Mid-chain scission

F. 10 Site-specific activation, (a) Mid-chain scission of molecular bottlebrushes occurs selectively at the disulfide linkage, (b) In a short bottlebrush, tension is focused to the middle of the backbone, resulting in its mid-chain scission, (c) By focusing txmd tension at the branching center, molecular stars with bottlebrush arms undergo sequential scissimi of covalent bonds upon adsorption onto an aqueous substrate... 

Third, molecular branched architecture can be tuned to ensure maximum concentration of mechanical tension at a specific chemical bond. For example, star-like [134] and pom-pom structures [125] as well as short bottlebrushes [135] may be used to introduce site-specific activation at the branching center. Figure 10b displays the concentration of tension in the middle of the backbone resulting in its mid-chain scission. In a similar fashion (Fig. 10c), molecular stars with a spoke wheel core and bottlebrush arms exhibit preferential dissociation of the arms followed by the scission of covalent bonds in the bottlebrush backbones... 

The Gaussian scission probability distribution function, with a preference for mid-chain scission, is frequently encountered in shear-induced mechanochemical degradation. The parabolic distribution, on the contrary, indicates a preference for chain-end degradation. This phenomenon has been reported in the hydrolysis of dextran, as a result of chain branching. 

The molecular degradation in turbulent flow is also explainable by the theoretical model of Brostow (62-64) (see Fig. 7). The mid chain scission in turbulent flow has been experimentally and theoretically explained (143-145,147). Recently, Kim and co-workers (153) studied the mechanical degradation of high molecular weight polystjmene under turbulent flow using a rotating disk apparatus for three various solvent systems. They found that the extent of the... 

Thermal degradation of plastics can be classified as depolymerization, random decomposition and mid chain degradation [54, 55], In the process of depolymerization, the conjunction bonds between monomers are broken up, which leads to the forming of monomers. Depolymerization type plastics mainly include a-polymethyl styrene, polymethyl methacrylate and polytetrachloroethylene. In the random decomposition process, scission of carbon chains occurs randomly, and low-molecular hydrocarbons are produced. Random-decomposition-type plastics include PP, PVC and so on. In most cases, both decompositions take place. To be more specific, the degradation of polyolefins can be classified as the following three types ... 

Various theoretical approaches have been developed to describe mechanical degradation. One of the earlier studies was made by Frenkel [39] and Kauzmann and Eyring [40]. They proposed that linear macromolecules are extended in a shear field in the direction of motion. The strain of the molecules is primarily concentrated at the middle of the chain. No degradation is expected when the degree of polymerization is below a certain critical value. Bueche [41] predicts that entanglements produce preferential tension in the mid-section of macromolecules. Thus, chain scission is more likely to occur in the center of the chain. He also predicts that main chain rupture increases dramatically with increasing molecular weight. 

Scheme 4.13. -Scission of butyl acrylate mId-chaIn radical...

Scission events can occur in any system where mid-chain radicals are formed. However, scission is more temperature-activated than H-abstraction and thus becomes important only at elevated temperatures. The reaction is not believed to occur during butyl acrylate polymerization at 75 C [37], but is shown to be important at 140°C [29, 45], Scission is a dominant mechanism in styrene polymerizations at 260-340°C [26], and also occurs during LDPE production [30]. Scission of midchain radicals formed via intermolecular transfer to polymer can have a significant effect on the breadth and the shape of polymer MWD [46]. 

Termination of the mid-chain radical (Q ) is also considered in the scheme. While the mechanism provides improved tmderstanding of this complex system, many questions still remain does the mid-chain radical terminate at the same rate as chain-end radicals what is the reactivity of the unsaturated chain end and does the raid-chain radical scission with equal probability in each direction The reaction engineering challenge is to consider the set of mechanisms required to represent the basic rate behavior and polymer architecture of the system without introducing unneeded complexity. 

Scheme 6.6 Proposed mechanism for producing macromonomers by mid-chain radical formation by backbiting and P-scission. For an acrylate, Z COjR.

The mechanism by which polymers interact with cavitation-induced shear fields informs the design and interpretation of mechanochemical experiments making use of sonochemistry. First, it is observed that scission occurs preferentially around the mid-point of the polymer chain, as the solvodynamic forces are greatest at this point... 

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