Chemistry Reference
In-Depth Information
place between a radical and a parent initiator molecule. This can lead to the formation of different
initiating species. It can, however, also be a dead end as far as the polymerization reaction is
concerned.
After the initiating radical has diffused into the proximity of the monomer, the capture of the free
radical by the monomer completes the step of initiation. This is a straightforward addition reaction,
subject to steric effects:
R
3
R
1
R
3
R
3
R
1
R
3
R
+
R
R
3
R
3
R
3
R
3
R
3
R
2
2
, carbon
The unpaired electron of the radical is believed to be in the pure p-orbital of a planar,
sp
3
configuration appear to form [
42
-
44
].
Using quantum chemical calculations it was demonstrated that nucleophilic and electrophilic alkyl
or aryl radicals attack alkenes following a tetrahedral trajectory [
45
,
46
]:
atom. Occasionally, however, radicals with
sp
R
X
Z
H
H
Y
This means that only substituents Y at the attacked olefinic carbon exert large steric effects [
47
]. In
addition to the steric effects, the rates of addition of strongly nucleophilic or electrophilic radicals are
governed mainly by polar effects of the substituents R
x
, Y, and Z [
48
]. In borderline cases, however,
the stabilities of the adducts and products tend to dominate [
47
].
Also, it was demonstrated that acyclic radicals can react with high stereoselectivity [
45
]. In order for
the reactions to be stereoselective, the radicals have to adopt preferred conformations where the two
faces of the prochiral radical centers are shielded to different extents by the stereogenic centers. Giese
and coworkers [
49
] demonstrated with the help of Electron Spin Resonance studies that ester-
substituted radicals with stereogenic centers in
-positions adopt preferred conformations that mini-
mize allylic strain [
49
] (shown below). In these conformations, large (L) andmedium sized substituents
(M) shield the two faces. The attacks come preferentially from the less shielded sides of the radicals.
Stereoselectivity, because of A-strain conformation, is not limited to ester-substituted radicals [
50
].
The strains and steric control in reactions of radicals with alkenes can be illustrated as follows [
50
]:
b
R
H
L
OR
C
C
H
C
N
C
C
O
C
H
OR
R
M
O
M
L
Cram
Felkin-Anh
A-strain
Auxilliary Control
Substrate Control
The above considerations can be illustrated on initiation by benzoyl peroxide, a commonly used
initiating compound. The half-life of the initial benzoyloxy radicals from decompositions of benzoyl
