Chemistry Activity: Modelling Polymers with Paperclips
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Chemists often build molecular models using plastic rods and balls to represent different atoms and their bonding. It can often be easier to visualise the symmetry, shape and potential bonding interactions of a molecule when you have the 3-D model on hand to rotate.
In order to aid your understanding of how different types of polymer can be formed we are going to use paperclips to represent individual monomers. When joined together they will form different variations of polymer chains.
The following items will be required:
- A large number of paperclips, preferably of different size and colour.
First take a set of paperclips all of the same size or colour. These all represent the same molecule. Joining them together shows how many of the same monomer can be used to form a long, polymer chain. If you had many of these chains adjacent to each other they would pack closely together and so you would have a high-density polymer.
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Add some paperclips at regular intervals to act as side chains to the main polymer chain. This would represent a lower-density polymer as the steric hindrance of the side chains prevents the polymer chains from packing as closely together.
Let’s start a new polymer chain and this time include two different types of monomers. You may differentiate your monomer molecules through using paperclips of different types, sizes or colours. If you have a large enough collection of paperclips at your disposal then you can make all of the types of polymer listed below and then compare them.
First put your paperclips together in a random order. This would indicate two monomers which are able to react with both themselves and the other available monomer. Next create a chain of alternating monomers. These monomers might be bi-functional. The functional groups they possess can only react with the functional groups on the other monomers. Finally create a block co-polymer which consists of a length of a single type of monomer followed by another length of the other monomer. As each monomer might have different properties these are carried into the properties of the polymer. Most commonly one monomer will be hydrophilic and the other monomer will be hydrophobic. This creates a polymer with a hydrophilic section and a hydrophobic section.
As we have seen, polymers with discrete hydrophilic and hydrophobic sections can aggregate into micelles when placed in water. They will have a hydrophilic exterior and hydrophobic interior. The interior can be used to house drugs for targeted delivery or to “capture” oil and dirt when used in soaps and washing-up liquids.
If you have any remaining paperclips then you can investigate the effects of cross-linking between chains. Connect separate polymer chains together through the regular placement of cross-linking bond paperclips and you should get a sheet of paperclip chains. Is the sheet strong enough to hold the weight of a tennis ball? Can you join the chains close enough to hold a marble?
Adapted from: Y. Umar, J. Chem. Educ., 2014, 91, 1667.
Polymer structure and properties
When designing a polymer for a particular application, there are a number of factors that affect the physical properties of a polymer, and so need to be considered. For example, the viscosity, strength and toughness of a polymer depends on its molecular weight (this is determined by the number of repeating units in the polymer chain and the molecular weight of the repeating unit). The lower the molecular weight of a polymer, the lower the viscosity and the mechanical properties. On increasing molecular weight, due to increased entanglement of chains, a polymer becomes more viscous and tougher, which makes the processing of the polymer more difficult.
Also, polymer chains can attract one another encouraging them to stick together, using secondary bonds or intermolecular forces. Take, for example, the polyamide Kevlar. Its strength comes from strong intermolecular forces between adjacent chains of the Kevlar polymer. One intermolecular force is hydrogen bonding, which describes the electrostatic attraction between some hydrogen atoms in –NH– groups in one chain and the oxygen atoms in –C(=O)– groups in another chain. Additional strength is derived from stacking interactions between the benzene rings in different chains.
Another factor that needs to be considered is tacticity. This term is used to describe the way pendent groups on a polymer chain are arranged on a polymer backbone. The tacticity of a polymer is determined by what side of the polymer chain the pendant groups are on (i.e., are all of the groups pointing towards us, or are some pointing away from us). This is important because the relative position of the groups can have dramatic effects on the physical properties of the polymer.
Tacticity arises when there are chiral carbons in the polymer chain backbone. For example, this occurs in free radical polymerisation of propene, H2C=CH(CH)3, to form poly(propene), [–CH2–CH(CH)3–]n. When a propene monomer adds to the end of the growing polymer chain, the monomer can either join the pendant CH3 group on the same side as all of the other CH3 groups, or it can join the pendant CH3 group on the side away from the nearest pendant CH3 group. If propene adds to the polymer backbone with the pendant CH3 group on the same side as the previous CH3 group, this is called isotactic. If propene adds so that its CH3 group adds to the opposite side of the previous CH3 group, it is called syndiotactic. If there is no order to the way the CH3 group adds, (completely random) the polymer is said to be atactic (see the pdf in the downloads section below).
The tacticity has a dramatic effect on the physical properties of poly(propene). Atactic poly(propene) has little order in the polymer backbone and it is called an amorphous polymer. The polymer chains move across each other when it is pushed or pulled, so it has some flexibility and elasticity. Isotactic poly(propene) has long-range order, which adds mechanical strength and crystallinity. It is called a semicrystalline polymer and it is a stiffer material. The demand for these polymers resulted in the development of a range of catalysts (by Karl Ziegler and Giulio Natta) that can selectively prepare atactic or isotactic poly(propene).
Is it possible to illustrate intermolecular forces and/or tacticity in your paperclip model?
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