Which reactions can stop the methane chlorination chain reaction?

In the photochemical reaction between methane and chlorine, the chain reaction is sustained by highly reactive free radicals. Termination occurs when these reactive species collide and combine, effectively removing them from the propagation cycle.1

According to the reaction mechanism, the possible chain-termination steps include:

• The combination of two methyl radicals ($·{\text{CH}}_3$) to form ethane (${\text{CH}}_3{\text{CH}}_3$).1
• The combination of a methyl radical ($·{\text{CH}}_3$) and a chlorine radical ($·\text{Cl}$) to produce chloromethane (${\text{CH}}_3\text{Cl}$).1

Additionally, as noted in the initiation phase, some chlorine atoms may simply recombine to form chlorine molecules (${\text{Cl}}_2$), which also effectively removes them from the propagation steps.1