# Recent Approaches to the Plasticization of Poly(lactic Acid) (PLA) (A Review)
## 2 Copolymerization with Flexible Polymers  
An effective method for modifying PLA is through the creation of its copolymers. Copolymerization with more flexible polymers makes PLA elastic. In some cases, creating such copolymers allow one to regulate the physicochemical characteristics and increase thermal stability, improve the rheological properties of the melt and impart greater biodegradability. It is important to note that, thanks to copolymerization, it is possible to create materials based on incompatible polymers when mixing results in weak interfacial  
Figure 2: Basic approaches to the plasticization of PLA. interaction [20]. To obtain copolymers of PLA with other copolymers, two main approaches are widely used: directional polymerization with the production of copolymers of a given composition and structure and graft-copolymerization by reactive mixing (as a rule, extrusion) in a melt in the presence of an initiator and/or catalyst. Reactive extrusion is a fast and cost-effective method commonly used for the functionalization of PLA or ring-opening polymerization (ROP) of lactic (for example, [21; 22]). PCL, a linear semi-crystalline polyester, is often used to improve the properties of PLA (structural formulas of the monomer unit are in Figure 3). In addition to the plasticization effect, an increase in biodegradability in the natural environment can be achieved due to the lower crystallinity and greater susceptibility to hydrolysis of PCL. A long-chain branched block copolymer (LB-PCLA) of PLA and PCL was synthesized in [23]. When comparing the synthesized copolymers with pure PLA and PLA/PCL blends, it was shown that elongation at breakup increased 30-fold. In addition, the copolymers had more pronounced shear-thinning and longer relaxation times, which can be explained by their long-chain branched structure. Similar patterns were found for (AB)\({}_{n}\) multiblock copolymers based on PLA and PCL obtained by the chain extension reaction with hexamethylene diisocyanate [24], and random copolymers of poly(L-lactide-co-\(\varepsilon\)-caprolactone) (PLCL) (50/50 wt%) [25; 26; 27]. The copolymers demonstrated increased elongation at breakup to 800%. An increase in the thermal decomposition temperature compared to pure PLA was also shown, which is an advantage for the industrial process. In all studies, no noticeable deterioration in tensile strength was observed. Figure 3: Structural formulas of PLA (**a**), PCL (**b**), poly(ethylene glycol) (PEG) (**c**), PBA (**d**), PBAT (**e**), PBS (**f**), block copolymer of poly(L-lactide-co-\(\varepsilon\)-caprolactone) (PLA-PCL) (**g**) [27], graft-copolymer of poly(lactic acid)-g-natural rubber (PLA-g-NR) (**h**) [18] and triblock copolymer of poly(D-lactic acid-co-ethylene glycol-co-D-lactic acid) (PDLA-PEG-PDLA) (**i**) [28]. Along with directed copolymerization, the technique of reactive mixing of PLA with other polymers is used to create PLA copolymers. For example, a copolymer with a ratio of 95:5 was obtained by reactive mixing of PLA with poly(ethylene glycidyl methacrylate) (EGMA) in the presence of a DCP initiator [29]. The PLA/EGMA copolymer had an elongation at a break 58 times higher than that of pure PLA while maintaining high tensile strength and transparency. Natural rubber (NR) can be used as a plasticizing additive in PLA [30; 31]. Despite the ability of NR to exert a physical plasticization effect on PLA, when creating PLA/NR mixtures, NR is distributed in the PLA matrix in the form of dispersed particles that demonstrate poor adhesion to PLA. There are known works on the creation of graft-copolymers poly(lactic acid)-g-natural rubber (PLA-g-NR) [18] by grafting malleated natural rubber (MNR) onto low-molecular weight PLA. Two types of copolymers were prepared to have anhydride moieties of 10 and 20 wt%, the addition of which to PLA resulted in a 200% improvement in impact strength and a two-fold percentage rise in elongation at break.