Chemistry Reference
In-Depth Information
9.4.5
HO
2
•
/O
2
•−
-Elimination Reactions and
β
-Fragmentation of Oxyl Radicals
In peroxyl-free-radical chemistry, HO
2
•
/O
2
•
−
elimination reactions play a major
role (Chap. 8.4). In polymer free-radical chemistry, these reactions are of special
interest, because they lead to a conversion of slowly diffusing polymer-derived
radicals into the readily diffusing HO
2
•
/O
2
•
−
radicals. The HO
2
•
/O
2
•
−
-elimina-
tion typically proceeds from an
-hydroxyalkylperoxyl radical [reaction (22)].
In poly(vinyl alcohol), for example, such an structural element is formed by
H-abstraction and subsequent O
2
addition [reactions (18) and (19)]. The same
structural element may also be formed during the bimolecular decay of peroxyl
radicals which carry an H-atom in
α
β
-position [reactions (20) and (21)].
-HC(OH)-CR
2
- +
•
OH
→
-
•
C(OH)-CR
2
- + H
2
O
(18)
-
•
C(OH)-CR
2
- + O
2
→
-
•
OOC(OH)-CR
2
-
(19)
2 -C(H,OO
•
)-CR
2
-
→
2 -C(H,O
•
)-CR
2
- + O
2
(20)
-C(H,O
•
)-CR
2
-
→
-
•
C(OH)-CR
2
-
(21)
-C(OH,OO
•
)-CR
2
-
→
-C(O)-CR
2
- + HO
2
•
(22)
-C(OH,OO
•
)-CR
2
- + OH
−
-C(O
−
,OO
•
)-CR
2
- + H
2
O
→
(23)
-C(O)-CR
2
- + O
2
•
−
-C(O
−
,OO
•
)-CR
2
-
→
(24)
-C(O)H +
•
CR
2
-
-C(H,O
•
)-CR
2
-
→
(25)
The HO
2
•
elimination at reaction (22) is often slow, and at a high concentration
of peroxyl radicals this reaction may compete with the bimolecular decay of the
peroxyl radicals (leading to chain scission; Ulanski et al. 1994). However in the
presence of base, deprotonation speeds up the O
2
•
−
elimination [reactions (23)
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