A short fragment of a polyester chain (end groups shown) is drawn. It is formed by head-to-tail esterification of a single hydroxy acid monomer. How many molecules of water are eliminated when this one fragment is formed from its monomer molecules? Give the answer as an integer.

Numerical answer type, tolerance 0.5.

Answer: 3

The fragment is poly(lactic acid), PLA, made from lactic acid (2-hydroxypropanoic acid). It contains four lactyl units joined by ester bonds; counting the ester bonds with RDKit gives 3. Each ester bond formed releases one water, so 4 monomers join with loss of 3 molecules of water. The answer is 3.

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