Microstructure of polyisoprenes

Fig. 11. Effect of different solvents on the microstructure of polyisoprene. (S. L. Aggarwal et al., Ref. 75 >)...

Fig. 18. Microstructure of polyisoprene obtained in cyclohexane at 30 °C with sec. BuLi as initiator. Conversion 10% (D. J. Worsfold, S. Bywater, Ref. 12S )...

The microstructure of polyisoprene prepared by lithium initiation in hydrocarbons is 95% 1,4 under all conditions. The trans 1,4 content however falls from about 20% to zero as the monomer/initiator ratio increases leading finally to a 95% cis 1,4 polymer. This variation can be explained with the following scheme. 

Figure 2. Microstructure of polyisoprene initiated with alkyllithium in cyclohexane...

The microstructure of polyisoprene prepared in a variety of solvents and solvent mixtures (113) has been determined. Various ethers and sulphides vary in their ability to reduce the 1,4 content of the polymer. The most effective ether was tetrahydrofuran. The presence of only two molecules per active chain was reported to reduce the 1,4 content to that observed in the pure ether. More recent investigations have failed to confirm that the requirement is as low as this 74,126) but relatively small amounts of tetrahydrofuran do markedly decrease the cis-1,4 content and increase the 3,4 content. Similar results have been obtained for butadiene 60) with respect to 1,4 and 1,2 structures. 

Fig. 3. Microstructure of Polyisoprene Prepared with organo alkali metals in bulk or in hydrocarbon solvent...

Table IV indicates the microstructure of polyisoprene obtained by the use of various catalyst systems.

As in the case of butadiene polymerization, the microstructure of polyisoprene is dependent upon the ratio of catalyst components, the reaction temperature, and the reaction medium. As shown in Table V, using an aluminum triethyl-titanium tetrachloride catalyst system, an Al/Ti ratio of 1.0 or higher yields a polyisoprene containing 96% cis-1,4-, 4% 3,4-, and essentially no trans-, A- or 1,2- units. Below this ratio, the trans-, A- structure is produced apparently at the expense of the cis-1,4- structure (I). An AlBua-TiCLt catalyst system is reported (7) to yield an essentially all- rarw-l,4-polyisoprene at an Al/Ti ratio of 0.67 and below. 

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). 

The <7-bonded lithium chain can be expected to predominate. In highly solvating solvents, such as ethers, the 7r-allyl structure is dominant leading to high 1,2 placements. Because the 2,3-bond is maintained, the above-shown equilibria should not be expected to lead to cis-trans isomerization. Such isomerizations do not take place for butadiene or for isoprene when they are polymerized in hydrocarbon solvents. They do occur, however, in polar solvents at high temperatures. This suggests that additional equilibria exist between the r-allylic structures and the covalent 1,2 chain ends. Table 3.3 shows the manner in which different polymerization initiators and solvents affect the microstructures of polyisoprenes. 

Table 3.3. Effect of the Counterions and Solvents on the Microstructures of Polyisoprenes ... 

An interesting effect of Lewis bases on diene microstructure is the fact that in the presence of strongly coordinating bases such as TMEDA, 1,2 units are observed for polyisoprene. For example, the microstructure of polyisoprene formed in the presence of TMEDA ([TMEDA]/[Li] = 1) in cyclohexane corresponds to 21% 1,4, 12% 1,2, and 67% 3,4 (137). The formation of 1,2 units requires the formation of the less stable 1,4 chain ends versus 4,1 chain ends as shown in equation 52 ... 

The microstructure of polybutadiene was studied via IR spectroscopy on a Shimadzu IR Prestige spectrometer. Analysis was performed with the use of polymer films applied on KBr glasses. The films were cast from toluene solutions. The microstructure of polyisoprene was studied via H NMR spectroscopy on a Bruker AM-300 spectrometer. Deutero chloroform was used as a solvent. 

The microstructure of polyisoprene was determined via high-resolution IH NMR spectroscopy on a Bruker AM-300 spectrometer (300 MHz). 

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