Addition polymerization exclusively occurs with monomers that possess a carbon-carbon double bond (C=C), such as alkenes. This unsaturation is the key feature that allows the monomers to react and join together.
During the reaction, one of the bonds within each C=C double bond breaks, typically the weaker pi () bond. This creates reactive sites on the carbon atoms, allowing them to form new single bonds with adjacent monomers.
The resulting polymer chain consists solely of carbon-carbon single bonds along its backbone, as all the double bonds from the original monomers have been converted. This saturation contributes to the stability and chemical inertness of many addition polymers.
The polymerization process usually requires specific conditions, including high pressures and the presence of a catalyst. These conditions facilitate the initiation and propagation of the chain reaction, ensuring efficient conversion of monomers into polymers.
The naming of addition polymers follows a straightforward convention based on the name of the monomer. The prefix 'poly-' is added to the name of the monomer, which is typically enclosed in parentheses.
For example, if the monomer is ethene, the resulting polymer is named poly(ethene). Similarly, the monomer propene forms poly(propene), clearly indicating its origin.
This systematic naming helps to quickly identify the monomer from which a particular addition polymer was derived. It provides a clear link between the starting material and the final polymeric product.
To draw the repeat unit from a given monomer, the carbon-carbon double bond in the monomer is converted into a single bond. This reflects the bond breaking that occurs during polymerization.
Extension bonds (also known as continuation bonds) must be added to each end of the single-bonded carbon chain, extending outside a pair of brackets. These bonds signify that the unit is part of a much longer chain.
The entire repeat unit, including the extension bonds, is enclosed in square brackets [], with a subscript 'n' placed on the bottom right. The 'n' indicates that a large, indefinite number of these units are linked together to form the polymer.
All other atoms or groups originally attached to the carbons of the double bond in the monomer must be maintained in their correct positions within the repeat unit.
Conversely, to deduce the monomer from a polymer's repeat unit, one must first identify the repeating segment. This segment will typically be a two-carbon chain with single bonds and extension bonds.
The single bond between the two carbons in the repeat unit is then converted back into a carbon-carbon double bond. This reverses the polymerization process conceptually.
The extension bonds and the subscript 'n' are removed, as they are features of the polymer chain, not the individual monomer. The resulting structure is the original unsaturated monomer.
Sometimes, the 'n' can be placed in front of the monomer to indicate that 'n' number of monomers combine to form the polymer.
Addition polymers are generally characterized by their high molecular mass, resulting from the extensive linking of numerous monomer units. This large size contributes to their macroscopic properties, such as strength and rigidity.
The polymer backbone is composed of strong carbon-carbon single bonds, which are chemically stable and resistant to many forms of degradation. This inherent stability makes addition polymers unreactive and chemically inert under typical conditions.
The inertness of addition polymers means they do not easily biodegrade, posing environmental challenges for disposal. Their resistance to chemical attack is a direct consequence of the saturated carbon backbone formed during polymerization.
Master Monomer-Repeat Unit Conversion: A common exam question involves drawing the repeat unit from a given monomer or vice-versa. Practice converting between the two, paying close attention to the double bond breaking and the addition of extension bonds.
Accurate Naming: Ensure you can correctly name addition polymers by applying the 'poly-(monomer name)' rule. Be careful with monomers that have substituents, as their full name should be used within the parentheses.
Identify Polymerization Conditions: Remember that addition polymerization typically requires high pressure and a catalyst. While the specific catalyst might not always be required, knowing the general conditions is important.
Recognize Unsaturated Monomers: Only monomers containing a C=C double bond can undergo addition polymerization. If a molecule lacks this feature, it cannot form an addition polymer. Always check for the presence of unsaturation.
Forgetting Extension Bonds: A frequent error when drawing repeat units is omitting the extension bonds outside the brackets. These bonds are crucial for indicating that the unit is part of a continuous chain, and their absence implies a discrete molecule.
Incorrect Bond Conversion: Students sometimes fail to correctly convert the C=C double bond to a C-C single bond in the repeat unit, or vice-versa when deducing the monomer. The double bond is the site of reaction and must be handled precisely.
Confusing Monomer with Repeat Unit: It's a common mistake to draw the monomer when asked for the repeat unit, or to include the 'n' subscript with the monomer. Understand that the monomer is the starting material, and the repeat unit is the structural segment within the polymer.
Ignoring Substituents: When drawing or naming, ensure all atoms or groups attached to the carbons of the double bond in the monomer are correctly transferred to the repeat unit. Do not simplify or omit them.
