Polymers Large molecules diene

The alkene and diene polymers discussed in Sections 7.10 and 14.6 are called chain-growth polymers because they are produced by chain reactions. An initiator adds to a C=C bond to give a reactive intermediate, which adds to a second alkene molecule to produce a new1 intermediate, which adds to a third molecule, and so on. By contrast, polyamides and polyesters are called step-growth polymers because each bond in the polymer is formed independently of the others. A large number of different step-growth polymers have been made some of the more important ones are shown in Table 21.2. 

Most monomers contain only one polymerizable group. There are some monomers with two polymerizable groups per molecule. Polymerization of such monomers can lead to more than one polymer structure. The polymerization of 1,3-dienes, of large industrial importance, is discussed in Sec. 8-6. 1-Substituted-l,2-dienes (allenes) undergo polymerization through both the substituted and unsubstituted double bonds... 

It is well known that, in the cyclopolymerization of unconjugated dienes which can form 5- and 6-menbered ring structural units, these units often largely prevail over the linear ones, that is the units deriving from monomer molecules which have contributed to the polymer chain growth with only one of their unsaturations. To account for this fact, several hypotheses have been suggested, which will be reviewed and discussed in the present paper. 

The other class of molecules that self-reacts consists of monomers that readily form polymers -often these are alkenes or dienes. The polymerization process is usually free-radical generated and with polymeric products sometimes involving thousands of monomer linkages that produce viscous materials. The very large amount of heat cannot be dissipated quickly and runaway reactions can occur, leading to fires and explosions. (This is also briefly discussed in Section 5.3.2.)... 

Figure 3.7 Hole formation via a packing arrangement of four participating polymer chains [37]. (a) Natural rubber, (b) ethylene-propylene-diene rubber. The holes are just large enough to allow diffusion of another polymer molecule of the same chemical species. However, the distance between the participating chains forming the hole needs to increase by 15% for natural rubber but by 27% for ethylene-propylene-diene rubber, compared to their respective close-packed chain conformations.

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