Hydrogen, bond dissociation energy polymers

Several factors contribute to the high heat stability of these compounds. There are extremely strong bonds between the carbon atoms in the polymer backbone and the attached fluorine atoms [13]. These factors help the polymer resist chain scission. In addition, the high fluorine to hydrogen ratio and saturation of the backbone increase the strength and stability of that polymer backbone [13]. Table 8.9 shows some bond dissociation energies that must be exceeded to rupture the bond. 

This oxidation model, however, ignores factors such as relativity differences between polymers and differences in oxygen permeability. Also, it does not account for the fact that the nature and character of the propagating radicals is not the same for aU polymers. Polymers show a wide variation in susceptibility to thermal autoxidation. The ease of autoxidation depends primarily on the relative C-H bond dissociation energies for the component parts of the polymer structure. Once the free-radical process is initiated, autoxidation proceeds. The abstraction of the most labile hydrogen atom in the polymer by alkylperoxy radical predominates in the propagation step. This follows this order ... 

Carbon-hydrogen bonds, such as those in hydrocarbons like methane and ethane, have dissociation energies close to 400 kj-mol whereas single bonds between carbon and fluorine have dissociation energies close to 500 kj-mol-1. The greater strength of a carbon-fluorine bond helps to explain why fluorocarbon polymers are very resistant to chemical attack. They are used to construct valves for corrosive gases and to line the interiors of chemical reactors. 

Hydrogen fluoride vapors contain monomeric HF molecules, dimers (HF)2 and polymeric species, (HF) , with n varying from 3 to 8. The stmctures of the polymers remain unknown, but the stmcmre of the dimer has been determined by spectroscopic measurements and by high-level quantum chemical calculations. The structure indicated in Fig. 18.8 represents a recommended structure based on information from both experiments and calculations [12]. The complex consists of two monomeric units with essentially the same H-F bond distance as in an isolated monomer. The monomers are joined through a H- F bond which is twice as long as bond distance in the monomer, but nearly 100 pm shorter than the sum of the van der Waals radii of H and F (Table 8.4). Note the similarity to the structure of HFICI (Fig. 18.6). The dissociation energy of the dimer at zero kelvin is Do = 13 kJ mol [13]. 

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