**2. N-heterocyclic carbenes (NHCs) overview**

Nowadays, developing new polymeric material that possesses biocompatible properties has been strongly emerged. Using organic metal-free catalysts became an inevitable approach in today's environmental mindset. Hence, these catalysts can be easily removed from polymers unlike toxic metals in other types of catalytic systems. Carbenes, in particular N-heterocyclic carbenes (NHCs), are considered as a class of organic metal-free catalysts for different types of the polymerization process. Ever since the first successful isolation of N-heterocyclic carbenes by Arduengo, in the early 1990s, the chemistry richness of these compounds has been revealed in many applications. Their chemical structure can be described as heterocyclic moiety having at least one nitrogen atom and of course carbene carbon [7]. These neutral divalent species of carbon owing only four electrons have participated in σ-bonds and two remained at the central carbon. The presence of nitrogen atoms elevates the stability of carbenes by their ability of π donation to the empty carbon π orbital along with σ withdrawing (**Figure 2**).

This behavior leads to a huge gap of σ-pπ (**Figure 3**) that precedes the strong nucleophilic feature of NHCs. Nevertheless, some NHCs would have amphiphilic character. By substituting the amino with σ-donating alkyl group, an increase of electrophilicity and also nucleophilicity is observed. Also, the incorporation of carbonyl groups into the backbone augmented electrophilicity over the nucleophilicity as they compete with the carbene center for the π donation of the nitrogen atom [8].

Therefore, by studying NHCs ability to donate the electron pair (Lewis basicity) it was found that the triazole-ylidene is less nucleophilic by 103 than Imidazole and imidazoline-type (**Figure 3**). Many studies of proton affinity of NHCs, by evaluating the pKa of their conjugated acid, have been employed. They revealed the great impact of the electron-donating substituent on the nitrogen atom as well as the bulkiness of NHCs on their Bronsted basicity. Also, the increase from 5 to 6 membered ring increases the carbenes angles, and leads to an increase in pKa [9–11].

Besides, their distinctive coordination chemistry, N-heterocyclic carbenes have other advantages one of them is they can be easily be modulated bearing in mind the large library of heterocyclic chemistry as shown in **Figure 4**. However, several methods of preparation can be categorized in imidazolium deprotonation, imidazole-thione reduction, and NHCs-adducts thermolysis [12–16].

NHCs have been heavily exploited as ligands for transition metals [17–20]. However, their superiority in metal-free transformations is well recognized in organocatalytic chemistry [21] as well as in macromolecular chemistry [22, 23].

**Figure 2.** *Ground-state electronic structure of one class of N-heterocyclic carbenes.*

*Carbene*

**Figure 3.** *Energy (eV) of border orbitals of classical NHC.*

**Figure 4.** *Examples of N-heterocyclic carbenes polymerization catalysis.*
