Polymers are used all across chemistry and materials science. With a growing need for plastics recycling and increasing interest in medical and solar research, “polymer” will likely become a word you’ll hear more, not less, over the coming decade.
So what exactly is a polymer?
What is a polymer?
A polymer is a molecule made up of repeating units. These units, called “monomers”, form a long molecular chain. It might have branches, or it might just be one straight line of molecules, each connected in the same way.
Polymers can range in simplicity from polyethylene, which has monomers of one carbon atom connected to two hydrogen atoms, all the way through to more complex proteins (polypeptides), which are chains of various amino acids. These chains can become extraordinary structures, like antibodies and enzymes.
The name comes from the Greek poly, meaning “many”, and mer, meaning “mer”*.
Because they’re such a diverse class of molecule, polymers have a range of different chemical and physical properties. But they usually have one thing in common: they’re fairly stable molecules, making them tricky to digest, dissolve or turn into other things. And slight variations in a polymer can have big effects.
*Mer actually comes from meros, which means “part”. There’s a mer of you that thought that joke was funny.
Polymer roll call
Polymers are everywhere, both synthetic and naturally occurring. Here are a few polymers you interact with on a daily basis.
All plastics are polymers of one form or another. The long molecular chains of polymers can make them resistant to water and very difficult to break down – which are exactly the properties that make plastics so useful, and so damaging to the environment.
In Australia, plastic is classified in seven ways: six of them correspond to a specific polymer. For instance, category 1 is polyethylene terephthalate, or PET, which is made of monomers of ethylene terephthalate. (All other types of plastic polymer fall into category 7: other. The recyclability of the different types of plastic depends on your local council.)
The shape of the polymer chain can also make a difference: high-density polyethylene (2) and low-density polyethylene (4) are both made of the same monomer, but the low-density version has a lot more branches along its chain, making it less strong and (surprise) less dense.
Polysaccharides: starches and sugars
There are plenty of naturally occurring polymers as well. Polysaccharides – made from linked sugar molecules – are a useful food source because of how well they store energy. Starch is composed of two different polymers: amylose, made of linked glucose monomers, and amylopectin, made of the same monomers, but with more branches.
This is where the exact shape of the polymer chain becomes important. Humans can digest starch, because we have a specific enzyme in our bodies that can dismantle the two polymers. But we can’t digest cellulose, even though it is made from a nearly identical glucose monomer to amylose – because they connect up in a slightly different way.
Because of their regular structure, starches have been flagged as potential replacements for traditional plastic polymers. They’re much more digestible and can be broken down in the environment faster than plastics. Polylactic acid, made from corn starch, is one example. Cellulose is also a popular candidate – especially when it comes from fast-growing sources, like bamboo.
Note that not all plastics labelled “bioplastic” or “biodegradable” will be starch plastics, though – and they might not even be truly biodegradable.
DNA, RNA & proteins
It’s bizarre to think that DNA can hold so much information in a single molecule, but it does. The reason? It’s a really, really long molecule.
The monomers in DNA are more complicated than those in plastics: including a phosphate and a sugar molecule, as well as one of four different nitrogen bases that form our genetic code. A full strand of human DNA contains 3 billion of these monomers!
At the monomer level, RNA is similar to DNA, with a slightly different sugar and different base. Proteins – polypeptides – are made of connected chains of amino acids. There are 20 naturally occurring amino acids, and proteins can be any combination of these, at just about any length.
Polymers to watch
In addition to common plastics, there are a huge range of synthetic polymers making waves in research.
Organic polymers (“organic” here meaning “containing carbon” – they’re developed in labs) are making waves in solar cell research. They’re lighter and cheaper than traditional silicon-based solar panels, and they could even be printed using an inkjet printer. Other light-sensitive polymers could turn to face the Sun.
Because they’re difficult to break down, there’s a lot of focus on recycling in the polymer world – or polymers that can self-heal. Some Australian researchers are exploring green chemistry with polymers.
Changing the length of the chain can change the properties of the polymer – a team of chemists has made a polymer that glows different colours depending on the number of monomers in its chain.
Soft robotics is dependent on flexible polymers that can mimic muscles.
And finally, polymer gels are driving exciting breakthroughs in medicine.