Poly isoprene, chain structure

Successive 1,4 units in the synthetic polyisoprene chain evidently are preponderantly arranged in head-to-tail sequence, although an appreciable proportion of head-to-head and tail-to-tail junctions appears to be present as well. Apparently the growing radical adds preferentially to one of the two ends of the monomer. Which of the reactions (6) or (7) is the preferred process cannot be decided from these results alone, however. Positive identification of both 1,2 and 3,4 units in the infrared spectrum shows that both addition reactions take place during the polymerization of isoprene. The relative contributions of the alternative addition processes cannot be ascertained from the proportions of these two units, however, inasmuch as the product radicals formed in reactions (6) and (7), may differ markedly in their preference for addition in one or the other of the two resonance forms available to each. We may conclude merely that structural evidence indicates a preference for oriented (i.e., head-to-tail) additions but that the 1,4 units of synthetic polyisoprene are by no means as consistently arranged in head-to-tail sequence as in the naturally occurring poly-isoprenes. 

A similar comparison can be made with cis-poly(isoprene), natural rubber, by taking advantage of the fact that the polymer is very slow to crystallize [164], Consequently, the comparison can be made between the supercooled, noncrystalline polymers at 0°C and the semi-crystalline polymer (31% crystalline) at the same temperature. The Tlc values for each of the five carbons involved were again found to be the same for the completely disordered polymer and the semicrystalline one, so that a similar conclusion can be made with regard to their chain structure. 

The melting of a crystalline-amorphous block copolymer of poly(tetrahydro-furan)-poly(isoprene) (PTHF-PI) was investigated using DSC by Ishikawa et al. (1991). They found a double melting peak, which was proposed to result from the semicrystalline structure of the crystalline PTHF layer, with less-ordered crystallites melting before those with well-ordered domains of chain-folded PTHF. Alternative explanations include fractionation of the polydisperse block copolymer or melting of crystals with different fold lengths. 

A more detailed analysis of the NMR signal from elastomer samples, addresses finer details such as the chemical structure of chain segments. In general a hierarchy of dipolar interaction between protons exists instead of a single chain-averaged dipolar interaction [34, 35]. For example, in cis-l,4-poly(isoprene) these different dipolar interactions can... 

The chemical structure of a polymer determines whether it will be crystalline or amorphous in the solid state. Both tacticity (i.e., syndio-tactic or isotactic) and geometric isomerism (i.e., trans configuration) favor crystallinity. In general, tactic polymers with their more stereoregular chain structure are more likely to be crystalline than their atactic counterparts. For example, isotactic polypropylene is crystalline, whereas commercial-grade atactic polypropylene is amorphous. Also, cis-pol3nsoprene is amorphous, whereas the more easily packed rans-poly-isoprene is crystalline. In addition to symmetrical chain structures that allow close packing of polymer molecules into crystalline lamellae, specific interactions between chains that favor molecular orientation, favor crystallinity. For example, crystallinity in nylon is enhanced because of... 

Two possible scenarios can be envisaged for the structure of the hybrid material (see Fig. 11). The poly(ethylene oxide) block, albeit strongly interacting and partially penetrating, forms a pure PEO layer at the interface to the hydrophobic poly(isoprene) (Fig. 11, left-hand sketch) ( three-phase system). The other possibility is the complete dissolution of the PEO chains in the aluminosilicate, which results in the two-phase system depicted in the right-hand sketch of Fig. 11. Spin-diffusion NMR experiments showed that there appears to be no dynamic heterogeneity in the poly(ethylene oxide) chains, as would be expected for a three-phase system, giving rise to the conclusion that the hydrophilic... 

For the purpose of illustration, the way in which structural factors of apparently minor importance may be decisive for the existence of high-elasticity may be pointed out. Referring to p. 35, where the chemical formulae of rubber and gutta percha are shown to be different only, in that one is the cis and the other the trans form of poly-isoprene, it may be asked why the first is rubbery elastic at room temperature, while the other only possesses this property after being heated above 60 C. Gutta is crystalline at room temperature and melts at 60° C, proving that at this temperature the micro-Brownian motions of the chain elements become so intense, that the lattice forces are insufficient to keep these parts fixed. 

Stupp and coworkers reported on rod-coil systems containing a monodisperse rod part basing on an azo dye bound to a rigid monomer [151-153]. For these systems, the authors synthesized poly-isoprenes anionically and terminated the living chains with carbon dioxide resulting in carboxylated polyisoprenes that were then coupled to the rigid block (see Figure 42(a) for the end structure). 

The polymorphism in poly(l, 4-trans-isoprene) (gutta percha) has been studied in detail. Based on a detailed analysis of chain stereochemistry Bunn (261) predicted the possibility of four different crystalline modifications of this polymer, each with a different chain structure. Two of these, crystallized solely by cooling the polymer to an appropriate temperature, have been identified and their crystal structures determined.(261-263) A third form, that crystallizes upon stretching, has also been identified.(264) However, its structure has been questioned.(264)... 

The most striking result of this work was to show the effect of solvating solvents, e.g., ethers, in delocalizing the electrons of the c Lrbanionic chain end, from a largely covalent carbon-lithiiom bond to a TT-allylic type. This correlated very well with the activity of the y-carbon in generating side-vinyl structures in the polymer chain (l,2 units of polybutadiene and 3,i+ units of poly-isoprene). 

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