# Barriers and Chemistry in a Bottle: Mechanisms in Today's Oxygen Barriers for Tomorrow's Materials
## 4 Active Barriers
### Co-Based
#### 4.4.1 Polyamide Sacrificials  
MXD6 was the first polymer that was employed for its scavenging ability [170]. It has already been discussed as a passive barrier in PET, both as a blend and a multilayer. The polymer owes its popularity to similar thermal and rheological properties compared to PET, which is beneficial for the processing of a PET-MXD6 combination. Additionally, the low permeability for gases would enhance the functionality as a passive barrier, both for oxygen and CO\({}_{2}\). The disadvantage of haze creation in blends has already been stated in Section 3.3. Additionally, another downside to MXD6 is the potential yellowing effect on the final packaging. At high processing temperatures, a discoloration has been assigned to the condensation of aldehyde-groups to amine end-groups, forming azomethine-groups and eventually creating a conjugated oligoenimine structure, yielding the typical yellow color. The concerning aldehydes can be generated by either oxidation of the polyamide itself or the acetaldehyde elimination of PET [177; 178]. The consequence of this effect is that nylon blends are not recyclable and all MXD6 has to be removed prior to post-consumption solid state polycondensation, leading to higher recycling costs. The autoxidation reactions of polyamides as MXD6 have been investigated to a wide extent [177; 172; 178]. Under the influence of temperature, light or certain transition metal ions, activated oxygen will be able to attack the polymer chain, creating organic radicals. The \(\alpha\)-carbon to the amide functionalities has been identified to be the main site of H-abstraction due to conjugation of the radical charge [172; 178]. Figure 11 provides visual representation of the initiation (a) and the following breakdown reactions (b-i). Propagation may continue through a variety of parallel reactions. After H-abstraction, the organic radical species (R\({}^{\circ}\)) provides a high affinity for capturing triplet oxygen and forming a peroxyl group (ROO\({}^{\circ}\)) (b). This peroxyl group in turn abstracts another hydrogen atom from an unreacted functional site (RH), propagating the chain to both a peroxide (ROOH) and a new organic radical (c). On the other hand, when the peroxyl-radical (ROO\({}^{\circ}\)) encounters an organic radical before it has captured molecular oxygen, termination may occur, leading to the creation of an N-carbonylimine (d), able to later undergo hydratation to an alcohol. Additionally, peroxyl radicals can recombine with each other or stable peroxide functionalities to create a variety of different products, conform the exemplary decay mechanism in Section 2.1 (Figure 5). After recombination or Fenton-catalysis of peroxide groups, the O-O bond splits. Oxyl radicals are formed (R-O\({}^{\circ}\)) that will again lead to multiple pathways, depending on the available functionalities in their environment (e). Whereas the oxyl function could be the target of an H-abstraction, forming an imide function (f), it could also be the acting H-abstractor to create an alcohol (g). Additionally, \(\beta\)-scissions are also common in oxyl-decay. An internal electronic shift will break the bond next to the oxyl-function. For MXD6 this is usually the weaker C-N bond (h), but the side of the aromatic ring is also possible (i). Scissions usually lead to aldehyde formation, even though these aldehydes can also oxidize further to carboxylic acid functions [171; 172; 178]. The role of cobalt is still very unclear. [PERSON] and [PERSON] have both examined the oxidation of MXD6 as an oxygen scavenger in PET and found that Co is immobile in the polymer blend [171; 172]. The immobility of cobalt should eliminate all pathways where cobalt requires direct contact with the reactive site of the polyamide in the catalytic cycle. This is where the two researchers disagree on whether Co will be either an initiation catalyst, a propagation catalyst or both. The discussion is fueled by the suggested ligation of cobalt to MXD6.