Bromine, backbone structure

Exxon Mobil in brochures argues that butyl elastomer (HR) apparently is miscible with chlorobutyl elastomer (CIIR) and bromobutyl elastomer (BUR). This occurs because they have the same backbone structure of polyisobutylene and polyisoprene with an occasional different functional group of chlorine and bromine, respectively [33,34]. 

From information presented earlier in this chapter, it will be clear that three types of molecular structures in polyphosphazenes are known to give rise to rubbery or elastomeric properties. These are summarized in Table 3.1. First, if the side groups are single atoms, such as fluorine, chlorine, or bromine, the inherent flexibility of the backbone dominates the materials flexibilty and gives rise to elastomeric character. Unfortunately, the polymers (NPF2) , (NPC12) , and (NPBr2) are slowly hydrolyzed in contact with atmospheric moisture. Hence, they are of interest as reaction intermediates but not as usable materials. 

With regard to reactive flame-retardants, two routes can be followed to improve thermal stability and fire behavior of PU foams use of brominated or phosphorus-containing polyol or, for rigid foams, the introduction inside polymer backbone of more thermally stable structure than urethane, mainly isocyanurate, but also uretidione rings or carbodiimide.19... 

Figure 1.7. Structural formula of 2-methyl-2,3-dibromopentane (a) pentane is the major chain therefore, there is a straight saturated five-carbon chain as the major backbone (b) 2-methyl- there is a methyl group on the number two carbon (c) -2,3-dibromo dibiomo means two bromine atoms, and they are on the number 2 and 3 carbons (d) the rest of the bonds are not specified therefore, they are all bonded to hydrogen thus we have 2-methyl-2,3-dibromopentane.

NciBrasym is the number of chlorine and/or bromine atoms which are not symmetrically attached to the same backbone atom. The symmetry of backbone substitution affects the value of e. There is considerable mutual cancellation of dipole moment vectors when substitution is symmetrical. This cancellation results in smaller net dipole moment vectors for structural units consisting of the backbone carbon atoms and their substituents, explaining why the term proportional to N(C1 or Br) is only added if Cl and/or Br atoms are not attached symmetrically to the same backbone atom. See Figure 9.2 for three examples, explained in the next paragraph. 

Cationic grafting from PVC often occurs at structural irregularities on the backbone. A similar uncertainty regarding the nature and concentration of the active centres in the grafting reaction surrounds the use of chlorinated and brominated butyl rubber backbones, chlorinated polycyclopentadiene, and chlorinated st rene-butadiene copolymers." The graft reaction of st3o ene into chlorosulphonated polyethylene appears to occur at tertiary chlorine sites and not at the sulphonyl chlorides. ... 

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