Polybutadiene rubber stereospecific

Catalysts. Iodine and its compounds are very active catalysts for many reactions (133). The principal use is in the production of synthetic rubber via Ziegler-Natta catalysts systems. Also, iodine and certain iodides, eg, titanium tetraiodide [7720-834], are employed for producing stereospecific polymers, such as polybutadiene rubber (134) about 75% of the iodine consumed in catalysts is assumed to be used for polybutadiene and polyisoprene polymerization (66) (see Rubber CHEMICALS). Hydrogen iodide is used as a catalyst in the manufacture of acetic acid from methanol (66). A 99% yield as acetic acid has been reported. In the heat stabilization of nylon suitable for tire cordage, iodine is used in a system involving copper acetate or borate, and potassium iodide (66) (see Tire cords). 

Cisdene . [Am. Syn. Rubber] Stereospecific polybutadiene used in mixtures with SBR or NR to enhance properties. 

To a 2-liter, round-bottom, glass flask equipped with a reflux condenser, drying tube, dropping tunnel, thermometer, and stirrer, is added 300 ml of tert-butyl alcohol. While stirring, lOgm (0.25 mole) of potassium metal (as small pieces) is slowly added at a rate to keep the reaction under control. The excess alcohol is removed under reduced pressure and to the residual cake of potassium fert-butoxide is added 300 ml of dry n-pentane. Then a solution of 60 gm of stereospecific polybutadiene rubber in 800 ml cyclohexane is added. The resulting mixture is cooled to 0°C while 63 gm (0.25 mole) of bromoform is added dropwise and when complete the temperature is allowed to rise to 25°C. Then 2,6-di-/err-butyl-4-methyl phenol (5 ppm of rubber product) is added and then the mixture poured into methanol to precipitate the product. The solid is filtered, washed with alcohol, water, aleohol, and then dried at 50°C under reduced pressure. The produet on analyses showed 2 wt% of chemically bound bromine. 

The most spectacular case of products arising from a catalyst invention is that of the stereospecific hydrocarbon polymers made possible by the Ziegler-Natta work on aluminum alkyl/transition metal halide combinations around 1950. Until these catalysts existed, polypropylene, polyiso-prene, and cis-polybutadiene could not be made, and linear polyethylene could not be made cheaply. For each of these products, very large investments were needed in big plants and in market development before they were competitive with the established, big thermoplastics and rubbers. Entrance fees ran into tens of millions of dollars. 

Normal rhombic sulphur has differing degrees of solubility in the different rubber types. In NR and SBR the required proportion for crosslinking dissolves relatively rapidly at room temperature. In stereospecific rubbers such as polybutadiene and nitrile it does not solubilise so readily. As one would expect, the solubility of the sulphur within the rubber increases with temperature increase. 

Crystallization of oriented chains is, in various respects, important for the polymer properties. The fact has been mentioned before, that stereospecific rubbers such as cis-1,4 polybutadiene can crystallize when under strain. The spontaneously formed crystals contribute strongly to the strength of the vulcanizate. A vulcanized natural rubber has, without carbon black reinforcement, a tensile strength of about 40 MPa, whereas an unreinforced SBR breaks at about 3 MPa. (With SBR a high tensile strength can only be reached with carbon black.)... 

However, no method of polymerisation known before 1954 allowed one to obtain polymers with a high regularity of structure from the most common conjugated dienes. A true breakthrough in the development of conjugated diene rubbers took place after the discovery of stereospecific polymerisation with transition metal-based coordination catalysts. From the late 1950s, a rapid development of industrial production of solution types of polybutadiene by means of polymerisation with Ziegler-Natta catalysts was observed. 

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. 

While earlier attempts to produce satisfactory synthetic rubber from iso-prene were unsuccessful, in 1955 American chemist Samuel Emmett Horne Jr. (b. 1924) prepared 98 percent czr-l,4-polyisoprene via the stereospecific polymerization of isoprene. Home s product differs from natural mbber only in that it contains a small amount of rfr-l,2-polyisoprene, but it is indistinguishable from natural mbber in physical properties. First produced in 1961, BR (for butadiene mbber), a mbberlike polymer that is almost ex-clnsively czr-1,4-polybutadiene, when blended with natural or SBR mbber, has been nsed for tire treads. 

Budene solution polybutadiene (solution polymerized) is cis-1,4-poly(butadiene) produced with stereospecific catalysts which yield a controlled MWD, which is essentially a linear polymer. Butadiene rubber, polybutadiene, is solution-polymerized to stereospecific polymer configurations " by the additional polymerization of butadiene monomer. The following cis- and trans-1,4-polybutadiene isomers can be produced cis-1,4-polybutadiene with good dynamic properties, low... 

Interest in straight polybutadienes was revived with the discovery of the stereospecific catalysts. Commercial processes using both co-ordination and alkyllithium catalysts were established by 1960 and polybutadiene is now a significant general purpose rubber. (See Table 18.2.)... 

Whilst for various reasons these rubbers, often referred to as stereo rubbers because of the stereospecific action of the catalysts during polymerization, and which include not only polyisoprenes and polybutadienes but also ethylene-propylene rubbers, have not yet seriously challenged the predominance of SBR and natural rubber, they have become important and successful materials. 

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