Frontal polymerization is self-propagating: a reaction zone advances through monomer, converting it to polymer as it goes. The heat released by the reaction drives the next layer to react. The front moves at a speed set by the balance between heat generation and thermal diffusion. At least, that's the conduction-only picture.
Experiments on dicyclopentadiene (DCPD) reveal that gravity breaks the symmetry (arXiv:2603.20835). Trigger the reaction from the bottom and the front propagates upward roughly 50% faster than a top-triggered front moving downward. The mechanism is buoyancy-driven convection. Hot reacted material is less dense than cold monomer. When the front moves upward, buoyant plumes from the reaction zone rise into unreacted material ahead of the front, preheating it. The monomer arrives at the front already warm, needing less energy to react, so the front advances faster.
Trigger from the top, and the hot material rises away from the front — convection carries heat backward, not forward. The front gets no preheating and relies on conduction alone.
As monomer viscosity increases — either by changing formulation or by aging — convection weakens. At high viscosity, buoyancy can't drive flows fast enough to matter, and both directions converge to the same conduction-limited speed. The transition from convection-dominated to conduction-dominated propagation is smooth and predictable from the viscosity.
The structural insight: the reaction doesn't know about gravity. The chemistry is identical regardless of orientation. But the heat transport that enables the chemistry does know about gravity, and heat transport controls the front speed. The rate-limiting step is not the reaction but the delivery of thermal energy to the unreacted zone. When delivery has a directional bias, the process inherits that directionality. The asymmetry is in the transport, not the transformation.