## 2 Results and Discussion
### Chain Microstructures of Synthesized Copolymers  
Microstructural analysis of the synthesized copolymers, both in terms of their monomer sequencing and average monomer block length, were conducted on the basis of the attribution of the peaks in the \({}^{13}\)C-NMR spectra, since this technique is very sensitive to monomer sequencing. Moreover, it is a powerful tool for determining the average sequence length for each type of monomer unit [26]. Monomer sequencing was determined on the basis of the expanded carbonyl carbon signals from \(\delta\) = 168-174 ppm at \({}^{13}\)C-NMR spectra as previously described by [PERSON] [27] and [PERSON] and [PERSON] [28]. We used various mixtures of co-monomers because reactivity ratios between these two monomers are quite different. Copolymers containing mainly incidlyl units are expected to be formed at an initial stage of a reaction because lactide monomer is generally preferentially incorporated in the polymer chain [25]. However, as the reaction proceeds, especially with long reaction times, a monomer sequencing tends to randomize. The process takes place as a result of transesterification reactions, which play an important role in the redistribution of monomer sequences and affect a polymer chain microstructure [24; 29]. [PERSON] and [PERSON] for the first time defined two models of transesterification referred to as the first and the second modes. In the first transesterification mode, a cleavage of the polylactide block occurs between monomeric units and is connected to the appearance of a triad signals of CLLC. In the second mode of transesterification, an ester bond opening occurs between monomeric units as well as within lactidyl units [28]. Figure 3 shows the assignment of peaks in \({}^{13}\)C-NMR spectra of the copolymers synthesized in [bmim][NTf\({}_{2}\)] at 110 \({}^{\circ}\)C.  
The presence of the signals between 169.6-173.4 ppm indicates the occurrence of the intermolecular transesterification between caroyl (C) and lactydyl (LL) units of the polymer chains [30]. Since at the first reaction stage, mainly the first mode of transesterification proceeded, we recognized one transesterification type, referred to as the first mode. However, it is necessary to assume that the presence of the signal at 170.8 ppm, assigned as the carbonyl group of CLC triad (or CLLLC chain sequences at 170.3 ppm), resulted from the attack of active caroyl ends on the formed lactyl unit sequences (L), which could also indicate the presence of the second mode of the transesterification. However, by the end of the copolymerization, CLC triads were not detected, so we concluded that only the first mode of transesterification occurs at 110 \({}^{\circ}\)C. A similar effect was observed for [bmim][BF\({}_{4}\)] as a solvent (spectra not shown). Interestingly, in the analyzed microstructures of copolymers synthesized in ILs at 80 \({}^{\circ}\)C (samples A2, A5, B2, B5, with 0.50% and 0.67% CL in the feed), we observed low intensities of CLC sequences attributable to the second mode of transesterification (Figure 4). It is well known from the ROP of lactones and lactides that the catalyst or the initiator causes transesterification reactions at high temperatures, or with long reaction times. Moreover, two different types of transesterification also compete with each other: (1) intermolecular transesterification between different polymer chains and (2) intramolecular transesterification (back-biting) within the same chain [31]. Probably, higher temperatures led to the formation of cyclic polymer chains at first, so intramolecular transesterification are dominating. [PERSON] et al. revealed that macrocyclic products were initially formed and gradually disappeared as the reaction proceeded [25]. In a case with a lower temperature, in the presence of the high feed ratio of a more active monomer, we recognized the existence of both the first and the second modes of transesterification.