Rubber peroxidation chain mechanism

It was seen in Chapter 3 that hydrocarbon polymers, of which natural rubber is a naturally occurring example, degrade both abiotically and biotically by a peroxidation chain mechanism. This process is accelerated by the introduction of photosensitive... 

Solomon (3, h, 5.) reported that various clays inhibited or retarded free radical reactions such as thermal and peroxide-initiated polymerization of methyl methacrylate and styrene, peroxide-initiated styrene-unsaturated polyester copolymerization, as well as sulfur vulcanization of styrene-butadiene copolymer rubber. The proposed mechanism for inhibition involved deactivation of free radicals by a one-electron transfer to octahedral aluminum sites on the clay, resulting in a conversion of the free radical, i.e. catalyst radical or chain radical, to a cation which is inactive in these radical initiated and/or propagated reactions. 

Co-reaction of blends to improve the performance has for decades been a practice in the rubber industry. In high-shear mixers some of the chains in rubbers are broken and are reformed by the free-radical mechanism. A similar phenomenon occurs during intensive mixing of polyolefins (see Table 4.34). To enhance this process, sometimes a free-radical source, e.g., peroxides, can be added. 

When attempts are made to cross-link conventional butyl rubber with peroxides such as dicumyl peroxide it is found that degradation occurs, a technique sometimes deliberately employed for some sealant compositions. It has been suggested that the mechanism occurring is hydrogen abstraction from the isobutene methyl group followed by scission and chain termination (Loan, 1964) ... 

As the ceiling temperature is approached, some rubber loses chemical stability. The initial reaction is crosslinking, creating long-branches. When the reaction is extensive both chain-scission and crosslinking occur. Such a reaction may be intensified by the mechanical action like milling and by the presence of oxygen or peroxide. 

Radicals that have been formed from the peroxide decomposing are reactive toward hydrogen atoms in chains. Hydrogen abstraction is a process where the radical removes a hydrogen atom from another nearby atom, and is a very important step in the peroxide curing reaction, as it is the mechanism by which radicals are transferred from peroxide molecular fragments to the rubber backbone." ... 

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