Propagation polyethylene oxide

Figure 3. Linear dependence of the apparent bimolecular rate constant of living polyethylene oxide propagation on the fraction of free ions a with cryptates as counterions in THF at 20°C K + 222 ( ) with + [222] ( ) + [ ] ) with 4>kBCs + [ ] -...

The polyethylene radical would be expected to react readily with oxygen and contribute to oxidative propagation as another peroxy radical ... 

In the auto-oxidation of polyethylene or polypropylene, both chain initiation and chain propagation, chain branching and chain termination consist of several potential chemical reactions. In the following, only the most significant reaction steps will be indicated. P denotes here a polymer chain. is used as a symbol for an unsaturated chemical bond with an unpaired electron. Polymer molecules, where such unpaired electrons occur, are called macro radicals. The remaining letters denote the chemical elements in the common way. 

Several historic publications that provided early evidence into the chemistry involved in this process were published between the 1920s and 1940s. Christiansen [4] and Backstrom [5] provided data in 1924 and 1927, respectively, that a chain reaction was involved. Next, in the early 1930s, Backstrom [6] and Ziegler and Ewald [7] demonstrated that the oxidation of hydrocarbons was initiated by free radicals. In 1942, Farmer and coworkers reported that an autocatalysis process involving free radicals and the formation of hydroperoxides was involved [8]. This 1942 paper identified the three steps responsible for the auto-oxidation of polyethylene that result in the degradation of polyethylene properties. These three steps were free-radical initiation, propagation and termination. 

From the analyses of NMR and electron-spray ionization mass (ESI-MS) spectroscopy, the polymers obtained from the polymerization of cyclic disulfides were found to be a cyclic structure [202], The cyclic structure consisting of poly(DT) is assumed to be formed by a backbiting reaction of propagating species [203]. Thermal and mechanical properties of the polymers, and decomposition behaviors of the polymers demonstrate that the polymers obtained from thermal polymerization of cyclic disulfides include a polycatenane structure. From polymerization of cyclic disulfides in the presence of cyclic polyethylene oxide), a polycatenane consisting of two different cyclic polymers was obtained [199]. Thus, poly(DT) contains spatial entanglements of cyclic polymers with each other (a polycatenane structure was presumed) (Fig. 61). 

The function of an antioxidant is to prevent the propagation of oxidation. Because many plastics are susceptible to oxidative degradation, the role of antioxidants in slowing down the process and imparting longer life to many products has taken on added importance. Depending on the aging environment, most plastics can benefit from antioxidants. Plastics especially susceptible to oxidation such as polypropylene, polyethylene, ABS, and polystyrene often include antioxidants in their formulations. 

Table 72.7 Propagation reactions of oxidation and corresponding value of the rate constant iki) at ambient temperature in some common hydrocarbon polymers polypropylene (PP), polyethylene (PE), polybutadiene (PBD) and polyisoprene (PIP)...

Zolotova NV, Denisov ET. Mechanism of propagation and degenerate chain branching in the oxidation of polypropylene and polyethylene. J Polym Sci Polym Chem Ed 1971 9 3311-20. 

These radical species are responsible for radical reactions (crosslinking, initiation, and propagation of the oxidative cascade). A number of EPR studies investigating the radical species formed in polyethylene following irradiation can be found in the literature. However, the attribution of the EPR signal is often controversial. 

The oxidative degradation of polyethylene consists of four stages initiation, propagation, branching, and termination. The principal reactions involved in each step are illustrated in Figure 1. The process as a whole is often referred to as autoxidation or auto-oxidation. Many reactions other than those illustrated in this simplified scheme can also occur. 

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