Polymerization temperature trans-polybutadiene

Chemical methods for structure determination in diene pol3 mers have in large measure been superseded by infrared absorption techniques.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,... 

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 graft polybutadiene containing both styrene and acrylonitrile prepared in a separate experiment was isolated by a double precipitation. The experimental value for the degree of grafting was calculated on the basis of the trans isomer of polybutadiene (Figure 17). Except for two fractions, the styrene contents are within the shaded band. We note that both the styrene and the acrylonitrile contents increase slightly. This phenomenon might be caused by the polymerization temperature which was lower than in the previous two experiments (100°C. instead of 110°C.). 

Another type of emulsion (co)polymer that has been analysed in toms of compositional sequence distributicHi is, for example, polybutadioie. By means of carbon-13 NMR, it is possible to determine the vinyl-1,2, cfr-1,4 and the trans-1,4 units from the olefinic resonances [142], Alternatively, the alifAiatic carbon-13 resonances of polybutadiene have been analysed in the same microsctructural sequences by means of low MM model compounds [143]. The fractions of the microstructural units of a polybutadiene pr rared in emulsirm are typically 18% vinyl-1,2,20% cfr-1,4 and 62% mons-1,4 and depend only on the polymerization temperature [144]. 

Figure 15.2 Effect of polymerization temperature upon the microstructure of emulsion-polymerized polybutadiene (Hampton [13]) ( ) 1,2 units (A) cw-1,4 units ( ) trans-lA Units...

Nickel compounds can also be employed as catalysts [161-170]. A three-component system consisting of nickel naphthenate, triethyl-aluminum, and boron trifluoride diethyletherate is used technically. The activities are similar to those of cobalt systems. The molar Al/B ratio is on the order of 0.7 to 1.4. Polymerization temperatures range from -5 to 40 °C. On a laboratory scale the synthesis of 1,4-polybutadiene with allylchloronickel giving 89% cis, 7.7% trans, and 3.4% 1,2-structures is particularly simple [8]. In nickel compounds with Lewis acids as cocatalysts, complexes with 2,6,10-dodecatriene ligands are more active than those with 1,5-cyclooctadiene (Table 4) [171]. 

Fig. 31.—A plot of the log of the trans/cis ratio for polybutadiene against the reciprocal of the absolute temperature of polymerization by a free radical mechanism. (Results of Richardson and Sacher obtained by infrared analysis.)...

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). 

Comparison of results from the polymerizations of butadiene and isoprene with an AlR3-TiCl4 catalyst system reveals some interesting features. The 1 to 1 Al/Ti ratio yields a cis-l,4-polyisoprene and a mns-1,4-polybutadiene. Kinetic studies have, in fact, indicated that in both cases, the rate of polymerization at this ratio is proportional to the first power of the monomer pressure 6, 21). At lower Al/Ti ratios, higher trans-l,A- content is observed in both polyisoprene and polybutadiene. At comparable Al/Ti ratios, lower temperatures increase the trans-l,A-structure in both polymers. Although essentially all-cis-1,4-poly isoprene and all-... 

Alcock and coworkers studied the polymerization of butadiene (as well as of monoolefins, acetylene and aromatic olefins) trapped within the tunnel clathrate system of tris((9-phenylenedioxy)cyclotriphosphazene, induced by Co-y-radiation. The host was used in order to find if the concatenation and orientation of the monomer molecules under the steric forces generated within the host crystal lattice will lead to stereospecific polymerization. The clathrate was prepared by addition of liquid butadiene to the pure host at low temperature. The irradiation was conducted at low temperatures. Irradiation of pure butadiene (unclathrated bulk monomer) leads to formation of a mixture of three addition products f,2-adduct, cis- and trons-f,4-adducts. In contrast, the radiation-induced polymerization within the tunnel system of the host yielded almost pure trans-1,4-polybutadiene. A small percentage of f, 2-addition product was observed, but no evidence for the formation of c/s-f,4-adduct was found, confirming the earlier observation by Fin ter and Wegner. The average molecular weight was about 5000,... 

In hydrocarbons, a polybutadiene is obtained containing about 35 % 1,4-cis, 54 % 1,4-trans and 11 % 1,2 units, while isoprene is polymerized under the same conditions with a cis selectivity of more than 90 %. By addition of polar ligands, such as tetrahydrofuran, dimethylglycol ether, tetramethylethylendiamine, or dipiperidylethane in the butadiene polymerization, the 1,2-selectivity can be enhanced by up to 100%. The effect increases with the coordination power and probably also with the space-filling ability of the ligand, and decreases with a rise in temperature. 

Medium-c/5 lithium-polybutadiene was first developed by Firestone Tire and Rubber Company in 1955 [86]. Solution polymerization using anionic catalysts is usually based on butyllithium. Alkyllithium initiation does not have the high stereospecificity of the coordination catalysts based on titanium, cobalt, nickel, or neodymium compounds. Polymerization in aliphatic hydrocarbon solvents such as hexane or cyclohexane yields a polymer of about 40 % cis, 50 % trans structure with 10 % 1,2-addition. However, there is no need for higher cis content because a completely amorphous structure is desired for mbber applications the glass transition temperature is determined by the vinyl content. The vinyl content of the polybutadiene can be increased up to 90 % by addition of small amounts of polar substances such as ethers. 

Free-radical polymerizations of 1,3-butadiene usually result in polymers with 78-82% of 1,4-type placement and 18-22% of 1,2-adducts. The ratio of 1,4 to 1,2 adducts is independent of the temperature of polymerization. Moreover, this ratio is obtained in polymerizations that are carried out in bulk and in emulsion. The ratio of trans-1,4 to cis-1,4 tends to decrease, however, as the temperature of the reaction decreases. Polybutadiene polymers formed by free-radical mechanism are branched because the residual unsaturations in the polymeric chains are subjects to free-radical attacks ... 

There is evidence, however, that lithium still exerts a cis orienting effect on the course of the polymerization. If it did not, one might expect results similar to those involving a free propagating species, like a free-radical polymerization. Hart and Meyer (1949) showed that under these conditions predominately trans-1,4-polybutadiene was obtained. Furthermore, the isomeric composition would be expected to be temperature-dependent as in the free-radical reaction. To the contrary, Kuntz and Gerber (1960) observed no change in isomeric composition over the reaction temperature range 4 to 80°C. 

The microstructures are influenced primarily by the nature of the alkylaluminum compound. With triethylaluminum the portion of trans-, 4 double bonds reaches a relatively high level of 10%, while tris(2-methylpropyl)aluminum and bis(2-methylpropyl) aluminum hydride yield cis-, A contents as high as 99% [190]. Similarly, high cis-1,4 portions are obtained in the polymerization of 1,3-butadiene with j -allyluranium complexes. The osmometric measured mole mass ranges from 50 to 150 000, the molecular mass distribution between 3 and 7. The extremely high temperature-induced crystallization rate of uranium polybutadiene in comparison with titanium or cobalt polybutadiene corresponds to a greater tendency toward expansion-induced erystallization. A technical application, however, is in conflict with the costly removal of weakly radioactive catalyst residues from the products [132],... 

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