Active Species and its Formation

In scientific literature many speculations on the nature of the active Nd species can be found. Unfortunately, most of these speculations are not supported by experimental or analytical evidence. In the following, the discussions on this issue are summarized for catalyst systems which are based on different Nd-precursors. At the end of this subsection special attention is given to the state of discussions about the fraction of active Nd centers in the catalyst mixture. 

Though NdX3-based catalyst systems are among the first Nd Ziegler/Natta systems, studies directed at the elucidation of the active species became more numerous since 1986. Essentially there are the research groups of Hsieh, Iovu and Monakov who made major contributions to this topic. 

Hsieh and co-workers focused on the catalyst system NdCl3/EtOH/TEA. They formulated several Nd allyl species which are supposedly present during the polymerization of dienes [139]. 

These intermediates contain Nd-Nd and Nd-Al species which are clustered by Cl/alkyl-bridges as well as by Cl/Cl-bridges. In various papers Monakov et al. emphasize the oligomeric and polymeric nature of these species [609- 

Also mathematical models for the polymerization of dienes with catalyst systems of the type NdX3 TBP/TIBA were developed [615,616]. 

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Another factor is the solvation of (PtCU) by ligands other than C), in fact. (PtCIjtj) is the more active species and its formation is facilitated by hard ligands. 

The following scheme describes the formation of the trinudear green pigment and polyindophenols from (I) and resorcinol and is consistent with the general mechanism described earlier. This scheme suggests that the dii-minium ion (IX) is the actual active species, and its formation has already been described. In step 2, this electrophilic species attacks a resorcinol anion, para to the phenolic group, forming (XXVII), which is oxidized to indophenol in step 3. 

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