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Learning about biodegradable polymers and non-biodegradable polymers. What exactly sets them apart though? Here's the formula from wikipedia for the biodegradable PHB:

here

and here for polypropylene, it's non-biodegrable counterpart

here

As far as I understand, biodegradable just means that bacteria will degrade it into other constituent parts. Is it just that bacteria is a picky eater and won't eat certain polymers? But they seem to be made of similar elements, and the bonds between are generally similar. Any help is helpful.

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    $\begingroup$ The former contains oxygen and the bonds are not very similar at all. The polar nature of PHB suggests that it is more reactive, at least in a biochemical sense. $\endgroup$
    – canadianer
    Nov 23, 2014 at 7:35

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The difference between biodegradable polymers and 'regular' polymers is the bonds that make up the backbone. The backbone is the part of the polymer that extents the brackets. Anything attached to that chain is a side group and is less important.

The two examples you show here are very dissimilar. The polypropylene is only made up of Carbon-Carbon bonds. These bonds are very stable: that holds for chemical and 'biological' stability. The PHB how ever bonds monomers with an Ester functional group (Ester). This group is far more reactive that a C-C bond.

The reactivity of the groups the polymer is made up of, is very, very important. That being said, I cannot claim that all poly-esters are biodegradable. Depending on the groups close to the esters and the enzymes (probably from bacteria) involved, the bond might be cleaved, or might be inaccessible.

Hope this helped!

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    $\begingroup$ It is also useful to see that terms like "biodegradable" are not chemical or biological terms which are quantifiable. They are terms used to describe the relation of a chemical in the context of the world. Often "non-biodegradable" plastics could be broken down, it is just that these species are in relatively low frequencies. $\endgroup$
    – user85426
    Sep 7, 2020 at 11:02
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An all-carbon backbone is generally less biodegradable, but what really makes polymers badly degradable are high crystallinity and/or hydrophobicity and/or high glass transition temperature. Wood for example is highly crystalline, and can only be degraded by a few specialists (which have had half a billion years to evolve), unless some small animal gives mechanical help.

Same with many polyesters, polyamides and other fiber-forming polymers. They hardly swell in cold water, and so any enzyme would have to break the crystalline order and the backbone bonds at the same time. That kind of energy is usually not available in cold water.

Polystyrene is hydrophobic and has a high Tg, sunlight, oxygen and water have to make the surface a bit more polar chemically before anything biological can attack. The good thing is PS is most often foamed and has a huge surface, plus it is so brittle that it cracks easily, further increasing the surface.

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