## 2 Results and Discussion
The values of transesterification coefficient (\(T\)) were calculated from the proportion of _iss_ tetrad in \({}^{1}\)H- or \({}^{13}\)C-NMR data using Bernoullian statistics [54]. Figure 1: MALDI TOF MS spectrum of PLA obtained in the presence ZnEt\({}_{2}\)/PGAc catalytic system (**PLA 26**). \(T\) was calculated using the following equation:  
\[T\,=\,(isi_{0}-isi)/(isi_{0}-0.125) \tag{1}\]  
The experimental _isi_ relative weight can essentially vary from 0.125 (random linkage of lactyl units) to 0.25 (Bernoullian addition of pairs). It is known that \(T\) values varying from 0 to 1 and in a stereoselective process the upper limit related to the _isi_ tetrad relative weights is higher [11]. In our study, a racemic mixture of LA was polymerized (for the ratio of enantiomers, k = 1). It is possible to assume that the probabilities of the enantiomer addition to the growing chain terminated with the same enantiomer are equal \(p_{\text{PR/RR}}=p_{\text{SS/SS}}=p_{1}\). The probabilities of the enantiomer's addition to the growing chain terminated with opposite enantiomers are equal \(p_{\text{PR/SS}}=p_{\text{SS/RR}}=p_{2}\) (because \(p_{\text{FR/RR}}+p_{\text{SS/RR}}=1\) and \(p_{\text{SS/SS}}+p_{\text{RR/SS}}=1\)) [45]. It is therefore possible to calculate the intensity values of the individual sequences:  
* for tetrads \[(iii)\,=\,{p_{1}}^{3}\,+\,1.5{p_{1}}^{2}p_{2}\,+\,0.5{p_{1}}{p_{2}}^{2}\] (2) \[(iis)\,=\,(iii)\,=\,0.5{p_{1}}^{2}p_{2}\,+\,0.5{p_{1}}{p_{2}}^{2}\] (3) \[(isi)\,=\,0.5{p_{1}}^{2}p_{2}\,+\,p_{1}{p_{2}}^{2}\,+\,0.5{p_{2}}^{3}\] (4) \[(sis)\,=\,0.5{p_{1}}{p_{2}}^{2}\,+\,0.5{p_{2}}^{3}\] (5)
* for hexads \[(iiiii)\,=\,{p_{1}}^{3}\,+\,0.5{p_{1}}^{2}p_{2}\] (6) \[(iiiis)\,=\,(iiiii)\,=\,0.5{p_{1}}^{2}p_{2}\] (7) \[(iiisi)\,=\,(isiii)\,=\,0.5{p_{1}}^{2}p_{2}\,+\,0.5{p_{1}}{p_{2}}^{2}\] (8) Figure 1:0 (9) Figure 1:1 (10) Figure 1:2 (11)  
The coefficient probabilities \(p_{1}\) and \(p_{2}\) were calculated from the above equations using the intensities of signals in the \({}^{13}\)C-NMR spectrum [45]. In this work, the influence of the types of catalytic systems, as well as the reaction time and temperature on the chain microstructure was investigated. As shown in Table 3 and Figures 2 and 3 when _rac_-LA was employed using a ZnEt\({}_{2}\)/PGAc catalytic system (40 \({}^{\circ}\)C within 16-48 h or 60 \({}^{\circ}\)C within 16-24 h), the intermolecular transesterification process was not observed (\(T\) = 0). For example, for a polyester obtained in the presence of ZnEt\({}_{2}\)/PGAc catalytic system (60 \({}^{\circ}\)C within 16 h, Table 3), the calculated coefficient of stereoselectivity \(p_{2}\) was 0.58 (**PLA 25**), whereas the average length of lactyl units \(L_{i}\) = 3.38 (when no \(ss\) sequences were present in the polymer chain, this coefficient may be defined as \(p_{2}\) = 2/\(L_{i}\), where \(L_{i}\) is the average length of the isotactic microblocks). In our research, the \"predominantly isotactic\" PLA (**PLA 25**) was obtained in the conditions stated above. As is commonly known, the ROP process of _rac_-LA enables the following to form:  
Figure 1:3  
\"predominantly isotactic\" PLA (\(T\) = 0, \(p_{1}\) = 0.5, \(p_{2}\) = 0.5, \(L_{i}\) = 4)...SSSRRRSSSSRR...  
\"completely disrudiotactic\" (heterotactic) PLA (\(T\) = 0, \(p_{1}\) = 0, \(p_{2}\) = 1, \(L_{i}\) = 2)...SSRRSSRR...  
atactic PLA (\(T\) = 1)...RRSSSSRR... (Scheme 2) [46]. In the \({}^{13}\)C-NMR spectra of PLA obtained in the presence of ZnEt\({}_{2}\)/PGAc catalytic system (40 \({}^{\circ}\)C within 16-48 h or 60 \({}^{\circ}\)C within 16-24 h), no lines due to tetrads and hexads containing the \(ss\) sequenceswere present (Figures 2 and 3). In contrast, when ROP of _rac_-LA was carried out in the presence of ZnEt\({}_{2}\)/PGAc catalytic system at 40-80 \({}^{\circ}\)C within 48 h (Table 3), in the presence of ZnEt\({}_{2}\)/GAc catalytic system at 40 \({}^{\circ}\)C within 24-48 h, or at 60-80 \({}^{\circ}\)C within 6-48 h (Table 1), the intermolecular transesterification process was observed (\(T\
eq 0\)).