[Music] in today's video we're going to look at how we can make addition polymers using alkenes with a focus on how to draw the monomers the repeating units and the polymers the feature that makes a molecule an alkene is the fact that it has a carbon-carbon double bond like we can see here between the two carbon atoms in this ethene molecule because of this double bond we say that alkenes are unsaturated and under the right conditions the double bond can break into a single bond which allows the two carbons to form new bonds with something else so if we had three ethene molecules and all the double bonds opened up then they'd be able to form new bonds with each other and so combine to form one long chain which would then call a polymer now one way to represent this reaction would be like this where we show all the ethene molecules which are our monomers on the left and draw out the entire polymer that they form on the right the problem with this technique though is that these reactions can sometimes involve hundreds of monomers so to draw out all of them would take forever a better way to do it is like this where we show a single monomer on the left and a single repeating units of our polymer on the right and when we say repeating units all we mean is the group of atoms that are repeated throughout the polymer and contain exactly the same group of atoms as i ran the monomer so in this case two carbons and four hydrogens when it comes to writing out these equations yourself there's a few points that you need to remember one is that you need to draw the bonds of the monomer facing up and down rather than out at angles and on the repeating unit you need to draw these empty bonds pointing out to the left and right that go through the brackets the other important feature are the ends which represent how many of the monomers and thus how many repeating units there are in the reaction for example if n was 10 then that would mean that there were 10 ethene molecules on the left and so the polymer would be 10 repeat units long to put all of this together let's try and write an equation for the addition polymerization of which is this four carbon alkene the first thing we need to do is redraw the butane so that it's in the same format as this diagram of ethene that we just saw to start we need to identify the double bond which is here between the first two carbons next we need to arrange all of these atoms as separate groups because they're the groups attached to the carbons of our carbon-carbon double bond and if you look back at our ethene example we need to place them directly above and directly below the two double bonded carbons to do this for butane let's start with our two double bonded carbons and then we'll rearrange each of the separate groups one by one so that it's easy to follow along with what's happening this first hydrogen needs to change from being shown sticking out at an angle to being shown like this pointing straight up from the carbon and we do exactly the same thing for this hydrogen so that it points straight down rather than sticking out an angle then this hydrogen over here is already pointing straight down so we just redraw it exactly the same way finally for this big group on the right we use a little trick and simplify it before moving it about to do this we just count up all the carbons and hydrogens and write them out in shorthand so because there are two carbons and five hydrogens we write this group as c2 h5 and then we just do exactly the same things before and rewrite the group so that it's directly above our double bonded carbon one thing to be aware of though is that it's important that this bond is drawn going from carbon to carbon rather than to the h5 part now that it's drawn in the right format we can add the brackets and put the n in front of it next we can draw our reaction arrow and then draw the other side of the equation that shows the repeating unit we draw pretty much exactly the same thing but notice that we've taken out the double bond and instead put the carbon's bonds out to the sides extending through the brackets and remember for a repeating unit the n goes here on the bottom right just outside the brackets the last thing we need to cover is how to name the polymers which is actually really easy because all we have to do is put the word poly in front of the monomer's name and put the monomer's name in brackets so in our example here where we're drawing the polymer of butane the monomer is butane and therefore the polymer is called polybutane or if we go back to the first example we did the polymer would be called polyethylene or if we had this monomer which is called chloroethene we'd make the polymer polychloroethylene the very last thing we need to mention is that these addition polymerization reactions require high pressures and a catalyst to work so you might sometimes see pressure and catalyst written on the reaction arrow you don't have to put this on the arrow yourself but you might be asked for these two conditions anyway that's everything for this video so hope that all made some sense and cheers for making it through to the end