Free radical polymerization of vinyl chloride is a chain-growth polymerization process that leads to the formation of polyvinyl chloride PVC with high molecular weight. The mechanism involves three main steps: initiation, propagation, and termination.1. Initiation:The initiation step begins with the formation of a free radical, which can be generated by various methods such as heat, light, or chemical initiators like peroxides or azo compounds. For example, when using a peroxide initiator, it decomposes into two radicals:R-O-O-R 2 R-O where R is an alkyl group The generated radical R-O then reacts with a vinyl chloride monomer, breaking the double bond and forming a new radical:R-O + CH2=CHCl R-O-CH2-CHCl2. Propagation:The newly formed radical reacts with another vinyl chloride monomer, breaking its double bond and generating a new radical:R-O-CH2-CHCl + CH2=CHCl R-O-CH2-CHCl-CH2-CHClThis process continues, with the radical reacting with more vinyl chloride monomers, leading to the growth of the polymer chain. The propagation step is highly exothermic, and the reaction rate depends on the concentration of the radicals and the monomers.3. Termination:The polymerization process eventually terminates when two radicals react with each other, forming a covalent bond and ending the chain growth. Termination can occur through various pathways, such as:a Combination: Two growing polymer chains with radicals at their ends combine to form a single, longer polymer chain:PVC + PVC PVC-PVCb Disproportionation: One radical abstracts a hydrogen atom from another, forming a double bond in one chain and a saturated end in the other:PVC + PVC PVC=CH2 + PVC-CH2ClThe molecular weight of the resulting PVC depends on the ratio of the rate of propagation to the rate of termination. A higher propagation rate relative to the termination rate leads to the formation of PVC with a higher molecular weight.In summary, the free radical polymerization of vinyl chloride involves the generation of radicals, which initiate the polymerization process. The radicals react with vinyl chloride monomers, propagating the polymer chain growth. The reaction terminates when two radicals react with each other, forming a covalent bond. The overall mechanism leads to the formation of PVC with high molecular weight, depending on the relative rates of propagation and termination.