Recycling of plastics enabled by dynamic covalent diketoenamine bonds (2018) [pdf]

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.

> 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