In cyclic polymerisation, alkyne molecules add to each other resulting in the formation of a cyclic compound. For eg. when ethyne is passed through a red hot iron tube at 873 K it undergoes cyclic polymerisation. In cyclic polymerisation, three molecules of ethyne polymerise to form benzene

Cyclic polymerization of ethyne, also known as acetylene, involves the formation of a cyclic polymer by linking multiple ethyne monomers together. This process occurs through a series of chemical reactions and is often catalyzed by a transition metal catalyst.

The Process of Cyclic Polymerization of Ethyne:

Polymerization Initiation:
- The cyclic polymerization of ethyne begins with the initiation step, where a transition metal catalyst initiates the reaction by coordinating with the ethyne molecule.
- Common catalysts used in this process include nickel, palladium, or copper complexes.
- The catalyst facilitates the formation of a metal-alkyne complex, which is an intermediate compound necessary for the subsequent steps.

Monomer Activation:
- Once the metal-alkyne complex is formed, the monomer activation step takes place.
- In this step, the metal-alkyne complex undergoes a ligand exchange reaction, where one or more ligands attached to the metal center are replaced by the ethyne molecule.
- This process activates the ethyne monomer by making it more reactive and prone to undergo polymerization.

Polymerization Propagation:
- The activated ethyne monomer can now participate in the polymerization propagation step.
- During this step, the activated monomer reacts with another ethyne molecule to form a new metal-alkyne complex.
- The metal center coordinates with the new ethyne molecule, leading to the formation of a longer polymer chain.
- This process continues as more ethyne monomers react with the growing polymer chain, resulting in the formation of a cyclic polymer.

Polymerization Termination:
- The cyclic polymerization reaction can be terminated through several mechanisms.
- One common termination mechanism is the addition of a terminating agent, such as a small molecule or a reactive species, to the growing polymer chain.
- This terminates the polymerization process and prevents further growth of the polymer.
- Alternatively, the termination step can occur through the coordination of a ligand to the metal center, which disrupts the catalytic cycle and stops the polymerization reaction.

Overall Reaction:
- The overall reaction for the cyclic polymerization of ethyne can be represented as follows:

n C2H2 + Catalyst → (C2H2)n

Applications:
- Cyclic polymerization of ethyne is a valuable process in the synthesis of various cyclic polymers, which have unique properties and applications.
- These polymers can be used in the production of specialty materials, such as high-performance fibers, coatings, adhesives, and electronics.
- The ability to control the polymerization conditions and catalyst choice allows for the customization of the polymer's properties, such as its molecular weight, structure, and thermal stability.

In conclusion, cyclic polymerization of ethyne involves the coordination of ethyne monomers with a transition metal catalyst, leading to the formation of a cyclic polymer. This process proceeds through initiation, monomer activation, polymerization propagation, and termination steps. The resulting cyclic polymers have various applications in different industries.