Diene radical polymerization

The elastomer produced in greatest amount is styrene-butadiene rubber (SBR) Annually just under 10 lb of SBR IS produced in the United States and al most all of it IS used in automobile tires As its name suggests SBR is prepared from styrene and 1 3 buta diene It is an example of a copolymer a polymer as sembled from two or more different monomers Free radical polymerization of a mixture of styrene and 1 3 butadiene gives SBR... 

The reversible addition of sodium bisulfite to carbonyl groups is used ia the purification of aldehydes. Sodium bisulfite also is employed ia polymer and synthetic fiber manufacture ia several ways. In free-radical polymerization of vinyl and diene monomers, sodium bisulfite or metabisulfite is frequentiy used as the reduciag component of a so-called redox initiator (see Initiators). Sodium bisulfite is also used as a color preventative and is added as such during the coagulation of crepe mbber. 

Thiol-ene polymerization was first reported in 1938.220 In this process, a polymer chain is built up by a sequence of thiyl radical addition and chain transfer steps (Scheme 7.17). The thiol-ene process is unique amongst radical polymerizations in that, while it is a radical chain process, the rate of molecular weight increase is more typical of a step-growth polymerization. Polymers ideally consist of alternating residues derived from the diene and the dithiol. However, when dienes with high kp and relatively low A-, monomers (e.g. acrylates) are used, short sequences of units derived from the diene are sometimes formed. 

Of the major methods for living radical polymerization, NMP appears the most successful for polymerization of the diene monomers. There are a number of reports on the use of NMP of diene monomers (B, I) with TEMPO,188,1103 861 4, cw and other nitroxides.127 High reaction temperatures (120-135 °C) were employed in all cases. The ratio of 1,2- 1,4-cis 1,4-trans structures obtained is similar to that observed in conventional radical polymerization (Section 4.3.2). 

Most addition polymerizations involve vinyl or diene monomers. The opening of a double bond can be catalyzed in several ways. Free-radical polymerization is the most common method for styrenic monomers, whereas coordination metal... 

Chemical methods for structure determination in diene pol3 mers have in large measure been superseded by infrared absorption techniques. By comparing the infrared absorption spectra of polybutadiene and of the olefins chosen as models whose ethylenic structures correspond to the respective structural units, it has been possible to show that the bands occurring at 910.5, 966.5, and 724 cm. are characteristic of the 1,2, the mns-1,4, and the m-1,4 units, respectively. Moreover, the proportion of each unit may be determined within 1 or 2 percent from measurements of the absorption intensity in each band. The extinction coefficients characteristic of each structure must, of course, be known these may be assigned from intensity measurements on model compounds. Since the proportions of the various units depend on the rates of competitive reactions, their percentages may be expected to vary with the polymerization temperature. The 1,2 unit occurs to the extent of 18 to 22 percent of the total, almost independent of the temperature, in free-radical-polymerized (emulsion or mass) poly butadiene. The ratio of trans-1,4 to cfs-1,4, however,... 

In the free radical polymerization of 1,3-dienes, 1,4 addition dominates 1,2 addition. The proportion of 1,2 (and 3,4 )units decreases in passing from butadiene to its methyl and chlorine substitution products isoprene, 2,3-dimethylbutadiene and chloroprene. The trans configuration of the 1,4 unit from butadiene is formed preferentially, the proportion of trans increasing rapidly with lowering of the polymerization temperature. 

Significant improvement in controlled polymerizations of a variety monomers, including styrene, acrylates, acrylamide, acrylonitrile, 1,3-dienes, and maleic anhydride has been achieved when alkoxyamines have been used as initiators for living, free radical polymerization.(696c, 697) Alkoxyamines can be easily synthesized in situ by the double addition of free radicals, generated by thermal decomposition of an azo-initiator, such as 2,2 -azo-h/.s-/.so-butyronitrile (AIBN), to nitrones (Scheme 2.206). 

Many of the synthetic elastomers now made are still polymerized by a free radical mechanism. Polychloroprene, polybutadiene, polyisoprene, and styrene-butadiene copolymer are made this way. Initiation by peroxides is common. Many propagation steps create high molecular weight products. Review the mechanism of free radical polymerization of dienes given in Chapter 14, Section 2.2. 

Stereospecific polymerization has particular significance for the preparation of stereoregular polymeric dienes. In the radical polymerization of butadiene or isoprene the molecular chains always consist of varying proportions of adjacent cis- and trans-1,4-units as well as 1,2- and 3,4- linked units, depending on the polymerization conditions but it is now possible, using particular ionic initiation systems to make a synthetic natural rubber that contains more than 90% cfs-l,4-isoprene repeating units (see Example 3-21). 

In this section the occurrence of LCB in several other of the more important polymers during free-radical polymerization will be discussed branching due to irradiation or the use of multifunctional monomers (including dienes) will not be... 

The most important feature of ionizing radiations is, as the term implies, ionization to give ionic intermediates in irradiated systems. Though radiation-induced radical polymerization had long been studied, it is only a decade since radiation-induced ionic polymerization was first found. In 1957, Davison et al. obtained polymer from isobutene, which is known not to be polymerized by radical catalysts, by irradiating at low temperature with y-rays (7). Before long, the radiation-induced polymerization of styrene was proved to proceed as an ionic mechanism in suitable solvents (2,3,4). Since these pioneering researches, the study of the chemical kinetics of radiation-induced ionic polymerization has been extended to several vinyl, diene and cyclic monomers. 

The substituted carbon atoms in the polymer chain are asymmetric. Stereoregular polymers are produced if all these carbon atoms have the same configuiation (all d or all I) or if the d and 1 configurations occur alternately pronounced stereo-regularity is seldom achieved in radical polymerizations except perhaps at very low temperatures. When dienes are polymerized by a radical mechanism, the resulting polymers contain several distinct types of monomer unit, thus butadiene can give rise to -CH2 C(CH CH2)-, -CH CH CH CH2- cis, and -CH CH.CH CH2- trans. 

Combining whole-cell biocatalysis and radical polymerization, researchers at Imperial Chemical Industries (ICI) published a chemoenzymatic route to high-molecular-weight poly(phenylene) [86], This polymer is used in the fibers and coatings industry. However, since it is practically insoluble, the challenge was to make a soluble polymer precursor that could first be coated or spun, and only then converted to poly(phenylene). The ICI process starts from benzene, which is oxidized by Pseudomonas putida cells to cyclohexa-3,5-diene-l,2-diol (see Figure 5.17). The... 

---