**1. Introduction**

In the breathtaking development of polymers, the preparation of biocompatible polymers in the economic and environmental friendliness rout has achieved great interests. The synthesis of polymers is usually considered as macromolecular architecture that provides versatile materials in a different application. This includes sophisticated design by different polymerization or post-polymerization techniques [1]. Besides, polymeric materials provided by free radical polymerization, most of the time, the well-known commercial polymers were developed using metalbased catalysts. In this regard, hazardous residual metals and by-products arise as a precarious issue in the biomedical and electronic applications. The cost of precious rare metals, as well as the purification steps, increases the expenditure in large-scale production. In this context, metal-free polymerization by organocatalysts overcomes this obstacle and offers a variety of new synthetic strategies. N-heterocyclic carbenes (NHCs) are classified as one of the most reactive compounds in organocatalysis. In the late 19th and early 20th centuries, NHCs were described as reactive intermediates because the isolation of carbenes was not achieved [2]. N-heterocyclic carbenes have their roots back. Mizuhara et al. [3] reported, in 1954, a natural nucleophilic carbene existence was a catalytically active species of the coenzyme thiamine (**Figure 1**). Ever since the successful isolation of stable NHCs in the early 1990s [2], their contribution has been enlarged rapidly in synthetic chemistry. They

**Figure 1.** *Coenzyme thiamine.*

offer a variety of catalysis and reaction pathways. Besides, their estimated impact on organic synthesis, NHCs are considerable catalysis in the polymer chemist toolbox.

#### **1.1 Polymerization and organocatalysis**

In the history of chemistry reactions, catalysis was performed by enzymes and transition metal species. In yet, organocatalysis has emerged to play an integral part in catalysis systems. With regard to other catalytic systems, organocatalysis has been inescapable for many reasons. Beyond their derivation from a variety of organic reagents with plenty of chiral forms, organocatalysis systems are ecofriendly reagents having a low toxicity. Therefore, much of the molecular and macromolecular synthesis relies on it. They were developed to catalysis or initiate polymer synthesis for a variety of sensitive applications like biomedical application, food preserving or packaging, and sophisticated electronic species.

Mainly, polymerization is known to be performed by two categorically mechanisms chain growth and step-growth polymerization. Chain growth polymerization (CGP) is distinguished by the formation of reactive intermediate (anion, cation or radicals) throughout the initiation step. These reactive species transfer the reactive center by reacting with a monomer molecule which is called the propagation stage. The progress of polymer chains is contingent by the continuous reaction of monomer molecules with the formed active center until termination occurs by consuming the active center. In step-growth polymerization (SGP), polymerization starts with the reaction between two molecules that compromise two functional groups. Then another molecule reacts with the formed dimer and so on. Consequently, polymers chain formation depends on the reaction between molecules and/or the formed small chains [4, 5]. Although the difference between these two polymerization mechanisms, they all share using catalytic or initiating systems not only to establish a polymerization process but sometimes to design the macromolecule structure.

Various types of organocatalysts have been employed either in chain growth or step-growth polymerization. It is true that excessive use of organocatalysts was in chain polymerization, in particular, the ring-opening polymerization. However, very recently, many researchers were motivated to use organocatalysts in step-growth polymerization. Given the constantly similar nature of functional groups of ringopening polymerization, a true example of chain polymerization, with step-growth polymerization, it is nearly to have the same catalytic system for both polymerization mechanisms [6]. Across the field of metal-free polymer preparation catalysis, N-heterocyclic carbenes (NHCs) have affirmed the potential of organocatalysis. This will be presented by revealing NHCs capability to activate certain groups which impact the synthesis of metal-free polymers that are commercially important.
