What is the relationship between the monomer and polymer in PGA?

[soopo]

Esters have the form R-CO-OR'.

PGA has the form [-O-CH3-CO-].
It is a polyester that means an ester according to Wikipedia.
I cannot see why it is an ester.
It does not have two oxygens bind to one carbon.

Why is PGA an ester?

 

[chemisttree]

Esters have the form R-CO-OR'.

PGA has the form [-O-CH3-CO-].

No it doesn't. Review that part again.

 

[soopo]

No it doesn't. Review that part again.

The form of the ester is right: http://en.wikipedia.org/wiki/Ester

One part of PGA has the above form.
I had only the given part of PGA in the exam. It was my first time to see PGA.

I am trying to deduce somehow that PGA has an ester -part.
PGA is poly glycosidic acid.
This means that there is a glycosidic bond in the polymer, probably many.
Glycosidic bond is within an ester: http://en.wikipedia.org/wiki/Glycosidic_bond

Does an ester have always a glucosidic bond?

It seems that the key to solve the problem was to think about the abbreviations of PGA, not the single unit of the polymer.

 

[chemisttree]

5 bonds to carbon. It's wrong.

You gave a general formula for an ester as R-CO-OR' but you chose to show it in a different way for the polymer. Does this help?

PGA (polyglycolic acid) = [-CH2-COO-]
which is equivalent to [-OCH2-CO-]

 

[soopo]

5 bonds to carbon. It's wrong.

You gave a general formula for an ester as R-CO-OR' but you chose to show it in a different way for the polymer. Does this help?

PGA (polyglycolic acid) = [-CH2-COO-]
which is equivalent to [-OCH2-CO-]

WOOT!
Thank you!
I did not get it that it is the monomer of the PGA.
It seems that PGA does not have different monomers: only the one.

I now see that the monomer really make the polymer an ester.