Ziegler-Natta catalysts polyisoprene polymerization

The revolutionary development of stereospecific polymerization by the Ziegler-Natta catalysts also resulted ia the accomplishment ia the 1950s of a 100-year-old goal, the synthesis of i7j -l,4-polyisoprene (natural mbber). This actually led to the immediate termination of the U.S. Government Synthetic Rubber Program ia 1956 because the technical problem of dupHcating the molecular stmcture of natural mbber was thereby solved, and also because the mbber plantations of the Far East were again available. 

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). 

For instance, in the field of elastomers, alkyllithium catalyst systems are used commercially for producing butadiene homopolymers and copolymers and, to a somewhat lesser extent, polyisoprene. Another class of important, industrial polymerization systems consists of those catalyzed by alkylaluminum compounds and various compounds of transition metals used as cocatalysts. The symposium papers reported several variations of these polymerization systems in which cocatalysts are titanium halides for isoprene or propylene and cobalt salts for butadiene. The stereospecificity and mechanism of polymerization with these monomers were compared using the above cocatalysts as well as vanadium trichloride. Also included is the application of Ziegler-Natta catalysts to the rather novel polymerization of 1,3-pentadiene to polymeric cis-1,4 stereoisomers which have potential interest as elastomers. 

Only single insertions into an Al-C bond occur for propene and higher alkenes and this is utilized for catalytic dimerization of propene as illustrated in Scheme 3. Insertion of propene into an Al-C bond of "PrsAl followed by )3-hydride elimination yields an aluminum hydride and 2-methylpent-l-ene. Insertion of propene into the Al-H bond regenerates "PrsAl. Thermal cracking of 2-methylpent-l-ene gives isoprene, which is subsequently polymerized with a Ziegler-Natta catalyst to form the synthetic rubber, cA-1,4-polyisoprene. 

As in the polymerization of butadiene, the presence of polar solvents aflFects the microstructure of polyisoprene obtained with a Ziegler-Natta catalyst in a hydrocarbon reaction medium (1, 21). 

Polyisoprene is produced by solution polymerization using Ziegler-Natta catalysts. The cis-1,4-polyisoprene is a synthetic equivalent of natural rubber. However, the synthetic polyisoprenes have cis contents of only about 92-96% consequently, these rubbers differ from natural rubber in several ways. The raw synthetic polyisoprene is softer than raw natural rubber (due to a reduced tendency for a stress-induced crystallization be cause of the lower cis content) and is therefore more difficult to mill. On the other hand, the unvulcanized synthetic material flows more readily this feature makes it easier to injection mold. The synthetic product is somewhat more expensive than natural rubber. 

The polymerization of 2-methyl-l, 3-butadiene (isoprene [CH2=C(CH3)CH= CH2]) by radical processes produces a rubber-Uke material. Indeed, a synthetic rubber very close to natural rubber (all cis -l,4-polyisoprene. Figure 6.9) can be prepared from 2-methyl-l 3-butadiene (isoprene [CH2=C(CH3)CH=CH2]) with a Ziegler-Natta catalyst vide supra). 

Natural rubber (hevea) is 98% c/ -l,4-polyisoprene with 2% 3,4-structure. It can be synthesized by anionic polymerization with alkyllithium compounds or with Ziegler-Natta catalysts [220-225]. The polymerization is carried out in solvents. Impurities such as acetylenes, carbonyl compounds, hydrogen sulfide, and water have to be removed [217,226-228]. 

Polymerization of 2-methyl-l,3-butadiene (isoprene. Section 4-7) by a Ziegler-Natta catalyst (Section 12-15) results in a synthetic rubber (polyisoprene) of almost 100% Z configuration. Similarly, 2-chloro-1,3-butadiene furnishes an elastic, heat- and oxygen-resistant polymer called neoprene, with trans chain double bonds. Several million tons of synthetic rubber are... 

For reasons of space diolefin polymerization has not been included in this Chapter. Some information and pertinent references are summarized here. 1,3-E>ienes can be polymerized by lithium alkyls or by Ziegler-Natta type catalysts, containing titanium or cobalt, nickel, and neodymium. Industrially important products are 1,4-cis-polybutadiene (>2 Mt/a) and 1,4-cis-polyisoprene (>1 Mt/a). They are... 

Figure 1. Scheme for polymerization of isoprene to cis-1,4-polyisoprene with Ziegler-Natta type catalyst... 

In summary, cationic polymerization gives polymers of structure P8a Ziegler-Natta complexes mainly lead to blocks of structure P8b. This study gives further evidence for cyclization reactions of high-molecular-weight polyisoprenes. This point of view has been confirmed by the study of the cyclization of model polyisoprene molecules with two, three, or four monomer units toward the catalysts able to initiate such a reaction (24). 

IV to VIII metals and base metal alkyls of Group II or III metals (Penczek and Premia, 2012 Boor, 1979 Ciardelli, 1992). It arose from the spectacular discovery of Ziegler et al. (1955) that mixtures of titanium tetrachloride and aluminum alkyls polymerize ethylene at low pressures and temperatures and from the equally spectacular discovery by Natta (1955) that the Ziegler catalysts can stereospecifically polymerize monoolefins to produce tactic, crystalline polymers. As can be imagined, these systems can involve many combinations of catalyst components, not all of which are catalytically active or stereospecific. However, we shall be concerned here only with polymerizations involving the commercial elastomers, principally polyisoprene, polybutadiene (Duck and Locke, 1977 Zohuri et al., 2012 Teyssie et al., 1988), and the ethylene-propylene copolymers (Schobel et al., 2012 Ver Strate, 1986 Davis et al., 1996 Noordermeer, 2003 Baldwin and Strate, 1972). 

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