## 3 \"Click\" Chemistry
### Thiol-Ene Click Reactions in Monolithic Column Functionalization  
[PERSON] _et al_. prepared poly(GMA-_co_-EDMA) monolithic columns for chiral selector modification (see Figure 4) [87]. The column was successfully used in the separation of _N_-3,5-dinitrobenzoyl-leucine enantiomers, however, relatively low efficiencies were obtained (figures not published) and attributed to the inherent macro-porosity of the monolithic porous structure. Later, the monoliths were used for thiol functionalization [50]. The available content of epoxide groups was only 10%, due to the incorporation of the large majority of epoxides within the monolith structure, as reported by [PERSON] _et al_. [44]. The column was employed for the separation of enantiomers of mefloquine tert-butylcarbamate, with the highest efficiency obtained for an eluent containing 20% methanol (MeOH) (>47,000 N/m).  
[PERSON] _et al._ produced poly(isocyanoethyl methacrylate-_co_-methyl methacrylate-_co_-EDMA) monoliths (poly(IEM-_co_-MMA-_co_-EDMA)) to facilitate modification using hydroxyl, amine, or thiol functionalities [88]. The longer the alkyl chain length of the respective modifier, the lower was the ligand density. Later, using poly(GMA-_co_-EDMA) monoliths, thiol-ene reactions were performed in both UV and thermal initiated addition reactions. A reversed-phase column (LMA) and a hydrophilic interaction liquid chromatography (HILIC) column (N,N-dimethyl-N-metacryloxyethyl-N-(3-sulfopropyl) ammonium betaine, MEDSA) were produced upon thiolated columns [89]. The UV initiation method outperformed the thermal initiation with a ten-fold increase in plate count for uracil (3,000 N/m to 30,000 N/m, respectively), and a lower pressure drop across the column. A zwitterionic column was prepared using a betaine type methacrylate providing an efficiency of 19,000 N/m for guanosine in the  
\begin{table}
\begin{tabular}{l l l l l l} \hline \hline \multicolumn{1}{c}{**Monolith**} & \multicolumn{1}{c}{**Reaction**} & \multicolumn{1}{c}{**\(\lambda\)**} & \multicolumn{1}{c}{**Temperature**} & \multicolumn{1}{c}{**Time**} & \multicolumn{1}{c}{**Reference**} \\ \hline \hline
**Poly(proparyl)** & Click addition of 1-azidootacane and & & N/A & 30 & 120 h & [91] \\
**methacrylate-_co_-EDMA)** & 1-azidootadecane using a Cu(1) catalyst. & & & & \\ Poly(GMA-_co_-EDMA) & Azide modified surfaces clicked with & & & & \\ poly(VBC-_co_-DVB) & 1-decyne using a Cu(1) catalyst. & N/A & 30 to 60 & 48 h & [92] \\ \hline
**Poly(3- & & & & & \\ (Trimethoxysly)propyl & Active surface modified with cinnidyl & & & & \\ acrylate-_co_- & azide or & & N/A & 84 & 16 h & [93] \\
**propagaryacrylate-_co_-AMPS- & 6-azido-6-deoxy-beta-cyclodextrintrim. & & & & \\ _co_-TRIM-_co_-PETRA)** & & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 2: _Cont._  
Figure 4: Schematic of a thiol-ene click reaction, with radical (AIBN) addition of O-9-tert-butylcarbamoyllquinine to the available surface bound thiol groups. Reprinted from [87]. Copyright (2004), with permission from Elsevier.  
separation of nucleotides, which was greater than the column prepared by direct co-polymerization of MEDSA and EDMA [30].  
[PERSON] _et al_. [90] prepared a poly(NAS-_co_-EDMA) monolithic column, modified with allylamine, after which thiolated oligoethylene glycol (OEG) was grafted to the column via surface alkene groups, _i.e._, thiol-ene clicking. A yield of 50% was achieved due to the large size of OEG units. The column was used in the separation of DMF, acrylamide, and toluene and showed dual retention mechanisms dependent on the percentage of organic modifier in the mobile phase, typical of HILIC columns. Additionally, a reversed-phase (RP) column was prepared using the thiol-ene click pathway, resulting in a C\({}_{18}\) bonded stationary phase.