**2.6 ROP of anhydride**

N-carboxyanhydrides have been polymerized using NHC to produce linear poly(-peptoids) in THF [35]. One of the biggest advantages of this polymerization is the ability to prepare a definite structure with low molecular weight distributions in the range of 1.04–1.12and the molecular weight ranges (3000–40,000 gmol−1). The authors found that small N-substituents of NHC enhance the reaction rate. They also revealed that the control of molecular weight is strongly dependent on the solvent and the NHC structure. The mechanism of the polymerization followed the ROP mechanism under the loss of CO2. Side reactions are significantly suppressed in low dielectric solvents due to the reduced basicity and nucleophilicity of the negatively charged chain ends of the zwitterions, resulting in quasi-living polymerization behavior.

## **2.7 Step-growth polymerization**

Virtually all high-performance polymers (80%) that are currently utilized are products of chain-growth polymerization along with step-growth polymerization. The top valued polymers, polyether ketones, polysulfones, polyimides are stepgrowth polymerization products. Normally, step-growth polymerization (SGP) compromises the reaction between two different bi-functional groups that might present in one monomer or two different monomers. Amidation, esterification, nucleophilic aromatic substitution, transesterification, and urethane formation with isocyanates are the conventional reaction in step-growth polymerization. They almost proceed with the high conversion that is suitable for polymerization. However, the hard condition, high pressures and temperatures, and side reaction leading to monomers decomposition and limiting the molecular weight [6]. Therefore, almost all step-growth polymerizations require a catalyst to increase the rate of reactions and consequently reduce the potential side reactions.

(NHCs) have been used in step-growth polymerization to achieve high molecular weight polymers. Mostly**,** they were in-situ developed through deprotonation of imidazolium salts with a base.

Bearing in mind their potential in transesterification reaction, NHCs catalysis was implemented in step-growth polymerization of 6-hydroxyhexanoate, bis(2 hydroxyethyl) terephthalate as well as the polycondensation of dimethylcarbonate (DMC) and a number of diols.

Hedrick et al. polymerized bis(2-hydroxyethyl) terephthalate using only NHCs as a catalytic agent in THF. The polymerization process accomplished almost full conversion within one hour at 250°C. They also, succeeded to prepare aliphatic polyesters by polytransesterification reactions of ethyl 6-hydroxyhexanoate and ethyl glycolate [28]. Poly-(6-hydroxyhexanoate) with dispersity of 1.57 and Mn of 21,000 gmol−1was obtained by carrying out the SGP at 60°C for 24 h. The polymer in 95% yield was obtained by removing EtOH at low pressure. By this procedure, polyesters (with Mn ranging from 8000 to 20,000 gmol−1) were similar to poly(εcaprolactone) (PCL) and poly-(glycolide) synthesized by ring-opening polymerization (ROP).

NHCs activate the monomers by attacking their carbonyl carbon. This feature was also implemented to prepare a variety of industrial polymers. Plasseraud et al. reported their success to prepare metal-free aliphatic polycarbonates [36]. Dimethylcarbonate and diols in molar mass equal 3:1, respectively, were reacted in bulk at 150°C under reduced pressure. The reactions were conducted at 100°C for 15 min in the first stage to liberate the active NHC by decarboxylation of the NHC– CO2 adduct that was used as precatalyst. Thereafter, the temperature was elevated to 150°C for one hour under reduced pressure to remove methanol which forceful the polymer formation. Random copolymer with moderately controlled molecular weight distributions and molecular weight (19,000 gmol−1) and homopolymers were produced. Employing a molar equivalent 1: 2 of DMC and aliphatic diols, respectively, hydroxy-terminated polycarbonates could also be achieved.

Umpolung reactions have their influence on polymer chemistry. The benzoin condensation reaction motivated Pinaud, et al. to synthesis polybenzoin [37]. In this case, the carbonyl group in bis-aldehyde is activated by NHC in THF or DMSO at 40°C to form alkoxide that triggers the formation of "Breslow intermediate". This intermediate attack the electrophilic carbon of another aldehyde molecule (**Figure 9**). Thereafter, C-C bond formation leads to the step-growth polymerization of bis-aldehyde and cyclic polymers by-products.

In another pathway, NHCs have been used for activation of the alcohol for developing interesting polyurethane (PU) from isocyanates and polyols reaction. A study performed by Bantu et al. showed that the order of addition is a key for successful formation of PU [38, 39]. Hence, first, the alcohol was deprotonated

**Figure 9.** *The proposed mechanism of the step-growth polymerization of bis-aldehyde.* by the NHC before the addition of the di-isocyanate monomer. In this investigation, the synthesis of cross-linked polyurethanes was conducted in CH2Cl2 at 60–70°C affording in-situ generation of NHC catalyst from NHC–CO2 adducts. The resulting alkoxides from the reaction of NHC catalyst and ethylene glycol or polyol in a 1/1 ratio at 70°C were detected quantitatively by 1H NMR analysis. The C2H imidazolium proton and pyridinium proton were detected confirming the proposed mechanism of alcohol activation. Not only the order of addition of reactants is vital but also the nature of the diisocyanate monomer. Coutelier et al., found that when linear aliphatic diisocyanates are employed, soluble, linear PUs (2000–5000 gmol−1) might be derived [40] otherwise crosslinked PU is formed. The SGP polymerizations were carried out in THF using 1 mol% catalyst relative to monomer between 30 and 50°C. The 1/1 ratio was employed for a selected diol and two aliphatic diisocyanates (isophorone diisocyanate and 1,6-diisocyanatohexane). Despite the potency of NHCs as catalysts for the cyclo di or trimerization reaction of phenyl monoisocyanate (70% cyclodimer and 30% cyclotrimer) [41], traces of such uretdione or isocyanurate were detected with alkyl isocyanates. This provides another confirmation of the alcohol activation through H-bonding before nucleophilic addition onto the isocyanate species.

This activation mechanism was utilized by Marrot et al*.* for the polycondensation of disilanols [42]. In a closed schlenk tube, α,ω-Dihydroxy oligodimethylsiloxanes was mixed with a catalytic amount of isolated NHCs at 80°C for 16 h to yield almost 90%. Interestingly, the water released from the dehydration of the silence did not depress the catalytic activity of NHC. The hydrophobic nature of the developed polydimethylsiloxane seems to prevent direct contact with NHC. Nevertheless, removing the produced water leads to increasing molecular weights of the resulting silicone polymers. This observation suggests another role for NHCs as a catalyst for depolymerization reactions in the presence of H2O. Therefore, the catalytic amount of NHC and water withdrawal have an effect on regulating the produced polymer molecular weight.
