Technological Aspects of the Polymerization in Solution

In Sect. 2.2 important technological aspects of the neodymium-catalyzed solution processes are addressed. In this context a book chapter on large-scale BR production in solution by Taube and Sylvester is recommended [81]. 

The first report on higher polymerization activities in hexane than in benzene dates back to Throckmorton s pioneering work on ternary Ce-based catalyst systems [34], To the present day there are several reports available in the literature in which the influence of solvents on polymerization activities, microstructure of the resulting polymers and molar masses are compared. The relevant results of these investigations are summarized in this section. 

For ternary catalyst systems polymerization activities are also higher in aliphatic and cycloaliphatic hydrocarbons than in aromatic solvents. These features were observed for various catalysts systems. Detailed studies are available for NdzO/TIBA/DEAC [161,165,166], NdV/DIBAH/EASC [205,422], Nd(N(SiMe3)2)3/TIBA/DEAC [318,320], NdV/DIBAH/fBuCl [423] and NdV/ MAO/fBuCl [175]. 

In this equilibrium the Nd-species to which a diene is coordinated is active in polymerization, whereas the Nd-species to which an arene is coordinated is inactive. According to the authors the experimentally determined ranking of activities toluene mesitylene toluene (+ 7% hexamethylbenzene) correlates with the electron richness (i.e. Lewis basicity) of the aromatic compounds. The polymerization activity decreases with increasing Lewis basicity of the aromatic compound as the equilibrium is shifted and the concentration of the active species is reduced. These considerations were supported by the following experimental results (Table 18). 

In the already quoted publication by Cabassi et al. the use of alkenes as potential solvents was also investigated. As alkenes are not incorporated into the polymer chain the authors suggested the use of alkenes, such as cis-butene, 1-butene and isobutene as appropriate solvents for the polymerization of BD. Also, the technical potential of the C4 cracking fraction as a suitable solvent for the industrial production of BR was recognized. This suggestion was later followed up by Laubry who succeeded to selectively polymerize IP in the C5 cracking fraction [264,265]. 

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