New strategy for recyclable polyolefins
© Wiley-VCH, re-use with credit to 'Angewandte Chemie' and a link to the original article.
It is hard to imagine our daily lives without plastics made out of
polyolefins. Unfortunately, practical methods for recycling of
polyolefins are lacking. In the journal Angewandte Chemie, a
research team has now introduced a new approach for making novel
polyolefins that can be chemically deconstructed and re-polymerized
without a loss of quality. The secret to the method is masked double
bonds introduced to the polymer chain by means of a so-called “Trojan
horse” functional group in the polymer chain.
Polyolefins are stable, light, versatile, and inexpensive plastics
made of very long hydrocarbon chains. However, their high stability and
durability come with a drawback: after use, polyolefins are extremely
persistent in the environment. Mechanical recycling forms products with
inferior properties. The high chemical stability of polyolefins also
inhibits chemical depolymerization to get back the monomers.
It would be more sustainable to have a circular economy based on
alternative, chemically recyclable polyolefins that could be
disassembled into smaller fragments, purified, and polymerized again.
These smaller fragments are also more likely to be biodegradable if they
accidentally enter into the environment. Unlike conventional
polyolefins, such polymer chains would need to contain cleavable
functional groups. A team led by Geoffrey W. Coates at Cornell
University (Ithaca, NY, USA) has now taken a step further down this
road. Their novel approach is based on a small number of unsaturated
bonds that are built into the polyolefin chains with the help of a
“Trojan horse” protecting group.
To do so, the team polymerizes ethylene to polyethylene in the
presence of a special cyclic co-monomer (oxa-norbornadiene). Heating the
copolymer causes the Trojan horse to “open up”; the rings are then
cleaved with a retro Diels–Alder reaction, leaving behind a double bond
in the polymer chain. The distance between the double bonds in the
polymer backbone can be controlled by varying the amount of co-monomer
in the feed.
In a reaction known as olefin cross-metathesis, the chains are split
apart at the double bonds and attached to another molecule
(2-hydroxyethyl acrylate) forming smaller chains with reactive groups at
the ends. These macromonomers are then linked back together to form PE
with low amounts of ester bonds.
By this method, the team was able to produce polymers with thermal
and mechanical properties corresponding to those of high-density
polyethylene (HDPE), a very important and widely used plastic. In
contrast to HDPE, the new material has ester linkages along the polymer
backbone. When recycled, the polymer chains can be chemically split at
the ester bonds to regenerate the original macromonomers. After
purification, these can be repolymerized again, without a loss in
quality.
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About the Author
Geoffrey W. Coates
is the Tisch University Professor in the Department of Chemistry and
Chemical Biology at Cornell University. He has worked in the area of
sustainable polymers for over 25 years, and is a member of the US
National Academy of Sciences.
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