Stereoregulation, origin

A reasonable expectation for a transition metal catalyzed process would be the ability to control the micro structure of the product and in this case the polysilane, EI(RSiEI )VE 1, that is produced. An unusual selectivity seemed to be apparent in the condensation of PhSiH3 by bis(indenyl)- and bis(tetrahydroindenyl)zirconium dimethyl catalysts and was originally attributed to stereoregulation in the polymerization but was later proposed to be due to an unusual selectivity in the production of cyclics.36,79 In an earlier report17 of the condensation of PhSiH3 with rac- E BI)ZrCl2/2"BuLi... 

Let us recall that the origin of stereospecificity in the syndiospecific polymerisation of styrene lies in a chain end stereochemical control mechanism [52,70]. Key features of the stereoregulation mechanism are stereorigid rf coordination of the growing chain end and diastereoselective rj2 coordination of the styrene... 

The origin of the stereoregulation in conjugated diene coordination polymerisation was recognised by Porri et al. [7,41] who explained the different modes of formation of the isotactic and the syndiotactic polymers in the presence of various transition metal-based catalysts. 

The mechanism of stereoregulation in the stereoselective polymerisation of propylene oxide with zinc dialkoxide and related zinc dialkoxide-ethylzinc alkoxide complexes has been satisfactorily explained by the enantiomorphic catalyst sites model prepared by Tsuruta et al. [52,75], According to this model, the presence of chiral sites with a central octahedral zinc atom, bearing the polymer chain and coordinating the monomer, was assumed to be the origin of the stereoregulation mechanism. 

Developments Since 1965 Catalyst Systems and Chemistry Types of Olefin Monomers and Polymers Polymerization Mechanisms Origin of Stereoregulation... 

Still another new approach was the combined use of fractionation, osmometry and viscometry to study the influence of the polymerization conditions Ctemperature, extent of polymerization, solvent) on the infrastructure of addition polymers it was originally exemplified with three polystyrene samples prepared at different temperatures but has since been used in refined form and with such improved fractionation techniques (GPC) to study such problems as branching, stereoregulation and grafting. 

Cationic group 3 metal catalysts of the type [Ln(T -C5Me4R )R], which are isostructural with the trivalent titanium system [Ti(r -C5Me4R )R]+, clearly indicate that the structure of the active species as well as the stereoregulating mechanism are related, supporting the original proposal by Zambelli et al. [32],... 

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