10.4.2. Mechanisms of Polymerization

1. Initiation: Radical initiator (e.g., benzoyl peroxide, AIBN) decomposes thermally to form radicals. Radicals add to the monomer (alkene) to produce a new radical at the chain end.

2. Propagation: The chain-end radical reacts with additional monomer molecules, extending the chain one monomer unit at a time, generating a new radical at the end after each addition.

3. Termination: Two chain-end radicals combine (coupling) or disproportionate (a hydrogen atom transfers from one chain to another), quenching radical activity.

4. Chain transfer: The radical center transfers to another molecule (monomer, solvent, or polymer), which can lead to branched or lower-molecular-weight chains.

● Control of radical polymerization: The degree of polymerization (chain length) depends on the ratio of propagation to termination rates. Techniques such as RAFT (reversible addition–fragmentation chain transfer), ATRP (atom transfer radical polymerization), and NMP (nitroxide-mediated polymerization) provide more controlled (“living”) radical polymerizations with narrow molecular weight distributions.

Radical polymerization is a method used to create long chains of molecules by linking together smaller units known as monomers. The process starts with initiation, where a compound known as a radical initiator breaks down to form reactive radicals. These radicals turn the first monomer into a free radical. As the radical attacks more monomers, the chain grows through a process called propagation, where each new addition creates another radical at the end of the chain. Eventually, the process stops either when two radicals meet and combine or when one radical changes into a more stable form through disproportionation. Chain transfer happens when the radical portion of the chain passes to another molecule, which may result in shorter chains. Control of the process can lead to specific properties and behaviors in the final polymer product.

Think of radical polymerization like building a long train with Lego blocks. Each red Lego block is a monomer that you connect one after another using a special connector (the radical). Once you build a long train, if two trains collide and stick together, you get longer trains (like termination). Or if you pass one block to another build (chain transfer), you might end up with a shorter train. By carefully controlling how you build the train, you can determine its length and how it behaves when you play with it.

1. Cationic polymerization: Initiated by a strong acid or Lewis acid generating a carbocation on the monomer (e.g., isobutene), which then adds to another monomer to propagate the cation. Termination occurs when the cation reacts with a nucleophile (e.g., water) or via rearrangement. Sensitive to substituents that stabilize carbocations (e.g., isobutene >> ethene).

2. Anionic polymerization: Initiated by a strong base or organometallic reagent, which generates a carbanion on the monomer (e.g., styrene, 1,3-butadiene). Propagation continues by carbanion addition to monomers. Termination occurs upon protonation or reaction with electrophiles. “Living” anionic polymerizations can be achieved in rigorously anhydrous, oxygen-free conditions, producing polymers with very narrow molecular weight distributions and precise architectures (block copolymers, star polymers).

Ionic polymerization involves either cationic or anionic mechanisms to grow polymer chains. In cationic polymerization, a strong acid generates a positively charged carbocation from the monomer, which can then react with more monomers leading to chain growth. The polymerization continues until the cation meets a nucleophile (like water) that terminates the chain. Conversely, in anionic polymerization, a strong base creates a negatively charged carbanion that reacts with unsubstituted monomers to form longer chains. This method can produce very uniform polymers, as controlled conditions can lead to exact chain lengths and architectures, such as block or star polymers.

Consider ionic polymerization like a team of kids alternatively adding blocks to a tower while playing a game. In the cationic game, one child starts with a bright colored block (the carbocation) and adds blocks of different colors until they run out of their turn. In the anionic game, a child starts with a regular block (the carbanion) and continues adding until someone interrupts by adding something else (terminating). The kids have to be careful not to introduce destabilizing blocks (like water) in their game to keep the tower strong and intact.