The polymerization reactions of polyethylene and polypropylene are both examples of addition chain-growth polymerization, which involves the formation of long polymer chains through the successive addition of monomers. However, the mechanisms behind their polymerization reactions differ due to the different structures of their monomers and the catalysts used.Polyethylene Polymerization:Polyethylene is synthesized from the polymerization of ethylene C2H4 monomers. The mechanism involves the following steps:1. Initiation: A catalyst, typically a transition metal complex like Ziegler-Natta or metallocene catalyst, reacts with the ethylene monomer, breaking one of the carbon-carbon double bonds and forming a bond between the catalyst and the monomer.2. Propagation: The catalyst-ethylene complex reacts with another ethylene monomer, breaking the second carbon-carbon double bond and forming a bond between the two monomers. This process is repeated, with the growing polymer chain continuously adding more ethylene monomers.3. Termination: The polymerization reaction is terminated when the active catalyst-polymer complex is deactivated, either by reacting with a terminating agent or through a transfer reaction with another molecule.Polypropylene Polymerization:Polypropylene is synthesized from the polymerization of propylene C3H6 monomers, which have a similar structure to ethylene but with an additional methyl group CH3 on one of the carbons. The mechanism also involves initiation, propagation, and termination steps, but with some differences:1. Initiation: Similar to polyethylene, a catalyst e.g., Ziegler-Natta or metallocene catalyst reacts with the propylene monomer, breaking one of the carbon-carbon double bonds and forming a bond between the catalyst and the monomer.2. Propagation: The catalyst-propylene complex reacts with another propylene monomer, breaking the second carbon-carbon double bond and forming a bond between the two monomers. The presence of the methyl group in propylene leads to the possibility of different orientations isotactic, syndiotactic, or atactic in the resulting polymer chain, depending on the stereochemistry of the catalyst and the polymerization conditions.3. Termination: Similar to polyethylene, the polymerization reaction is terminated when the active catalyst-polymer complex is deactivated.In summary, the mechanisms behind the polymerization reactions of polyethylene and polypropylene are similar, as both involve initiation, propagation, and termination steps. However, the presence of the methyl group in propylene leads to different stereochemistry possibilities in the resulting polymer, which can affect the properties of the final product. Additionally, the choice of catalyst and reaction conditions can influence the polymerization process and the properties of the resulting polymers.