Trans-l,4-polybutadiene

The polymerization of butadiene with an aluminum trialkyl-titanium tetra-iodide catalyst system apparently yields a polybutadiene containing more than 85% cis-1,4- structure (19). The limited amount of data available does not cover a sufficiently wide range of Al/Ti ratios to permit the drawing of conclusions analogous to those drawn in the case of the trans-l,4-polybutadienes. However, as shown in Table III, over the range of Al/Ti ratios from 1.5 to 5.0, the polybutadiene prepared in benzene at 30° C. contains 85 to 93% cis-1,4-, 3 to 11% tram-1,4-, and 3 to 5% 1,2- structures. 

In the 1960s a number of publications on aqueous polymerizations of different monomers appeared. Rinehart et al. and Canale and co-workers independently reported aqueous polymerization of butadiene catalyzed by rhodium salts. Utilizing Rh "Cl3 3H2O as a catalyst precursor, semicryslallirie trans-l,4-polybutadiene was obtained stereoselectively [Eq. (4) >99% trans] [20, 21]. 

The sulfonation of low molecular weight model olefins was undertaken to determine the feasibility of this approach. Competitive sulfonations using acetyl sulfate were carried out on the model compounds below, representing the repeat structures of cis-l,4-polyisoprene (PIP), cw-l,4-polybutadiene (c-PBD), and trans-l,4-polybutadiene (Z-PBD), respectively. It was necessary to model both the cis and trans isomeric forms of 1,4-polybutadiene, since ttey have a nearly equal probability of occurrence when the anionic polymerization (Ii counterion) is conducted in a nonpolar hydrocarbon medium... 

Fig. 4.10. Thermal analysis and proton NMR second moment curves of trans-l,4-polybutadiene. The heat capacity for the solid (glass and low temperature crystal form I) have been computed by fitting at low temperature to an approximate skeletal vibration spectrum. The gradual decrease of the second moment of the proton resonance spectrum below T comes mainly from increasing mobility in the amorphous phase, Ref. The melting curve is of an approximately 90% crystalline sample. Ref.. Compare also to Fig. 1.3...

Iwayanagi S, Miura J (1965) Nudear magnetic resonance study of sdid phase transition of trans-l,4-polybutadiene. Rept Progr. Polymer Phys. Japan 8 303... 

If, however, the phenylene rings are para-oriented, the chains retain their axial symmetry and can crystallize more readily. Similarly, double bonds in trans configuration maintain the chain symmetry thus allowing for crystallite formation. This is highlighted by a comparison of the amorphous elastomeric ds-polyisoprene (=28°C) with highly crystalline tra s-polyisoprene T =74°C), which is a nonelastomeric rigid polymer, or ds-l,4-polybutadiene (Tm= — 11°C) with trans-l,4-polybutadiene (T, = 148°C). 

The preparation and characterization of 1,3-butadiene monomer is discussed extensively elsewhere (1 4) (see Butadiene). Butadiene monomer can be purified by a variety of techniques. The technique used depends on the source of the butadiene and on the polymerization technique to be employed. Emulsion polymerization, which is used to make amorphous /n j -l,4-polybutadiene (75% trans-1 4 , 5% kj -l,4 20% 1,2), is unaffected by impurities during polymerization. However, both anionic and Ziegler polymerizations, which are used to prepare kj -l,4-polybutadiene, mixed cis-1 4 and... 

Polybutadiene and polyisoprene are produced and used mainly as synthetic rubber on an industrial scale by using transition metal catalysts, especially titanium- and nickel-based ones. By contrast, only minor attention has been paid to the palladium-catalyzed polymerization of butadiene. A mixture of 1,2-polybutadiene and trans- and c/s-l, 4-polybutadiene was obtained by using PdCl2 as a catalyst (7, 2). 

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. 

Polyisoprene is composed of four structures as shown in Equation 5.48. As in the case of polybutadiene, it is the cis-, A structure that is emphasized commercially. The cA-1,4-polyisoprene is similar to the cw-l,4-polybutadiene material except it is lighter in color, more uniform, and less expensive to process. Polyisoprene is composition-wise analogous to NR. The complete cw-1,4 product has a Tg of about —71°C. Interestingly, isomer mixtures generally have higher Tg values. Thus, an equal molar product containing cA-1,4 trans-, A, and 3,4 units has a Tg of about —40°C. 

The polymer ra-l,4-polybutadiene (Tg = 170 K) is more flexible and has a lower Tg than trans- 1,4-polybutadiene (Tg = 190 K), and in general this is true of all geometric isomers. 

In this section information on possible condis states of the following macromolecules are reviewed polyethylene, polytetrafluoroethylene, poly(vinylidene fluoride), poly-chlorotrifluoroethylene, polypropylene, trans-1,4-polybutadiene, cis-l,4-poly(2-me-thylbutadiene), polyoxybenzoate, polyethylene terephthalate), nylon, poly(diethyl siloxane), and polyphosphazene. There is no reason to assume that this selection is complete. Station ni) has shown, for example, already in 1959 on a list of 29 macromolecules that longitudinal and lateral disorder may exist. Similarly, textbooks18> u2)... 

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). 

Catalyst complexation with a Lewis base or other electron donor may affect the polymer microstructure in different ways. If the added component occupies one coordination site, a monomer coordinates to another site of the active species with one double bond, i.e. as an s-trans-rf ligand, which gives rise to the formation of trans-1,4 monomeric units via the pathway (a)-(b) [scheme (10)]. Depending on the lifetimes of metal species complexed with the monomer and with the Lewis base or the other donor [scheme (11)], mixed cis-1,4/trans- 1,4-polybutadienes or an eb-czs-1, 1 A trans-1,4-polymer can be formed. One should mention in this connection that equibinary cis-l,A/trans- 1,4-butadiene polymers can also be formed in systems without the addition of a Lewis base or other electron donor in such cases, the equilibrium of the anti-syn isomerisation is not shifted and there are equal probabilities for the reaction pathways involving coordination of a transoid monomer and a cisoid monomer [7]. 

Polymerization was carried out in benzene in the presence of bis-(7r-allylnickel halides). The latter were prepared from nickel carbonyl and allyl halide (allyl bromide, crotyl chloride, bromide, or iodide etc.). The results of the polymerization runs are reported in Table I. The data indicate that all of the bis(7r-allylnickel halides) initiate by themselves the stereospecific butadiene polymerization yielding a polymer with 97-98% 1,4-units. The cis-l,4/trans-l,4 ratio depends on the halide in the dimeric r-allylnickel halide but not on the nature of allylic ligand. The case of bis(7r-crotylnickel halides) shows the effect of halide on microstructure, for whereas (C4H7NiCl)2 initiates cis- 1,4-polybutadiene formation, trans-1,4 polymers are produced by (C4H7NiI)2. The reactivity increase in the series Cl < Br < I. 

Fig. 1.3. Entropy of trans-1,4-polybutadiene that shows a coixlis phase between Tj and T( and of ds-l,4-polybutadiene that does not show a condis jdiase, but mdts in one stqj at T,...

That the reactivity of lanthanide active centres depends on the nature of AIR3 follows also from the microstructure of polydienes. Varying AIR3 component results in changes in the relative number of c/s-1,4 and trans-l,4-units in polybutadiene. At the same time, the content of 1,2-units remains the same (abont 0.6% at 25 °C). The only exception is a catalyst with diisobutylalnminnm hydride, in which case, approximately a three-fold increase in the content of 1,2-imits is observed. As the concentration of butadiene decreases (<0.5 /o mol/1) and the polymerisation temperatnre increases (from 25 to 80 the dependence of the number of cis-l,4-units in the polymer on the structure of AIR3 becomes more pronounced [10]. 

At Al/Ti > 2.0 the catalytic behavior was quite different. The amorphous precipitate was catalytically active without mother liquor owing to the presence of trapped alkylaluminum compounds. Although the polymers prepared with the washed precipitates were about 60% iraws-1,4-polybutadiene, they nevertheless exhibited, at room temperature, crystallinity characteristic of sterically pure trans-1,4 poylmer. After ether extraction a very pure (> 90%) tmns-l,4-polybutadiene was left. The ether extract afforded very pure (94-95%) cis-l,4-polybutadiene. 

The latest more extensive work on the chlorination of PVC was part of an overall study of the chlorination of polyalkenamers. The chlorinated butadiene polymer (poly-butenamer) was found to be crystalline or at least had crystalline chain segments a unit cell for chlorination products of trans-1,4-polybutadiene was determined and for the crystalline material a structure of a diisotactic poly(erythro-l,2-dichlorobutadiene) was proposed. This proposal required the assumption that the addition of chlorine is stereospecific. It was also mentioned that the number of ordered units of about ten is sufficient to display crystallinity, sufficient to allow the determination of the structure by x-ray analysis. 

---