Mega-supramolecules for safer, cleaner fuel

Guided by the statistical mechanics of ring-chain equilibrium, we designed and synthesized polymers that self-assemble into ``mega-supramolecules'' ($\ge $5,000 kg/mol) at low concentration ($\le $0.3{\%}wt) in hydrocarbon liquids. Experimental results accord with model predictions that end-functional polymers, which distribute among cyclic and linear supramolecules, can form a significant population of mega-supramolecules at low total polymer concentration---if, and only if$,$ the backbones are long (\textgreater 400 kg/mol) and end-association strength is optimal (16-18\textit{kT}). Hydrocarbon liquid fuels are the world's dominant power source (34{\%} of global energy consumption).~~Transportation relies heavily on such liquids, presenting the risk of explosive post-impact fires. The collapse of~the~World Trade Center~on September 11, 2001~inspired us to revisit polymers for mist control to mitigate post-impact fuel explosions. Rheological and both light and neutron scattering measurements of long end-functional polymers having polycyclooctadiene backbones and acid or amine end groups verify formation of mega-supramolecules. Post-impact flame propagations experiments show that mega-supramolecules control misting. Turbulent flow measurements show that mega-supramolecules reduce drag like ultra-long covalent polymers. With individual building blocks short enough to avoid hydrodynamic chain scission (400\textless $M_{w}$ [kg/mol] $\le $1,000) and reversible linkages that protect covalent bonds, they respond reversibly to flow through pumps and filters without degradation. Mega-supramolecules had no adverse effect on power output, fuel efficiency or emissions in diesel engines. In fact, they gave a 12{\%} reduction in diesel soot. Thus, long end-associative polymers may open the way to fuel additives that reduce pollution and improve transportation safety and security.