Isospecific catalysts

In general, these stereochemical events are completely independent. For instance, isospecific catalysts like the heterogeneous ones (based on TiCl3 or on TiCU supported on MgCI2) as well as C2 symmetric metallocenes [e.g., based on rac-C2H4(H4-l-Ind)2ZrCl2] can present extremely poor cis trans... 

In a system with site control ideally the mistake leads to a single odd insertion in the chain the site enforces for instance the growth of an isotactic chain with all m configurations and after one mistake has occurred it will return to producing the same configurations. In other words, an isospecific catalyst will produce a polymer chain with all methyl groups pointing towards us (as in... 

The different mode of insertion is attributed to the different polarities of the Ti—C and V—C bonds. The regioselectivity of the syndiotactic catalysts, however, is much lower than that of isospecific catalysts inversion, namely, head-to-head and tail-to-tail additions (type 1-2 addition), are more frequent.359... 

It is worth noting that, to obtain efficient supported Ziegler-Natta catalysts of the third and fourth generation, it is TiCU rather than preformed TiCL that is supported. When TiCl3 was supported on MgCl2 (e.g. by deposition from the solution of a TiCl3.3Py complex) and then activated with an alkylaluminium compound, the obtained catalyst appeared to be aspecific for propylene polymerisation [69], However, it could be converted into an isospecific catalyst by adding a Lewis base such as ethyl benzoate [70],... 

Dimeric homochirotopic [rac.-Me2Si(Me3Si, t-BuCp)2YH]2 is the first singlecomponent isospecific catalyst it is suitable to study the subtle steric factors that govern the remarkably high stereospecificities exhibited in the polymerisation of a-olefins by this and related two-component class III catalysts of C2 symmetry based on group 4 metallocenes. 

Many attempts have been made to synthesise ethylene/propylene block copolymers, referred to as polyallomers, with isospecific Ziegler-Natta catalysts. However, true block copolymers can hardly be synthesised. This is due to the short life of the growing polymer chains [68,241]. Therefore, only in a few cases, when the copolymers are synthesised by adding two comonomers sequentially and under very specific conditions in order to reduce chain transfer reactions, does unambiguous evidence for true block copolymer formation with isospecific catalysts exist [457]. 

The different orientation of the monomer in systems with Ziegler-Natta catalysts based on Ti or Nd and Co or Ni precursors has been suggested [41] to be determined by the presence in the isospecific catalysts (Ti- and Nd-based ones) of anionic ligands bound to the transition metal and the absence of any anionic ligand in the syndiospecific catalysts (Co- and Ni-based ones). In the... 

Recently, a series of soluble, highly isospecific catalysts were developed for propylene polymerization. These materials are zirconium, titanium, or hafnium based metallocenes, such as... 

Methylaluminoxane is required by these rigid metallocenes to form highly isospecific catalysts capable of very high isotactic placement. They are also very active, yielding very large quantities of polypropylene per each gram of zirconia. 

Initially, a series of soluble, highly isospecific catalysts were developed for propylene polymerization [168]. These materials are zirconium, titanium, or hafiiium based metallocenes, such as racemic 1,1-ethylene-di-ri -indenylzirconium dichloride. The term metallocene applies to complexes of transition metals sandwiched between two aromatic rings, usually two cyclopentadienyl. They are rigid structures, due to ethylene bridges between the two five-membered rings. Syntheses of these compounds yield racemic mixtures of two enantiomers. Both produce isotactic polypropylene ... 

Preparation of elastomeric polypropylenes was also reported by Chien et al. [64]. Two metallocene catalysts of different stereospecificities were used. The isospecific catalyst precursors were either rac-ethylene bis-(l-r -indenyl)zircoifium dichloride or rac-dimethylsilylene bis(l-T] -indenyl)zirco-nium dichloride. The unspecific one was ethylene bis(9-T] -fluorenyl)zirconium dichloride. The precursors were activated with triphenyl carbenium tetrakis(pentafluorophenyl)borate and triisobutylaluminum. The resultant catalysts exhibit very high activity, yielding products that range from tough plastomers to weak elastomers [64]. 

Catalysts that form ethylene-propylene copolymers can be used to produce a material known as ethylene-propylene rubber (EPR). If an isospecific catalyst for the formation of polypropylene incorporates a small amount of ethylene, a crystalline copolymer is formed that has a lower melting point than isotactic PP. If an aspecific catalyst is used, or if more ethylene is incorporated into the polymer, an amorphous ethylene-propylene rubber (EPR) is formed. EPR generally has a lower glass transition temperature than atactic PP, and is a useful material for low-temperature applications. Catalysts that form isotactic or syndiotactic polypropylene can also generate polymers possessing defined stereochemistry within the propylene units in an EP copolymer. 

Reetz, M. T. WUluhn, M. Psiorz, C. Goddard, R. Donor complexes of bis( 1-indenyl)phenylborane dichlorozirconium as isospecific catalysts in propene polymerization. Chem. Commun. 1999, 1105-1106. 

It should be pointed out that, in contrast to the case with the isospecific catalysts, the sense of the enantioface selectivity for olefin insertion has not been determined for the syndiospecific catalysts IV. The model shown below is based on that offered by Corradini and coworkers that the orientation of the growing polymer chain is the primary stereodifferentiating element. ... 

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