## 3 Results  
Table 1 shows the solubility parameters that are associated with the 1% DVB crosslinked PS and each ether used in the precipitation step after cleavage. It should be noted that the solubility parameter values are additive components [36]. Thus, the solubility parameter value of the ethers was revised based on the percentage of the ether in the final mixture [ether-TFA (5:1)] (Figure 2). It is important to highlight that, when the ethers are considered alone (Table 1), the solubility parameters diverge more from the corresponding of PS. SEM was used to monitor the morphological structure of the CTC resin at several stages: (i) before starting the synthetic process (native resin); (ii) after the synthetic process (protected peptide anchored on the resin); (iii) after the cleavage step and the addition of the chilled precipitating ether (resin and \begin{table}
\begin{tabular}{c c c} \hline
**Entry** & **Resin/Solvent** & **Solubility Parameter (cal\(\text{{\bf cal}}^{\text{1/2}}\text{ cm}^{\text{-3/2}}\))** \\ \hline
1 & 1\% DVB crosslinked PS & 9.1 [9; 31] \\
2 & TFA & 12.5 [32] \\
3 & DEE & 7.6 [7] \\
4 & CPME & 8.4 [33] \\
5 & MTBE & 7.4 [34] \\
6 & 2-MeTHF & 8.5 [33; 35] \\ \hline \end{tabular}
\end{table} 
Table 1: Solubility parameters of 1% DVB crosslinked polystyrene (PS) and the ethers used in this study. Figure 2: Solubility parameters of PS and ethers used in SPPS protocol. Solubility parameter of each ether was revised based on the ratio of each mixture, [ether-trifluoroacetic (TFA) (5:1)]. the cleaved peptide); and, (iv) after the regeneration step. SEM analyses are interpreted in terms of the solubility parameter of 1% divinylbenzene (DVB) crosslinked PS and the corresponding ethers, in which, the values must be compared to estimate the resin-solvent compatibility and interpret the SEM data accordingly. The dried resin beads of each mentioned stage were coated with a thin layer of gold using Quorum Q150 RES sputter coater (Quorum Technologies Ltd, East Sussex, UK), and photographed using SEM, where the electron beam was set to 5.00-6.00 kV (Zeiss ultra plus FE-SEM, Jena, Germany). The SEM images proved that the full elongation process itself has no effect on the morphology of the beads. Thus, the morphological characteristics of the beads before the cleavage step was similar to the native resin before the synthetic process starts (Supplemental Figure S2A,B). CTA resin (with no previous syntheses) was regenerated/activated and the SEM images have also proved the absence of the osmotic shock phenomenon. Thus, the morphological properties of its beads were not affected (Supplemental Figure S2C), which is apparently attributed to the closeness between the solubility parameters of the activation solution (SOC\({}_{2}\) in dichloromethane (CH\({}_{2}\)Cl\({}_{2}\)), 1:1) (9.6 cal\({}^{1/2}\)cm\({}^{-3/2}\)) [6] and that of the resin (9.1 cal\({}^{1/2}\)cm\({}^{-3/2}\)) [9; 31]. From a theoretical point of view and based on the solubility parameters values (Figure 2), CPME and 2-MeTHF could be considered as good solvents of PS (CTC) resin due to their close solubility parameters (9.1, 9.2 and 9.1 cal\({}^{1/2}\)cm\({}^{-3/2}\), respectively). On the other hand, DEE (8.4 cal\({}^{1/2}\)cm\({}^{-3/2}\)) and MTBE (8.2 cal\({}^{1/2}\)cm\({}^{-3/2}\)) are considered to be \"bad\" solvents of CTC resin due to the high differences in their solubility parameters. Accordingly, the chance of osmotic shock occurring upon the introduction of DEE or MTBE as precipitating ethers is expected to be higher than in the case with CPME or 2-MeTHF. The SEM images have shown that CTC resin has suffered from the osmotic shock to different extents upon treating it with three of the precipitating ethers used in this study: DEE, MTBE (Figure 3A,B), and, surprisingly, 2-MeTHF (Figure 3C).