1.1. Catalytic Upcycling Strategies for Polyolefins
Plastics are unquestionably one of the most important technologies of the last century. They have transformed sectors as diverse as health care, food safety, textiles, electronics, transportation, machinery, and construction, and have made a vast array of consumer goods more broadly accessible to people in all sectors of society. Since the early days of mass production, the global production of plastics has soared, from less than 2 million metric tonnes (MMT) in the early 1950s to nearly 450 MMT today. (1,2) On our current trajectory, it is possible that by 2050, the world will be producing 1100 MMT plastics each year. (3) By 2050, the plastics industry is predicted to consume as much of 20 % of remaining oil production, while generating 15% of annual carbon emissions. (4)
The commodity polyolefins (POs) are polymers of simple alkenes or substituted alkenes, namely, ethylene (PE), propylene (PP), vinyl chloride (PVC) and styrene (PS). These four POs represent the majority of modern plastics. In 2021, PE accounted for approx. 27 % of plastic production, including low-density polyethylene (LDPE) and high-density polyethylene (HDPE), while PP constituted about 20 %, PVC about 13 %, and PS about 5 %. (5) Thus POs represent two-thirds of all plastic production and a similar fraction of the plastic waste. (6)

This immense manufacturing scale, combined with the low cost of POs, has led to a proliferation of short-term uses and a corresponding surge in waste, with an estimated 353 MMT plastic waste generated in 2019 alone. (2) Globally, approx. 49 % ends up in landfills. About 19 % is incinerated, which can achieve a modicum of energy recovery but which also releases CO2 and (potentially) toxic organic compounds into the environment. Conventional recycling (9 %) has not thus far proven to be a viable solution to address these issues. (7) In addition, ca. 22 % of the waste is not responsibly managed, leading to leakage into the natural environment including the rivers and oceans. This leakage contributes to microplastics and nanoplastics in water, causing widespread public concerns about health impacts. (Note however that an early and highly publicized estimate of the amounts of these microscopic plastic particles that humans ingest has now been shown to be many orders of magnitude too high, (8) providing an important cautionary tale (9,10) about hasty and/or unreproducible science... (11))