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What is a polymer and when a substance is not a polymer?

  1. Feb 2, 2013 #1
    Polymers
    Polymers are defined as susbtances (natural or artificial) that are composed of smaller units called monomers. These monomers are molecules.
    Molecules are an aggregation between two or more (similar or different) atoms.

    That said, why are not all substances polymers? After all, all sustances are made of molecules connected to each other. That sounds like what a polymer is....I am missing the difference. Is a polymer a looong chain of molecules? Ok, but that chain will need to connect to another chain.We are still looking at molecules connected to other molecules like in all materials.....

    In summary, a polymer is made of monomers (which are made of molecules). Monomers are "connected" to each other via chemical bonds instead of intermolecular forces.
    What is the difference between chemical bonds (ionic, covalent, metallic) and intermolecular forces?
    SiO2, for example, has covalent bonds but it is not a polymer, right?

    thanks
    fisico30
     
  2. jcsd
  3. Feb 2, 2013 #2

    mfb

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    In polymers, those monomers form chains or chain-like structures. Something you won't find in many other materials. The forces between chains can be van-der-Waals-forces, which are weaker than "true" chemical bonds.

    Most crystals do not have anything you could call "molecule" (water ice is an exception).
     
  4. Feb 2, 2013 #3

    epenguin

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    Well for several very basic questions like the difference between chemical bonds frankly this is not the place because we clear up confusions but if we substituted ourselves with offhand explanations for textbooks or other sources, even wiki, which have been thought out carefully we would cause more confusion than we cleared. E.g. Just look up the commonest artificial polymer, polystyrene, on wiki and you will get the idea.

    There is maybe some scope for confusion in that 'polymer' is generally understood but not quite rigidly defined - e.g. How long does a molecule have to be before you call it a polymer?

    Chemically unite a molecule with one of the same kind and you have a dimer, add another and you have a trimer, etc. etc. A few is an oligomer. Many is a polymer. Can you tell me exactly how many you need to pass from 'few' to 'many'? 10 would still be oligo and 100 poly roughly.

    The repetitions don't have to be exact. That is they can be the same sort of molecule, the same chemistry of the uniting bond, but the monomers may vary, that is the case in most of the biological examples like proteins (polypeptides) where the -NHCHRCO- repeats : NHCHR1CONHCHR2CONHCHR3CO-... but the R groups vary among a repetory of 20 different, or nucleic acids (polynucleotides) or polysaccharides.
    One chain does not need to connect with another, mostly they start and end somewhere. But chains can be linked to others ('crosslinks') generally by a different kind of chemical than the main one, often this has to do with making them tough.
     
  5. Feb 3, 2013 #4
    thank you.

    But consider the two cases:

    1) a molecular compound

    2) a polymer

    both are made of molecules connected to each other. some explain the polymer with the necklace analogy: the necklace is the polymer and the beads are the monomers (molecules). These necklaces are connected each other to form the polymeric material.
    In essence, we are still talking about molecules connected to molecules, but maybe the main difference is presence of this large structures that repeatedly found in the material.
    WE can have amorphous or crystalline polymers.

    A molecular substance has molecules connecte to each other. Consider methane: we have many molecules tied to other molecules but we cannot identify large identical structures that can be called polymers in methane.

    thanks
    fisico30
     
  6. Feb 3, 2013 #5
    As already said, a polymer is just a sort of a long molecule which may be folded and crosslinked. In normal conditions methane is a gas and therefore it consists of separate molecules (no bonding between them). You can not make a polymer out of it, since it has no double or triple bonds to break, nor any free electrons to use for linking multiple molecules. If you solidify it by freezing, you will get either a glass or a crystal made of molecules bonded by the weak Van der Waals forces, but nothing similar to a polymer. However, if you take e.g. ethylene (ethene) which does have a double bond, you can obtain from it either a molecular crystal by cooling it down or polyethylene by breaking the double bonds and linking numerous molecules to form a chain.
     
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