Application of Stable Radicals as Mediators in Living-Radical Polymerization - Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds - page 418

11.4.3 Other radicals as mediators

This section describes radical mediators that are best describes as persistent, rather than truly stable, since
they are formed in situ and, although they are relatively long lived, they cannot be isolated.

11.4.3.1 Borinate radicals

Borinate radicals are relatively stable species formed by the decomposition of peroxyborinate precursor
molecules. 153 Peroxyborinates decompose quite readily at ambient temperatures by homolytic cleavage
of their peroxide bond to yield an alkoxy radical and a borinate radical, as for example 21 , the latter
exhibiting stability due to the back-donation of electron density into the empty p -orbital of boron.

O

B

21

Peroxyborinates can be used as initiators in a similar fashion to the nitroxide-derived alkoxyamines. The
alkoxy radical produced from the homolytic cleavage of the peroxyborinate initiates polymerization while
the borinate radical forms a weak, reversible bond with the propagating radical chain.
Polymerizations performed using peroxyborinate initiators with ethyl methacrylate give polymers that
increase in molecular weight with time; however, the molecular weight distributions are typically broad
(M w /M n 5), in part because the system does not allow for a simultaneous initiation of all polymer

chains. 153 An 8-boro-indane/oxygen system containing a boron bridging atom between a six-membered
ring and a more reactive five-membered ring undergoes reaction with oxygen to expand the five-membered
ring to a seven-membered ring containing a peroxide moiety. A reaction of the resulting peroxyborinate
with methyl methacrylate showed improvement in the molecular weight distributions of the resulting
homopolymer, (M w /M n ∼

∼ 2 .

3 - 1.6). 154 1 .

11.4.3.2 (Arylazo)oxyl radicals
(Arylazo)oxyl radicals, such as 22 , are formed in situ either by an electron transfer reaction between an
activated carbon - halogen compound and an (arenediazo)oxyl anion or by the one-electron oxidation of
an arenedihyponitrite or cyanate anion with an arenediazonium ion. Both reactions give carbon-centered
radicals as byproducts which can be used to initiate the polymerization, while the (arylazo)oxyl radicals
serves to reversibly terminate the growing polymer chain. 155

N

N O

22

Polymerizations of methyl methacrylate and n -butyl acrylate at temperatures between 25 and 40 ◦ C with
PhN 2 BF 4 /NaN 2 O 2 as the initiating/mediating system produce polymers that increase in molecular weight
with conversion but which exhibit broad molecular weight distributions (M w /M n >

3). 156

Block copolymers

prepared using this system contained significant amounts of homopolymers.