[dalmatiner]

Many people say that a polymer is a kind of macromolecule where always the same unit is repeated. Since there are copolymers that have different units within their structure I find that quite confusing.

If you go by the IUPAC definition a polymer consists of macromolecules and macromolecules refers to a molecule of high relative molecular mass, which itself comprises the multiple of units from molecules of low relative molecular mass. There is also a side note in the IUPAC definition that synthetic polymers can be regarded as having high relative molecular mass if the addition or removal of one or a few units has negligible effect on the molecular properties.

I tried to understand this by using an example but it hasn't really helped me out either. Now I specifically tried to figure out, if synthetic plastic, where I ask myself again if plastic now means polymer or something else, can be referred to as a macromolecule or/ and a polymer and what the difference are.

I thought I'd try to understand it thinking of Polyethylene:

I thought that the macromolecule in Polyethylene is ethylen because it's the basic structural unit, but it actually has a low relative molecular mass. Then I thought maybe that's the exception where taking away or adding a unit doesn't change that much but since it is only one unit itself I can quite understand how that should work.

I just wanted to know what a macromolecule and a polymer exactly means since also in the IUPAC definition they are two different things but I can't really work out what it means if you really look at substances like PE or PET and where the differences are.

[chenbeier]

Macro means big or large in opposit of mikro which means small. A macromolecule is a very large molecule, which mostly made of a bunch of smaller molecules. In nature the DNA is a macromolecule made of a lot of aminoacids and others.

Other examples: Glucose to starch, Ethylen to Polyethylen, Di-isocyanate and Diol to Polyurethan, Diamins and Dicarbonacids to Polyamide,  Diol and Dicarbonacids to Polyester, etc.
Generally x Monomers become (monomer unit)_x = Polymer

[P]

Yep - from the latin,   mono (single) mer (unit) and poly (many) mer (unit) - give us definitions for monomers and polymers....  so most polymers are macromolecule (very big molecules) but a macromolecule doesn't have to be a polymer.

Plastic is a broad definition, like wood or metal. Most plastics are polymers of some kind (I can't think of an example of a plastic not being a polymer off the top of my head).

A co-polymer is still a polymer and a macromolecule if large and most are probably considered plastics (plastic is a definition of a property of material). You can get ter-polymers also and presumably polymers with more that 3 repeat units (poly-polymers?)

[mjc123]

Greek, to be pedantic.

[Corribus]

From a chemical materials standpoint, "plastic" isn't very precise. "Polymer" is preferred. Colloquially and historically we often call them plastics, referring to their general property of plasticity.

To understand polymers, think of a piece of string containing blue-colored beads. If you start with one bead (monomer) and add another (dimer) and another (trimer), and so forth, you can perceive a large difference in the way the string looks with each additional bead you add. But once you get up to, say, 50, adding one additional bead doesn't really mean much. This might be a loose distinction between a polymer and an oligomer. In a polymer, adding or subtracting one or two units just doesn't influence the look and feel of the chain very much. Although, to be sure, the properties DO change, but it takes a larger number of beads to affect an equivalent degree of change once the strand is already large.

There's no real rigorous definition of a macromolecule. IUPAC provides a definition, but it's not really that useful. Virtually everything that would be considered a macromolecule is a polymer of some sort. Proteins are, for example, polymers that have a large variation in the modifications to the primary polymer chain. Likewise, all polymers would be considered macromolecules.

[wildfyr]

This issue has personally plagued me (a polymer chemist), because a "plastic" is a very colloquial term to most people, but is a bastardization with good reason.

The most common consumer polymers, PET, polyethyelene, and polypropylene are thermoplastics. That is, they can be heated until they soften and eventually flow without the process significantly damaging their mechanical or chemical properties after they return to room temperature.

A thermoplastic is a very specific scientific term, a plastic is what regular people extracted from the term, and use with somewhat decent accuracy considering most of what they encounter is a thermoplastic.

[Enthalpy]

Is this query for exact definitions making things even more complicated?

We should distinguish polymers from polycondensates like PA, polyesters...

The "repeating unit" fails with copolymers. Their unitS are irregularly arranged. Don't expect A, B, S to alternate periodically.

My impression is that life is simpler if accepting some fuzziness in the terms here.

[wildfyr]

Even in a copolymer there are repeating units, it's just a statistical distribution of them with gaps in between where other monomers are.

[Corribus]

Plus many copolymers (block, grafted, periodic, etc.) are not random.

[Enthalpy]

True, the unitS repeat, just not periodically, so they can provide a manageable definition.

[P]

The "repeating unit" fails with copolymers. Their unitS are irregularly arranged. Don't expect A, B, S to alternate periodically.

Not really - you can have Alternating co-polymers, block copolymers, statistical co-polymers, branched co-polymers, dendric forms etc... they are still repeat units but arranged statistically according to the reactivity ratios of the co-monomers in the system.

Greek, to be pedantic.

Correct - sorry - my mistake. Have a snack!

[P]

Plus many copolymers (block, grafted, periodic, etc.) are not random.

I guess 'random' is kinda right depending on how you view statistics. It depends upon the reactivity ratios of the monomers involved...  so it is 'statistically distributed according to reactivity' rather than random I would say.