1,3-Pentadiene, trans isomer polymerization

By polymerizing the trans isomer of 1,3-pentadiene two different types of crystalline cis-1,4 polymers have been obtained, one with an isotactic, the other with a syndiotactic structure. The isotactic polymer was obtained by homogeneous systems from an aluminum alkyl chloride and a cobalt compound, the syndiotactic one by homogeneous systems from an aluminum trialkyl and a titanium alkoxide. Some features of the polymerization by Ti and Co catalysts are examined. IR and x-ray spectra, and some physical properties of the crystalline cis-1,4 polymers are presented. The mode of coordination of the monomer to the catalyst, and possible mechanisms for the stereospecific polymerization of pentadiene to cis-1,4 stereoisomers are discussed. 

The cis-1,4 syndiotactic polypentadiene has been obtained by soluble catalysts prepared from an aluminum alkyl chloride and a cobalt compound. Only the trans isomer of pentadiene has been polymerized by these catalysts. However, mixtures of the cis and trans isomers can be used, the cis isomer remaining seemingly unaltered. The presence of the cis isomer was found to have no appreciable influence on the stereospecificity when its percentage in the mixture of the two isomers is lower than about 30%. For higher percentages a decrease in the stereoregularity of the syndiotactic polymer obtained was observed. 

Shortly after the cis-1,4 syndiotactic polypentadiene was obtained and characterized, it was observed that the homogeneous systems prepared from an aluminum trialkyl and a titanium tetralkoxide can polymerize pentadiene to polymers predominantly cis-1,4. Both the cis and trans isomers of pentadiene are polymerized by these systems. However, while the crude polymers obtained from the cis isomer were found to be amorphous by x-ray examination, those obtained from the trans isomer were found to be crystalline. The type of crystallinity of these polymers appeared different from that of the polymers... 

Let us examine separately the case of the cis and trans isomers of pentadiene. For the trans isomer the cis conformation is permissible so that one cannot assume a priori that this isomer won t coordinate to Ti by the two double bonds. This hypothesis, however, can be easily rejected by the following considerations. If the steric situation around Ti during the polymerization were to permit the coordination of the trans isomer of pentadiene by the two double bonds, in the cis conformation, butadiene or isoprene should also coordinate the same way. In this case, however, cis-1,4 units should be obtained both from butadiene and isoprene, and not 1,2 and 3,4, respectively, as observed. It seems reasonable to conclude, therefore, that the trans isomer of pentadiene coordinates to Ti by the vinyl group only, as butadiene or isoprene, before it is incorporated as a cis-1,4 unit. 

The Co catalysts polymerize only the trans isomer of pentadiene. 

This monomer is usually obtained as a mixture of the cis and trans isomers both of which have been polymerized with coordination type catalysts. Polymerization of the cis form is considered to be preceded by isomerization, since those catalysts which do not isomerize the cis monomer (e.g. cobalt salt—organo aluminium halide) selectively polymerize the trans isomer. A kinetic study of the polymerization of cis 1,3-pentadiene using Ti(OBu-n)4/AlEt3 (Al/Ti = 1.3—6) as catalyst has been published [267]. This gives a polymer containing ca. 73% cis 1,4 15—16% trans 1,4 and 11—12% 3,4 microstructure. 

With these eatalysts a mixture of cis- and tm y-l,3-pentadienes in a wide range could be polymerized. But the polymer obtained from the trans isomer is more clean and crystalline. In the titanium catalyst the Al/Ti ratio plays an important role for the molecular weight. With an increasing Al/Ti ratio, the molecular weight of the polymer deereases [370,371]. An optimal values is Al/Ti = 7. 

It is known that CoCl2/Al(C2H5)Cl can polymerize trans-l,3-pentadiene but not the cis isomer [263]. This suggests that a two-point coordination is required. Several reaction schemes that provide for an attack at either Ci or at C4 positions were proposed over the years [263]. One mechanism for polymerization of butadiene suggests that complexes of the catalysts in solvents of low dielectric constant will either act as ion pairs or as independent solvated entities. Also, the growing chain may be bound by either a 71 or a a linkage, and it is suspected that a crnitinuous u n isomerism is possible [264] ... 

CpTiCbactivated by MAO polymerizes (Z)-l,3-pentadiene, affording a polymer with high cis content (>99%) at room temperature. In this case, the polymer does not show crystallinity, suggesting an atactic arrangement of the methyl groups." Notably, in this case the (A) isomer is more reactive, but its polymerization results in a mixture of 1,4-cis (50%), 1,4-trans (40%), and 1,2 (10%) units. Similar results have also been obtained using bis(phenoxyimino) titanium catalysts 2 activated by MAO. ... 

Cationic polymerization provides, independent of the isomer of the 1,3-pentadiene, high trans-, A and trans-1,2 microstructures (Table 17). Studies on the insertion mechanism and the various side reactions have been carried out [373,374],... 

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