Well-Defined Transition Metal Catalysts for Metathesis Polymerization - Late Transition Metal Polymerization Catalysis - page 162

Scheme 5.7 Synthesis of Mo-based Schrock-catalysts. Ar =phenyl, 2,6-
Me 2 -C 6 H 3 , 2,6-i-Pr 2 -C 6 H 3 , etc.; R=ethyl, phenyl, trimethylsilyl, CMe 2 Ph or
t -butyl and R =CMe 3 , CMe 2 CF 3 , CMe(CF 3 ) 2 , C(CF 3 ) 2 , aryl, etc. Tf = triflate
(OSO 2 CF 3 ) 2 .

)(CHCMe 2 R)(OTf) 2 ·DME
possess a pseudooctahedral conformation with the imido- and alkylidene ligand in
a cis - and the two triflate groups in a trans -position. The final catalysts of the type
Mo(NAr

The key intermediate of the general formula Mo(NAr

) 2 possess a tetrahedral geometry. More recently, even
silsesquioxanes have been reported as ligands [87]. It is worth noting, that such a
ligand exchange may also be performed in the course of the polymerization.
Thus, after starting the polymerization of 2,3-bis(trifluoromethyl)norborn-5-ene
with Mo(NAr

)(CHCMe 2 R)(OR

)(CHR)(OCMe 3 ) 2 , addition of HOCMe(CF 3 ) 2 leads to an alkoxide-ex-
change and forms a living polymer containing the Mo(NAr )(CHR)[OCMe(CF 3 ) 2 ] 2

moiety at the chain end. This approach offers access to block -copolymers with dif-
ferent cis - trans- configurations for the resulting double bonds [88]. Generally speak-
ing, tungsten and in particular molybdenum Schrock carbenes are highly reactive
initiators for the ROMP of a vast variety of cyclic alkenes such as substituted
NBEs, NBDEs, 7-oxanorbornenes, cyclooctatetraenes (COTs), cyclooctadienes
(CODs), etc. and polycyclic alkenes such as quadricyclanes [89]. Despite their sen-
sitivity towards traces of oxygen or moisture, these systems nowadays hold a
strong place due to their stability versus functional groups as well as the ease of
catalyst tuning.