# Barriers and Chemistry in a Bottle: Mechanisms in Today's Oxygen Barriers for Tomorrow's Materials
## 4 Active Barriers
### Co-Based
#### 4.4.2 Unsaturated (Co-)Polymer Sacrificials  
As a reaction to Cochran's polyamide solution, several alternatives came onto the market carrying polydienes and other unsaturated sacrificials. In contrast to MXD6, the oxygen scavenging for unsaturated compounds has been found to not entail an induction phase; instead the compounds are able to immediately capture oxygen. However, the total capacity per active site appears to be much smaller, so only shorter shelf lives can be achieved with this functional barrier. In-house experiments have shown that in the case of bottle technologies, commercial oxygen scavenging containers with polyamides are able to obtain shelf lives where oxygen will not exceed degradative concentrations of 6 months to several years. Commercial PBD-functionalized containers are shown to obtain 1-8 months of oxygen-free content [188; 189]. There have so far been no publications explaining these differences, but it is often assumed that the high scavenging activity will exhaust the capacity already during injection and/or empty shelf life.  
The general mechanism of polybutadiene oxidation is very similar to MXD6. A hydrogen abstraction is also the most accepted initiation for the polymer. Moreover, PBD is reported to be more susceptible to initiation than polyamides [190]. It is possible that this hails from the generation of singlet oxygen during early stages of oxidation, which is highly selective towards unsaturated compounds as seen in Section 2.1. Further radical propagation through the same mechanism as MXD6 will eventually lead to carbonylation or \(\beta\)-scission. There also have been reports of additional propagation steps for PBD. Firstly, the presence of conjugating double bonds next to the hydrogen-abstracted methylene can transfer the radical along the chain to realize many different arrangements. Aside from this, reports also state the occurrence of epoxidations and intermolecular additions in PBD degradation [191; 192; 193; 194; 195]. These additions cause PBD to have a much wider range of degradation pathways and products as MXD6 of which some have been listed in Figure 13.  
The immiscibility of PET and nylon has already been proven to be the limitation to the concentrations due to its hazy look. The blending of highly hydrophobic olefins such as PBD will evidently suffer the same consequences. Effectively, the mass-based concentrations of PBD in PET are claimed to be much lower than of MXD6 for the acceptable visual appearance. Increasing the interactions between the two polymers is actually indispensable [189; 196]. One improvement for this problem has been to bind the PBD covalently to PET. The addition of anhydrides (especially maleic anhydride) or epoxide-containing polymers is common in this area [188; 197]. The PBD itself can be functionalized before injection as evidenced by the broad application of hydroxyl-terminatedpolybutadiene (HTPBD). These end groups can enter in the polycondensation of PET by reacting with carboxylic end groups and producing a block-co-polymer [192; 196]. Other inventions attempt to bypass PBD altogether by implementing unsaturated sacrificials with higher hydrophilicity. Polyseters from terephthalatic acid and unsaturated diol monomers are produced for this cause. Examples are cyclohexenyl diol and 2-butene 1,4-diol [198; 199]. These types of molecules have also been linked to inorganic materials by silylation as part of a multilayered structure for OLED protection [14]. It has therefore already proven its merit in these applications, but the water-dependence of the reaction should also be an issue here. Further investigation will prove valuable here as well, even though the amide-based barriers are known to own higher capacities.