Thermally initiated depolymerization

The thermal degradation of irradiated isotactic PMMA has also provided information on the structure of the end groups formed at the site of main-chain scission [408]. The weight loss of non-irradiated PMMA at 250°C has been shown by Grassie and Melville [409] to be mainly due to depolymerization initiated at the carbon—carbon double bonds situated at the chain ends. A small proportion of randomly initiated depolymerization also occurs at this temperature. In agreement with this mechanism, the rate of volatilization has been found to be much higher for atactic than for isotactic PMMA, the latter having no double bonds at the chain ends. If 4.3 scissions per chain are produced by 7-irradiation in the isotactic sample, the rate of monomer evolution is identical to that of the initial unirradiated isotactic sample. This proves that chain ends of the type... 

Depolymerization is the reverse reaction to polymerization it consists of unzipping the monomeric units after initiation of the reaction either at random or at the chain ends. The initiation can be the consequence of a photochemical reaction but depolymerization itself is a purely thermal reaction which is discussed in Chapter 1 of this book. It is usually negligible at room temperature. Photochemically initiated depolymerization has been studied in the case of polymethylmethacrylate [9], poly-a-methylstyrene [10] and polymethylisopropenylketone [11]. 

The main difference between homo- and copolymers is the fact that copolymers exhibit higher thermal-oxidative resistance. Figure 5.196. Thermal-oxidative degradation in polyoxymethylene leads to the formation of unstable end groups that initiate depolymerization under the formation of formaldehyde. In homopolymers, this results in total decomposition. In copolymers, depolymerization proceeds only to the next comonomer unit (mostly polyethylene, comonomer content approx. 0.5 to 5 wt.%). That widens the processing window for copolymers and reduces the risk of mold fouling [771]. 

That implies that aU the PS— > bonds are broken in the low temperature step, confirming that they are less stable than the C—C bonds in the polystyrene chain. Therefore, the decomposition mechanism can be rationalized by taking into account the well-known thermal decomposition of PS through a radical initiated depolymerization (Scheme 5.21). 

Degradation pathways of polystyrene derivatives also depend on stmctural conditions (Scheme 4). Substitution with one single methyl group can substantially influence the direction whether depolymerization or H-transfer is preferred Figure 3 mediates an impression on the formation of transformation products and particularly on the reconstitution of corresponding monomers during thermally initiated degradation of important technical polymers. 

TABLE IV Depolymerization of TBMS Initiated by Thermal and Photochemical Generators of Acid at 120 C... 

In the depolymerized scrap mbber (DSR) experimental process, ground scrap mbber tires produce a carbon black dispersion in oil (35). Initially, aromatic oils are blended with the tire crumb, and the mixture is heated at 250—275°C in an autoclave for 12—24 h. The oil acts as a heat-transfer medium and swelling agent, and the heat and oil cause the mbber to depolymerize. As more DSR is produced and mbber is added, less aromatic oil is needed, and eventually virtually 100% of the oil is replaced by DSR. The DSR reduces thermal oxidation of polymers and increases the tack of uncured mbber (36,37). Depolymerized scrap mbber has a heat value of 40 MJ/kg (17,200 Btu/lb) and is blended with No. 2 fuel oil as fuel extender (38). 

Other degradation processes in addition to depolymerization can be initiated thermally. Thermal dehydrohalogenation of poly(vinyl chloride) is one such example [5]. 

When the substituent R stabilizes radicals as in (A) and (C), chain scission is more likely than termination by coupling. Radicals (C) then propagate the depolymerization process with volatilization of polypropylene and polystyrene at a temperature at which these polymers would not give significant amounts of volatile products when heated alone. Moreover, unsaturated chain ends such as (B) would also initiate the volatilization process because of the thermal instability of carbon-carbon bonds in P position to a double bond (Equation 4.23). 

Diffusion of solutions of Cr, Co and Mn ions through a PTFE membrane allows for separation of Cr from the remaining ions. Thermal stability of polymeric materials is a significant consideration in many poly(phosphazene) applications. The kinetics of the thermal degradation of PTFE are best fit with a model requiring a two step initiation for depolymerization. These steps involve formation of defect units, such as =P(0)NH- and =P(0)N(CH2CFj)-, which become active centers for depolymerization. Mixed... 

Polyfluoral made by pyridine or trimethylamine initiation was thermally not very stable. It depolymerized above 70°C in three minutes to monomer and initiator. Evidently the polymei—monomer equilibrium is very mobile, suggesting that the growing ends retained their activity. 

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