# Recent Approaches to the Plasticization of Poly(lactic Acid) (PLA) (A Review)
## 2 Copolymerization with Flexible Polymers
Based on these results, it was also found that PLA-g-NR can improve the properties of PLA to a greater extent than NR due to its higher compatibility. Numerous studies have shown the effectiveness of using poly(ethylene glycol) (PEG) to create highly flexible PLA copolymers. PEG is a polyether compound and has exceptional elasticity. Among various plasticizers and impact modifiers, PEG is one of the most effective due to its wide range of molecular weights, non-toxicity, miscibility and biodegradability (for example, [32]). PEG plasticization can effectively increase the chain mobility of PLA as well as improve its ductility and stretchability, thereby expanding the range of potential applications. Using biodegradable block-copolymers with both hydrophilic and relatively hydrophobic functions allows one to create flexible, partially hydrated and biocompatible materials for medicine. The creation of PLA-PEG copolymers gives the possibility of controlling the properties of PLA more effectively. Directionally synthesized via ring-opening polymerization, poly(l-lactide)-b-polyethylene glycol-b-poly(l-lactide) (PLLA-PEG-PLLA) in the presence of an extender chain exhibited greater flexibility than PLLA thanks to the flexibility of the PEG middle blocks [33]. [PERSON] et al. [34] and [PERSON] et al. [28] showed similar patterns for PLLA-b-poly(ethylene glycol)-b-PLLA (PLLA-PEG-PLLA) block copolymers synthesized by polymerization of PLA with ring-opening and chain extender based on epoxy resin. In his work, [PERSON] synthesized triblock copolymers of PEG (block B) type ABA in combination with PLLA (blocks A) with different molecular weights of PEG and different degrees of polymerization of PLLA [35]. The works showed the positive effect of the chain extender on the plasticization of PLA, so PLLA-PEG-PLLA samples with an extended chain did not have phase separation. As the length of the PEG blocks increased, the elasticity of the materials rose, the hardness decreased, and water absorption increased. In another work, the graft-copolymerization of low-molecular-weight poly(ethylene glycol) acrylate (PEGA) to PLA through reaction mixing was successfully carried out. The presence of acrylate PEG grafted onto PLA resulted in a decrease in the glass transition point by more than 20 \({}^{\circ}\)C and an increase in elongation at the breakup from 4.7% to 17.9% [36]. PBS is another important biodegradable thermoplastic polyester available in the market. There are known works on the creation of graft-copolymers of PLA with PBS using reactive mixing in the presence of oligo(butylenesuccinate) and an initiator [19]. The introduction of flexible PBS chains reduced the glass transition point, leading to a decrease in hardness and elastic modulus. It is important to note that it is proposed to use PLA-co-PBS copolymers as individual polymers and as additives that improve compatibility in these PLA-PBS blends, which leads to decreased interfacial tension and improved impact strength and elongation. In order to impart increased plasticity to PLA, polyesters containing flexible adipic acid units, such as PBSA [14] and poly(propylene adipate) [15; 37], are widely used. As a rule, the synthesis is carried out by the ring-opening polymerization of lactides in the presence of oligomers of the modifying component, resulting in block-copolymer formation. It was shown that the introduction of flexible adipic acid units successfully increased the elongation and the rate of PLA biodegradation.