PerspectiveChemistry

Plastics recycling with a difference

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Science  27 Apr 2018:
Vol. 360, Issue 6387, pp. 380-381
DOI: 10.1126/science.aat4997

Since the synthesis of the first synthetic polymer in 1907, the low cost, durability, safeness, and processability of polymers have led to ever-expanding uses throughout the global economy. Polymers, commonly called plastics, have become so widely used that global production is expected to exceed 500 million metric tons by 2050. This rising production, combined with rapid disposal and poor mechanisms for recycling, has led to the prediction that, by 2050, there will be more plastic in the sea than fish (1). On page 398 of this issue, Zhu et al. (2) report an important step toward addressing this problem with the synthesis of a plastic with mechanical properties comparable to those of commercially available plastics, but with an intrinsically infinite recyclability.

Repeatedly recyclable polymers

Zhu et al. report production of a plastic that can be recycled repeatedly through chemical methods without loss of function. Blending of the two enantiopure polymers yields a plastic that can withstand higher temperatures, expanding its usefulness further.

GRAPHIC: N. DESAI/SCIENCE

The production of synthetic plastics is far from being sustainable (3, 4). Most plastics are produced for single-use applications, and their intended use life is typically less than 1 year. Yet the materials commonly persist in the environment for centuries. More than 40 years after the launch of the recycling symbol, only 5% of plastics that are manufactured are recycled, mainly mechanically into lower-value secondary products that are not recycled again and that ultimately find their way to landfills or pollute the environment (1). With these materials being lost from the system, there is a constant need for the generation of new plastics, mostly from petrochemical sources, thus further depleting natural resources. Although there has been a substantial effort to develop biodegradable plastics, with polylactide arguably the most successful example, the mechanical and thermal properties of these materials still need to be improved to be substitutes for a wider range of existing materials properties (3).

In the past decade, an alternative sustainable strategy has been proposed in which the plastic never becomes waste. Instead, once used, it is collected and chemically recycled into raw materials for the production of new virgin plastics with the same properties as the original but without the need for further new monomer feedstocks (5). This strategy not only helps to address the environmental issues related to the continual growth of disposed plastics over the world but may also reduce the demand for finite raw materials by providing a circular materials economy (6).

Chemical recycling methods have been classified into two groups. In the first, plastic waste is converted directly into products with high added value (7). For example, Hedrick and co-workers have pioneered the depolymerization of aromatic polycarbonates and polyesters directly into bisphenol-type monomers that can be used for the preparation of high-value poly(aryl ether sulfones) (8, 9). In the second, plastic waste is depolymerized back to the starting material and then repolymerized to yield virgin-like plastics (10, 11). For example, Hong and Chen have shown that poly(γ-butyrolactone) can be quantitatively depolymerized back into the initial γ-butyrolactone by simply heating the bulk material (12). However, plastics that can be so easily depolymerized lack suitable mechanical and thermal properties to be widely useful. This is the central paradox of the plastic problem: Although we desire our plastics to be readily recyclable or not to persist in the environment, they must also be sufficiently robust to function in their desired application.

In the search for an intrinsically recyclable plastic with robust mechanical properties, Zhu et al. have designed a variant of the five-membered γ-butyrolactone that bears a fused cyclohexyl ring with defined stereochemistry (see the figure). The steric hindrance and stereoregularity that result after polymerization yield a semicrystalline polymer that has better thermal and mechanical properties than the unsubstituted derivative. Furthermore, retention of the five-membered γ-lactone core ensures that the thermal recyclability is preserved. The monomer incorporates the cyclohexyl ring fused to the α- and β-positions but leaves the γ-position unsubstituted. The results suggest that maintaining the γ-position unsubstituted is essential to obtain a monomer with sufficient ring strain to be readily polymerized.

Moreover, by judicious choice of catalyst, the authors were able to retain high stereo-regularity and prepare highly isotactic polymers in which all stereocenters are aligned along the same side of the polymer chain. These polymers are semicrystalline and possess mechanical properties that compare well with those of polylactide.

Furthermore, the authors found that by blending two enantiomerically pure polymers of opposite stereochemistry in a 1:1 stoichiometric ratio, superior materials could be obtained. Because of a phenomenon known as stereocomplexation (13), the materials that result from the simple blending process have melting temperatures that are more than 75°C higher than those of either constituent polymer, potentially enabling use in high-temperature applications.

Plastics will continue to be critical for addressing the continuing demands of our society. New polymeric materials will, for example, be needed for energy generation and storage, to address healthcare needs, for food conservation, and for providing clean water. The circular materials economy will require implementation of an appropriate infrastructure to underpin collection and sorting of plastic that has reached the end of its first life before it reaches the environment (4). Beyond the design of new materials, this will require collaboration across scientific and nonscientific disciplines as well as political and public will to ensure success.

Studies such as that of Zhu et al., in which disposed plastics can be infinitely recycled without deleterious effects on their properties, can lead to a world in which plastics at the end of their life are not considered as waste but as raw materials to generate high-value products and virgin plastics. This will both incentivize recycling and encourage sustainability by reducing the requirement for new monomer feedstocks. Current chemical recycling processes are expensive and energetically unfavorable, and further advances in monomer and polymer development and catalyst design are required to facilitate the implementation of economically viable sustainable polymers (14).

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