in today's video we're looking at condensation polymers and in particular add a type called polyesters which are made using these ester links to make things easier we're going to run through an example first to give you an idea of how it works and then we'll explain exactly what's going on afterwards like all polymers condensation polymers are made up of lots of individual monomers with polyesters though a common combination is to use two different monomers a dicarboxylic acid monomer which contains two carboxylic acid groups and a diol monomer which contains two alcohol groups and as a quick aside if you haven't seen these coloured rectangles within molecules before they just represent the rest of the molecule and we tend to use them in general examples like here where we don't want to make things complicated by showing the specific molecules in order for the two monomers to combine the dicarboxylic acid has to give up its oh group and the diol gives up a hydrogen atom from its oh group and when these three atoms combine they form a water molecule this leaves this carbon from the dicarboxylic acid to bond directly to this oxygen from the dial and it's this bond that we call the ester link at the moment this is technically just a dimer because there's only two monomers combined to show it as a repeating unit though we need to remove this o h and h from the ends and then point the empty bonds out to the sides so that they can bond on to other repeating units and importantly because we removed an o-h and hydrogen again it will form another water molecule then to finish we add these big brackets to either end that cut through the bonds if we now put all of this together we effectively had a dicarboxylic acid monomer plus a diol monomer so to form a condensation polymer which in this case was a polyester because this group in the middle is an ester link and we also formed two molecules of water in real life this process often happens with hundreds or thousands of monomers and so instead of writing the exact number of molecules we have in front of each molecule we instead use the letter n to represent how many there are and we put the n in front of each of our reactants and in the bottom right corner of our repeat units and we need to put a 2n in front of our h2o because we formed two water molecules per repeat unit this is actually why we call these polymers condensation polymers because the process forms water molecules like when gaseous water condenses in order for molecules to be able to combine in condensation polymers there's a few important things that they need to have one is that each of the monomers has to have at least two functional groups for example our dicarboxylic acid has two carboxyl groups and our dial has two alkyl groups second there needs to be at least two different functional groups overall so here we have the carboxyl and the alcohol group and finally a small molecule is given off in the process which is generally water to show you a real example ethan diaric acid can combine with ethan diol to form poly ethyl ethanoate and water the very last thing we need to say is that polyesters are generally biodegradable which means they can break down naturally because bacteria and other microorganisms can break down the ester links importantly this is a big difference to addition polymers like plastics which generally aren't biodegradable and so stay in the environment for ages anyway that's everything for this video so hope you found that useful if you did then please do give us a like and subscribe and we'll see you again soon