Which among the following is not a polyester?

The term “polyester” is reserved for polymers whose backbone contains the characteristic ester linkage $$\left( -\,\mathrm{COO}\,- \right)$$ repeated again and again. So, to decide whether a given polymer is a polyester, we only have to inspect the type of linkage present in its main chain.

We have, first of all, the definition of an ester linkage:

$$\text{Alcohol + Carboxylic acid} \xrightarrow{\;\text{condensation}\;} \text{Ester} + \mathrm{H_2O}$$

and the corresponding polymerisation (step-growth) reaction produces an entire chain of $$\left( -\,\mathrm{COO}\,- \right)$$ units.

Now let us examine the four polymers one by one.

1. Glyptal: It is obtained by condensation of glycerol $$\left(\mathrm{HO{-}CH_2{-}CH(OH){-}CH_2OH}\right)$$ with phthalic anhydride $$\left(\mathrm{C_6H_4(CO)_2O}\right)$$. Every time an -OH of glycerol reacts with a -COOH of phthalic acid, an ester linkage $$\left( -\,\mathrm{COO}\,- \right)$$ is produced. Hence the entire backbone is full of $$-\,\mathrm{COO}\,-$$ groups. Therefore Glyptal is definitely a polyester.

2. PHBV (poly(3-hydroxybutyrate-co-3-hydroxyvalerate)): The individual monomers $$\mathrm{HO{-}CH(CH_3){-}CH_2{-}COOH}$$ and $$\mathrm{HO{-}CH(C_2H_5){-}CH_2{-}COOH}$$ combine through their -OH and -COOH ends to give the repeating unit

$$[-\mathrm{O{-}CH(R){-}CH_2{-}CO}-]_n$$

where $$R = \mathrm{CH_3}$$ or $$\mathrm{C_2H_5}$$. Each repeat contains the $$-\,\mathrm{COO}\,-$$ linkage, so PHBV is a biodegradable polyester.

3. Novolac: This polymer results from the condensation of phenol with formaldehyde under acidic conditions. The linkage formed is a methylene bridge $$\left( -\,\mathrm{CH_2}\,- \right)$$ connecting two aromatic rings:

$$\mathrm{Ph{-}OH + HCHO \;\;\longrightarrow\;\; Ph{-}CH_2{-}Ph + H_2O}$$

Repeating this gives

$$[-\,\mathrm{Ph{-}CH_2{-}}\,-]_n$$

Throughout the backbone, there is no $$-\,\mathrm{COO}\,-$$ group at all. The network is made of $$-\,\mathrm{CH_2}\,-$$ bridges, so Novolac is a phenol-formaldehyde resin, not a polyester.

4. Dacron (polyethylene terephthalate, PET): Formed by condensation of ethylene glycol $$\mathrm{HO{-}CH_2{-}CH_2OH}$$ with terephthalic acid $$\mathrm{HOOC{-}C_6H_4{-}COOH}$$, the resulting repeating unit is

$$[-\,\mathrm{O{-}CH_2{-}CH_2{-}O{-}CO{-}C_6H_4{-}CO}\,-]_n$$

in which the ester linkage $$-\,\mathrm{COO}\,-$$ clearly appears. Therefore Dacron is a classic polyester.

Summarising, Glyptal, PHBV and Dacron all possess ester linkages in their main chains, while Novolac does not. Hence the only polymer in the list that is not a polyester is Novolac.

Hence, the correct answer is Option C.