Chain structure scission

The polymers of rubber plastics have unsaturated hydrocarbon chain structure, since they are polymerized from alkadienes. The general formula of poly(l,3-butadiene) or butadiene rubber (BR) and polyisoprene or natural rubber (NR) is drawn in Scheme 12.5, where X is hydrogen in BR and methyl group in synthetic polyisoprene or NR. The free radical mechanism of thermal decomposition starts by homolytic scission of the alkyl C-C bonds. Two primary macroradicals (4 and 5) are formed for which the rearrangement... 

The breaking of chemical bonds under the influence of heat is the result of overcoming bond dissociation energies. Organic substances such as polymers are highly heat sensitive due to the limited strength of the covalent bonds that make np their structures. Scission can occur either randomly or by a chain-end process, often referred to as an unzipping reaction. 

Figure 11.6. Schematic illustrations of brittle fracture, (a) Idealized limiting case of perfectly uniaxially oriented polymer chains (horizontal lines), with a fracture surface (thick vertical line) resulting from the scission of the chain backbone bonds crossing these chains and perpendicular to them. This limit is approached, but not reached, in fracture transverse to the direction of orientation of highly oriented fibers, (b) Isotropic amorphous polymer with a typical random coil type of chain structure. Much fewer bonds cross the fracture surface (thick vertical line), and therefore much fewer bonds have to break, than for the brittle fracture of a polymer whose chains are perfectly aligned and perpendicular to the fracture surface, (c) Illustration of a defect, such as a tiny dust particle (shown as a filled circle), incorporated into the specimen during fabrication, which can act as a stress concentrator facilitating brittle fracture.

The influence of polymer chain structure on scission and crosslinking reactions is discussed. 

The relationship between fatigue life and carbonyl content can be explained as follows according to the photooxidation mechanism of PE, carbonyl groups result mainly from a Norrish type II reaction, i.e., for each carbonyl formation, there is a scission of a segment of a molecule chain. Such scission creates a defect in the structure which can grow and propagate into a microcrack under application of a load. Under cyclic loading, it is understandable that the number of cycles the sample can sustain will be directly related to the number of defects (such as microcracks, microvoids. ..), as is clearly described by equation (2). ... 

When poly(vinyl chloride) (PVC) is pyrolyzed, no such oligomeric pattern occurs. Instead of undergoing random scission to produce chlorinated hydrocarbons, PVC produces aromatics, especially benzene, toluene, and naphthalene, as shown in Figure 1.2. This is the result of a two-step degradation mechanism that begins with the elimination of HCl from the polymer chain (structure V), leaving the polyunsaturated backbone shown as structure VI. 

In a study on PTT recycling, Ramiro and co-workers [24] noted that the structure of the polymer did not change despite lowering of molecular weight. This suggests that chain-end scission is probably the correct first step. They noticed that, despite the lack of new structures, the polymer did yellow considerably. This was a paradoxical observation unless the colour is attributable to small fragments or polymerised unsaturated species. 

One can arrive at an intuitive acceptance of the maxim that ladder polymers should be more thermally stable than single strand polymers by a consideration of the factor that confers their desirable properties on polymers, i.e., long chain structure (or. alternatively, high molecular wei t). Macromolecules lose their desirable characteristics as soon as degradation (chain scission) becomes extensive. If this process can be prevented, or at least, slowed down, stability should be concomitantly enhanced. Any rupture in a single strand chain is effective in... 

Benzene rings in both the skeleton structure and on the side groups can be subjected to substitution reactions. Such reactions do not normally cause great changes in the fundamental nature of the polymer, for example they seldom lead to chain scission or cross-linking. 

They are chemically as reactive as their straight-chain counterparts. Cycloalkenes can lose their double bond in addition reactions. In scission or cleavage reactions, the ring structure opens up into a straight chain. 

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