Making plastic more degradable

Despite its low cost, ease of production, and useful mechanical properties, plastic has at least one big problem. Polyethylene, the most abundant type, takes hundreds of years to decompose and has polluted the environment in various ways (see, for example, “It takes a global village to track plastic waste,” Physics Today online, 7 May 2021 ).

Polyethylene is manufactured by linking together, or polymerizing, monomers of ethylene via a catalyst, typically a transition metal compound. Copolymerizing the ethylene monomers with carbon monoxide fits ketone functional groups—molecules with a carbon–oxygen double bond—into the polymer chain in an alternating fashion, which yields polymers with high melting points and entirely different properties than polyethylene.

Now Maximilian Baur, Stefan Mecking, and their colleagues at the University of Konstanz in Germany have tweaked the copolymerization process using a nickel-based catalyst. That choice produces nonalternating ketone groups in the polyethylene chain, a patterning that maintains polyethylene’s typical structure and mechanical properties yet allows the material to break down at ambient conditions when exposed to sunlight.

The catalysts the researchers considered are based on metals with eight d-orbital electrons. Previous efforts showed that metal catalysts with that electron configuration work well with CO in other polymerization reactions. Earlier papers had considered a palladium-based catalyst for CO–polyethylene copolymerization, but its performance was disappointing. (Pd lies in the same group as nickel in the periodic table.) That was one reason, among others, why Baur and his colleagues decided to explore nickel-based catalysts.

Baur and coauthor Tobias Morgen experimented with different mixtures of ethylene and carbon monoxide, using low concentrations of the latter. Too much CO blocks coordination sites, which restricts the polymer yield and favors alternating structures. Too little CO yields an undesirably resilient chain. The reaction temperatures had to be kept above 70 °C to produce the nonalternating pattern.

An ethylene-to-CO ratio of about 100:1 formed polyethylene chains that had structures containing an ideal fraction of nonalternating ketones yet retained polyethylene tensile strength. In short-term irradiation experiments, the plastic film samples became brittle and broke into smaller pieces after they were exposed to the equivalent of five months of natural sunlight.

Making big pieces of plastic into smaller ones doesn’t immediately solve the pollution problem. But according to a previous paper on the topic, the smaller polyethylene chain segments remaining after degradation may be of a size that could more fully deteriorate by mineralization through natural geochemical processes. (M. Baur et al., Science 374, 604, 2021 .)