End-group depolymerization

For initial end-group depolymerization followed by complete unzipping... 

Thus for end-group depolymerization, DP is a linear function of time for partial unzipping and independent of time for complete unzipping. For random bond breakage partial unzipping is a complicated logarithmic function of DP related to reaction extent whereas complete unzipping is related to a linear function of 1/DP with time. 

Figure 1. Idealized plots relating molecular weight ratio to depolymerization mechanisms. Key A, end-group depolymerizationfollowed by complete unzipping B, random scission followed by complete unzipping C, end-group depolymerization followed by incomplete unzipping and D, random depolymerization followed by incomplete...

Polyester composition can be determined by hydrolytic depolymerization followed by gas chromatography (28) to analyze for monomers, comonomers, oligomers, and other components including side-reaction products (ie, DEG, vinyl groups, aldehydes), plasticizers, and finishes. Mass spectroscopy and infrared spectroscopy can provide valuable composition information, including end group analysis (47,101,102). X-ray fluorescence is commonly used to determine metals content of polymers, from sources including catalysts, delusterants, or tracer materials added for fiber identification purposes (28,102,103). 

Because very rapid depolymerization occurred at higher temperatures, it was necessary to control the temperature within the narrow range of 50 10°C. Even so, the of the polymer was no greater than 15,000 because of rapid degradation by the living cationic end group. 

Besides the main depolymerization reactions, side reactions should also be considered in the kinetic description of a PET recycling process. This is emphasized by the results obtained from a PET extrusion model [85] shown in Figures 2.19-2.23. The complete set of reactions summarized below in Table 2.10 have been used, but shear effects have not been taken into account. Chain degradation, accompanied by a significant reduction of intrinsic viscosity, occurs even within residence times of a few minutes. Carboxyl end groups, vinyl end groups and acetaldehyde are formed in amounts depending on residence time, temperature and initial moisture content of the PET flakes. 

Backbone chain scission degradation can be divided as occurring via depolymerization, random chain breakage, weak-link or preferential site degradation, or some combination of these general routes. In depolymerization, monomer is split off from an activated end group. This is the opposite of the addition polymerization and is often referred to as unzipping. ... 

Interestingly, it should not be assumed that a polymer will be useless above its ceiling temperature. A dead polymer that has been removed from the reaction media will be stable and will not depolymerize unless an active end is produced by bond cleavage of an end group or at some point along the polymer chain. When such an active site is produced by thermal, chemical, photolytic, or other means, depolymerization will follow until the monomer concentration becomes equal to [M]c for the particular temperature. The thermal behavior of many polymers, however, is much more complex. Degradative reactions other than depolymerization will often occur at temperatures below the ceiling temperature. 

The industrial synthesis of polyformaldehyde [poly(oxymethylene)] occurs by anionic polymerization of formaldehyde in suspension. For this the purification and handling of monomeric formaldehyde is of special importance since it tends to form solid paraformaldehyde. After the polymerization the semiacetal end groups have to be protected in order to avoid thermal depolymerization (Example 5-13). This is achieved by esterfication with acetic anhydride (see Example 5-7). As in the case of trioxane copolymers (see Sect. 3.2.3.2) the homopolymers of formaldehyde find application as engineering plastics. 

The thermal depolymerization of polyoxymethylene starts from the unstable hydroxy end groups, but the oxidative and acid-catalyzed hydrolytic degradation takes place within the main chain. Flence, if polyoxymethylene is heated in air or in the presence of strong acids samples with blocked end groups will also degrade. 

Based on the rate data, it is possible to rank the relative importance of these reactions. Therefore, it follows that reactions 1 and 3 are of key importance, followed by reaction 2 and possibly reactions 4, 5 and 6. Unfortunately, little or no data exists for the latter set of reactions under the conditions employed for poly(amic acid) preparation. However, if isoimide formation were to occur to any significant degree, then reactions 5 and 6 are possible with reaction 6 being the more dominant pathway, since amine concentration due to end-groups or depolymerization would be quite low [21, 22],... 

Although several telechelic polymers of 1,3-dioxolane have been prepared by cationic polymerization, their application is limited due to their susceptibility to acid-catalyzed hydrolysis and/or depolymerization. By termination of living mono- and difunctional poly(l,3-dioxolane) with amines or phosphines, polymers containing one or two stable ionic (ammonium, phosphonium) end groups has been prepared [129,274],... 

End-chain scission the polymer is broken up from the end groups successively yielding the corresponding monomers. When this polymer degrades by depolymerization, the molecules undergo scission to produce unsaturated small molecules (monomers) and another terminal free radicals. (Polymethylmethacrylate, polytetrafluorethylene, polymethacrylonitrile, polyethylstyrene, polystyrene, polyisobutene)... 

The main depolymerization reaction consists of a carboxyl group catalysed elimination of lactam from the amine end group, and the rate of lactam formation follows the equation [217]... 

Peeling. Peeling is the term applied to the stepwise depolymerization of polysaccharides from the reducing end groups under alkaline conditions. In conjunction with the formation of new end groups by alkaline hydrolysis of glycosidic bonds, the degradation of the polysaccharides can be extensive. 

---