## 3 The Advantages of Polycaprolactone for Advanced Polymer Architecture Formation  
Since the discovery of the first synthetic polyesters in the 1930s, poly(\(\varepsilon\)-caprolactone) (PCL) has remained one of the most important and widely studied biodegradable polymers in various fields. PCL is formed by a saturated aliphatic polyester chain with repeating hexanoate units. This semi-crystalline polymer has varying molecular weights, typically ranging from 3000 to 800,000 g mol\({}^{-1}\). At higher molecular weights, crystallinity is reduced due to chain folding, resulting in a crystallinity of 33% at Mw = 200,000 g mol\({}^{-1}\), which can reach 70%. In addition to this variation in crystallinity, degradation by bacterial and fungal enzymes is one of the parameters that has highlighted the interest in PCL, making it particularly attractive for biodegradable material applications. In addition to the expected esterase degradation, there is also evidence of PCL sensitivity to enzymatic degradation by lipases [78]. It should be noted that several parameters affect the degradation of PCL by hydrolysis, such as the rate of crystallization and the number of carbon atoms in the chain. These factors are known to affect the hydrolysis rate of esters under physiological conditions, which decreases significantly as chain length increases. PCL chains exhibit an elastic behavior at room temperature due to their very low glass transition temperature, which can be below \(-70\)\({}^{\circ}\)C. In addition, the relatively low melting temperatures of around 60 \({}^{\circ}\)C make PCL materials easy to manufacture or transform into highly structured forms [79]. If we examine the production of scientific research regarding PCL applications, we can find that PCL-based materials are increasingly being used as specific drug release vectors in cell culture, regenerative medicine implants, and drug release [80]. In addition, the rheological properties of PCL have proven very useful for studying the principles of crystallization of complex and poorly understood polymers [81]. Although it is possible to produce PCL by the direct condensation of 6-hydroxycapric acid, the most common method for large-scale synthesis of high molecular weight and low dispersity polymers is the ring-opening polymerization (ROP) of \(\varepsilon\)-caprolactone. However, there are reports that have used enzymatic means to produce PCL with a molecular weight of up to 10,000 g mol\({}^{-1}\). ROP polymerization chemistry of PCL largely mirrors that of other common lactones, and a great deal of research has been performed to optimize procedures for this polymerization. The most commonly used method involves using a metal-based catalyst to polymerize \(\varepsilon\)-caprolactone through a coordination-insertion mechanism (Figure 2) [82; 83]. Tin-based metal complexes (tin octanoate II) are particularly effective in inhibiting retro-inhibition and enabling the production of PCL chains with very high molecular weights up to 800,000 g mol\({}^{-1}\) and a polydispersity of 1.1. These have certainly been widely used in ROP lactone polymerization chemistry. Obviously, PCL polymerization is a very rich field where efforts are being made to optimize reactions by using specific mechanisms with either organisms or transition metal-based or rare earth-based catalysts to increase yield and effectiveness. Synthetic methods for PCL have been more extensively covered in recent reviews [84]. The combination of orthogonal polymerization reactions, such as ring-opening polymerization, ATRP, NMP, RAFT, and click conjugation, has allowed for the development of complex polymeric architectures based on PCL. This has led to the observation of very attractive physical-chemical properties of this polymer.