2012: Advent Chemistry: Superglue

by on December 16, 2012

Superglue is marvellous. It sticks a lot of things to a lot of things. It’s not the best adhesive there is – it’s possible to drive four hundred miles with an engine held together by Araldite, for instance, which I doubt is possible with superglue – but it’s pretty good. It’s a polymer, or at least it’s a polymer once it’s exposed to the air and allowed to set.

The base molecule of superglue is cyanoacrylate, which can come with a range of substituents – bits added on to the main structure, like methyl (CH3), ethyl (CH3CH2) and so on. The structure below is that of methyl cyanoacrylate, the simplest version.

The structure of cyanoacrylate, a small molecule with several functional groups.

Superglue has a very interesting polymerisation mechanism. That is, the way the atoms and electrons move around when it goes from individual molecules to one big polymer is unusual.

Mostly, carbon-carbon double bonds – alkenes – undergo addition reactions, but even when they don’t, they almost always act as nucleophiles. Some of the electrons from the double bond latch onto something that’s a bit positive (like a nucleus, whch is why they’re called nucleus-loving)  and make a new bond, breaking the double bond in the process.

Superglue is special. There are so many ways to shuffle electrons around and get them out of the way that the polymerisation of superglue starts when something negative (like a water molecule on the surface it’s been spread on) pushes electrons into the double bond and it breaks, putting a negative charge on the inner carbon. I was quite gleeful when I learned that in a lecture, because it was the first time I’d ever heard of an alkene behaving that way.

Even with the nitrile group on one side and the ester on the other (you should be able to figure out which is which by now, I hope? Nitrogen and oxygen, respectively), carbon isn’t happy to have a negative charge. It’s not a stable situation. That negative charge needs to go somewhere, and where it goes is into the double bond on the next cyanoacrylate molecule.

And the cycle just keeps repeating, until there aren’t any more cyanoacrylate molecules to add to the chain and the reaction stops, leaving a wodge of intertwined polymer strands in between and bound to the surfaces it’s touching. And thus, it acts as a glue.

Superglue isn’t permanent, not really, which is why you shouldn’t glue a car engine together with it. It’s soluble in water and steam, if not very soluble, and also in acetone. If you use it in place of stitches it will work and gradually wear away letting the skin heal together, but unfortunately it releases formaldehyde as it does so, which is carcinogenic, so it’s not a great thing to put into your body if you’ve got better options.

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