Application of Stable Radicals as Mediators in Living-Radical Polymerization - Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds - page 419

11.4.3.3 Cobalt-mediated radical polymerizations
Cobalt-mediated polymerization was first reported by Wayland in 1994 for the polymerization of acrylates
using a (tetramesitylporphyrinato)cobalt (TMPCo • ) complex ( 23 ). 157,158 The polymerizations proceed by
a reversible mechanism involving a dormant cobalt(III)-capped polymer chain and the active free radical
propagating polymer chain to give polymers with M w /M n

= 1 . 25. Much higher molecular weights (M n =

and faster reaction rates were realized using the octabromo derivative of TMPCo • . 158
The addition of the initiator 2,2 -azo-bis(4-methoxy-2,4-dimethyl valeronitrile (V-70) aids the formation
of the organo - cobalt mediating agent by serving as a hydrogen source for the TMPCo • to TMPCoH
transformation. This latter compound reacts readily with acrylate monomers to form an organocobalt
complex which then initiates and controls the polymerization. 159

10 5

1 − 5 . 5 × )

R

L

R

N

O O

Co
O

Co N

N

O

N

L

R

R

24

23

An advantage of the cobalt-mediated radical polymerization is its ability to control the polymerization
of reactive monomers, such as vinyl acetate. Cobalt bis(acetylaceotnate) complexes of general structure 24
allow for the production of polymers with M n up to 99 000 g mol − 1 and M w /M n between 1.1 and 1.3. 160
A detailed account of the role of the vazo initator in vinyl acetate polymerization has been presented
by Wayland. 161
The ability to polymerize acrylonitrile 162 allows the synthesis of block copolymers of poly(vinyl
acetate- b -polyacrylonitrile), which can be readily hydrolyzed to give water soluble blocks of poly(vinyl
alcohol- b -poly(acrylic acid). 163

The cobalt(II) tetramesitylporphyrin complex also enables the poly-

20 %. 164 On the other hand the
Co(acac) was shown not to be as efficient for n -butyl acrylate polymerization as the porhorincobalt
complex. 165
Acrylate polymerizations with organo - cobalt porphyrin complexes have been shown to proceed by
both a reversible transfer and a degenerative transfer mechanism. 166,167 For vinyl acetate polymerization
the mechanism has been shown to vary according to the reaction conditions. In bulk, a degenerative
chain transfer mechanism predominates but in the presence of water or pyridine a reversible termination
mechanism prevails. 168,169

merization of vinyl acetate but the yields are generally low, ∼

11.4.3.4 Organotellurium-mediated living-radical polymerizations
First introduced by Yamago et al ., 170 - 172 the organotellurium-mediated LRP (TERP) was shown to
work well with a variety of monomers, including styrene, acrylates vinyl acetate and methacrylates,
although the molecular weight distributions for methacrylates were broad. The addition of a small
amount of dimethyl ditelluride (MeTe) 2 , 171,173 which serves to deactivate the propagating chain in
the methacrylate polymerizations, was shown to enable better control and much improved molecular
weight distributions. 174
The tellurium structures, R - TeL, contain an R group, which acts as the initiator for the polymerization,
and an L non-transferable substituent, which remains with the tellurium. The TeL moiety reacts reversibly