Catalyst dependence trans-polybutadiene

Polymerization Temperature. The stereoregularity of polybutadienes prepared with the BuLi-barium t-butoxide-hydroxide catalyst in toluene is exceedingly temperature dependent. Figure 6 compares the trans-1,4 dependence for polybutadiene prepared with BuLi, alone, and with the BuLi-barium t-butoxide-hydroxide complex in toluene (the molar ratio of the initial butadiene to BuLi concentration was 500). The upper curve demonstrates that the percent trans content increased rapidly from 627. to 807. trans-1,4 as the temperature decreased from 75°C to 22°C. From 22°C to 5°C, the microstructure does not change. The increase in trans-1,4 content occurred with a decrease in cis-1,4 content, the amount of vinyl unsaturation remaining at 5-87.. For the polybutadienes prepared using BuLi alone, there is only a very slight increase in the trans-1,4 content as the polymerization temperature is decreased. 

The stereoregularity of butadiene based polymers prepared in cyclohexane with Ba-Mg-Al catalysts depends on polymerization temperature and catalyst concentration. Trans-1,4 content increases nonlinearly with a decrease in polymerization temperature over the range of 80° to 30°C (Figure 11) and/or a decrease in the initial molar ratio of butadiene to dialkyl-magnesium from 3400 to 400 (Figure 12). For polybutadienes prepared with relatively large amounts of catalyst at 30°C, the trans-1,4 content approaches a limiting value of about 907.. 

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 dependence of the polymer microstructure on the ratio of catalyst components is related to the nature of these components. The structure of polybuta-diene obtained with an aluminum triisobutyl (AIBU3)-titanium tetrachloride catalyst system is a function of the Al/Ti molar ratio (Table II). Polybutadiene prepared at Al/Ti ratios of 0.5 to 8 in benzene or heptane and at 3° or 25° C. contain at least 90% 1,4- units. Polymerizations carried out at ratios of 1.0 and less at 25° C. in heptane and at ratios of 1.25 or less at 3° C. in heptane or benzene give crystalline polymers containing more than 96% trans-, A- structure (6). A similar temperature dependence of polymer structure has been reported in the polymerization of butadiene with a diethylcadmium-titanium tetrachloride catalyst system (3). Polybutadiene obtained with a triethylaluminum-titanium tetrachloride catalyst system at a 0.9 Al/Ti ratio at 30° C. in benzene is reported to contain 67% cis-1,4- units (19). 

Butadiene can form three repeat units as described in structure 5.47 1,2 cw-1,4 and trans-, A. Commercial polybutadiene is mainly composed of, A-cis isomer and known as butadiene rubber (BR). In general, butadiene is polymerized using stereoregulating catalysts. The composition of the resulting polybutadiene is quite dependent on the nature of the catalyst such that almost total trans-, A, cis-, A, or 1,2 units can be formed as well as almost any combination of these units. The most important single application of polybutadiene polymers is its use in automotive tires where over 10 t are used yearly in the U.S. manufacture of automobile tires. BR is usually blended with NR or SBR to improve tire tread performance, particularly wear resistance. 

Catalysts based on 7r-allylic derivatives of transition metals supported on alumina, silica or silica-alumina gels exhibit generally enhanced activity by comparison with their unsupported counterparts, while the stereospecificity depends on the nature of the catalyst carrier. For instance, Cr(All)3, which predominantly produces 1,2-polybutadiene [137], becomes a stereospecific catalyst for the formation of trans- 1,4-polybutadiene when supported on silica or silica-alumina gel and for the formation of cis- 1,4-polybutadiene when supported on alumina [148]. However, an increase in the content of cis-1,4 monomeric units in polybutadiene with increasing silica concentration in n-allylnickel-alumina-silica catalysts has been observed [149]. 

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]. 

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]. 

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