# Lastingly Colored Polylactide Synthesized by Dye-Initiated Polymerization
## 1 Introduction  
Biodegradable polymers are considered as a green alternative to protopolymers and a long-term solution for the environmentally damaging factor of plastics pollution [1; 2; 3]. In this class of commercially valuable polymers, polyesters are a prominent group. This set contains polylactide (PLA), which is currently the most important one on the market, because of the ecological profile of the industrial production, and also, it presents with a wide spectrum of various applications [4; 5]. PLA is often referred to as _double green_ because, apart from biodegradability, it is obtained from renewable raw materials. Among others, the most developing sectors are technologically advanced bioapplications of PLA for medicine and pharmacy [6; 7; 8; 9; 10; 11; 12]. However, it is necessary and at the same time still insufficient to search for new technologies for the production of environmentally friendly short-time commercial products for a sustainable future. Aromatic polyesters, mainly polyethylene terephthalate (PET) is dominant in the apparel industry and packaging applications; however, they are not readily degradable or recyclable [13]. In contrast, the not yet so popular PLA meets all the requirements for greenpolymers in terms of sustainability and degradation, which constitutes the key factors in the full cycle of eco-profile assessment of polymers using the LCA (Life Cycle Assessment) tool [4]. PLA is a material with a fairly high tensile strength; however, it is characterized by a quite high Young's modulus (little ability to deform and low impact strength). These, among other disadvantages, are often compensated by the application of various types of chemical and physical modifications [14; 15]. Chemical methods include the use of epoxy, anhydride, isocyanate, and carbodiimide (chain extenders) during processing [16], or the formation of various types of statistical, block, or grafted copolymers at the synthesis stage [17; 18]. In industrial practice, physical modification methods are much more popular by introducing various types of additives that lower the price (isotropic fillers), improve strength (anisotropic fillers), increase flexibility (plasticizers), or adjust the color of compounds (dyes and pigments) [19; 20]. One of the examples of a targeted application is the use of PLA in fabrics for apparel or filaments for 3D printing. PLA fiber is beneficial to the environment and is well described in the literature [21]. On the contrary, the classical process of coloration of PLA is not so green, and it requires improvement. The disperse dyeing of hydrophobic polymers such as PLA is performed in the presence of an appropriate dispersant, but that process is also not so ecological [21]. Dispersant-free dyeing with temporarily solubilized disperse dyes could extend the environmental friendliness. As a consequence, the proposed processes do not ensure color fastness or resistance to changing conditions during use, and they are prone to the uncontrolled release of dyes into the environment [21]. The alternative greener conception could be the ring-opening polymerization (ROP) of lactide (LA) by the \"dye-initiated polymerization\" method, which would be used for the synthesis of colored PLA, with tailor made properties. That method allows for the synthesis of well-defined PLA-Dye conjugates, which consist of PLA chains with precisely planned lengths and with dye end groups that were obtained in a controlled fashion. In products that were obtained in this manner, the dyeing would be much longer lasting than in classic physical dyeing systems, because there will be no adverse phenomenon of migration of dyes [22; 23]. The most important issue in the synthesis of PLA-Dye or other conjugates is the selection of a suitable catalyst.