Addition (Chain-Growth) Polymerization
Break the double bond, build the chain. Master the Free Radical Mechanism.
In the world of polymers, building a massive macromolecule is essentially a game of snapping chemical links together. Addition Polymerization is the most direct way to do this. Monomers simply add together, one after another, to form a long chain without the loss of any small molecules (like water or alcohol).
1. What is Addition Polymerization?
Also known as Chain-Growth Polymerization, this process involves the repeated addition of monomer molecules possessing double or triple bonds (unsaturated compounds like alkenes or alkynes) to form a polymer.
- No Byproducts: The empirical formula of the polymer is exactly the same as that of the monomer. Nothing is lost during the reaction.
- Requirement: The monomer must have unsaturation (a $\pi$-bond) that can break open to form two new single $\sigma$-bonds linking to other monomers.
2. The Free Radical Mechanism
How do alkenes magically start linking together? They need a "spark" to start the chain reaction. This is typically achieved using a Free Radical Mechanism, which is catalyzed by a small amount of a radical initiator like Benzoyl Peroxide, Acetyl Peroxide, or tert-Butyl Peroxide. The mechanism involves three distinct steps:
Step 1: Chain Initiation
The peroxide initiator breaks apart symmetrically (homolytic cleavage) when heated or exposed to light, generating highly reactive free radicals ($R^\bullet$). This radical attacks the double bond of the first monomer (e.g., Ethene), transferring the radical to the carbon atom and starting the chain.
Step 2: Chain Propagation
The new carbon radical attacks another ethene molecule. The chain grows longer with each addition, continuously regenerating the free radical at the growing end of the polymer chain.
Step 3: Chain Termination
The chain stops growing when two free radicals bump into each other and pair up their unpaired electrons to form a stable covalent bond (Coupling), or when a hydrogen atom transfers between them (Disproportionation).
Figure 1: The free radical ($\color{red}{\bullet}$) is the active site that continuously breaks open new double bonds.
3. Crucial Addition Polymers for Exams
These five polymers and their specific synthesis conditions are heavily tested in board exams and competitive entrance tests.
A. Polythene (Polyethylene)
Formed by the addition polymerization of Ethene ($CH_2=CH_2$). It exists in two vastly different commercial forms based on how it is synthesized.
- Conditions: Extreme high pressure (1000-2000 atm) & high temp (350-570 K) with a trace of peroxide initiator.
- Structure: Free radical addition leads to a highly branched structure. Branches prevent chains from packing closely, hence Low Density.
- Uses: Squeeze bottles, flexible pipes, plastic bags.
- Conditions: Low pressure (6-7 atm) & temp (330-340 K) using a Ziegler-Natta Catalyst (Triethylaluminum + Titanium tetrachloride).
- Structure: Linear chains with almost no branching. Chains pack tightly, giving High Density and higher melting point.
- Uses: Buckets, dustbins, tough plastic bottles.
| Polymer Name | Monomer | Structure of Monomer | Properties & Uses |
|---|---|---|---|
| Teflon (PTFE) | Tetrafluoroethene | $CF_2=CF_2$ | Chemically inert and highly resistant to heat. Used for non-stick coating on cookware and gaskets. |
| Polyacrylonitrile (PAN) / Orlon | Acrylonitrile (Vinyl Cyanide) | $CH_2=CH-CN$ | Hard, horny, and high melting. Used extensively as a synthetic substitute for wool in making blankets and sweaters. |
| Polyvinyl Chloride (PVC) | Vinyl Chloride | $CH_2=CH-Cl$ | Tough and electrically insulating. Used for manufacturing water pipes, vinyl flooring, and wire insulation. |
| Polystyrene | Styrene (Vinyl Benzene) | $CH_2=CH-C_6H_5$ | Transparent, brittle. Used to make disposable cups, toys, radio/TV cabinets, and packaging material (Styrofoam). |
Mastery Check: Addition Polymerization
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