# Lastingly Colored Polylactide Synthesized by Dye-Initiated Polymerization
## 1 Introduction
The industrial standard for the production of PLA is Sn(Oct)\({}_{2}\), which operates for lactide polymerization in bulk conditions at elevated temperatures (110-180 \({}^{\circ}\)C) and, which is worth underlining, causes the number of transesterification reaction acts, so the distribution of molar masses is broad. Similarly, it is the most popular choice in conventional studies involving the ROP of cyclic esters. The versatility of this catalytic system is obvious; however, it induces also disadvantages and needs some improvements. The problem is particularly visible for the synthesis of polymers with low or ultra-low molecular weight and for those with functional or bulky end groups. Therefore, the synthesis of PLAs with well-defined properties matched to specific applications in the presence of commercial Sn(Oct)\({}_{2}\) is difficult or, in some cases, impossible. Although Sn(Oct)\({}_{2}\) is approved by the FDA as a food additive, the tin content in polymers should be lower than 20 ppm, as it has been found to have some cytotoxicity [24; 25; 26; 27]. For example, the soluble tin compounds such as Sn(Oct)\({}_{2}\) can be harmful at the nutrition level even at the concentration of 0.1%. In this context, the polymer obtained in the presence of tin catalyst should be handled with great care in biomedical applications. It is connected with the fact that the bulk polymerization promoted by Sn(Oct)\({}_{2}\) implies that the catalyst residue remains inside the polymer material. On the other hand, the Scientific Panel on Contaminants in the Food Chain (at European Food Safety Authority) established a group-tolerable daily intake of 0.25 \(\mu\)g per kg of body weight for tri- and di-alkyl tin compounds [28]. This is an issue from a toxicity point of view, and the catalysts residue can also dramatically alter the polymer properties during thermal treatments at higher temperatures, thermo-modeling, or extrusion, for example [29; 30; 31; 32]. Such rigorous requirements concerning PLA, especially for biomedical applications, are the reason for the constant search for new biocompatible lactide ROP catalysts. Therefore, even if the results obtained with Sn(Oct)\({}_{2}\) are to be valued, new catalysts operating in solution under mild conditions are highly desirable to develop the further use of functional polyesters. In this aspect, the rational alternative to commercial catalysts is the catalytic systems based on biomedical coordination compounds that are suitable to a given targeted application. In this context, group 1 and 2 metals, as well as zinc,are still promising candidates, and here, the most perspective and effective are single-site initiators. The synthesis of PLA-Dye conjugates is possible by using alternative appropriately designed ROP initiators, which are based on the coordination compounds of metals with a single-site motif L-M-OR (L: ancillary ligands, OR: initiating group) [33; 34; 35]. In such a system, dye molecules could be the fragment of the ancillary ligands or the initiating group. Moreover, this approach requires the synthesis of a new initiator for every used dye molecule. The proposed better solution here is based on the use of bifunctional catalytic systems with homoleptic coordination metal compounds, which contain an external initiating group. Catalytic systems containing homoleptic simple aminophenolate metal compounds are selectively able to produce linear alkylesters or polyesters from extra-low to high molecular weight polymer systems [36; 37; 38; 39; 40]. For this purpose, we studied our new binary catalysts composed of the (L\({}^{\text{dmp}}\))\({}_{2}\)M compounds (L\({}^{\text{dmp}}\)-2,6-dimethylpiperidine (dmp), aminophenolate ancillary ligands, M-Zn, Mg), and the dye molecule possessing the hydroxyl functionalization DR1/13 (Scheme 1). Herein, we report a new catalytic system containing precisely design ancillary ligands that prevent the metal center against aggregation, and additionally, ligands redistribution reactions in the presence of the bulky external initiating group.