Propagation reactions addition

The initiation reaction yields an imide moiety, which constitutes a growth center for propagation reaction. Addition of certain imides such as acyl lactams as coinitiators essentially eliminates the initiation reaction and makes possible the polymerization at relatively low reaction temperatures. Mechanistic and kinetic aspects of the anionic polymerization of lactams have been treated quite extensively [16b]. The discussed subjects relate to the various equilibria governing the polymerization process. They comprise equilibria allied to monomer conversion, to the formation of cyclic oligomers, and to the effect of initiator concentrations. 

Radicals are employed widely in the polymer industry, where their chain-propagating behavior transforms vinyl monomers into polymers and copolymers. The mechanism of addition polymeri2ation involves all three types of reactions discussed above, ie, initiation, propagation by addition to carbon—carbon double bonds, and termination ... 

As the quinone stabilizer is consumed, the peroxy radicals initiate the addition chain propagation reactions through the formation of styryl radicals. In dilute solutions, the reaction between styrene and fumarate ester foUows an alternating sequence. However, in concentrated resin solutions, the alternating addition reaction is impeded at the onset of the physical gel. The Hquid resin forms an intractable gel when only 2% of the fumarate unsaturation is cross-linked with styrene. The gel is initiated through small micelles (12) that form the nuclei for the expansion of the cross-linked network. 

As shown, ia the case of chlotination of aEyl chloride, the resonance states of the chloroaEyl radical iatermediates are not symmetrical and their propagation reactions lead to the two different dichloropropene isomers ia an approximate 10 90 ratio (26). In addition, similar reactions result ia further substitution and addition with products such as trichloropropanes, trichloropropenes, tetrachloropropanes, etc ia diminisbing amounts. Propylene dimerization products such as 1,5-hexadiene, benzene, 1-chloropropane, 2-chloropropane, high boiling tars, and coke are also produced ia smaE amounts. 

In addition, other additives may exist which react with R and RO2, introducing new propagation reactions which lead to a slower chain reaction. Such materials would be referred to as oxidation retarders. 

Some inorganic fillers are used as flame retardants in rubber base formulations. Flame retardants act in two ways (1) limiting or reducing access of oxygen to the combustion zone (2) reacting with free radicals (especially HO ), thus acting as terminator for combustion-propagation reaction. The additives most widely used as flame retardants for polymers are antimony oxides and alumina trihydrate. 

Copolymerization of methacrylic acid with butadiene and isoprene was photoinitiated by Mn2(CO)io without any halide catalyst [28,29]. The po]ymerization system is accompanied by a Dieis-Alder additive. Cross propagation reaction was promoted by adding trieth-y]aluminum chioride. 

Propagation reactions involving the fluoro-olefins, vinyl fluoride (VF)6Q 7 vinylidene fluoride (VF2)69 7""74 and trifluoroethylene (VF3),75 show relatively poor rcgiospccificity. This poor specificity is also seen in additions of small... 

The experiments with 2-(3-butenyloxy)benzenediazonium ions (10.55, Z = 0, n = 2, R=H) and benzenethiolate showed a significant shift of the product ratio in favor of the uncyclized product 10.57. They also indicated that the covalent adduct Ar — N2 — SC6H5 is formed as an intermediate, which then undergoes homolytic dissociation to produce the aryl radical (Scheme 10-83). Following the bimolecular addition of the aryl radical to a thiolate ion (Scheme 10-84), the chain propagation reaction (Scheme 10-85) yielding the arylphenylsulfide is in competition with an alternative route leading to the uncyclized product 10.57. 

Radical addition to conjugated systems is an important part of chain propagation reactions. The rate constants for addition of cyclohexyl radical to conjugated amides have been measured, and shown to be faster than addition to styrene. In additions to RCH=C(CN)2 systems, where the R group has a chiral center, the Felkin-Ahn rule (p. 148) is followed and the reaction proceeds with high selectivity. Addition of some radicals, such as (McsSijaSi-, is reversible and this can lead to poor selectivity or isomerization. ... 

The photo-oxidation of a solid branched alkane can be expected to proceed in localized domains, new oxidation chains being generated from the photo-cleavage of -00H products, and chain propagation (reactions 2 and 3) being concentrated close to each initial site in a given domain to produce a zone of high -00H concentration. Thus the distribution of an additive in and around these domains is of special importance. 

Equations 4.2.3 and 4.2.4 are the elementary reactions responsible for product formation. Each involves the formation of a chain carrying species (H- for 4.2.3 and Br- for 4.2.4) that propagates the reaction. Addition of these two relations gives the stoichiometric equation for the reaction. These two relations constitute a single closed sequence in the cycle of events making up the chain reaction. They are referred to as propagation reactions because they generate product species that maintain the continuity of the chain. 

Propagation head-tail. As usually considered, a propagation reaction leads to the addition of a monomer molecule, with the same linear orientation as... 

In addition to cross-propagation reactions, one cross-termination reaction is introduced in addition to two self-termination reactions, viz,... 

The introduction of hydroxylamine into oxidizing hydrocarbon adds the new cycle of chain propagation reactions to the traditional R —> R02 —> R cycle. This scheme is similar to that of hydrocarbon oxidation with the addition of another hydroperoxide (see earlier). 

The chain unit in the thermal and photochemical oxidation of aldehydes by molecular dioxygen consists of two consecutive reactions addition of dioxygen to the acyl radical and abstraction reaction of the acylperoxyl radical with aldehyde. Experiments confirmed that the primary product of the oxidation of aldehyde is the corresponding peroxyacid. Thus, in the oxidation of n-heptaldehyde [10,16,17], acetaldehyde [4,18], benzaldehyde [13,14,18], p-tolualdehyde [19], and other aldehydes, up to 90-95% of the corresponding peroxyacid were detected in the initial stages. In the oxidation of acetaldehyde in acetic acid [20], chain propagation includes not only the reactions of RC (0) with 02 and RC(0)00 with RC(0)H, but also the exchange of radicals with solvent molecules (R = CH3). 

The reaction temperature is above the critical temperature of ethylene so that the ethylene is in gas phase. High pressures are needed for propagation reaction. Only about 6-25 per cent of ethylene is polymerised. Rest of monomer is recycled. Extensive chain transfer reactions takes place during polymerisation to yield a branched chain polyethylene. In addition to long branches, it also contains a large number of short branches of upto 5 carbon atoms produced by intra-molecular chain transfer reactions. A typical molecule of Low density polyethylene contains a short branch for about every 50 carbon atoms and one or two long branches per molecule. 

Copolymerizations of nonpolar monomers with polar monomers such as methyl methacrylate and acrylonitrile are especially comphcated. The effects of solvent and counterion may be unimportant compared to the side reactions characteristic of anionic polymerization of polar monomers (Sec. 5-3b-4). In addition, copolymerization is often hindered by the very low tendency of one of the cross-propagation reactions. For example, polystyryl anions easily add methyl methacrylate but there is little tendency for poly(methyl methacrylate) anions to add styrene. Many reports of styrene-methyl methacrylate (and similar comonomer pairs) copolymerizations are not copolymerizations in the sense discussed in this chapter. 

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