Addition Polymers

Bond Reorganization: The mechanism of addition polymerization relies on the presence of a carbon-carbon double bond ($C=C$) in the monomer. During the reaction, one of the two bonds in the double bond breaks, allowing each carbon atom to form a new single covalent bond with an adjacent monomer unit.

Saturation Shift: The monomer starts as an unsaturated molecule, but the resulting polymer is a saturated chain consisting only of single carbon-carbon bonds ($C-C$). This change in bonding type is responsible for the transition from reactive monomers to highly stable, unreactive polymer chains.

Conservation of Mass: Because no small molecules are eliminated during the reaction, the mass of the polymer is the sum of the masses of all the monomers used. This makes addition polymerization an atom-efficient process where every atom from the starting material ends up in the final product.

Converting Monomer to Repeat Unit: To represent a polymer, identify the $C=C$ in the monomer and redraw it as a $C-C$ single bond. Ensure all side groups attached to the original carbons remain in their relative positions to maintain the identity of the specific polymer.

Standard Notation: Enclose the repeat unit in large square brackets. Crucially, the horizontal single bonds on either side must extend through the brackets; these are known as continuation or extension bonds. A subscript $n$ is placed outside the bottom right bracket to signify the repetition of the unit.

Naming Rules: The IUPAC name for an addition polymer is derived by placing the name of the starting monomer in parentheses and adding the prefix poly-. For instance, the monomer 'chloroethene' yields the polymer 'poly(chloroethene)', also known commercially as PVC.

Unsaturated vs Saturated: Monomers are alkenes (unsaturated), while the resulting addition polymers are essentially long-chain alkanes (saturated). This distinction explains why polymers do not undergo the typical addition reactions of alkenes, such as decolorizing bromine water.

Atom Economy: Unlike condensation polymerization, which releases a small byproduct like water, addition polymerization involves 100% atom economy. The polymer is the only product of the reaction, ensuring no mass is 'lost' to byproducts.

Chemical Inertness: The strength of the $C-C$ bonds in the polymer backbone makes addition polymers exceptionally stable. While this is beneficial for long-term use, it means they are non-biodegradable because microorganisms cannot easily break these strong covalent bonds.

Landfill Issues: Because they do not decompose, waste polymers persist in landfills for centuries, consuming space and potentially leaching additives into the soil. This has led to a global crisis in waste management and a push for more sustainable materials.

Incineration Hazards: Burning polymers can generate energy, but it releases $CO_2$, contributing to the greenhouse effect. Furthermore, specific polymers like poly(chloroethene) release toxic hydrogen chloride ($HCl$) gas when incinerated, necessitating advanced gas scrubbing technologies to prevent acid rain and health hazards.

Check the Bonds: A common error is leaving the double bond inside the polymer brackets. Always ensure the $C=C$ from the monomer is converted to a $C-C$ in the repeat unit, otherwise the structure is chemically impossible.

The 'n' Factor: Never forget the subscript $n$ on the repeat unit. Without it, you have drawn a specific molecule rather than a polymer chain. Similarly, ensure the extension bonds clearly exit the brackets to indicate the chain continues.

Mapping Groups: When drawing complex monomers like propene or chloroethene, map the side groups ($CH_3$, $Cl$, etc.) exactly as they appear in the monomer. If a group is on the 'top-left' of the monomer's double bond, it must be on the 'top-left' of the repeat unit's single bond.