Transcript CH 2 - C = CH

CHEMICAL DEGRADATION

Whilst all polymers will be attacked by certain
chemicals it is the reactive chemicals in the
atmosphere which must be considered.
 Of these probably the most important are:
 1) Oxygen which leads to oxidative degradation.
 2) Ozone - which leads to ozonolysis
 3) Water
- which leads to hydrolytic
degradation.
 Protection
 A range of ANTIOXIDANTS etc. Have been
developed. Combinations of antioxidants often
used to obtain a synergistic effect.
OXIDATIVE DEGRADATION

Oxidative degradation normally initiation by
 1) radiation e.g. UV
 2) heat
 3) direct O2 attack (not too important with
saturated polymer)
 4) initiator residues (proxides).
UNSATURATED POLYMERS

1) Can be easily initiated
 2) Can be sujected to repeated O2 attack
 3) Must always be protected
- CH2 - C = CH - CH2
CH3
H2 allylic hydrogen’s – easily replaced
- CH2 - C = CH - CH
CH3
- CH2 - C - CH = CHCH3
PROTECTION

By preventing the production of free radicles.
MECHANISMS

1- system which cross-link.
 Example
 Polyethylene.
 Changes observed on exposure to radiation are:
 a) H2 and low molecular mass hydrocarbons
evolved.
 b) cross – linking occurs.
 c) unsaturation Increases.
 d) crystallinity decreases.
 e) polymer yellows.
 f) in air surface oxidises.
hv
-----CH2--------
----CH------ + H
H + ----CH2----
----CH------ + H2
H + ----CH2----
----CH------ + CH4
CH3
branch
----CH------ cross – link forms
2 ----CH----
----CH------ by radical combination
H
H
----CH--CH------CH---- + O2
hv
unsaturation increases
----CH = CH---- + H2
----CH------
ocidative
----O – O ----- breakdown

2- System which show reduction in molecular
mass.
 Mechanism still obscure, two main theories
 a) fracture of main chain followed by
disproportionation of radicles.
CH3
CH3 hv
----CH2 – C - CH2 – C CH3
CH3
CH3
CH3
+
CH3
----CH2 – C + CH2 – C -----
CH3
CH2
----CH2 – C
CH3
CH3 – C ---CH3
CH3
disproportionation

B) Multiple bond scission
CH3
----CH2 – C
CH3
hv
complex rearrangement.
OXIDATIVE DEGRADATION
MECHANISM

Initiative:- Formation of free radicales.
 e.g. from catalyst residue, effect of radiation and heat.
 Propagation: radical formed on polymer chain as
consequence of initiation step (Ro) is attacked by oxygen.
R
+
O2
ROO
ROO
+
RH
ROOH + R
RO
+ OH
Hydroperoxide
decomposition
Rearrangement to stable product
i.e. termination
example

Saturated polymer (polystyrene)
----CH2 - CH --CH2 – CH-
hv
----CH2 - C
--CH2 – CH---
O2
O-O
----CH2 – CH----
OH +O
----CH2 – C-- -------CH2 – C-- ---
rearrangement
----CH2 - CH
O OH
----CH2 – C-- --- + ----CH2 – C-- --ROOH
hydroperoxide
Farther chain
activated
Chain
rearrangement
scission
O
----CH2 - CH
OH
+
----CH2 – CH--
Stable products
OXIDATIVE DEGRADATION

Unsaturated polymer
 Effect of oxygen attack far more serious
with unsaturated polymers
 e.g. natural rubber.
MECHANISM
CH3
CH3
--------CH2 – C = CH – CH2 - CH2 – C = CH – CH2---------
-H
CH3
CH3
--------CH2 – C = CH – CH2 - CH2 – C = CH – CH--------Rearrangement to stablilise radical
CH3
CH3
--------CH2 – C = CH – CH2 - CH2 – C - CH = CH--------O2attack
CH3
CH3
--------CH2 – C = CH – CH2 - CH2 – C - CH = CH--------Hydroperoxide
cyclization
O-O
formation
CH3
CH2 – CH2
--------CH2 – C - CH
CH3
C - CH = CH---------
O-O
further O2attack
CH3
CH2 – CH2
CH3
--------CH2 – C - CH
C - CH = CH--------Further
O-O
O-O
cyclization
Hydroperoxide formation
CH3
CH2 – CH2
--------CH2 – C - CH
HO - O
O-O
CH3
C - CH = CH---------
Hydroperoxide decomposition
CH3
CH2 – CH2
--------CH2 – C - CH
HO + O
O-O
CH3
C - CH = CH---------
Chain scission
CH3
H - C – CH2 - CH2 – C - CH = CH-----O
O
Further chain scission
CH3
+
CH2 - C –
O
CH3
H - C – CH2 - CH2 – C
O
O
+
CH = CH-----To stable
product
ANTIOXIDANTS

It is a must in some way to stop the
breakdown process.
 1- inhibitors
 2- light absorbers
 3- hydroperoxide decomposing agents
 4- propagation interrupturs.
 5- metal deactivators
INHIBITORS

React rapidly with radicals, therefore it
stops the initiation stage.
 Example
 Benzoquinone
R
+
O
O
hv
RO
LIGHT ABSORBERS

Again prevent initiation caused by
radiation.
 Example
2 hydroxybenzophenones
H
O
O
hv
C
H
O
C
O
HYDROPEROXIDE DECOMPOSING
AGENTS

Example
 Sulphides.
R2S + ROOH
R2SO + ROH
PROPAGATION INTERRUPTURS

Example Phenols and amines
OH
tBu
tBu
+ ROO
CH3
2, 6 – ditertiary butyl 4 methyl phenol
O
tBu
+
ROOH
tBu
CH3 OOR
CH3
O
O
tBu
tBu
+
ROO
tBu
tBu
CH3
METAL DEACTIVATORS

Multivalent metal ions accelerate oxidation and
generate the unwanted peroxy radical.
 This must be prevented by complexing out the
metal ions using chelates.
 If this is not done the hydroperoxide breakdown
is accelerated and occurs as shown below.
M2+ +
M3+ +
ROOH
ROOH
Overall 2 ROOH
M3+ + RO + OHM2+ + RO O + H+
RO
+ RO O
+ H+
Thank You
See You Next Lecture