In objects made of plastic (and other polymers), how many molecules would one generally expect to find?

Intuitively, it seems like, say, a plastic shopping bag or PET bottle couldn't be a single molecule, but on the other hand, that's one of the strengths of polymers.

So the question is, I suppose, do items made of plastic consist of a single molecule, or many molecules fused together by way of glues or mechanical action?

Edit: Following a comment, I realise that the core of my question could be clearer, so here's an alternate phrasing:

If I chose an atom in a typical plastic object and gave it a colour, and then gave every atom connected to it by way of covalent bonds the same colour by flood fill, and then chose a colourless atom in the object and repeated the process until no more colourless atoms existed in the object, how many colours would a typical plastic object have?

  • $\begingroup$ Most plastic items are composed of many molecules. If you stretch a plastic bag, the molecules line up beside each other (like uncooked spaghetti). Since the bag is not one molecule thick or wide, there must be a lot of molecules. OTOH, under the right conditions the rubber in a tire could be one molecule. $\endgroup$ – LDC3 May 10 '15 at 1:13
  • $\begingroup$ @LDC3 "[A] bag is not one molecule thick or wide[.]"? Molecules come in a wide variety of thicknesses and widths, which is why I'm asking. Although I guess my question could do with a bit of clarification. $\endgroup$ – Williham Totland May 10 '15 at 1:45

The short answer is a very large number and far more than you could reasonably count

Polymerization reactions typically produce very large molecules but not so large that a single molecule would make up as much material as a whole plastic bag or item of any significant size. This source on LDPE tells us that the typical molecular weight of the industrial product is 90,000 (some polymers are much bigger than this).

For non-chemists molecular weights are (simplifying slightly) the weight relative to a single hydrogen atom. Carbon is 12 on this scale and ethylene (the monomer from which the plastic is made which consists of 2 carbons and 4 hydrogens) is 32. This mean that a typical molecule of the plastic is (crudely) made from a little under 3,000 ethylene molecules. The other fact you need to know is that there are about 6 x 1023 hydrogen atoms in a gramme of hydrogen (see the wikipedia definition of Avogadro's number). Simple mathematics tells us that a gramme of LDPE will contain roughly 6.7 x 1018 molecules so that is the order of the number in small plastic item which, I expect, weighs just a few grammes.

So, the bottom line is that even in a small plastic item consisting of a long chain polymer (LDPE is relatively short but other polymers might have chains 10-100 times more at most) there will be something like 1016 to 1018 molecules of polymer, which is a lot.

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    $\begingroup$ Very heavily cross-linked polymers could conceivably create single molecules of macroscopic size. As LDC3 mentions, vulcanised rubber is one possibility. I think the molar mass range for polymers can go from anywhere between some $\rm{10^3\ g\ mol^{-1}}$ for oligomers to $\rm{10^{25}\ g\ mol^{-1}}$ and beyond. $\endgroup$ – Nicolau Saker Neto May 10 '15 at 11:23
  • $\begingroup$ There is nothing very useful or well defined between 10000 kg/mol and one network molecule for the whole object. $\endgroup$ – Karl Mar 31 '16 at 1:17

Some plastics have their molecular weight (m.w.) specified, e.g. polyethylene and ultra-high-molecular-weight polyethylene. Knowing the type of plastic, and therefore its m.w., and the mass of the object, using Avogadro's constant, you should be able to calculate how many molecules there are.

Measuring a low-density polyethylene sandwich bag would be a convenient place to start.

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