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9 Comments:
apo said 11 days ago:

The idea is to construct plastics using reversible polymerization "click" reactions. Polymerize the monomers under mild conditions. Recover them again through acid degradation. Build entirely new kinds polymers (if you want) with the monomers you recover.

Notable features include:

1. synthesis involves ball-milling the starting materials, which eliminates the need for solvent

2. decomposition in sulfuric acid (0.5-5 M) gives one component of the pair of monomers (triketone) cleanly, but amine recovery is less clear. Quantitative recovery wasn't clear from my reading.

3. the mechanical properties of the small set of possible polymers produced from this system appear consistent with at least certain kinds of industrial use.

4. the chemistry lends itself to the production of a wide range of different polymer structures, but the sensitivity of the key bond forming/breaking step to starting monomer modification remains to be shown.

Iv said 11 days ago:

Let me do a tl;dr because that's an important subject that could cange plastic from environmentally problematic material to a highly renewable one.

Recycling plastic is hard because we have different types of plastic for different uses. If you mix them in a trash, it is hard to take them apart and recycling them together leads to very poor materials.

Nowadays, there is active research in making a more intensive recycling that retransforms plastics into their precursors, which are basically the same even for different types of plastic, so this is a process that do not require to sort plastics by type.

Only scanning through the article, it looks like this is a solution to separate these precursors from the remaining impurities (coatings, ink, colorant) and get pure monomers. That's an important step in the process.

softwarelimits said 11 days ago:

Because that institution also seems to be heavily engaged in the area of "Intellectual Property" I would like to mention:

one big reason for the problems with plastic recycling is the fact that producing companies do want to "protect" their "intellectual property" and do not provide a list of ingredients.

This is an area where IP does have a very big impact in the global contamination process.

userbinator said 11 days ago:

one big reason for the problems with plastic recycling is the fact that producing companies do want to "protect" their "intellectual property" and do not provide a list of ingredients.

That has to do with the additives, not the base --- and you are correct that separating these otherwise contaminants from the base material is important for recycling. On the other hand, everyone knows what the base materials for Teflon and polyethylene are, for example.

rumanator said 11 days ago:

Thank you for the summary. Very insightful and very well put in layman's terms.

twic said 11 days ago:

I don't think this is quite right. You say:

> Nowadays, there is active research in making a more intensive recycling that retransforms plastics into their precursors, which are basically the same even for different types of plastic, so this is a process that do not require to sort plastics by type.

This paper is about poly(diketoenamine)s, a specific kind of polymer made from a specific kind of precursors - a combination of a beta-triketone (whatever that is - seems to be a carbon ring with three ketone groups somewhere on it) and a substituted amine. The key advance is that it's really easy to break this polymer back down into its precursors by acid hydrolysis (boiling it in concentrated sulphuric acid for a while), after which you can separate and purify the monomers.

From the paper:

> PDKs hydrolyzse in strong aqueous acid (0.5–5.0 M H2SO4) at ambient temperature to yield pure, immediately reusable triketones (Supplementary Fig. 2); amine monomers are then recovered using a regenerative resin-based process, closing the loop (Fig. 2a and Supplementary Fig. 3). In 5.0 M H2SO4, complete depolymerization occurs in less than 12 h, and pure triketone and amine monomers are recovered in >90% isolated yields (Fig. 2b–d); 5.0 M HCl was similarly effective in PDK depolymerization. [...] From complex mixtures, triketone monomers are easily separated from additives, and other polymers, by extracting into aqueous base, filtering off insoluble components and subsequently precipitating triketones upon re-acidification (pH < 3)

The "amine monomers are then recovered using a regenerative resin-based process" feels like it's sweeping some complexity under the rug; maybe it's really easy to recover the triketones, but not the amines.

Also:

> Under the same conditions, depolymerization is not observed for common plastics in use today, allowing PDKs to be easily separated from mixed plastic waste streams (Fig. 2e).

A lot of the paper is about showing that other plastics, and other ingredients in manufactured articles that might be made of PDKs, won't get drawn into the recovery stream, which is nice.

Also:

> Notably, ß-triketone monomers are accessed in a single step from widely available polytopic carboxylic acids and 1,3-diones.

So this stuff is pretty easy to make.

Another nice detail is about this idea of "vitrimers":

> In particular, dynamic covalent polymers known as vitrimers have been recently proposed as sustainable replacements for non-recyclable thermoset materials. Vitrimers undergo associative bond exchange reactions in the solid state, allowing crosslinked materials to be thermally processed and recycled like thermoplastics while maintaining high crosslink density

Thermoset polymers tend to be stronger and tougher than thermoplastics, and so are used for all sorts of important things [1], so having a class of recyclable alternatives is particularly cool. There's a lovely example in the paper where they make fibreglass with PDK, then recover the PDK by hydrolysis, leaving clean glass wool behind!

The one big thing i don't see in the paper is anything about the mechanical and thermal properties of the bulk polymer (although you can read the modulus of elasticity off some of the graphs). Those properties have to be competitive with existing polymers for PDKs to take off. I imagine you can tune the properties quite a lot by the choice of amine, though, so perhaps that is an area for future work.

[1] https://en.wikipedia.org/wiki/Thermosetting_polymer#Examples

Iv said 10 days ago:

Yeah, looks like I read too quickly and skipped too many terms I did not know. Thanks!

softwarelimits said 11 days ago:

Using the big searchbox it is not possible to find this document, you need to use the small searchbox on top.

Then you can find this post that seems to be related.

https://foundry.lbl.gov/2019/06/05/new-polymers-close-the-lo...

mrnobody_67 said 10 days ago:

Bunch of interesting stuff being done to turn plastic into diesel: https://www.cbc.ca/news/canada/nova-scotia/sustane-technolog...