Poly-butadienes 1.4- trans

On the contrary, butadiene and methacryloyl monomers (1,3,4, 10,11) can also be polymerized in the liquid expanded phase. The butadiene lipids have previously been shown to form 1,4-trans-poly(butadiene)s (40j in the monolayer (Eqn. II.). 

Finter, J. and Wegner, G. The relation between phase transition and crystallization behavior of l,4-trans-poly(butadiene). Makromol. Chemie 182, 1859 (1981) (see here and Ref. 3 for older data)... 

The configuration equilibrium observed in solution can be displaced when one isomer can be removed from the equilibrium, by, for example, crystallization. For example, when 1,4-poly(butadienes) of high trans content are dosed with trace amounts of an all-trans poly(butadiene), a decrease in the trans content is initially observed. Subsequently, however, the trans content increases again. It is assumed that a trans isomerization occurs at the crystalline-amorphous interface, whereby the longer trans sequences are incorporated into the crystal lattice and thereby removed from the equilibrium. A new equilibrium is then established by producing more new trans sequences. 

The l,4-poly(butadienes) corresponding to the butene-2 compounds, however, differ in their melting points by almost 140°C (Table 1-2). 1,4-c/s-Poly(butadiene) is an elastomer l,4-trans-poly(butadiene) is thermoplastic. The other isomeric poly(butadienes) likewise show a marked difference in their properties. Aromaticized l,2-poly(butadiene), although not an isomer of poly(butadiene), shows semiconductor properties because of its conjugated double bonds. 

In practice, cis- and trans-poly(butadiene) can be subjected to the same... 

As a class the aliphatic polyalkenamers have low values due to a combination of low chain stiffness and low interchain attraction. The presence of double bonds has the effect of increasing the flexibility of adjacent single bonds (see Chapter 4) and overall this leads to a reduction in. Thus in the sequence from polydecenamer down to polypentenamer an increase in the double bond concentration leads to a lowering of Tg. On the other hand the Tg of polybutenamer, i.e. poly butadiene, is somewhat higher than that of polypentenamer, presumably because the proportion of stiff links, i.e. double bonds, becomes sufficiently high to override the flexibilising effect on adjacent chains. Consequently the polypentenamers have the lowest Tg values known for hydrocarbon polymers (cis- -114°C, trans- -97°C). 

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,... 

Unlike polybutadiene, polyisoprene prepared at low temperatures shows little or no inclination to crystallize either on stretching or cooling. This may seem surprising in view of the even greater preponderance of trans-1 4 units in polyisoprene than in poly butadiene. The explanation for the contrasting behavior in this respect between low temperature synthetic polyisoprene, on the one hand, and guttapercha and low temperature polybutadiene, on the other, probably is to be found in the appreciable occurrence of head-to-head and tail-to-tail sequences of 1,4 units of the former. 

Figure 15. Differential Scanning Calorimetry (DSC) curve of 90% trans-i, 4-poly butadiene...

Figure 7. Stress optical coefficient X temperature as a function of temperature (3). PBD is poly butadiene. Key O, p-xylene A. toluene , benzene CClt all of swollen trans-/,4 PBD.

White91 used a 1,3-butadiene-urea canal complex to produce all-trans- 1,4-poly butadiene. The complex is formed only at temperatures in the range — 55 °C to 25 °C and needs a small amount of methanol to be formed. 

Diene polymers refer to polymers synthesized from monomers that contain two carbon-carbon double bonds (i.e., diene monomers). Butadiene and isoprene are typical diene monomers (see Scheme 19.1). Butadiene monomers can link to each other in three ways to produce ds-1,4-polybutadiene, trans-l,4-polybutadi-ene and 1,2-polybutadiene, while isoprene monomers can link to each other in four ways. These dienes are the fundamental monomers which are used to synthesize most synthetic rubbers. Typical diene polymers include polyisoprene, polybutadiene and polychloroprene. Diene-based polymers usually refer to diene polymers as well as to those copolymers of which at least one monomer is a diene. They include various copolymers of diene monomers with other monomers, such as poly(butadiene-styrene) and nitrile butadiene rubbers. Except for natural polyisoprene, which is derived from the sap of the rubber tree, Hevea brasiliensis, all other diene-based polymers are prepared synthetically by polymerization methods. 

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... 

During the polymerization of the styrene, the poly(butadiene) or butadiene copolymer is grafted onto the styrene polymer chain. To increase the grafting efficiency of the poly(butadiene), it is desirable for the poly(butadiene) to have end segments having a high vinyl content rather than cis- or trans-configurations. 

The structure of the live lithium chain ends is a matter of controversy and will be discussed in a later section. After the lithium-polybutadiene is terminated with protic material, the isolated poly butadiene polymer exhibits a mixed microstructure (—35% cis-1,4, 54% trans-1,4, and -11% 1,2). 

Soon after, it was reported that allylnickel halides could polymerise butadiene to crystalline polymers consisting of cis-1,4 or trans-1,4 units [36] and that trisally lehr omium polymerised it predominantly to 1,2-poly butadiene but bisal-lylcobalt iodide polymerised it predominantly to cis- 1,4-poly butadiene [37],... 

The activity and stereospecificity of rc-allylic catalysts for conjugated diene polymerisation depend both on the kind of metal and on the nature of the ligand attached to this metal. For instance, Cr(All)3 [137] and Co(f/3-C8Hi3)(C4H6)-CS2 [103] catalysts yield 1,2-polybutadiene, while Cr (A11)2C1 [120], Cr(All)2I [134] and U(A11)3C1 [147] catalysts produce cis-1,4-polybutadiene, but an Nd(All)3.DOX catalyst gives trans-1,4-polybutadiene [146] and a Co(fj3-C4H7)3—I2 catalyst yields eb-c/.v-l, 4/1,2-poly butadiene [137,145] (Table 5.5). 

The reactions presented in scheme (10) also account for effects exerted by the addition of Lewis bases or acids (as well as other electron donors and acceptors) to the polymerisation system on the microstructure of the polymers formed. As shown in Tables 5.4 and 5.5, some catalysts that are highly stereospecific for the formation of cis- 1,4-polybutadiene yield trans- 1,4-poly butadiene (or eb-m-1,At trans-1,4-polybutadiene) after the addition of a Lewis base or other electron donor to the catalyst system. A plausible explanation of the observed phenomena is that the added component occupies a coordination site at the transition metal, thus forcing the incoming monomer molecule to coordinate as an s-trans-rf ligand. When the additional catalyst component has a basicity comparable with that of the monomer, a competitive monomer/ Lewis base (electron donor) coordination takes place, as shown below [7] ... 

Nd-boranate-based catalysts were also used in the polymerization of dienes. Nd(BH4)3 (THF)3/TEA yields poly(butadiene) with a frans/cz s-ratio 50/50. If Nd(BH4)3 (THF)3 is combined with a stoichiometric amount of MgBu2 catalyst activities are increased, control of molar masses is improved and poly(diene)s with a trans- 1,4-content of up to 99% are obtained. [348, 349]. 

As can be seen from Table 9 an increase in the ( i/ Nd-ralio from 0.5 to 3.0 results in an increase of the cis- 1,4-content. A further increase of the ci/ Nd-ratio to 4.0 and 10.0 decreases the cis- 1,4-content. BR which is obtained without halide donor (at a very low catalyst activity) exhibits a unique mi-crostructural composition 71.9% czs-1,4, 21.2% trans-1,4 and 6.9% 1,2. This observation corroborates the fact that high-czs-l,4-poly(butadiene) products only can be obtained in the presence of halide donors. 

The mechanism is complicated by the possibility of anti-syn-isomerization and by n - a-rearrangements (it - r 3-allyl Act - r 1 -allyl). In the case of C2-unsubstituted dienes such as BD the syn-form is thermodynamically favored [646,647] whereas the anti-isomer is kinetically favored [648]. If monomer insertion is faster than the anti-syn-rearrangement the formation of the czs- 1,4-polymer is favored. A higher trans- 1,4-content is obtained if monomer insertion is slow compared to anti-syn-isomerization. Thus, the microstructure of the polymer (czs-1,4- and frazzs-1,4-structures) is a result of the ratio of the relative rates of monomer insertion and anti-syn-isomerization. As a consequence of these considerations an influence of monomer concentration on cis/trans-content of BR can be predicted as demonstrated by Sabirov et al. [649]. A reduction of monomer concentration results in a lower rate of monomer insertion and yields a higher trans-1,4-content. On the other hand the czs-1,4-content increases with increasing monomer concentration. These theoretical considerations were experimentally verified by Dolgoplosk et al. and Iovu et al. [133,650,651]. Furthermore, an increase of the polymerization temperature favors the formation of the kinetically controlled product and results in a higher cis- 1,4-content [486]. l,2-poly(butadiene) can be formed from the anti- as well as from the syn-isomer. In both cases 2,1-insertion occurs [486]. By the addition of electron donors the number of vacant coordination sites at the metal center is reduced. The reduction of coordination sites for BD results in the formation of the 1,2-polymer. In summary, the microstructure of poly(diene) depends on steric factors on the metal site, monomer concentration and temperature. 

Poly (2-methyl-l,3-butadiene), trans Poly (2-methyl-1,3-butadiene), cis Polyethylene, high density low density... 

The type of alkylaluminum compound has only a secondary influence on structure in the polymerization of diolefins in contrast to its strong effect on the structure of polypropylene. An exception is AlEtCb, which, apparently in connection with its cationogenic character, with j3-TiCl3 induces polymerization to trans-1,4 polybutadiene and, even without transition metal compound, leads to formation of cyclized polyisoprene. Incidentally, this indicates that poly-butadiene is much more stable towards cyclization than polyisoprene. 

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