Lithium polybutadiene

The distinction between the rates of homo- and copolymerization apparently is misapprehended by some workers. For example, a recent review 141) discusses the results of McGrath 142) who reported butadiene to be more reactive in polymerization in hexane than isoprene, whether with respect to lithium polybutadiene or polyisoprene, although the homopropagation of lithium polyisoprene in hexane was found to be faster than of polybutadiene. The miscomprehension led to the erroneous statement14l) McGrath 142) results regarding the rate constants for butadiene and isoprene place in clear perspective the bizarre assertion 140) that butadiene will be twice as reactive as isoprene (in anionic co-polymerization). 

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). 

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. 

Effect of Initiator Concentration and Solvents on Chain Structure of Lithium Polybutadiene... 

The addition of styrene to lithium polybutadiene was shown to be by three orders of magnitude slower than the addition of butadiene to lithium polystyrene. Morton and Ells found this reaction to obey the internal first order kinetics in lithium polybutadiene, but the observed first order rate constant decreases linearily with the square root of the initial concentration of lithium polybutadiene. 

Sodium polybutadiene Emulsion polybutadiene Natural rubber Lithium polybutadiene... 

Sodium polybutadiene Emulsion polybutadiene Lithium polybutadiene Lithium polybutadiene Lithium butadiene-styrene, 89-11 LTP... 

The Young s bending modulus is a measure of the stiffness of a material—a higher value indicates a stiffer material. The sodium polybutadiene is, of course, considerably inferior to both of these polymers in this low temperature test. Table IV similarly illustrates the superiority, in compounded stocks, of lithium polybutadiene in low temperature shear recovery tests, also a measure of cold properties of a rubber. In this test the relative superiority of the lithium polymers to the emulsion and sodium polymer is even greater than that in the former test. 

Physical test properties on some cured rubber stocks prepared from lithium-catalyzed butadiene polymers are listed in Tables V and VI with appropriate controls. The results are only roughly indicative of the potential properties of rubbers made from lithium-catalyzed butadiene polymers because of the limited quantity of polymer available. The tensile data in Table VI indicate that compounded stocks from the lithium polymers are about equal or slightly inferior to the emulsion and sodium polymer controls in regard to these properties however, a hot tensile (lOO C.) on a cured compound from lithium polybutadiene was 325 pounds per square inch compared to 200 to 250 for an emulsion polybutadiene control. The internal friction of cured stocks from the lithium-catalyzed butadiene polymers is similar in magnitude to the emulsion or sodium polymer controls at 50 C. but higher at 100 °C. All lithium polymers, even those with low Mooney viscosities, gave cured compounds with high values of dynamic modulus. 

Anionic polymerization of 1,3-butadiene allows for a wide range of heterotactic vinyl enchainment levels. The structures obtained in hydrocarbon solvents with alkali metal initiators are shown in Table 9. Lithium polybutadiene maintains the lowest vinyl, highest 1,4 content possible via anionic polsrmerization. The microstructure produced via a free-radical emulsion polymerization is provided as a reference. 

Composite proplnts, which are used almost entirely in rocket propulsion, normally contain a solid phase oxidizer combined with a polymeric fuel binder with a -CH2—CH2— structure. Practically speaking AP is the only oxidizer which has achieved high volume production, although ammonium nitrate (AN) has limited special uses such as in gas generators. Other oxidizers which have been studied more or less as curiosities include hydrazinium nitrate, nitronium perchlorate, lithium perchlorate, lithium nitrate, potassium perchlorate and others. Among binders, the most used are polyurethanes, polybutadiene/acrylonitrile/acrylic acid terpolymers and hydroxy-terminated polybutadienes... 

The same approach supposedly demonstrated the dimeric nature of lithium polyisoprene and polybutadiene. A tenfold decrease of viscosity was claimed 97), contrary to the findings of Worsfold and By water 115) who reported a 15 fold decrease of viscosity for lithium polyisoprene on protonation of their hydrocarbon solutions. 

The correct explanation of the peculiar behaviour of the butadiene-styrene system was provided by O Driscoll and Kuntz 144). As stated previously, under conditions of these experiments butadiene is indeed more reactive than styrene, whether towards lithium polystyrene or polybutadiene, contrary to a naive expectation. This was verified by Ells and Morton 1451 and by Worsfold 146,147) who determined the respective cross-propagation rate constants. It is germane to stress here that the coordination of the monomers with Li4, assumed to be the cause for this gradation of reactivities, takes place in the transition state of the addition and should be distinguished from the formation of an intermediate complex. The formation of a complex ... 

Hydrogenation was carried out with the assistance of an n-butyl lithium/cobalt octoate catalyst (6). It was necessary to determine the proper conditions for efficTent hydrogenation with minimal degradation (7). For the BIB polymer the Li/Co ratio used was 5/1 to obtain selective hydrogenation of the polybutadiene, while for the total hydrogenation of the BBB polymer, a ratio of 2.2/1 was satisfactory. NMR analysis showed better than 99% hydrogenation. 

Microstructure of Polybutadienes. Microstructure strongly influences the viscosity of the CTPB prepolymer. The viscosity of CTPB increases with increased vinyl content, but for CTPB prepolymers of the required molecular weight, an upper limit of 35% vinyl groups is satisfactory from the standpoint of propellant processing characteristics. It has also been found that the microstructure changes markedly with the synthesis process. Lithium-initiated polymerization yields prepolymers with slightly higher vinyl content than those produced by free radical initiation. 

It is apparent from these data that all of the polymers, including butadiene, exhibit an association as dimers, and that there is no reason to expect any higher states of association for polyisoprene or polybutadiene. This is confirmed not only by the viscosity data on the active vs. terminated "capped" polymers, but also by the fact that there was no significant increase in viscosity when the polystyryl lithium was "capped" by butadiene or isoprene, i.e., all three types of chain ends are associated in the same way, as dimers. 

These efforts coupled with the much earlier work on sodium and lithium initiated polymerizations led to an appreciation of the stereospecificity of the alkyllithium initiators for diene polymerization both industrially and academically. Polymerization of isoprene to a high cis polyisoprene with butyllithium is well known and the details have been well documented 2 Control over polybutadiene structure has also been demonstrated. This report attempts to survey the unique features of anionic polymerization with an emphasis on the chemistry and its commercial applications and is not intended as a comprehensive review. 

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