Poly-cis-1,4-butadiene

Experimental support to the above conclusions, although of a semi-quantitative nature, is provided by a series of careful X-ray investigations on cis-poly-1.4-butadiene stretched rub-... 

Polymerization of dienes can also produce polymers in which the carbon moieties are on the same side of the newly formed double bonds (cis) or on the opposite side (trans). The cis isomer is a soft elastomer the Tg of the cis isomer of poly-1,4-butadiene is —108 °C. The trans isomer of poly-l,4-bu-tadiene is hard, with a Tg of —83 °C. 

The trans-poly-1,4-butadiene isomer is a harder and less soluble rigid crystalline polymer than the cis isomer. As shown by the skeletal structures for the trans isomer (Figure 1.11), chain extensions on opposite sides of the double bonds allow good fitting of adjacent polymer chains, and this, results in a rigid structure. In contrast, the os-poly-1,4-butadiene isomeric polymer units do not permit such interlocking of alternate units. Even so, chain... 

Poly( 1,4-butadiene) (cis, see Fig. 6.13) Poly(dimethylsilylenemethylene) Poly[oxy(methylphenylsilylene)] (See Fig. 6.11) Poly(3-hexoxypropylene oxide) Polyoxytetramethylene Poly( 1,1 -dimethylsilazane)... 

Cis/trans isomerism of backbone double bonds (more correctly Z/E isomerism in modern nomenclature) is a structural feature of polymers that may affect NMR spectra in a manner similar to tacticity in vinyl polymers. We will take the prototype chain, poly (1,4-butadiene) (-CH2CH=CHCH2 ) as an example. Denoting cis and trans isomers by c and f, the possible dyad, triad, etc. isomeric sequences can be constructed from c and t in the same way as m and r were used for vinyl polymer stereosequences. The possible dyads are cc, ct, tc and tt. In the cc and tt dyads, the two CH2 carbons are equivalent whereas in the ct and tc dyads, the two CH2 carbons are not equivalent. 

Experimentally, the chemical shifts of both [68] and [69, 70] nuclei in poly (1,4-butadiene) are found to depend almost entirely only on the structure at the monomer level. In the CH2 region of the spectrum, cis units appear at 27.4ppm and trans units at 32.8 ppm [69]. In the olefinic region, there is a slight dependence on sequence structure [70] peaks from cis units at 128.8 ppm and trans units at 129.35 ppm are each split into a doublet of about 0.1 ppm due to a slight sensitivity to dyad structure. It should be noted that analysis of the spectra of poly(dienes) in terms of cis/trans isomerism is severely complicated by the occurrence of 1,2- as well as 1,4-addition. 

Figure 2.16 Maps of conformational energy of various cis and trans polydienes as function of torsion angles 9i and 0268 (a) cis-l,4-poly( 1,3-butadiene), (b) /ra .v-l,4-poly( J, 3-buladiene), (c) cis-l, 4-poly(isoprene), (d) trans-1,4-poiy(isoprenej, (e) cis-1,4-poly(2,3-dimethyl-1,3-butadiene), and (/) trans- 1,4-poly(2,3 -dimethyl-1,3-butadiene). Isoenergetic curves are reported every 2 kJ/mol of monomeric units with respect to absolute minimum of each map assumed as zero. (Reproduced with permission from Ref. 68. Copyright 1986 by the Societa Chimica Italiana.)...

Crystalline polymers characterized by disordered conformations of the chains are, for instance, polytetrafluoroethylene (PTFE), /ra .s-1,4-poly (1,3-butadiene), and cis-1,4-poly(isoprcnc). 

Poly-buten-(l) und Poly-4-methyl-penten-(l), das Poly-1,4-cis-butadien, Poly-1,4-trans-butadien,... 

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