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    Not been taught this topic yet but I made notes this week about Polymerisation for AQA AS CHEM2. It seems quite simple but I was wondering if plastics have to be created using their constituent poly(x) forms like poly(ethene) for plastic bags or could we use the simple straight alkane chains if they were readily available?

    Is there something special about poly(ethene)4 that's different to octane?

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    Well, it's a polymer. Octane has a eight carbon chain. Polymers could go on forever. As the chain length increases, boiling point increases (stronger intermolecular forces, more points of contact, more van der Waals' etc).

    You have n (monomer) -> (polymer)
    So, the difference is a polymer could go on forever, while octane has a limit of eight carbons! In petrol, you'll have multiple octane molecules. In a plastic bag, you'll have lots of poly(ethane) molecules.
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    Yes, but when we say poly(ethene)4 as in n = 4 you'd get a hydrocarbon chain that was essentially octane. Why do we bother using the n in shorthand repeating unit notation then, if we were just to classify it all as just poly(ethene) or polythene?
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    (Original post by AdamskiUK)
    Yes, but when we say poly(ethene)4 as in n = 4 you'd get a hydrocarbon chain that was essentially octane. Why do we bother using the n in shorthand repeating unit notation then, if we were just to classify it all as just poly(ethene) or polythene?
    Oh I see what you mean.
    I think it would be due to polyethene being built up from monomers. Yes, they will have the same molecular formula if n=4, but polyethene is made up from unsaturated ethene molecules. Whereas, octane is octane - you can only have one form of octane.

    I'm not really sure, to be honest. It's a good question!
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    Thanks anyway. It just seems strange that we're taught that you MUST polymerise alkenes in CHEM1 to get plastics yet I presume that when you find hydrocarbon chains that they're quite long before they undergo thermal cracking anyway. How long do they have to be to be used as a plastic? Oh well, if anyone has any insight, I'm interested
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    Do you know the mechanism for addition polymerisation across C=C? (Which is the only kind of addition polymerisation I know of to be fair.) If not, have a brief look at it: you'll see you need a "peroxide initiator", i.e. R(1)-O-O-R(2) (where R(1) and R(2) can be anything - any organic chain, in this case, they can even be metals, I think).

    What happens during the polymerisation is that you get the repeat units which, as you say, are a certain length. However, let me first point out a crucial difference in magnitude. You would rarely just have a plain old alkane of more than, say, 120 units long (C120H242). You will rarely have use for polymer chains less than 2000 units long. That should give you an idea. OK, so it's not necessarily so far apart that we classify them separately - but the synthesis is the point. We can create a 20,000-unit long alkane polymer if we want and put it to good use - from just the single monomer of commonly found length! But however hard we try, we will never find C20,000H40,002 sitting there waiting for us to pick it up and use it for something as mundane as x (you know, whatever you use these for). This becomes even greater when you think about the likelihood of randomly finding a molecule which happens to be a chain of 800 neatly separated units of chloromethane. Do you get what I mean?

    Now, remember that peroxide initiator I mentioned? That brings in the second difference. Polymers consist of repeat units; you tend to draw the C atoms, which were previously double-bonded, as having one bond to outside the repeat unit, to show the connectivity. But what ends those chains? What is at the end? The simple answer is that it depends on the initiator (and don't ask me what part of the initiator - I haven't studied this yet though I'm about to). It could be H atoms, in which case yes, your poly(ethene) will finally just be a massively long alkane which you would not find produced by any other means besides polymerisation as it's far too long just to crop up elsewhere in nature. Or it might be something else. Because addition polymers are usually relied on for their physical properties, it doesn't really matter: the vast majority of the chain will exhibit the properties that it must, so the polymer will behave in the right way (i.e. it will behave as a polymer).

    Hope this makes things clearer
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    (Original post by Big-Daddy)
    Do you know the mechanism for addition polymerisation across C=C? (Which is the only kind of addition polymerisation I know of to be fair.) If not, have a brief look at it: you'll see you need a "peroxide initiator", i.e. R(1)-O-O-R(2) (where R(1) and R(2) can be anything - any organic chain, in this case, they can even be metals, I think).

    What happens during the polymerisation is that you get the repeat units which, as you say, are a certain length. However, let me first point out a crucial difference in magnitude. You would rarely just have a plain old alkane of more than, say, 120 units long (C120H242). You will rarely have use for polymer chains less than 2000 units long. That should give you an idea. OK, so it's not necessarily so far apart that we classify them separately - but the synthesis is the point. We can create a 20,000-unit long alkane polymer if we want and put it to good use - from just the single monomer of commonly found length! But however hard we try, we will never find C20,000H40,002 sitting there waiting for us to pick it up and use it for something as mundane as x (you know, whatever you use these for). This becomes even greater when you think about the likelihood of randomly finding a molecule which happens to be a chain of 800 neatly separated units of chloromethane. Do you get what I mean?

    Now, remember that peroxide initiator I mentioned? That brings in the second difference. Polymers consist of repeat units; you tend to draw the C atoms, which were previously double-bonded, as having one bond to outside the repeat unit, to show the connectivity. But what ends those chains? What is at the end? The simple answer is that it depends on the initiator (and don't ask me what part of the initiator - I haven't studied this yet though I'm about to). It could be H atoms, in which case yes, your poly(ethene) will finally just be a massively long alkane which you would not find produced by any other means besides polymerisation as it's far too long just to crop up elsewhere in nature. Or it might be something else. Because addition polymers are usually relied on for their physical properties, it doesn't really matter: the vast majority of the chain will exhibit the properties that it must, so the polymer will behave in the right way (i.e. it will behave as a polymer).

    Hope this makes things clearer
    Excellent, top answer. I'll look into this initiator/catalyst stuff online. It's completely not needed for CHEM2 AQA but I think you need it for CHEM4 so it's always good to know for next year.

    Thanks a lot

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    (Original post by AdamskiUK)
    Excellent, top answer. I'll look into this initiator/catalyst stuff online. It's completely not needed for CHEM2 AQA but I think you need it for CHEM4 so it's always good to know for next year.

    Thanks a lot

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    You're welcome

    It's part of the free radical addition mechanism stuff. I think it's simple just to keep in mind that you have a peroxide initiator - my guess is that the O-O bond in the R(1)-O-O-R(2) molecule breaks into free radicals - you won't be asked to predict the termination (i.e. what's on the ends) of the molecule at any stage of A-level (I don't think), just draw the repeat units.

    But yeah, that's a good question. The crucial difference is that you wouldn't expect to find such massive molecules just lying out there, but in fact you can make them by quite a straightforward process of just multiplying smaller, common molecules known as monomers.

    Of course, if I do decide to polymerize my ethene and somehow stop it at n=4, with the ends being H atoms - then yeah, call it octane, why not?
 
 
 
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