Polymers dissociation energy

Secondary bonds are considerably weaker than the primary covalent bonds. When a linear or branched polymer is heated, the dissociation energies of the secondary bonds are exceeded long before the primary covalent bonds are broken, freeing up the individual chains to flow under stress. When the material is cooled, the secondary bonds reform. Thus, linear and branched polymers are generally thermoplastic. On the other hand, cross-links contain primary covalent bonds like those that bond the atoms in the main chains. When a cross-linked polymer is heated sufficiently, these primary covalent bonds fail randomly, and the material degrades. Therefore, cross-linked polymers are thermosets. There are a few exceptions such as cellulose and polyacrylonitrile. Though linear, these polymers are not thermoplastic because the extensive secondary bonds make up for in quantity what they lack in quahty. 

The range of high-temperature rubbers is very small and limited to the silicones, already considered in this chapter, and certain fluororubbers. With both classes it is possible to produce polymers with lower interchain attraction and high backbone flexibility and at the same time produce polymers in which all the bonds have high dissociation energies and good resistance to oxidation. 

The latter equation contains constants with well-known values and can therefore be used to predict the fracture stress of most polymers. For example, the bond dissociation energy Do, is about 80 kcal/mol for a C-C bond. For polystyrene, the modulus E 2 GPa, A. 4, p = 1.2 g/cm, = 18,000, and we obtain the fracture stress, o A1 MPa, which compares well with reported values. Polycarbonate, with similar modulus but a lower M. = 2,400 is expected to have a fracture stress of about 100 MPa. In general, letting E 1 GPa, p = 1.0 g/cm, and Do — 335 kJ/mol, the tensile strength is well approximated by... 

Much fewer experiments are available in solution where the few reported data are generally more concerned about the effect of molecular structure than about bond dissociation energy. In simple shear, it is generally agreed that chain flexibility dominantly influences the rate of bond scission, with the most rigid polymers being the easiest to fracture [157]. The results are interpreted in terms of the presence of good and poor sequences in the chain conformation. 

The average dissociation energy of bonds forming the structure of a macromolecule appears thus to be a first criterion for estimating the thermal stability of a given polymer. The fraction of bonds that reaches the energy equal to dissociation energy D is determined by the Boltzman s factor... 

Table 1 Dissociation energies of bonds A-B in kj/mol that may form the polymer structure... 

DY-x e dissociation energy of Y X bond probability of formed free radical pair to escape the cage of solvent or polymer kJ moU1... 

As is expected from these results, it is very difficult to control the polymerization of monomers other than St, e.g., that of MMA, because of the too small dissociation energy of the chain end of poly(MMA). In fact, the polymerization of MMA in the presence of TEMPO yielded the polymer with constant Mn irrespective of conversion, and the Mw/Mn values are similar to those of conventional polymerizations [216]. The disproportionation of the propagating radical and TEMPO would also make the living radical polymerization of MMA difficult. In contrast, the controlled polymerization of MA, whose propagating radical is a secondary carbon radical,has recentlybeen reported [217]. Poly(MA) with a narrow molecular weight distribution and block copolymers were obtained. 

The photochemistry of thietanes involves entirely different pathways from those encountered in azetidines the low bond dissociation energy of the C—S bond seems to be mainly responsible. The direct photolysis of thietane vapor with 213.9-228,8- and 253.7-nm light leads to ethylene and propylene, cyclopropane, and thiocyclopropene. A white polymer appeared as a constant by-product. ... 

Aromatic amines are known as to be efficient inhibitors of hydrocarbon and polymer oxidation (see Chapters 15 and 19). Aliphatic amines are oxidized by dioxygen via the chain mechanism under mild conditions [1,2]. Peroxyl and hydroperoxyl radicals participate as chain propagating species in the chain oxidation of amines. The weakest C—H bonds in aliphatic amines are adjacent to the amine group. The bond dissociation energy (BDE) of C—H and N—H bonds of amines are collected in Table 9.1. One can see that the BDE of the N—H bond of the NH2 group is higher than the BDE of the a-C—H bond in the amine molecule. For example, DN = 418.4 kJ mol 1 and DC H = 400 kJmol-1 in methaneamine. However, the BDE of N—H bond of dialkylamine is lower than that of the C—H bond of... 

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. 

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