The reactivity of monomers in addition polymerisation stems from the presence of the carbon-carbon double bond (C=C). One of the bonds within this double bond (the pi bond) is relatively weak and can be broken under specific reaction conditions.
When this double bond breaks, it creates two new bonding sites on the carbon atoms. These sites then form single covalent bonds with adjacent monomer molecules, initiating a chain reaction that links many monomers together.
This process continues, adding monomer units one after another, to form a long, continuous polymer chain. The final polymer contains only carbon-carbon single bonds along its backbone, meaning no atoms are lost during the polymerisation, and the polymer is considered saturated.
The specific properties of a polymer are not solely determined by the monomer's chemical structure but are also significantly influenced by the conditions under which polymerisation occurs, such as temperature, pressure, and the type of catalyst used.
Even with the same monomer, varying the polymerisation conditions can lead to polymers with vastly different physical properties. This is due to changes in the polymer's molecular structure, such as chain branching and overall chain length.
For instance, low-density poly(ethene) (LDPE) is produced using high pressure and moderate temperature with a specific catalyst. These conditions promote significant chain branching, which prevents polymer chains from packing closely together, resulting in a more flexible and less dense material.
In contrast, high-density poly(ethene) (HDPE) is formed under lower temperatures and pressures, often with a different type of catalyst (e.g., Ziegler-Natta catalysts). These conditions favor the formation of more linear polymer chains with minimal branching, allowing them to pack tightly and resulting in a more rigid, solid, and denser material.
Polymer molecules are extremely large, so chemists use repeat units to represent their structure concisely. A repeat unit shows the smallest repeating segment of the polymer chain.
To draw a repeat unit from a monomer, the first step is to change the carbon-carbon double bond (C=C) in the monomer to a single bond (C-C). This represents the bond breaking during polymerisation.
Next, add square brackets around the modified monomer unit. These brackets indicate that the enclosed structure is the repeating segment of the polymer chain.
Crucially, add continuation bonds extending outwards from each carbon atom that was originally part of the double bond, outside the brackets. These bonds signify that the repeat unit is connected to other identical units in a long chain.
Finally, a small subscript 'n' is placed on the bottom right-hand side of the closing bracket. This 'n' denotes a large, indefinite number of repeating units, indicating the macromolecular nature of the polymer.
Ensure that all other groups (e.g., hydrogen atoms, methyl groups) attached to the carbon atoms maintain their original positions relative to the carbon backbone, just as they surrounded the double bond in the monomer.
Monomer vs. Polymer: A monomer is a small, reactive molecule with a C=C double bond, serving as the building block. A polymer is a large macromolecule formed from many linked monomer units, characterized by a long chain of C-C single bonds.
Unsaturated Monomer vs. Saturated Polymer: Monomers are unsaturated due to their C=C double bond, which is the site of reactivity. The resulting addition polymer is saturated because the double bond breaks, forming only C-C single bonds along the backbone, thus having no remaining sites of unsaturation.
LDPE vs. HDPE: Low-density poly(ethene) (LDPE) is flexible and less dense due to branched polymer chains formed under high pressure and moderate temperature. High-density poly(ethene) (HDPE) is rigid and denser, resulting from linear, tightly packed chains formed under lower pressure and temperature with a different catalyst.
Master Repeat Unit Drawing: Practice converting common alkene monomers (like ethene and propene) into their correct polymer repeat units. Pay close attention to breaking the double bond, adding brackets, and including continuation bonds and the subscript 'n'.
Understand Property-Condition Links: Be prepared to explain how different polymerisation conditions (temperature, pressure, catalyst) can lead to variations in polymer properties, using examples like LDPE and HDPE.
Identify Monomers from Polymers: If given a polymer name like poly(chloroethene), you should immediately be able to identify the monomer (chloroethene) and vice-versa. This demonstrates a fundamental understanding of the naming convention.
Explain Saturation: Clearly articulate why addition polymers are considered saturated, emphasizing the breaking of the monomer's double bond to form single bonds in the polymer chain.
Forgetting to Break the Double Bond: A frequent error is drawing the repeat unit with the original carbon-carbon double bond still present. The double bond must break to allow for chain formation, resulting in a single bond in the polymer backbone.
Incorrect Continuation Bonds: Students sometimes forget to draw the continuation bonds extending outside the brackets, or they draw them incorrectly. These bonds are essential to show that the unit is part of a larger chain.
Misplacing the Subscript 'n': The 'n' indicating the number of repeat units should always be outside and to the bottom right of the closing bracket, not inside the brackets or elsewhere.
Confusing Monomer and Polymer Names/Structures: Ensure a clear distinction between the small, reactive monomer and the large, stable polymer, both in name and chemical structure.
