The synthesis of polyacrylonitrile PAN from acrylonitrile monomer typically involves a free radical polymerization process. Here is the step-by-step chemical reaction mechanism:1. Initiation:A suitable initiator, such as an organic peroxide or azo compound, is used to generate free radicals. The initiator undergoes homolytic cleavage, producing two free radicals.R-Initiator-R 2 R2. Propagation:The free radical R reacts with the acrylonitrile monomer, forming a new radical that is now part of the growing polymer chain.R + CH2=CH-CN R-CH2-CH-CNThe newly formed radical reacts with another acrylonitrile monomer, and the process continues, leading to the formation of a long polymer chain.R-CH2-CH-CN + CH2=CH-CN R-CH2-CH-CN-CH2-CH-CN3. Chain transfer:During the polymerization process, chain transfer reactions can occur, which limit the molecular weight of the polymer. Chain transfer can happen through various mechanisms, such as transfer to the initiator, transfer to monomer, or transfer to solvent.For example, transfer to initiator:R-CH2-CH-CN + R-Initiator-R R-CH2-CH-CN-Initiator-R + R4. Termination:The polymerization reaction terminates when two radicals react with each other, forming a covalent bond and ending the growth of the polymer chain. Termination can occur through two main mechanisms: combination and disproportionation.Combination:R-CH2-CH-CN + R'-CH2-CH-CN R-CH2-CH-CN-CH2-CH-CN-R'Disproportionation:R-CH2-CH-CN + R'-CH2-CH-CN R-CH2-CH-CN + R'-CH=CH-CNAfter the termination step, the polyacrylonitrile polymer is formed. The reaction conditions, such as temperature, initiator concentration, and monomer concentration, can be adjusted to control the molecular weight and polydispersity of the resulting polymer.