Coordination polymerization trans-1,4-polyisoprene

We have seen that the double bonds in a chain of polyisoprene can exist as cis and trans stereoisomers. Synthetic polyisoprene then has the added complexity of 1,2- versus 1,4-polymerization in addition to the possible existence of different stereoisomers about the double bond. As with butadiene, different coordination catalysts produce isoprene polymers with a preponderance of 1,2- or 1,4- polymer as well as different stereochemistry. Not unexpectedly, these different polymers possess strikingly different physical properties. 

Polybutadiene, CAS 9003-17-2, is a common synthetic polymer with the formula (-CH2CH=CHCH2-)n- The cis form (CAS 40022-03-5) of the polymer can be obtained by coordination or anionic polymerization. It is used mainly in tires blended with natural rubber and synthetic copolymers. The trans form is less common. 1,4-Polyisoprene in cis form, CAS 9003-31-0, is commonly found in large quantities as natural rubber, but also can be obtained synthetically, for example, using the coordination or anionic polymerization of 2-methyl-1,3-butadiene. Stereoregular synthetic cis-polyisoprene has properties practically identical to natural rubber, but this material is not highly competitive in price with natural rubber, and its industrial production is lower than that of other unsaturated polyhydrocarbons. Synthetic frans-polyisoprene, CAS 104389-31-3, also is known. Pyrolysis and the thermal decomposition of these polymers has been studied frequently [1-18]. Some reports on thermal decomposition products of polybutadiene and polyisoprene reported in literature are summarized in Table 7.1.1 [19]. 

The lanthanide coordination catalysts are known to be highly stereospecific for producing high-cis polybutadiene and high-cis polyisoprene as well as high-cis copoljnnerization of the two monomers. In addition, the polymerizations of other conjugated dienes such as trans-piperlene, 2,4-hexadiene,... 

As mentioned previously (section 1.4.2(b)), polymerization of isoprene may be accomplished by a variety of methods. The choice of method has a profound effect on the microstructure of the resulting polymer (Table 1.2) and, consequently, on the technological properties of the product. The only poly-isoprenes which have acceptable properties and which are of commercial importance are those with a high cis-1,4-content (although a small amount of high trans-1,4-polyisoprene is produced as a replacement for gutta percha). Three catalyst systems have been used for the production of cis-1,4-polyisoprene, namely coordination catalysts (e.g. titanium tetrachloride-tri-isobutylaluminium), lithium and alkyllithium compounds (e.g. butyllithium). The microstructures of the polymers produced with these catalysts are very similar (Table 1.2). 

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