Application of Stable Radicals as Mediators in Living-Radical Polymerization - Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds - page 413

The synthesis and application of a nitroxide containing one adamantyl substituent to living radical
polymerization has been reported. 89,90 Braslau concluded that steric bulk and an inherent instability of
the nitroxide were both factors in whether a nitroxide could mediate the polymerization of acrylate
monomers. This prompted the synthesis of 1-adamantyl-3-methyl-2-phenylazabutane-1-nitroxide, 13 ,an
acyclic nitroxide containing both an hydrogen and an adamantyl group. The corresponding alkoxyamine

was investigated as an initiator for the polymerization of styrene, N , N -dimethyl acrylamide, tert -butyl
acrylate and n -butyl acrylate.

α

N
O

N
O

12

13

Debuigne et al . 88

hydrogen
atoms, and found that while this nitroxide enabled low temperature (100 ◦ C) polymerization of styrene,
it failed to enable the polymerization of n -butyl acrylate. It was further shown that free nitroxide
accumulated in the reaction solutions over time in both acrylate and styrene polymerizations, but more
so in the acrylate polymerizations. These results prompted the authors to further argue that the more
important requirement for whether a nitroxide will enable the polymerization of acrylates is related to its
inherent stability or instability.

synthesized diadmantyl nitroxide 12 , a sterically bulky nitroxide with no α

11.3.4 Persistent radical effect
The main reaction that takes place in the stable free radical polymerization is the dynamic equilibrium that
occurs between the propagating chain end, the stable mediating radical, and the dormant polymer chain,
which allows for the repeated generation of a propagating chain end that can add monomer following a
reversible termination reaction with the mediating stable radical. The kinetics of this equilibrium operates
on a phenomenon known as the persistent radical effect (PRE). 47,91,92 In addition to the main reversible
termination reaction that takes place between the propagating chain end and the stable radical, other side
reactions, such as termination by chain - chain coupling, occur. In the early stages of polymerization, a
small number of the propagating chains undergo termination by coupling at diffusion-controlled rates,
since the radicals are small and the reaction medium is non-viscous, resulting in dead oligomeric chains.
Unlike the propagating chain radicals, the radical mediator is stable and does not terminate, resulting in an
increase in its concentration, leading to a more efficient formation of the dormant polymer species, which
in turn reduces the amount of termination by chain - chain coupling. Overall, the PRE gives rise to a small
excess of mediating radicals and ensures good control over the polymerization process. A small amount
of excess nitroxides is thus often added when alkoxyamines are used to initiate polymerization to help
establish this excess concentration of nitroxide. 61

11.3.5 Requirements of stable radicals as mediating agents
In order for a radical to be able to mediate a polymerization, it must be relatively stable under the reaction
conditions and must react with the propagating chain in a reversible manner. In addition, it must not react
with itself or with monomer to initiate new chains nor participate in side reactions, such as β -hydrogen

atom abstraction.