Question

What are the steps involved in polymer formation by chain reaction?

Short answer

Polymer formation by chain reaction involves initiation, propagation, and termination.

Step-by-step solution

Initiation

The chain reaction begins with the initiation step. In this step, an initiator molecule is used to generate free radicals. A common method is using a compound like benzoyl peroxide, which decomposes upon heating to form two benzoyl radicals. These free radicals are highly reactive and can start the formation of polymer chains by breaking the double bond in monomer molecules such as ethylene to create an active site.

Propagation

Once the first radical has reacted with a monomer, the growing polymer chain proceeds to the propagation phase. During this phase, the active site at the end of the polymer chain reacts with a new monomer, transferring the active site to this new monomer. The repetition of this reaction leads to the growth of the polymer chain. This stage is characterized by the continuous addition of monomer units to the active chain end, resulting in the formation of long polymer chains.

Termination

The chain reaction ends with the termination step. This can occur in several ways: two growing polymer chains with active free-radical ends can combine, neutralizing each other; an active chain can react with an impurity or a different radical quencher; or disproportionation can occur, where one chain transfers a hydrogen atom to another, ending the chain's growth. Termination stops the elongation of polymer chains and stabilizes the polymer molecules.

Key concepts

Initiator Molecules

Initiator molecules play a vital role in starting the polymerization process. Without these molecules, the entire chain reaction couldn't commence. Initiators are special compounds that create free radicals, which are essential in the beginning stages of polymerization.

• Often, compounds like benzoyl peroxide are used as initiators.
• When heated, these compounds break down to form free radicals.
• These free radicals are the sparks that ignite the creation of polymer chains.

Imagine initiator molecules as the match that lights a fire in the chain reaction process. They don't become part of the final polymer, but they are necessary to start the reaction that forms the polymer.

Free Radicals

Free radicals are highly reactive atoms or molecules that have an unpaired electron. This makes them eager to bond with other molecules, playing a crucial part in polymerization.

• In the context of polymerization, free radicals are generated by initiator molecules.
• Their reactivity helps break the double bonds in monomer molecules.
• This breaking creates active sites that lead to the formation of polymer chains.

Free radicals act like the energetic chemical agents that drive the chain reaction forward. Their penchant for reacting makes them powerful forces in the synthesis of polymers.

Monomer Units

Monomer units are the building blocks of polymers. Like the individual beads that make a necklace, monomers join together to form long, repeating chains.

• A common example of a monomer is ethylene, which combines to form polyethylene.
• Monomers contain a double bond that can be broken and linked in the polymerization process.
• During propagation in polymerization, monomer units continually add to the end of a growing polymer chain.

The relentless addition of monomer units during the polymerization chain reaction results in the creation of large polymer structures with desirable properties for various applications.

Chain Reaction

A chain reaction in polymerization is a sequence of reactions where the reactive intermediate generated by one reaction initiates the next. It describes how monomers link to form polymers, mainly occurring in three key phases.

• **Initiation:** The start of the chain reaction, where initiator molecules form free radicals.
• **Propagation:** The growth phase, where monomer units attach to the active end of the chain, extending it.
• **Termination:** The conclusion of the reaction, where the active chain ends its growth.

In this process, the chain reaction stages are interconnected, ensuring continuous polymer development until termination. This systematic step-by-step sequence is responsible for forming the vast arrays of polymers used in everyday life.