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5.1.9. Degradation by
free
radical mechanism
(oxidative-‐
reductive depolymerization – ORD)
Organic materials are generally degradable, also in the absence of enzymes.
One example is the slow decay of plastics, which become brittle and
fragmented, and eventually disappears. Such degradation is accelerated (i.e.
catalysed) by light, oxygen, and transition metal ions. ORD is a general, but
complicated cascade of reactions working on all kinds of polymers, including
biopolymers.
ORD is generally catalysed by autooxidable compounds such as ascorbic
acid, sulfite, phenols and many others. Also molecular oxygen (O
2
) and
transition metals such as Fe
2+
/Fe
3+
function as catalysts, and the reaction
seem in some cases to be catalysed by high pH. Also light catalyses ORD.
Thus, optimum stability towards ORD is obtained by avoiding alkaline
conditions, oxygen, light, and autooxidable compounds.
The reaction cascade generally follows three steps:
1.
A reducing compound (for example Cu
+
, F
2+
or ascorbic acid, see
figure) reacts with O
2
(the activated singlet state is particularly reactive)
to produce (directly or indirectly) a peroxide:
RH + O
2
→
R-O-O-H
2.
The unstable peroxide decomposes in the presence of catalysts such
as Fe
2+
to yield highly active radicals, including the extremely reactive
hydroxyl radical (
.
OH):
R-O-O-H
→
R-O
.
+
.
OH
3.
The free radicals (especially
.
OH) attack the polymer by abstracting a
hydrogen atom (
.
H):
-CH- +
.
OH
→
-C
.
- + H
2
O
The polymer radical (-C
.
-) is unstable, and subsequent (partly unknown)
reactions lead to chain scission.