Ester Linkage Formation: In the production of polyesters, a carboxylic acid group () from one monomer reacts with a hydroxyl group () from another. This chemical interaction results in a stable ester linkage () that joins the two monomers together.
Reaction Stoichiometry: For every linkage formed in the polymer chain, one molecule of the small byproduct is released. In a polymer consisting of repeat units, there will be water molecules produced because each repeat unit requires two linkages to join its neighbors.
Alternating Structure: Because the reaction typically involves two distinct monomers (such as a dicarboxylic acid and a diol), the resulting polymer chain features a perfectly alternating sequence of these units along its length.
| Feature | Addition Polymerisation | Condensation Polymerisation |
|---|---|---|
| Monomer Type | Alkenes with bonds | Bifunctional monomers (, ) |
| Byproducts | No byproducts formed | Small molecule (usually ) released |
| Atom Economy | 100% (all atoms kept) | Less than 100% (atoms lost as byproduct) |
| Structure | Single bond carbon backbone | Linkages like esters or amides |
Inertness vs. Reactivity: Addition polymers are typically chemically inert and do not biodegrade easily due to their strong carbon-carbon backbones. Condensation polymers often contain ester or amide links that can be targeted by chemical or biological processes.
Mass Distribution: The relative molecular mass of an addition polymer is exactly the sum of its monomers. In condensation, the final mass is the sum of the monomers minus the total mass of the eliminated byproducts.
Chemical Mechanism: Hydrolysis is the chemical addition of water to a polymer to break its linkages and restore the original monomers. During this reaction, the molecule is split, with the returning to the acid group and the returning to the alcohol group.
Analytical Use: Chemists use hydrolysis to break down complex polyesters into their constituent dicarboxylic acids and diols for identification and analysis. This technique is also the primary mechanism for the environmental degradation of bioplastics.
Conditions for Reaction: Hydrolysis typically requires heat and the presence of a catalyst, such as a strong acid or base, to proceed at a measurable rate. This stability ensures that synthetic polyesters do not fall apart immediately when they get wet in everyday use.
Drawing Repeat Units: To draw the repeat unit of a polyester, remove the from the acid and the from the alcohol, then connect the remaining fragments. Ensure that the extension bonds pass through the brackets to indicate the unit repeats infinitely.
Deducing Monomers: If provided with a polymer structure, identify the ester linkages and mentally 'cut' the single bond. Add a hydroxyl group back to the group to find the acid monomer and a hydrogen atom to the oxygen to find the diol monomer.
Byproduct Accounting: Always include in your balanced chemical equations for condensation polymerisation. A common exam error is treating these reactions like addition polymerisation and forgetting the second product.
Biopolyester Advantages: Unlike petroleum-based plastics, biopolyesters are derived from renewable resources like plant oils and sugars. They are designed to be biodegradable, meaning they can be broken down into harmless natural substances by environmental microorganisms.
Microbial Degradation: Microbes in the soil or compost produce specific enzymes that can catalyze the hydrolysis of ester links in biopolyesters. This biological 'digestion' prevents the accumulation of plastic waste in landfills and oceans.
Future Materials: Modern engineering focuses on creating biopolyesters that match the strength and durability of traditional plastics while maintaining their ability to vanish from the environment after their functional life is over.
