P-chain scission

Propagation is the second step in the free radical chain reaction. The free radicals generated by p-chain scission can eliminate the monomer by scission of another p-link and shorten the macromolecular radical chain by the reaction ... 

The pyrolysis of natural rubber takes place by a free radical mechanism as described in Section 2.6. The p-chain scission is the dominant initiation process and the bond dissociation reaction was estimated to be about 61.5-63 kcal mol ... 

Pyrolysis of polyisoprene takes place very similarly to that of polybutadiene. The p-chain scission (to the double bond) is the dominant initiation process, and the bond dissociation energy was estimated to be about 61.5-63 kcal mol V The cleavage of the carbon chain backbone has as the final result the elimination of some monomer, linear and cyclic dimers, trimers, etc. The yield of monomer depends on the heating conditions, degree of polymerization, etc. Table 2.1.1 indicates the formation of up to 58% monomer from natural rubber pyrolysis, while Table 7.1.4 shows only about 18% conversion. Some aspects regarding the thermal decomposition mechanism of polyisoprene were discussed in Section 2.1. 

In addition to providing fully alkyl/aryl-substituted polyphosphasenes, the versatility of the process in Figure 2 has allowed the preparation of various functionalized polymers and copolymers. Thus the monomer (10) can be derivatized via deprotonation—substitution, when a P-methyl (or P—CH2—) group is present, to provide new phosphoranimines some of which, in turn, serve as precursors to new polymers (64). In the same vein, polymers containing a P—CH group, for example, poly(methylphenylphosphazene), can also be derivatized by deprotonation—substitution reactions without chain scission. This has produced a number of functionalized polymers (64,71—73), including water-soluble carboxylate salts (11), as well as graft copolymers with styrene (74) and with dimethylsiloxane (12) (75). 

Photoinduced free radical graft copolymerization onto a polymer surface can be accomplished by several different techniques. The simplest method is to expose the polymer surface (P-RH) to UV light in the presence of a vinyl monomer (M). Alkyl radicals formed, e.g. due to main chain scission or other reactions at the polymer surface can then initiate graft polymerization by addition of monomer (Scheme 1). Homopolymer is also initiated (HRM-). 

Chemical reaction that results in the breaking of main-chain bonds of a polymer molecule. Note 1 See [2], p. 64 and Definition 3.24 in Chapter 1 for chain scission. 

Note 2 Some main-chain scissions are classified according to the mechanism of the scission process hydrolytic, mechanochemical, thermal, photochemical, or oxidative scission. Others are classified according to their location in the backbone relative to a specific structural feature, for example, a-scission (a scission of the C-C bond alpha to the carbon atom of a photo-excited carbonyl group) and P-scission (a scission of the C-C bond beta to the carbon atom bearing a radical), etc. 

The preferred route for reducing the molecular weight of PVA involves chain scission at the 1,3-diketone site (see Fig. 6). As the diketone element is chemically not very stable, a spontaneous degradation of oxidised PVA was also discussed [80]. Nevertheless, the preferred degradation pathway is most likely the biochemical process because enzymes were identified that showed high activity with diketone substrates [81], especially with oxidised PVA. The p-diketone hydrolase (BDH EC 3.7.1.7) hydrolyses aliphatic p-diketones to form methyl ketones and carboxylic acids in equimolar amounts [82]. The enzymatic cleavage of C-C bonds in p-diketones is not well studied [83]. BDH enzymes could be isolated from different PVA-degrading strains, purified, characterised and cloned [84]. 

Electron beam irradiation is one of the methods of cross-linking in fhis process. The other methods use peroxide, multifunctional azide, or an organofunctional silane. Polyethylene resins respond to electron beam irradiation well since the rate of cross-linking exceeds significanfly fhe chain scission. Polypropylene (PP) is prone to P-cleavage, which makes if difficult to cross-link by a free radical process. For fhaf reason, PP resins... 

Zinc complexes are important as additives for rubber polymers. Dithiocarbamate complexes are most commonly used here, but bis(8-hydroxyquinolinato)zinc inhibits the thermal decomposition of poly[(trifluoroethoxy)(octafluoropentoxy)phosphazene]. The zinc is thought to complex residual P—OH groups in the polymer chain, which would otherwise lead to rearrangement and chain scission.126... 

In carbohydrates, P-fragmentation of radicals adjacent to the glycosidic linkage [e.g reactions (14) and (15)] will also lead to chain scission. 

Singlet oxygen must attack the 2-position of a 3-hexylthienyl unit in order to abstract a p-hydrogen and form the hydroperoxide. Photochemical cleavage of the 0-0 bond yields the corresponding alkoxy radical and one hydroxyl radical. Chain scission then proceeds as a result of p-cleavage of the polymeric alkoxy radical. 

Chain scission (embrittlement) would result essentially from reaction 14.VIII, if radicals P° undergo j3 scission, whereas crosslinking (Tg increase) would result essentially from radical coupling (reaction 14.IV). 

The 355 nm emission is sharp and intense at the start of irradiation, and the intensity decreases with prolonged irradiation time. The 440 nm emission is weak and broad, and the intensity does not change with the irradiation time. Emission spectra of PMPrS obtained at ion fluences of 0.15,0.76, and 1.53 p,C/cm2 shows emission bands at 350 nm and 440 nm. The decrease in the intensity of the main peak indicates that main chain scission (photolysis) occurs under ion beam irradiation. Intense and sharp emission at 340 nm and weak broad emission at 440 nm for PDHS at 354 K are observed at the beginning of the irradiation and decrease on further irradiation. At 313 K and 270 K, sharp intense main emissions at 385 nm are seen. The 340 nm and 385 nm emission bands are assigned to a - a fluorescence. Experimental results have shown the presence of a phase transition at 313 K for PDHS.102,103 Below 313 K, the backbone conformation of PDHS is trans-planar, and above the solid-solid phase change temperature, a disordered conformation is seen. Fluorescent a -a transitions occur at 355 nm for PMPS, 350 nm for PMPrS, and 385 nm and 340 nm for PDHS. Emissions around 440 nm are observed at all temperatures examined and are assigned to defect and network structures induced by ion beams. 

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). 

Because of its relatively low activation energy and its involvement in a chain reaction, p-bond scission is often the dominant mode of bond breaking in the pyrolysis of large aliphatic hydrocarbons. Moreover, substitution of an oxygen atom for a methylene (CH2) group in the above structure will often lead to an even more rapid chain reaction because of the resulting lower endothermicity of reaction 4. 

One can consider that the kinetics carbonyl build-up is representative of the overall oxidation kinetics, at least when considered at the molecular scale (or monomer unit). It remains to establish a relationship between structural changes at this scale and molar mass changes. For the PE polymer understudy, random chain scission is predominant. It will be assumed that the main scission process is the rearrangement of alkoxyl radical (p scission). Then, every elementary reaction generating alkoxyl radicals will induce chain scission. In the chosen mechanistic scheme, both hydroperoxide decomposition processes and the nonterminating bimolecular peroxyl combination are alkoxyl sources. Thus, the number of moles of chain scissions per mass unit (s) is given by ... 

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