*3.2. Themes and Sub-Themes*

## 3.2.1. Synthesis and Characterisation of PCL-PEG Copolymers

Seven studies synthesised the PCL-PEG copolymers, except for Peng et al., which used pre-synthesized PCL-PEG copolymers. The copolymers were synthesised by using ring-opening polymerisation (ROP) of ε-caprolactone (ε-CL) [35–41]. In most studies, the PEG with the hydroxyl end group was used to initiate the ring opening of ε-CL in the presence of the catalyst stannous octoate (Sn(Oct)2) and under a nitrogen atmosphere (Figure 2) [35–37,39–41]. However, Mahdaviani et al., initiated the ROP of ε-CL by utilising N-protected 3-aminopropan-1-ol, and PCL-PEG copolymer was synthesised via an amidation reaction between the -COOH groups of PCL and the -NH3 end of PEG [38]. In addition, Zamani et al., added folic acid (FA) as a molecular probe to the synthesised PCL-PEG copolymers for targeted cancer treatment in the form of folate-lysine-PCL-PEG (FA-L-PEG-PCL) (Figure 3). To ensure sufficient conversion of ε-CL to PCL, the reaction was heated at 110 ◦C for 24 h [38]. However, most studies conducted the reaction at 120 ◦C to shorten the reaction period to 12 h. Various molecular weight copolymers were produced

by manipulating the ε-CL: PEG ratio. The summary of the reaction condition is tabulated in Table 4.

**Figure 2.** Schematic route for the preparation of PCL-PEG copolymers.

**Figure 3.** Preparation of (**A**) FA-L-PEG-OH and (**B**) FA-L-PEG-PCL. Reprinted/adapted with permission from Elsevier [40].

*Polymers* **2022**, *14*, 4847


**Table 4.** Summary of data obtained from the synthesis and characterisation of PCL-PEG copolymers.


vacuum-dried

Each study employed Fourier transform infrared spectroscopy (FTIR), proton nuclear magnetic resonance spectroscopy (1H NMR), and gel permeation chromatography (GPC) to characterise the synthesised copolymers. The successful formation of PCL-PEG copolymers in the studies was indicated by the sharp and intense bands around 1720.0 cm−<sup>1</sup> and 1100.0 cm−<sup>1</sup> of the FTIR analysis, representing the carboxylic ester (C=O) and ether (C–O) groups of the PCL-PEG linkage. The NMR analysis from the studies revealed that the peaks around 2.2–2.5 ppm of methylene proton –OCCH2- and 4.06 ppm of methylene proton –CH2OOC– belongs to the linkage between the PCL and PEG, which confirmed the formation of PCL-PEG copolymers. Mahdaviani et al., found that the synthesised copolymers had a 1:1 ratio of PCL and PEG by assessing the peak area associated with the PEG methylene group to that of PCL [38]. The ratio of the copolymers can be determined by equating the peak integration area at ~4.06 ppm (PCL block) to that at ~3.6 ppm (PEG block) [39]. Meanwhile, the number average molecular weight (Mn) of the PCL-PEG copolymers can be determined based on the equation:

$$\mathbf{M}\_{\mathbf{n}} = (\mathbf{1} + \mathbf{R}) \times \text{PEC}$$

where R = the ratio of the integration area of PCL:PEG and PEG = the number average molecular weight of PEG.

GPC analysis from all studies showed a polydispersity index (PDI) closer to one (<1.4), indicating the uniform distribution of the polymerisation of PCL-PEG copolymers. Hu et al., also stated that the study's unimodal curve of the GPC chromatogram suggested no impurities in the polymers [39].

Thermal analysis of the synthesised PCL-PEG was done using differential scanning calorimetry (DSC) analysis. The DSC thermograms in each study recorded an endothermic peak for the melting point of the copolymers. The melting transition temperatures of PCL and PEG homopolymers that were greater than those of respective blocks in copolymers indicated the interaction between the PCL and PEG blocks [39]. In addition, the formation of two peaks during the cooling process in the thermogram was most likely due to the length of the PCL block, which was half of the PEG block. The PEG block's higher crystallisation temperature compared to the PCL block resulted in the appearance of two peaks in the thermogram. However, when the PEG block's molecular weight and the PCL block's molecular weight were close enough, their peaks merged and became exothermic peaks. The fact that the crystallisation temperature of PEG was more significant when the block length was longer supports the conclusion that crystallisation temperature is positively related to the block length [39]. The same pattern was observed by Kheiri Manjili et al. [35–37,41] where the DSC thermogram copolymers showed two merged endothermic peaks of PCL and PEG (Figure 4). The characterisation of PCL-PEG copolymers is summarised in Table 4.

**Figure 4.** DSC thermogram of (a) CUR (b) mPEG-PCL, and (c) CUR/mPEG-PCL micelles. Reprinted/adapted with permission from Elsevier [35].
