Radical polymerization with double bonds

Radical reactions with double bonds usually give the more stable (more substituted) radical. Because additions are biased in this fasWon, the polymerizations of vinyl monomers tend to yield polymers with head-to-taU linkages. Vinyl polymers made by radical processes generally have no more than l%-2% head-to-head linkages. 

Various aromatic and heteroaromatic aldehydes were coupled with styrenes in the presence of catalytic quantities of CuClj and TBHP at 80 °C. The procedure is remarkably selective, and the products detected arise from the acylation of the a,p-unsaturated ketone or radical polymerization of double bonds. The mechanism proceeds via the Cu-assisted generation of an acyl radical that adds onto the double bond of the olefin. Oxidation by the Cu... 

The role of reactive centers is performed here by free radicals or ions whose reaction with double bonds in monomer molecules leads to the growth of a polymer chain. The time of its formation may be either essentially less than that of monomer consumption or comparable with it. The first case takes place in the processes of free-radical polymerization whereas the second one is peculiar to the processes of living anionic polymerization. The distinction between these two cases is the most greatly pronounced under copolymerization of two and more monomers when the change in their concentrations over the course of the synthesis induces chemical inhomogeneity of the products formed not only for size but for composition as well. 

The double bond of maleic anhydride may undergo free radical polymerization with the proper initiator. Polymers of maleic anhydride (or copolymers made with another monomer) are commercially available (Polysciences). They consist of a linear hydrocarbon backbone (formed from the polymerization of the vinyl groups) with cyclic anhydrides repeating along the chain. Such polymers are highly reactive toward amine-containing molecules. 

Diesel fuel made from the thermal cracking of plastics is more susceptible to oxidation and polymerization than refinery-made diesel fuels. This is because plastic-derived diesel fuels generally have terminal unsaturation (i.e. double bonds) at the ends of the diesel chains as a result of the P-scission chain cleavage. Over time free radicals that form in the plastic-derived diesel fuels during storage cause the diesel chains with double bonds (a-olefins) to polymerize resulting in a sludgy sediment also known as gum . 

Reactions (1) and (4) are essentially the same as the addition of reactive species to the monomer, which is the same as the initiation and propagation reactions in the free radical chain growth polymerization. However, the kinetic chain length in vacuum is very short, and in a practical sense these reactions can be considered to be stepwise reactions. Cycle I consists of reactions of reactive species with a single reactive site, and cycle II is based on divalent reactive species. Reaction (3) is a cross-cycle reaction from cycle II to cycle I. The growth via cycle I requires the reactivation of the product species, whereas cycle II can proceed without reactivation as long as divalent reactive species or monomers with double bond or triple bond exist. 

The mechanism of the polymerization reaction is presumed to be essentially that of a homogeneous bulk or solution free-radical polymerization. The concern is exclusively with the polymerization by double-bond opening of carbon compounds that contain at least one caibon-carbon double bond. The reactive species that propagates to produce the polymer chain is a free radical formed by opening of the rc-bond of the carbon-carbon double bond. The basic steps of the polymerization reaction are initiation, propagation, termination (by various means), and various transra reactions. Tbe structure of the polymer produced is determined by the balance of the propagation, termination, and transfer reactions. 

The copolymerization is performed in solution with radical-forming initiators at temperatures of about 70 °C. The silicone macromer contains polymerizable vinyl groups on both chain ends. Such silicone macromers are produced by Wacker GmbH and are available with different chain lengths. The copolymerization with vinyl acetate is very facile and take place statistically [2] after polymerization, no double bonds can be found. To avoid crosslinking, the molecular weight has to be adjusted by the use of chain transfer agents or by the solvent and its concentration. 

Additions of benzoyloxy radicals to double bonds" and aromatic rings (Scheme 3.79) are potentially reversible. For double bond addition, the rate constant for the reverse fragmentation step is slow (A 10"-10 s at 25 °C) with respect to the rate of propagation during polymerizations. Thus, double bond addition is effectively irreversible. However, for aromatic substrates, the rate of the reverse process is extremely fast. While the aromatic substitution products may be trapped with efficient scavenging agents (e.g. a nitroxide " or a transition... 

The ROMP of cyclooctene-5-methacrylate and its copolymerization with cyclooctadiene is catalyzed by Ru(=CHCH=CPh2)(Cl)2(PCy3)2 in the presence of p-methoxyphenol as radical inhibitor. The double bonds in the methacrylate groups are inert towards metathesis. After chain transfer with ethyl vinyl ether to release the polymer from the ruthenium centre, it can be cross-linked by radical polymerization through the methacrylate side-chains (Maughon 1995). 

Figure 3.3 shows graft polymerization occurs when the monomer reacts with a free radical on the surface of the fabric. The free radical attacks the double bond of the monomer, opening the bond and forming a new bond between the fabric and the monomer, through Reaction 1. This leaves the free radical on the monomer, which may... 

The cross-linking polymerization ceases when the two ends of the growing diradical combine with one another. Polyperoxides are exceptionally stable peroxides, but are decomposed by heat and light to alkoxy radicals (RC-0 ), which will in turn react with double bonds to form ether linkages (RC-O-CR). 

Amorphous copolymers of ethylene and propylene, EPM, also possess rubber-elastic properties. But they cannot be vulcanized with sulfur because of the absence of carbon-carbon double bonds, and so a special technique using peroxides as free radical sources for transfer reactions has had to be developed. However, polymerizing in a diene component such as, for example, cyclopentadiene or ethylidene norbornene, leads to the formation of what are known as EPDM rubbers with double bonds in the side chains. These can, on the one hand, be vulcanized in the classic way with sulfur, but, on the other hand, still have good aging properties. Consequently, EPDM rubbers are mainly used in automobile construction, the cable and construction industries, as well as for technical purposes. However, the EPDM rubbers have only slight self-adhesion, so that producing tires from cut sections is made more difficult. It is for this reason that EPDM rubbers are not used for tires. 

The chain addition polymerizations require monomers with double bonds. They require free radical or ionic initiators to open the double bond and form the polymerization path in the manufacture of polymers such as polyethylene, polypropylene, polystyrene, and polyvinyl chloride which together constitute the majority of polymers, about 70 % of all polymers produced. A wide range of copolymers or terpolymers are produced by chain addition polymerization of two or three different monomers with double bonds. 

Free-radical polymerization normally involves monomers with double bonds, generally represented by the following structural formula ... 

Three events are involved with chain-growth polymerization catalytic initiation, propagation, and termination [3], Monomers with double bonds (—C=C—R1R2—) or sometimes triple bonds, and Rj and R2 additive groups, initiate propagation. The sites can be anionic or cationic active, free-radical. Free-radical catalysts allow the chain to grow when the double (or triple) bonds break. Types of free-radical polymerization are solution free-radical polymerization, emulsion free-radical polymerization, bulk free-radical polymerization, and free-radical copolymerization. Free-radical polymerization consists of initiation, termination, and chain transfer. Polymerization is initiated by the attack of free radicals that are formed by thermal or photochemical decomposition by initiators. When an organic peroxide or azo compound free-radical initiator is used, such as i-butyl peroxide, benzoyl peroxide, azo(bis)isobutylonitrile, or diazo- compounds, the monomer s double bonds break and form reactive free-radical sites with free electrons. Free radicals are also created by UV exposure, irradiation, or redox initiation in aqueous solution, which break the double bonds [3]. 

In the first case (reaction (A)) this could form the initiation stage of a polymerization reaction through the double bonds of the diene polymer. In turn this reaction could involve those other reaction mechanisms associated with double bond polymerization such as chain transfer to solvent, to monomer and to polymer chain termination and inhibition by free radicals including those from antioxidants. 

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