Polymeric materials, in particular polyolefin plastics, are an irreplaceable part of the global economy, with countless uses associated with packaging, construction, transport, electronic equipment, and medicine. Their massive-scale manufacture, slow decomposition rates and incompatibilities with sensitive ecosystems, however, have led to a global plastics waste problem that traditional mechanical recycling has failed to address. Upcycling is a potential alternative solution to this problem wherein polymers are used as a feedstock for the generation of value added products. Nature achieves this upcycling regularly using a large array of macromolecules and always follows the same approach: 1) thread the macromolecule through a pore, 2) lock it in place when it reaches the active site, 3) perform a catalytic chopping reaction, 4) release the synthesized fragment and 5) repeat.
To guarantee the structural control necessary for generating a narrow product distribution we must first find a material that can favor the rapid threading of common polyolefin plastics. Using solid-state NMR we have studied the conformation of polyethylene that was adsorbed on a series of different silica materials. We were able to conclusively show that polyethylene readily threads itself into the pores of mesoporous silica which is then able to rigidly confine the chain’s conformation. Using an exchange NMR experiment we were then able to observe the threading motion of the polyethylene chain through the silica particle as a function of temperature and, surprisingly, found that not only is the energy barrier for this motion similar to that found in pure polyethylene but that the rate of the threading motion is of the same order of magnitude as that found in the melt. The first key step for the generation of a synthetic, enzyme-like, polymer upcycling catalyst is therefore met with the discovery of a material that promotes the rapid threading of polyolefins.
This research is sponsored by the Catalysis for Polymer Upcycling, a project funded by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, under Contract and DE-AC-02 07CH11358 (Ames Laboratory).
(1) F. A. Perras, A. L. Paterson, S. Patnaik, Y. Pei, A. M. LaPointe, S. C. Ammal, A. Heyden, W. Huang, I. I. Slowing, A. D. Sadow, M. Pruski, Submitted for publication.