that they proceed with the same rate constant as the C$_2$ + C$_2$H$_2$ reaction.

According to the model, reactions (\[reac:c2h\]) and (\[reac:c4h\]) account for the bulk of formation of polyynes in IRC+10216, while the radicals are mainly formed by photodissociation of the corresponding polyynes and by reaction (\[reac:c2\]). Other types of reactions, which are rapid and might be important synthetic routes in other environments, contribute little in IRC+10216 because they involve reactants which are not abundant enough in this source. For example, neutral carbon atoms and CH radicals have been experimentally found to react rapidly at low temperatures with C$_2$H$_2$ ([@cha2001] 2001; [@cla2002] 2002; [@can1997] 1997; [@loi2009] 2009) and they probably react rapidly with C$_4$H$_2$ and larger polyynes as well. These reactions tend to produce unsaturated carbon chains with an odd number of carbon atoms and in fact, in the chemical model, are major formation routes to species such as C$_3$H, C$_3$H$_2$, and C$_5$H, which are abundant in IRC+10216. However, here we focus on the chemistry of carbon chains C$_n$H and C$_n$H$_2$ with $n$ even, species for which reactions involving C atoms and CH radicals are not important.

Molecular anions have been found to play a non-negligible role in the 