Chemistry Reference
In-Depth Information
Formation of polyenes in polyvinylacetate [166] and formation of double bonds in CDA
[162] may be now explained by this sequence of reactions. In the case when RH has polyene
fragment R
polyene
H, reaction (7) will lead to the formation of low-active polyene radical.
(8)
Reaction (6) of photolysis, followed by fast reaction of polyene radical formation (8), is
unfolded 1
st
stage in the Scheme 3. Obtained values K
1
and K
3
(K
3
being larger than K
1
by 10
times) show that reaction (8) is not the main reaction of vinyl radicals. It can be explained by
low concentration of double bonds.
Thus, reactions (6) - (8) explain chemical mechanism of acetic acid chain formation at
CA photolysis and agree with kinetic scheme of the process. According to the scheme, length
of the chain of acetic acid formation depends on light intensity and at intensities, being used,
it is 10-30 units.
2.4.
K
INETIC
R
EGULARITIES OF
CDA
P
HOTOOXIDATION
Kinetic curves of O
2
absorption at CDA irradiation by light (253,7 nm) are given in
Figure 2.6. As it is seen from Figure 2.6 curves of O
2
absorption consist of two sections: first-
effect of photochemical preaction, characteristic for cellulose materials, is observed at initial
stages of photooxidation at photooxidation levels lower than 2·10
-5
mole/kg; second -
stationary, from the slope of which the rate of oxygen absorption is found in stationary mode.
As it is seen from Figure 2.6, oxygen absorption proceeds with the rate a bit higher zero from
the very beginning, however, at accumulation of radicals and achievement of stationary
concentration, the rate of oxygen absorption increases by 2,1 times.
Rise of the rate of oxygen absorption at initial stages is caused by increase of radicals
concentration up to its stationary value that is in conformity with increase of (R*) reaction
rate increases [172].
R
*
+ O
2
→ R
*
O
2
The fact, that at the very beginning of irradiation the rate of oxygen absorption is not
equal to zero, indicates the possibility of high-active radicals taking place in CDA
photooxidation. On the other hand, the time of achieving stationary rate of oxygen absorption
according to data of Figure 2.6 is 300 seconds, that shows fast achievement of stationary
radicals concentration.
As it is seen from Figure 2.6 (curve 2) at the moment of light source switching off the
rate of O
2
absorption decreases up to stationary rate in dark process and oxidation reaction
proceeds for some time. This effect is the effect of photochemical aftereffect or post-effect. It
is explained [173] by lasting radical-chain process of oxidation, consisting of two stages:
reactions of continuation and breaks of chain of photooxidation. Kinetic curve of oxygen
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