Boron-Containing Coinitiators

Other Lewis acids have been considered as alternatives to MAO for two reasons (1) one might avoid the cost of the large excess of MAO required to activate the metallocene and (2) simpler systems, which allow isolation of the product(s) from reaction of metallocene and coinitiator, would be useful to obtain a better understanding of metallocene-initiated polymerizations. 

Boron-based Lewis acids are useful coinitiators for metallocenes [Chen and Marks, 2000 Chen et al., 1998 Pedeutour et ah, 2001 Zhou et ah, 2001]. Organoboranes such as 

Cp2Zr(CH3)2 + (C6H5)3C+B(C6F5)4- [Cp2Zr(CH3)]+ [B(C6F5)4r + (QHsfaCCHa 

Unlike the case for MAO, one needs only an equimolar amount of the coinitiator to activate the initiator. In some reaction systems, higher polymerization activity is observed with an excess of initiator relative to coinitiator. Maximum activity is observed in some polymerization systems at a 2 1 initiator coinitiator ratio, which indicates that bimetallic species such as XXXVIII are the actual initiating species [Wang et al., 2003], A unique feature of the use of organoboranes and organoborates is that the complexes, both monometallic such as XL and XLI and the corresponding bimetallic complexes, have been isolated and characterized in a number of systems. 

The rate of polymerization is derived in a straightforward manner. The following derivation describes polymerization with a zirconocene (Zr) and MAO, but is general and applies to any metallocene coupled with any coinitiator. Reaction between the initiator zirconocene and MAO forms the active initiating species I, which subsequently adds monomer to yield the initial propagating species M  

For the case where Reaction 8-58 is irreversible and fast (i.e., [M ] = [I ]), the polymerization rate is 

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