Free radical chain polymerization mechanism .ppt

Free radical chain polymerization

1. CHAIN POLYMERIZATION
Free Radical Polymerization
Freeradicalareindependently-existingspeciesthathave
unpairedelectron.Normallytheyarehighlyreactive
withshortlifetime.
Freeradicalpolymerization’sarechainpolymerization’s
inwhicheachpolymermoleculesgrowsbyadditionof
monomertoaterminalfree-radicalreactivesiteknown
asactivecenter.
Aftereachadditionthefreeradicalistransferredtothe
chainend.
Chainpolymerizationischaracterizedbythreedistinct
stages,Initiation,propagationandtermination.

2. Example
Theformationofpolyvinylmonomer.
 CH
2=CHX

3. INITIATION
Thisstageisatwostepsstage
1.Theformationoffreeradicalsfroman
initiator.
2.Theadditionofoneofthesefreeradicalstoa
monomermolecules.
Freeradicalcanbeformedbytwoprincipal
1.Homolyticscission(homolysis)orbreakageof
asinglebond.
2.Singleelectrontransferredtoorfromanion
ormolecule(redoxreactions)
Homolyticcanbeachievedbyheat(thermolysis)
orbylightsuchasU.V.(photolysis).

4. Example
Sometimestheradicalsundergofurtherbreakdown(-
scissions)suchas
O O O
 -C-0-0-C- 2 -C-O
Benzolyperoxide Benzolyooxyradicals
(CH
3
)
2
C-N=N-C(CH
3
)
2
2(CH
3
)
2
C+N
2
 CN CN CN
Azobisisobutyronitril 2-Cyanopropylradicals
 O O
 -C-0 +C=O

(CH
3
)
2
-C-O CH
3
+(CH
3
)
2
-C=0
 CH3 Methylacetone
 radical

5. PHOTOLYSIS
Photolysisisthesecondprincipleoffree
radicalformation.
Theadvantageofthismethodisthatthe
formationoffreeradicalsbeginsatthe
instantofexposureandcasesassoonas
thelightsourceisremoved.

6. REDOX REACTION
Redoxreactiondefinedasthegenerationoffree
radicalsbyelectrontransferanditisusewhen
polymerizationperformedatlowtemperature.
Example
 CH
3
CH
3
 -C-0-0H+Fe
2+`
-C-O+OH+Fe
3+
 CH
3
CH
3
Cumyl Ferrous Cumyloyloxy
hydroperoxideion radical
O O O O O O
O-S-O-O-S-O+HO-S-O 0-S-O+O-S-O+OH-S-O
O O O O
PresulphateBisulphateSulphateSulphateBisulphate
ion ion ion radicalradical

7. Anactivecenteriscratedwhenafreeradical(R
o
)
whichisgeneratedfromaninitiatorattacksthe-bond
ofthemonomermolecules.
R+CH
2
=CH R-CH
2
-CHorR-CH-CH
2
X X X
Thisismore Thisismore
likely stable
Sometimefreeradicalreactwitheachothersuchas:
O O O
2 -C-0 -C-O- +C=O
OR
2

8. PROPAGATION
Theadditionofmonomermoleculestotheactivecentertogrow
thepolymerchain.
Therearetwomodesofchainpropagation
Againmode(1)aremoredominant.
Timeofadditionforeachmonomerisoftheorderofamillisecond.
Thusseveralthousandsofadditionscantakeplaceinafewseconds.
Thereforepolymerstructurearelike
-----CH
2-CH-CH
2-CH-CH
2-CH-CH
2-CH------
X X X X
1.HeadtoTail
R-CH
2
-CH+CH
2
=CH R-CH
2
-CH-CH
2
-CH
X X X X
2.HeadtoHead
R-CH
2
-CH+CH
2
=CH R-CH
2
-CH-CH-CH
2
X X XX

9. TERMINATION
Thelaststageofchainreactioninwhichthegrowthof
thepolymerchainterminated(orstopped).
Therearetwomechanismsoftermination
1.Combination
Couplingtogetheroftwogrowingchainstoforma
singlepolymermolecules.polystyrene
2.Disproportionation
whenahydrogenatommovefromonegrowingchainto
another
--CH
2
-CH+CH-CH
2
------- CH
2
-CH-CH-CH
2
---
 X X XX
 HH
--CH
2
-CH+C-C-------CH
2
-CH
2
+CH=CH
2
---
 X XH X X
 SaturatedendUnsaturatedend
 grouppolymergrouppolymer

10. Thank You
See You Next Lecture