Which repeating unit for poly lactic acid (PLA) is correct, picture 1 or picture 2?


I read somewhere that in polymerization, acids give $\ce{OH}$ and alcohol gives $\ce{H}$, creating water. According to this explanation, picture number 2 is correct but I have seen on websites like Wikipedia that picture 1 is correct. So, which is the correct picture and why?


1 Answer 1


In fact, both pictures show the same polylactic acid, only different disconnection points have been chosen:

enter image description here

The red disconnection (or your picture 1) is preferred because you can immediately see that it is a polyester.


Your teacher is right in that, during the polymerization, the $\ce{-COOH}$ group loses $\ce {-OH}$ and the $\ce{-OH}$ group loses $\ce {-H}$ to form $\ce{H2O}$. Therefore, your teacher prefers this arrangement:

enter image description here

If we draw it the other way, it are the wrong $\ce{-OH}$ and $\ce{-H}$ at the end of the chain, in the sense that it are not the ones that will give water:

enter image description here

However, note also that in either case, the part between brackets is not formed by identical fragments. The atoms are indeed the same, but since not all are connected to the same atoms, they are not all chemically equivalent. There are different functional groups. There are acid, alcohol and ester groups; mainly ester of course. So in reality, we are writing $\ce{[...]_n}$, but those $n$ are not all the same and thus it is not correct.

If you want to depict the whole chain including the polymer chain and the ends that can continue polymerizing, the most correct way is to draw the end monomers explicitly. At that point, you can place the brackets where you want, since the structures are absolutely equivalent.

enter image description here

  • $\begingroup$ Thanks for your answer, but my teacher don't accept picture 1 , he said that in polymerization,the acid gives OH and the alcohol give H so he chose the picture 2 because in this picture if you want extend repeating unit , you must add OH to acid (acid part in picture 2 without OH) and H to alcohol (alcoholic part in picture 2 without H) , so he said that pic 2 is correct.I said to him that both picture are correct but he don't agree with . on your opinion how can I convince him that both pictures are correct?If you have any resources , please write them,then I show the resources to my teacher. $\endgroup$ Jun 12, 2018 at 11:43

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