Reaction with polypropylene mechanism

The fundamental reaction mechanism for the free-radical oxidation of hydrocarbons has been used to relate the consumption of oxygen to the formation of oxidation products in polypropylene. A kinetic interpretation is based on the steady-state approximation equating the rates of the initiation and termination reactions. With this approach it is possible to derive mathematical equations describing the consumption of oxygen or the formation of specific oxidation products. To solve the equations it is necessary to determine the most likely route for initiation of oxidation. The initiation mechanism chosen is the bimolecular reaction of hydroperoxides, reaction (1 ) of Scheme 1.55, with a rate coefficient k. ... 

The rapid reaction between atomic oxygen and polymer films is discussed. This typical interface reaction is considerably influenced by the structure of the polymer. All polymers immediately react with atomic oxygen there is no evidence of even short induction periods or autocatalysis. Most readily attacked are highly branched polymers such as polypropylene and polymers with ether links for example, polyformaldehyde. Perfluorinated polymers, rubbers vulcanized with sulphur, and highly aromatic polymers are most resistant (Fig. 22). Oxidation of polymers by atomic oxygen occurs only at or near the surface of the polymer. For this reason the elucidation of the reaction kinetics and mechanism is very difficult. The conventional physico-chemical methods, UV and IR spectroscopy, are in this case inadequate. 

The mechanism on long-chain branch (LCB) formation with coordination catalysts was discussed briefly in Section 2.2 and illustrated in Figure 2.18. LCB formation with coordination catalysts is nothing more than a copolymerization reaction with macromonomers made in the reactor through -hydride elimination and transfer to monomer reactions for polyethylene, and -methyl elimination for polypropylene (Scheme 2.2). At this point, the population balances could be re-derived to include LCB-formation reactions and solved by the method of moments. For brevity, however, only the final results of this derivation will be shown, leading to an analytical solution for the instantaneous distribution of MWD for chains containing LCB derivation details are available in the literature [45-47]. 

Ethylene-vinyl acetate copolymers, usually known as EVA, are used in many applications, but especially for low voltage cables. These polymers are easily flammable and flame retardants are added to reduce their flammability. The classic solution is to incorporate aluminium hydroxide or magnesium hydroxide that develop endothermic reactions when heated. Nevertheless, large amounts have to be incorporated, often around 60% and this can lead to a loss of mechanical properties in the compound. Intumescent technology that works well with polypropylene has also been tried for EVA polymer systems. 

Thermodynamics is, similarly, well suited for the description of processes which lead to changes of the molecular structure, as just seen for phase changes. A reaction with unfavorable thermodynamics expressed by a positive AG does not occur. However, with a negative AG, a reaction may still fail kinetically, while another mechanism may succeed. A typical example is the preparation of polypropylene. Although the polymerization of propylene is possible thermodynamically, it was not achieved until the work of Ziegler (139) and Natta (140), who discovered the catalyzed mechanism with favorable kinetics. Thus, much effort has been devoted to understand the kinetics of polymerization (118). Early work concentrated on predicting molecular masses and their distribution. In this section the thermodynamics of polymerization is briefly discussed. Most attention is paid to addition (chain) polymerization, but the theory is also applicable to condensation (stepwise) polymerization. The subject is extensively reviewed (141-145). 

In the absence of light, most polymers are stable for very long periods at ambient temperatures. However, above room temperature many polymers start to degrade in an air atmosphere even without the influence of light. For example, a number of polymers show a deterioration of mechanical properties after heating for some days at about 100 °C and even at lower temperatures (e.g., polyethylene, polypropylene, poly(oxy methylene), and poly(ethylene sulfide)). Measurements have shown that the oxidation at 140 °C of low-density polyethylene increases exponentially after an induction period of 2 h. It was concluded that thermal oxidation, like photooxidation, is caused by autoxidation, the difference merely being that the radical formation from the hydroperoxide is now activated by heat. The primary reaction can be a direct reaction with oxygen (Van Krevelen and Nijenhuis 2009) ... 

Mechanical stresses in solid bodies can lead to chain cleavage and the formation of radicals. The alkyl radicals formed can create disproportionation, leading to unsaturated compounds or reactions with oxygen. For example, it was shown that mechanical load on polypropylene during extrusion at 230 °C created alkyl radicals [20], [69]. 

A composite consisting of a mixture of carbon particles (e.g., carbon black or graphite) and a polymer binder such as polyethylene or polypropylene with a surface layer of a carbon-black or carbon-felt flow-through structure serves as the Br2 electrode in Zn/Br2 batteries. Because of the low surface area of the carbon-polymer surface, an additional layer of carbon is necessary to obtain higher reaction rates. The mechanical deterioration of graphite-polymer composite... 

Polypropylene polymerized with triethyl aluminum and titanium trichloride has been found to contain various kinds of chain ends. Both terminal vinylidene unsaturation and aluminum-bound chain ends have been identified. Propose two termination reactions which can account for these observations. Do the termination reactions allow any discrimination between the monometallic and bimetallic propagation mechanisms ... 

Degradation of polyolefins such as polyethylene, polypropylene, polybutylene, and polybutadiene promoted by metals and other oxidants occurs via an oxidation and a photo-oxidative mechanism, the two being difficult to separate in environmental degradation. The general mechanism common to all these reactions is that shown in equation 9. The reactant radical may be produced by any suitable mechanism from the interaction of air or oxygen with polyolefins (42) to form peroxides, which are subsequentiy decomposed by ultraviolet radiation. These reaction intermediates abstract more hydrogen atoms from the polymer backbone, which is ultimately converted into a polymer with ketone functionahties and degraded by the Norrish mechanisms (eq. 

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