Synthesis of High Oxidation State Bimetallic Alkylidene Complexes for Controlled ROMP Synthesis of Triblock Copolymers

Abstract

An X-ray study of [(THF)(R F6 O) 2 (ArN)Mo CH] 2 (1,4-C 6 H 4 ) (OR F6 = OCMe(CF 3 ) 2; Ar = 2,6-diisopropylphenyl; THF = tetrahydrofuran; 1b ), which is closely related to known [(DME)(R F6 O) 2 (ArN)Mo CH] 2 (1,4-C 6 H 4 ) (DME = 1,2-dimethoxyethane; 1a ), showed it to be the expected bimetallic species in which each end is approximately a trigonal bipyramidal monoadduct of a syn alkylidene with the THF coordinated to the NOO face of the metal trans to the Mo C bond. Treatment of 1a with lithium tert -butoxide yielded [( t -BuO) 2 (ArN)Mo CH] 2 (1,4-C 6 H 4 ) ( 2 ). Addition of divinylferrocene to Mo(CHCMe 2 Ph)(NAr)(OR F6 ) 2 produced the bimetallic species {(R F6 O) 2 (ArN)Mo[ CH(C 5 H 4 )]} 2 Fe ( 3 ), which upon treatment with lithium tert -butoxide produced a related tert -butoxide complex ( 4 ). X-ray studies of 3 and 4 showed them to be “ syn/syn ” bimetallic species related to 1b . In solution two resonances can be observed in proton NMR spectra in the alkylidene region for the “ syn/anti” isomer of 1a, 2, 3, and 4; the total amount varies between 4 and 20% of the total. Bimetallic species 1a, 2, 3, and 4 have been shown to initiate at both ends and to produce homopolymers of 4,5-dicarbomethoxynorbornadiene (DCMNBD) and methyltetracyclododecene (MTD) in a living fashion. MALDI-TOF mass spectra of ferrocene-containing species have been obtained that are consistent with the polymerization process being living. Triblock copolymers (poly[(MTD) x /2 (DCMNBD) y (MTD) x /2 ]) were prepared by adding y equivalents of DCMNBD to the bimetallic initiators followed (after consumption of DCMNBD) by x equivalents of MTD. These triblocks were shown to be of relatively high purity (free of homopolymer and diblock copolymer) and to have a relatively low PDI (≤1.2).