This is a bit of a puzzler that I don’t have the answer to. I used to have the answer (or I thought I did), but not anymore. The question is this: what is the largest molecule? (By molecule, I am referring to something that has covalent bonds between the atoms. Metals and salts don’t qualify.)

If you’ve never thought about this before, you might be thinking about biochemicals, such as various proteins or DNA. As far as the molecules that most chemists work with go, they certainly are very large, but they are positively microscopic to what follows.

As a polymer person, I always like to shock people and suggest some kind of everyday object like a bowling ball. These types of objects are made from large polymers chains that have been crosslinked together with covalent bonds. If crosslinked enough, all the polymer chains become part of the same network. Maxwell’s Demon would be able to start at one atom and hop along the bonds to reach any other atom without having to jump to another molecule.

I used to think the largest molecule was a bowling ball, but given the core/shell construction used nowadays, I’m not so sure. Are outer and inner parts covalently bonded? I kinda doubt it. Another common suggestion are tires. The problem with that suggestion however, is that a tire is made up of more than just rubber. In car tires, there are large quantities of steel or other cords in the belts and also the rim bead that are not chemically bonded to the rubber. Also the carbon black isn’t even “bonded” to the rubber, only adsorbed.

So what is the largest molecule? You could argue that just the vulcanized rubber in a tire is one large molecule (I can’t disagree) and that therefore the largest tires would have the largest molecules as part of their construction. That’s a good start, but I can think of crosslinked rubber materials that are still bigger.

What about roll goods, like EPDM or other rubbers that are used as moisture liners or commercial and industrial flat roofs? You can buy rolls that are 30 feet wide and a 100 feet long. Ignoring the carbon black again, these rolls might suggest to you that least in one dimension, they could qualify as the largest molecule. But I can think of a bigger molecule yet.

How about crosslinked HDPE (PEX) tubing? A quick search came up with 1 1/2″ x 500 ft tubes – all a single molecule (except for the colorant). But since the EPDM and the PEX tubing are made in a continuous extrusion process, there really is no reason that you couldn’t make even larger molecules. You’re really only limited by your ability to roll up the finished goods.

Or am I wrong? Is there still something out there that would be bigger yet? If you have a suggestion, please leave it in the comments. (Yes you can post anonymously.)