Poly-butadienes isomerization

Figure 5-9. Conformations, lattice constants (fl, and c), density p, and crystal forms of the four isomeric poly(butadienes) (after G. Natta and P. Corradini).

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. 

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. 

The reader should note that stereoisomerism does not exist if the substituents X and Y in the monomer 4-14 are identical. Thus there are no configurational isomers of polyethylene, polyisobutene, or polyfvinylidene chloride). It should also be clear that 1,2-poly-butadiene (reaction 4-3) and the 1,2- and 3,4-isomers of polyisoprene can exist as isotactic, syndiotactic. and atactic configurational isomers. The number of possible structures of polymers of conjugated dienes can be seen to be quite large when the possibility of head-to-head and head-to-tail isomerism is also taken into account. 

Poly butadiene in the solid state undergoes photochemical cis—trans isomerization when irradiated in vacuo at 123.6 nm or 253.7 nm. Quantum yields have been estimated to be, respectively, 0.25 [48] and 0.036 [47] at these two wavelengths. The reaction proceeds in both cases towards a photostationary cis—trans ratio of about 60 40 (Fig. 12) [48]. Photo-isomerization at 253.7 nm was first assumed to be sensitized by... 

Fig. 12. Cis—trans photo-isomerization of poly butadiene, cis O medium pressure mercury source, lower scale, low pressure mercury source, upper scale trans A medium pressure mercury source, lower scale [reproduced with permission from Ref. 48].

Polymers of different geometric isomerism are also tactic. Cw-tactic (ct) and trans-idiCXic (tt) polymers can be distinguished according to the steric arrangement of chain components about double bonds in the main chain. Samples of this are cis- and 1,4-poly (butadiene) ... 

Such free radicals X are produced by irradiating organobromides, organo-sulfides, mercaptans, or Br2 by the action of uv light. Alternatively, the isomerization can proceed via charge transfer complexes with sulfur or selenium. In this way, cw-l,4-poly(butadiene) is isomerized at 25 C to an equilibrium product containing 77% trans bonds. Thus, with = 77/23 = 3.35, Equation (23-8) gives AG so = "3.0 kJ/mol. 

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 great influence exerted by the structure of macromolecular compounds on properties, in contrast to low-molecular-weight structures, becomes obvious in a comparison of the two 1,2-dimethylethylenes with the l,4-poly(butadienes). The melting point of the trans-butene-2 is about 34°C higher than that of the cis-butene-2, while the boiling points Tbp only differ by about 3°C. While butene-2 shows two isomers, poly-(butadiene) can occur in five isomeric forms. 

Butene-1 is converted to iso tactic polymers with Ziegler-Natta catalysts. Alternatively, cis- or trans-butene-2 can be used as starting material since, in the presence of certain catalyst systems, they isomerize before polymerization. st-Poly(butene-l) is obtained by hydrogenation of st-l,2-poly-(butadiene). 

Figure 2.10. Segmented polyacetylene copolymer with up to ten acetylene repeating units per segment is obtained from poly(butadiene) using a base-induced isomerization method.

The thermal reaction of cis or trans 1,4-poly(butadiene) with iron carbonyls results in geometrical isomerization of the alkenyl moieties and formation of polymers containing conjugated diene) iron tricarbonyl units. In the course of our continuing studies of the formation of stable colloidal iron dispersions by thermal decomposition of metal carbonyls in the presence of functional polymers, some aspects of this work have been repeated. Our objective was to isolate the soluble organometallic polymer which was intermediate to particle nucleation and independently examine the intramolecular condensation of metal atoms to yield metal clusters and metal particles. In this paper, the structure of the intermediate obtained on thermolysis of an excess of FeCCO) in a dilute xylene solution of c/5-poly(butadiene) has been described. 

C/5-2,6 octadiene would be the most appropriate small molecular model for the repeat unit in m-poly(butadiene). However, in light of the fact that FeCCO) catalyzes double bond migration and isomerization any isomeric octadiene monomer should serve as an appropriate model for the polymer reaction. We reacted 1,7-octadiene with FeCCO) and obtained a simple infrared spectrum which was nearly identical to that for the reaction product of c/5-poly (butadiene) with Fe(C0)5, i.e., both materials displayed two discrete peaks in the carbonyl region, a sharp peak at 2050 cm and a broad absorption ( "30 cm HWHH) centered at 198O cm" (See Fig. 2). The broad unresolved character of the 1980 cm" band, as compared to that in rj -butadiene iron tricarbonyl which shows discrete sharp peaks at 1980 and 1990 cm in addition to the sharp peak at 2056 cm", can be understood in terms of the variety of isomeric diene iron tricarbonyls which can be formed in the reaction of FeCCO) with either c/5-poly(butadiene) or 1,7-octadiene. 

Diene type polymers, prepared by either free radical or anionic methods, contain chain units that although chemically identical are isomeric to one another. Hence, from a crystallization point of view this class of polymers behave as copolymers. For example, polymers prepared from the 1,3-dienes are subject to several different kinds of chain irregularities. For poly (butadiene), the following structures are known to exist ... 

Since 1,4-polyisoprene has a secondary carbon atom at the double bond it follows that it is generally more reactive to both free radicals and to carbonium ions than 1,4-poly butadiene. The typical addition reactions associated with the double bond suggest that the ultimate hydrogenated, halogenated, hydrohalogenated and isomerized diene polymers would have the same structure irrespective of the initial cis-ltrans- ratio. 

Studies both with model compounds and deuterated polymers have shown that, as with the poly butadienes, isomerization is not accompanied by double bond migration. 

Poly butadiene is regarded as a copolymer consisting of three isomeric units, cis 1,4-, trans 1,4- and 1,2-. 

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... 

Butadiene and isoprene have also been studied in tetrahydrofuran (72). At 0° the rates are close to first order in polyisoprenyl or poly-butadienyllithium concentration which indicates that the rate constant for the ion-pair is being measured in the concentration range studied (> 10-s molar). The rate constants at 30°, k9 (butadiene) =1.8 litre/mole sec, kj, (isoprene) = 0.13 litre/mole sec, are appreciably lower than for styrene. For butadiene at —39° a kp value of 6 X 10 2 litre/mole sec can be derived from the results of Spirin (98). This value checks well with that extrapolated from Morton s data. The observed propagation constant for isoprene is rather low and is in fact equal to that of the mono-etherate in solvent mixtures of appreciably lower dielectric constant. At room temperature there is evidence for isomerization of polyisoprenyl-lithium in tetrahydrofuran which becomes particularly marked as the... 

In addition to copolymer composition, geometrical and steric isomerism play important roles in the determination of chain stiffness and thus of Tg. Polydienes, for example, can exist as cis and trans geometrical isomers. In the case of poly(l, 5-butadiene), the cis isomer has a Tg of-102 °C while the stiffer trans isomer shows a Tg of -48 °C. The effect is not nearly so marked in the... 

Other aldehydic compounds, such as glyoxal and chloral, also react in a similar way with polyisoprenes and unsaturated rubbers (e.g., poly(cfs-l,4-isoprenes), poly( fs-l,4-butadiene), and copolymers of isobutylene and iso-prene). The use of strong acids, or Lewis acids, causes complications, as the acids themselves, under suitable conditions, catalyze cyclization and cis-trans isomerization, and these reactions may occur simultaneously with the addition reactions. 

In conclusion, form II of l,4-tra s-poly(l,3-butadiene) represents a case of conformational isomerism of polymers in the crystalline state where different conformations occur at random within the same chain and long-range order is maintained among parallel chains [58]. 

Figure 1.9 Isomeric structure which can be obtained by the polymerization of butadiene and isoprene. Poly( 1,2-butadiene), poly(l,2-isoprene) and poly(3,4-isoprene) might be isotactic,...

Figure 1.13 Cis-trans isomerism of polyisoprene. The systematic name of isoprene is 1,3-butadiene, 2-methyl. Polyfisoprene 1,4-c s) is the main constituent of natural rubber. Poly Osoprene 1,4-trons) is the main constituent of gutta percha. Vinylic units (1-2) and (3-4) are present in synthetic rubbers.

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. 

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