Polymers double bonds, addition reactions

As a special case, polyvinyl alcohol is not obtained by the polymerization of ethylene alcohol, because ethylene alcohol has an unstable enol structure that can easily form aldehyde by isomerism therefore, vinyl alcohol is only a hypothetical monomer of polyCvinyl alcohol). For the majority of polymers prepared by means of a double bond addition reaction of alkene and diene monomers, this nomenclature method is simple, intuitive, and widely used. 

Another side reaction in the polymerization of acrylates could be the anti-Markovnikov addition of Co—H to the olefinic double bond. This reaction shown in Scheme 11 was suggested to explain the absence of any methyl group in the polymer backbone of acrylamide radically polymerized in the presence... 

In the case of BR or SBR, the efficiency can be much greater than 1.0, especially if all antioxidant materials are removed. A chain reaction is indicated here. It might be explained by steric considerations. In butadiene-based rubbers, double bonds are quite accessible. Radical addition to double bonds could give highly reactive radicals, which would be likely to add to other polymer double bonds. A chain of additions might be more likely in butadiene rubber than in the presence of hindering methyl groups in isoprene rubbers. 

Meanwhile, norbomene being a bicyclic olefin can be polymerized also via an addition scheme with opening of the double bonds. This reaction proceeds in the presence of Ni or Pd catalysts and results in polymers with entirely different structures and, hence, different properties (Scheme 3.1). 

An addition polymer ic a polymer formed by addition reactions between monomers that contain a double bond. For example, molecules of ethene can polymerize with each other to form polyethene, commonly called polyethylene. 

The word addition is derived from the kind of chemical reaction that is used to manufacture these polymers, namely the addition reaction of alkenes. Addition reactions of alkenes involve the adding of a reacting molecule across the double bond of the alkene, as shown in Figure 14.31. In this process, one of the two bonds in the double bond (the k bond) is broken. The carbons that were double bonded become single bonded, and the broken bond is replaced by bonds to the substituents from the reacting molecule, A and B. The molecule represented by A-B can be H-H, Cl-Cl, H-Cl, and so on, which results... 

The second mechanism corresponds to a heat-initiated, concerted electronic-transfer-type ene reaction (see Chapter 5), which explains how thermal and radical grafting produces different products. In the absence of initiators, the second reaction is believed most probable.The lack of effect of mercaptans (benzenthiol) on the reaction supports the mechanism, along with the clear evidence for double-bond movement during reaction. However, additional evidence shows that crosslinking and polymeric side-chain formation complicates the picture.In the ene grafting mechanism, each combined anhydride blocks a methylenic carbon in the position of the polymer double bond, precisely the one susceptible to oxidation. Thus, this mechanism more adequately explains how thermal grafting of MA can improve the oxidizability of rubber. 

The above approach of mechanochemically initiated addition of reactive antioxidants on different polymers, such as rubbers and unsaturated thermoplastics such as ABS is illustrated here for thiol-containing antioxidants. For example, using thiol compounds (37) and (38) as the reactive antioxidants, Kharasch-type addition of the thiol function to the polymer double bond takes place during melt processing to give bound antioxidant adduct (see reaction 7) the polymer becomes much more substantive under aggressive environments. 

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 ... 

Environmental Impact of Ambient Ozone. Ozone can be toxic to plants, animals, and fish. The lethal dose, LD q, for albino mice is 3.8 ppmv for a 4-h exposure (156) the 96-h LC q for striped bass, channel catfish, and rainbow trout is 80, 30, and 9.3 ppb, respectively. Small, natural, and anthropogenic atmospheric ozone concentrations can increase the weathering and aging of materials such as plastics, paint, textiles, and mbber. For example, mbber is degraded by reaction of ozone with carbon—carbon double bonds of the mbber polymer, requiring the addition of aromatic amines as ozone scavengers (see Antioxidants Antiozonants). An ozone decomposing polymer (noXon) has been developed that destroys ozone in air or water (157). 

Because no molecule is spHt out, the molecular weight of the repeating unit is identical to that of the monomer. Vinyl monomers, H2C=CHR (Table 2) undergo addition polymerization to form many important and familiar polymers. Diene (two double bonds) monomers also undergo addition polymerization. Normally, one double bond remains, leaving an unsaturated polymer, with one double bond per repeating unit. These double bonds provide sites for subsequent reaction, eg, vulcanization. 

The Michael addition of nucleophiles to the carbon—carbon double bond of maleimide has been exploited ia the synthesis of a variety of linear polymers through reaction of bismaleimide with bisthiols (39). This method has been used to synthesize ethynyl-terminated imidothioether from the reaction of 4,4 -dimercaptodiphenyl ether [17527-79-6] and A/-(3-ethynylphenyl)maleimide (40). The chemical stmcture of this Michael addition imide thermoset is as follows ... 

Fiber-Reactive Dyes. These dyes can enter iato chemical reaction with the fiber and form a covalent bond to become an iategral part of the fiber polymer. They therefore have exceptional wetfastness. Thein main use is on ceUulosic fibers where they are appHed neutral and then chemical reaction is initiated by the addition of alkaH. Reaction with the ceUulose can be by either nucleophilic substitution, using, for example, dyes containing activated halogen substituents, or by addition to the double bond in, for example, vinyl sulfone, —S02CH=CH2, groups. 

Polyethylene, a linear polymer, is made by an addition reaction. It is started with an initiator, such as FIjOj, which gives free, and very reactive —OFI radicals. One of these breaks the double-bond of an ethylene molecule, C2FI4, when it is heated under pressure, to give... 

BMI was also used as a crosslinking agent for poly(iminoethylene). The Michael addition takes place with the nucleophilic nitrogen of the imino group and the double bonds of the electrophilic BMI. The Michael addition of BMI is now adopted as a crosslinking reaction for polymers with amino end groups [2]. 

To accelerate the polymerization process, some water-soluble salts of heavy metals (Fe, Co, Ni, Pb) are added to the reaction system (0.01-1% with respect to the monomer mass). These additions facilitate the reaction heat removal and allow the reaction to be carried out at lower temperatures. To reduce the coagulate formation and deposits of polymers on the reactor walls, the additions of water-soluble salts (borates, phosphates, and silicates of alkali metals) are introduced into the reaction mixture. The residual monomer content in the emulsion can be decreased by hydrogenizing the double bond in the presence of catalysts (Raney Ni, and salts of Ru, Co, Fe, Pd, Pt, Ir, Ro, and Co on alumina). The same purpose can be achieved by adding amidase to the emulsion. 

Conjugated dienes can be polymerized just as simple alkenes can (Section 7.10). Diene polymers are structurally more complex than simple alkene polymers, though, because double bonds remain every four carbon atoms along the chain, leading to the possibility of cis-trans isomers. The initiator (In) for the reaction can be either a radical, as occurs in ethylene polymerization, or an acid. Note that the polymerization is a 1,4-addition of the growing chain to a conjugated diene monomer. 

Synthetic polymers can be classified as either chain-growth polymen or step-growth polymers. Chain-growth polymers are prepared by chain-reaction polymerization of vinyl monomers in the presence of a radical, an anion, or a cation initiator. Radical polymerization is sometimes used, but alkenes such as 2-methylpropene that have electron-donating substituents on the double bond polymerize easily by a cationic route through carbocation intermediates. Similarly, monomers such as methyl -cyanoacrylate that have electron-withdrawing substituents on the double bond polymerize by an anionic, conjugate addition pathway. 

In anionic and coordination polymerizations, reaction conditions can be chosen to yield polymers of specific microstructurc. However, in radical polymerization while some sensitivity to reaction conditions has been reported, the product is typically a mixture of microstructures in which 1,4-addition is favored. Substitution at the 2-position (e.g. isoprene or chloroprene - Section 4.3.2.2) favors 1,4-addition and is attributed to the influence of steric factors. The reaction temperature does not affect the ratio of 1,2 1,4-addition but does influence the configuration of the double bond formed in 1,4-addition. Lower reaction temperatures favor tram-I,4-addition (Sections 4.3.2.1 and 4.3.2.2). 

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