Polyethylene (PE) and isotactic polypropylene (iPP) account for nearly two-thirds of the world’s plastic—adding up to $200 billion in annual sales worldwide. When melted together, PE and iPP are immiscible and form a brittle material. So they tend to get recycled into lower-value products because of the expense associated with sorting them from one another. Now, researchers led by Cornell University’s Geoffrey W. Coates, Anne M. LaPointe, and James Eagan, along with the University of Minnesota’s Frank S. Bates, have come up with a way to turn a mix of PE and iPP into a more valuable plastic (Science 2017, DOI: 10.1126/science.aah5744). They found that adding a small amount—about 1% by weight—of a tetrablock copolymer made from PE and iPP will transform a blend of PE and iPP into a mechanically tough material. The copolymer essentially acts like glue and holds the two immiscible plastics together. The researchers make the tetrablock copolymer using a pyridylamidohafnium catalyst, which they previously reported in a patent (WO2008112133A2). Although development work still needs to be done, the researchers hope the discovery will affect the way plastics are recycled.