**2.1 Ring-opening polymerization (ROP)**

Ring-open polymerization has been devoted to developing interesting industrial polymers by synthesis of the analogs of natural as well as biocompatible polymers

#### *N-Heterocyclic Carbenes: A Powerful Catalyst for Polymerization DOI: http://dx.doi.org/10.5772/intechopen.102466*

by different methods. The sharp improvement in ROP is undoubtedly accelerated by organocatalysis. Mainly, organocatalysis of ROP proceeds according to four activation mechanisms; electrophilic monomer activation, nucleophilic monomer activation, base chain-end activation, or bi-functional activation mechanism. Both electrophilic and nucleophilic monomer activation starts by attacking the carbonyl group of the monomer to obtain a macromolecule that bears two ends having opposite charge starts what is called Zwitterionic ROP (ZROP) (**Figure 5**) [24]. However, they differ in their act for activating the carbonyl group. In electrophilic monomer activation, the carbonyl group is activated by protonation or H- bonding attachment that gives room for a chain end nucleophilic attack. While in nucleophilic monomer activation, the zwitterionic intermediate extends a deprotonation process of the alcohol. Then, the formed alkoxide proceeded with the acylation of the carbonyl group. Consequently, the catalyst is free to act again. The third activation mechanism is the chain-end activation where the nucleophilicity of the alcohol is elevated through deprotonation to form either alkoxide or H-bonding [6].

This chain-end attacks the carbonyl carbon triggering a ring-opening reaction to form an ester allowing the activated alcohol species to reform. The last mechanism for ROP is the bifunctional activation mechanism. It compromises activation of the monomer carbonyl carbon through electrophilic activation along with the activation of the chain end/initiator [25].

Ever since, knowing the benefits of NHCs in transesterification reactions [26–28], they were intensely employed in ring-opening polymerization (ROP). NHCs play a role in producing polymers with low disparities as they are able to provide living polymerization that control the polymer molecular weight. Furthermore, they facilitate the ROP for production of linear and cyclic aliphatic polyesters [29].

## **2.2 ROP of cyclic ester**

Thanks to Nyce et al. in 2002, through their navigation for an efficient nucleophilic catalyst, they discovered the effectiveness of NHCs as organocatalysts for ROP [28]. They also succeeded to polymerize cyclic monomers to deliver Poly (L-lactide) (PLA) (**Figure 6**), poly(ε-caprolactone) (PCL), and poly(b-butyrolactone) (PBL) with dispersity near to unity and definite chain ends which help to control the polymers molecular weight [26]. The polymerization was initiated by alcohols (benzyl alcohol or 4-(pyrene-1-yl)butan-1-ol) which provoke an α-end group address the ester from the initiating alcohol upon ring-opening a hydroxyl functional ω-chain end that propagates the chain. Hedrick's team first suggestion for the transesterification reaction mechanism was activated monomer mechanism. Considering the steric effect and the higher pKa of the alcohol compared to the conjugated acid of NHC in DMSO, deprotonation of less acidic alcohol by NHC is unlikely the beginning step of the catalysis act. Therefore, they assumed a direct attack of the monomer by the nucleophilic NHC to form a zwitterionic intermediate that interacts with the other monomer molecules pursued by the reaction with alcohol. Another initiation mechanism proposed by the theoretical study assumed the occurrence of an active chain-end mechanism. Lia *et al.* suggest the hydrogen bonding between NHC and alcohol, then deprotonation of alcohol which attacks the cyclic monomer [30]. This assumption was based on the lower energy of the H-bonded adduct than the zwitterionic intermediate. Several studies follow to

**Figure 6.** *Ring-opening polymerization of L-lactide through path: (A) monomer activation mechanism and (B) activechain end mechanism.*

#### *N-Heterocyclic Carbenes: A Powerful Catalyst for Polymerization DOI: http://dx.doi.org/10.5772/intechopen.102466*

find out the predominant mechanism. This dispute most likely has been resolved by Patel *et al.* manifesting the ability of NHC to act as bifunctional catalysts in the presence of alcohol and the two mechanisms are likely participating in [31].

The catalytic behavior NHCs in the absence of alcohol was investigated. At a relatively high LA concentration and ambient temperatures, a very fast polymerization was reported (5 s–900 s) yielding a cyclic polymer. In this case, NHC acts as an initiator that generates zwitterionic intermediate by a direct nucleophilic attack of NHC to the LA monomer. The ring-closure occurred by trapping the NHC within a zwitterionic NHC–CS2 adduct.

Engaging the spirit of the suggested mechanism of cyclic esters polymerization, remarkable turnovers were observed for the ROP of a variety of other cyclic monomers including cyclosiloxanes, epoxides, and N-carboxyanhydrides. NHCs proved extreme activeness, although the usage of low concentration and temperature.
