the reaction at low temperatures should be similar to that of the reaction C$_2$H + C$_2$H$_2$, with a rate constant of the same order of magnitude and the H loss channel strongly favored. Reactions of C$_2$H with polyynes larger than C$_4$H$_2$ can be reasonably expected to proceed in a similar way, although there are no experimental or theoretical studies on the kinetics of reaction (\[reac:c2h\]) with $n>2$.

Triacetylene (C$_6$H$_2$) and larger polyynes are also formed by another route involving the C$_4$H radical, $${\rm C_4H + C}_{2n}{\rm H_2} \rightarrow {\rm C}_{2n+4}{\rm H_2 + H}, \label{reac:c4h}$$ where the reaction of C$_4$H and C$_2$H$_2$ has been experimentally found to be rapid at low temperatures, with a rate constant of $(1.5-2.6)\times10^{-10}$ cm$^3$ s$^{-1}$ in the temperature range 39-300 K ([@ber2010] 2010). For polyynes larger than C$_6$H$_2$, routes similar to reactions (\[reac:c2h\]) and (\[reac:c4h\]) but involving radicals larger than C$_4$H, such as C$_6$H and C$_8$H, are likely to show a similar kinetic behavior (see, e.g., [@sun2015] 2015) and thus to contribute significantly to the formation of large polyynes in environments such as IRC+10216.

Once the polyynes have been formed, the corresponding radicals, such as C$_4$H and C$_6$H, form directly by photodissociation, although reactions involving C$_2$ 