Thermal depolymerization

Kawakami, Suzuki and Yamashita showed that compound 7, among many others, could be polymerized to derivatives of the corresponding open-chained species by treatment with boron trifluoride ether complex. Yamashita and Kawakami formed these same sorts of materials by heating the glycols and paraformaldehyde in the presence of toluenesulfonic acid. This led to prepolymers which were then thermally depolymerized to afford the cyclic oligomers which were separated by fractional distillation. 

Although the picture is far from complete, the available evidence suggests that PBPP is rather different from most polyphosphazenes. The polymer may be induced to be an electronic conductor, but perhaps as a result of this tendency to cross-link, the material is more sensitive and difficult to handle than most polyphosphazenes and the thermal depolymerization reaction is inhibited. 

Celanese came out a year later with a similar product under the trade name Celcon. Celanese circumvented DuPont s patent on the basis of employing a copolymer variation that allowed enhanced stabilization against thermal depolymerization (structure 4.74). The copolymer has a of 170°C ... 

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. 

Thermal Depolymerization of Poly(a-methylstyrene) and of Poly(methyl Methacrylate)... 

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. 

Monomeric l,3-dithiole-2,4,5-trithione, generated in situ from 268 by thermal depolymerization, is believed to be the active species participating in these reactions. Generally, excess 268 is refluxed with the dienophile in aptotic solvent until the dienophile disappears. 

Exercise 29-1 Write a reasonable mechanism for the thermal depolymerization of 1,3-cyclopentadiene tetramer. How could one chemically alter the tetramer to make thermal breakdown more difficult Explain. 

Pyrolysis is the process of thermal degradation of a substance into smaller, less complex molecules. Many processes exist to thermally depolymerize tires to salable products. Almost any organic substance can be decomposed this way, including rice hulls, polyester fabric, nut shells, coal and heavy crude oil. Pyrolysis is also known as destructive distillation, thermal depolymerization, thermal cracking, coking, and carbonization. 

The maximum conversion obtained was 21.7% with 5 mol% tetralin, and 6.0% liquid yield with pure toluene at 10.9 MPa and 623 K. These figures are rather small when compared to literature data, being necessary to investigate different levels of thermal depolymerization and different solvents to optimize the conversion of the high-ash sub-bituminous Butia-coal. High levels of sulfur reduction were observed - up to 44%, and it has been also shown that the dessulfurization of the coal under study is selective with regard to the solvent mixture. 

X-ray crystallographic studies during the course of the reaction have demonstrated that the reaction is a typical topochemical process involving a direct rearrangement of the monomer crystal to the polymer crystal having an extended rigid rod-like structure. By x-ray analysis and DSC on the thermal depolymerization of the polymer crystal, a reversible topochemical process has been demonstrated for monomer and polymer crystals. 

Morphological changes, which are classified into four groups, have been correlated to the degree of topotactic control. Thermal analysis has been studied on DSP poly-DSP in some detail. Two main endothermic peaks of as-polymerized poly-DSP crystals are characterized as thermal depolymerization in the crystalline state and crystal melting point followed by thermal depolymerization in the molten state. From the results of the studies on the heat treatment of as-polymerized polymer crystals, a reversible topochemical processe has been established. 

Of further interest is the thermal depolymerization behavior of these polymers in the crystalline state. This behavior has been investigated by observing the continuous changes of the X-ray diffraction pattern and of the TG-DSC diagram during the heat treatment of poly-DSP10,65). Changes of X-ray diffraction patterns on thermal treatment of as-polymerized poly-DSP crystals and photopolymerization of DSP crystals are shown in Fig. 16. 

The thermal depolymerization behavior in the crystalline state has been observed of most of polymers prepared by four-center photopolymerization, e.g. poly-p-PDA Me31. The X-ray diffraction patterns (d) and (e) in Fig. 16 will be discussed in Sect. Vl.a. 

Since two endothermic peaks, exemplified by the poly-DSP crystal in Fig. 1464 are generally observed for the as-prepared polymer crystals and since a weaker peak appears at the lower temperature which grows at the later stage of photopolymerization as shown in Fig. 1510, these two peaks are attributed to thermal depolymerization in the crystalline state and the processes involving degradation and/or crystal melting, respectively. 

Based on the results of crystalline-state depolymerization, a reversible topochemical process, which is a monomer crystal lattice-controlled photopolymerization and a polymer crystal lattice-controlled thermal depolymerization, is established65. ... 

Most of the polymer crystals are thermally depolymerized to monomer and then degraded to undefinable materials64 68). For example, an exothermal oxidation effect has been observed above 200°C in the DTA of poly-DSP645. In aqueous solution, fine crystals of poly-DSP are readily oxidized with potasium parmanganate at 70 °C for 10 h, and 2,5-pyrazinedicarboxylic add is identified as one of the reaction products containing a certain amount of benzoic acid135. 

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