What are Addition Polymers?
The naming convention for addition polymers is straightforward: the prefix "poly-" is added to the name of the monomer. If the monomer name is complex or consists of multiple words, it is typically enclosed in parentheses, for example, poly(ethene) from ethene or poly(chloroethene) from chloroethene.
To draw a repeat unit from a monomer, the carbon-carbon double bond in the monomer is converted into a single bond. Then, a single bond is added to each end of this newly formed single bond, extending outwards to represent its connection to adjacent units in the polymer chain.
These outward-extending bonds are known as extension or continuation bonds, and they signify that the unit is part of a much larger macromolecule. A subscript 'n' is typically placed outside the closing bracket of the repeat unit to indicate a large, unspecified number of such units.
To deduce the monomer from a polymer's repeat unit, the process is reversed. The single bond within the main chain of the repeat unit (which was originally the double bond) is converted back into a C=C double bond, and the extension bonds are removed. The subscript 'n' is also removed, and if representing the polymerization, it can be placed in front of the monomer.
The primary distinction between addition and condensation polymerization lies in the fate of the atoms from the monomers. In addition polymerization, all atoms from the original monomers are incorporated into the polymer chain, meaning there is no loss of any small molecules during the reaction.
In contrast, condensation polymerization involves the elimination of a small molecule, such as water, methanol, or hydrogen chloride, as the monomers link together. This means the polymer's molecular weight is less than the sum of the molecular weights of the original monomers.
Addition polymerization typically requires monomers with carbon-carbon double bonds, which open up to form the polymer backbone. Condensation polymerization, however, requires monomers with two different functional groups (or two of the same functional group on different ends) that can react with each other to form new linkages while releasing a small molecule.
Master Monomer-Repeat Unit Conversion: A common exam question involves converting between a monomer structure and its corresponding repeat unit, and vice-versa. Practice drawing the structures for common examples like ethene, propene, chloroethene, and tetrafluoroethene.
Understand Bond Changes: Always remember that the carbon-carbon double bond in the monomer becomes a single bond in the polymer's repeat unit. Forgetting this crucial transformation is a frequent error that leads to incorrect structures.
Correct Naming Conventions: Pay close attention to polymer naming. Ensure you use the "poly-" prefix correctly and enclose the monomer name in parentheses, especially for monomers with multiple words or complex structures, to avoid ambiguity.
Identify Extension Bonds: When drawing repeat units, clearly show the extension bonds (dashes extending outside the brackets) to indicate that the unit is part of a larger chain. Omitting these or drawing them incorrectly can result in lost marks.
Forgetting Double Bond Conversion: A very common mistake is to draw the repeat unit with a double bond still present, or to incorrectly place the side groups. The double bond must break and become a single bond to form the polymer chain.
Incorrect Extension Bonds: Students sometimes draw the extension bonds inside the brackets or fail to draw them at all. These bonds are critical for representing the continuity of the polymer chain.
Confusing Monomer and Repeat Unit: While structurally similar, the monomer and repeat unit are distinct. The monomer is the starting material with a double bond, while the repeat unit is the saturated segment within the polymer chain.
Misnaming Polymers: Incorrectly applying the "poly-" prefix or failing to use parentheses for complex monomer names can lead to errors. For instance, calling poly(chloroethene) simply "polychloroethene" might be considered incorrect in some contexts.
Addition polymers form the backbone of the modern plastics industry, with materials like polyethylene, polypropylene, and PVC being ubiquitous in everyday life. Their properties, such as flexibility, strength, and chemical resistance, are directly derived from the structure of their constituent monomers.
The specific side groups attached to the carbon atoms of the double bond in the monomer dictate the final properties of the polymer. For example, the presence of chlorine atoms in poly(chloroethene) (PVC) imparts rigidity and flame resistance, making it suitable for pipes and window frames.
Understanding addition polymerization is crucial for designing new materials with tailored properties. By selecting different monomers or modifying their structures, chemists can create polymers with desired characteristics for various applications, from packaging to medical devices.