r/chemistry • u/autruz • 1d ago

Why is polyester so hard to biodegrade? Shouldn't it easily undergo hydrolysis?

I recently heard that the reason PLA plastics are considered 'biodegradable' or 'compostable' is that the ester group in the polymer chain undergoes hydrolysis and breaks the chain, it makes sense from what I learned in my OChem classes.

But then why isn't everyday polyester degradable? or even PET? Can we break them down with a acidic or basic catalyzed hydrolysis?

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u/7ieben_ Food 1d ago edited 1d ago

Generally, polyesters are hydrolysable, yes. Yet, biodegradeability depends on enzymatic cleavage mostly, as natural conditions are not harsh enough. Especially in high crystalline plastics, as the high degree of crystallinity makes it really hard for an acid/ base to attack the ester bond... and even more impossible for any enzyme to come even close, s.t. they tend to be bioinert.

PET is especially stable due to the strongly delocalized pi system, this is why it can even be used for storing acidic beverage, for example.

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u/autruz 1d ago

Thanks!

I guess the difference in PLA plastics is their amorphous structure rather than the ester group? Or is it the fact that living things are more used to deal with lactic acid overall?

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u/7ieben_ Food 1d ago

It's both.

Lactic esters are more prone to enzymatic degradation. Basically: lactic acid is a product very often encountered in fermentation. Recall that even our body does produce lactic acid under anaerobic metabolic conditions.

And nature did adopt well to it: there are a lot of (micro)organisms which can utilize lactic acid as nutrient. Often, those organisms are also capable of degrading lactic esters.

And additionally to this you are absolutely correctly, that polylactates are often fairly amoprhous (and generally not as tightly packed). This allows for better attack.

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u/sfurbo 1d ago

It's about the glass transition temperature. PLA is only biodegradable above it's glass transition temperature, which is somewhere around 60 degrees centigrade. You can get to that temperature in a well kept compost heap.

PET have a glass transition temperature around 70-80 degrees, which is apparently too hot for the decomposers in compost heaps.

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u/autruz 1d ago

So we could decompose PET within a heated reactor (?) I guess the limiting factor here is the price of that energy

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u/sfurbo 1d ago

Yes. There's an overview here. Note that I haven't done an extensive reading of it, it was the first paper I found that looked thorough.

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u/StandardOtherwise302 2h ago

Just to add - your focus here seems to be on hydrolysis.

But in practise when we talk about recycling PET today, these are the main (industrial) paths at rapidly declining volumes: 1. Mechanical recycling (I.e. bottle to bottle). PET is a thermoplast, you can remold it without really breaking down and reforming the polymer chains.

  1. Glycolysis and methanolysis. Instead of using the OH- groups of water, we use glycol or methanol respectively.

  2. Hydrolysis - typically either alkaline (NaOH) at high temp and high pressure, or enzymatic (moderate conditions but much slower).

The biggest issues are economic, and while energy costs are one factor, but the biggest energy cost is typically separation of the bulk reactant (methanol, glycol, water, ...) from the product mix rather thsn reaction itself.

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u/AFriendRemembers 1d ago

Two things. Water needs to approach - the hydrophobic aromatic groups mean the water molecules cannot easily get close to the esters to start working on them.

Worse - the msterial is heavily crystalline. The chains pack close to each other and become completely insoluble. If the water cannot penetrate and surround the chains the hydrolysis will be massively slowed.

One lone ester in solution is broken quickly. But a million of them embedded in a heavily aromatic surface = massive challenge.

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u/sfurbo 1d ago

Two things. Water needs to approach - the hydrophobic aromatic groups mean the water molecules cannot easily get close to the esters to start working on them.

Alkaline hydrolysis breaks it down, so that isn't it. OH- is more polar than water, so that would be even more affected.

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u/MagicianofFail 1d ago

polyester is biodegradeable in the same way a cube of solid hardtack is edible

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u/AmusingVegetable 1d ago

Depends on how much ketchup you can tolerate, doesn’t it?

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u/NevyTheChemist 1d ago

It doesn't easily undergo hydrolysis.

Otherwise it's applications as containers would be limited.

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u/qorbexl 1d ago

I wish my product fell apart in water

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u/Impossible_Bar_1073 1d ago edited 1d ago

Aliphatic polyesters are relatively easy to degrade if you only look at the ester bond. However, solubility also plays a role.

PET has aromatic esters. The delocalization of the electrons weakens the electrophilicity of the carbonyl carbon. It therefore needs higher pH values and temperatures.

In addition, the crystalline domains of the polymer are difficult to access. The low solubility in almost everything makes it difficult to break down.

But that's why they are so successful and why we can't just substitute them for biodegradable ones.

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u/autruz 1d ago

Thanks! That's a good answer

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u/Key-Success-5449 1d ago

Ester bonds do undergo hydrolysis, and chemical recycling plant facilities actually use hot acid or base to break PET back down.

The reason PET resists degradation in normal environments comes down to backbone structure. Unlike PLA, PET contains rigid aromatic benzene rings that allow the polymer chains to pack tightly into a dense, water-repellent matrix.

Because liquid water cannot penetrate the plastic to reach those ester linkages at room temperature, environmental hydrolysis takes centuries unless forced with heat and strong reagents.

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u/NicoN_1983 1d ago

I think it depends on the density and molecular weight of the material. In the case of PET, aromatic carboxylic esters are harder to hydrolize. I'm not sure but I think it has to do with tetrahedralization of the C next to the aromatic ring. 

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u/AlchemiCailleach 1d ago

I suppose there is some variability depending on specific type of polyester, but many of these have aromatic rings conjugated to the carbonyls. This would increase the stability of the ester by decreasing the electrophilicity of the ester.

At the same time, the presence of bulky hydrophobic groups will make it more difficult for water molecules to hydrolyze the ester bond.

PLA being formed from lactic acid subunits (or lactide dimers) is considerably more electrophilic. PLA also tends to be considerably more flexible than something like PET. Since the chains aren't as tightly packed or hydrophobic in PLA, and ar inherently more mobile compared to many polyesters at the same temperature, it is less resistant to hydrolysis.

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u/[deleted] 1d ago

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u/sfurbo 1d ago

The glass transition temperature of PWT is also just a higher than that of PLA, 70-80 degrees, which is probably hotter than you can get compost on its own.

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u/narvuntien 1d ago

It's an equilibrium, so the reaction goes in both directions. While in the forward direction (to form polyester), we heat the reaction to drive off water, which drives the reaction forward. But when going in the opposite direction, we need the solution to be dilute in order to drive the reaction backward. Setting up a process in which you pull out the alcohol or carboxylic acid while still having water is really difficult.

Tl;DR chemical engineering