\)C within 6 h, macrocyclic products did not formed [40]. As shown in Tables 1-4 trends of macrocyclization process during ROP of _rac_-LA in the presence of ZnEt\({}_{2}\)/PGAc were similar. The macrocyclic content (_MC_) for PLAs obtained in the presence of ZnEt\({}_{2}\)/PGAc catalytic system was low when compared to the _MC_ of PLAs obtained in the presence of the ZnEt\({}_{2}\)/GAc catalytic system (Tables 1-4). For **PLA 23**, **PLA 1**, **PLA 29** and **PLA 7**, the corresponding _MC_ values were 2%, 3%, 3% and 6%, respectively. In summary, our results clearly show that ZnEt\({}_{2}\)/PGAc is a more effective catalytic system for the promotion of the polymerization of _rac_-LA, compared to ZnEt\({}_{2}\)/GAc. The _rac_-LA monomer had almost completely been consumed in the presence of ZnEt\({}_{2}\)/PGAc within 48 h at 80 \({}^{\circ}\)C (Table 3, **PLA 28**, conversion 91%). In comparison, the maximum conversion for ROP of _rac_-LA catalyzed by ZnEt\({}_{2}\)/GAc was 64% in the same reaction condition (Table 1, **PLA 6**). The same trend was observed in our previous experiments concerning ROP of CL in the presence of ZnEt\({}_{2}\)/PGAc or ZnEt\({}_{2}\)/GAc catalytic systems. This likely demonstrates that only -OZn- active species are formed in the first case (ZnEt\({}_{2}\)/PGAc) whereas in the second case (ZnEt\({}_{2}\)/GAc), -COOZn- species are also formed [40]. It has been established that the physico-chemical, biological and biodegradation properties of PLA are dramatically dependent on the stereochemistry of PLA. Although zinc compounds have been extensively studied, these are the highest stereoselectivity, achieved by zinc-based catalysts from _rac_-LA to date [47, 48, 49, 50, 51, 52, 53]. The microstructure of the PLA was evaluated by homonuclear-decoupled \({}^{1}\)H-NMR and \({}^{13}\)C-NMR spectra. The tetrad peaks in \({}^{1}\)H-NMR spectra were assigned as noted in the literature [49]. Tetrads (for the methine carbon) or hexads (for the carbonyl carbon) distribution were also observed in the \({}^{13}\)C-NMR [45]. The literature notes that when an intermolecular transesterification process does not occur during polymerization, the carbonyl carbon region exhibits several lines that correspond to 11 hexads, resulting from a pair addition of enantiomers of LA. When the transesterification process occurs, new lines can be observed in the spectrum of carbonyl region as a combination of 21 hexads containing _ss_ segment [45]. Moreover, when intermolecular transesterification process do not occur during polymerization, the resonanse lines due to _iss_, _ss_ and _ssi_ tetrads are not observed in the methine region [45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56].