Characteristics of Chain-Growth PolymerizationCharacteristics of Chain-Growth Polymerization
1. 1. Only growth reaction adds repeating units one at a Only growth reaction adds repeating units one at a time to the chaintime to the chain
2. 2. Monomer concentration decreases steadily throughout Monomer concentration decreases steadily throughout the reaction the reaction
3. 3. High Molecular weight polymer is formed at once; High Molecular weight polymer is formed at once; polymer molecular weight changes little throughout polymer molecular weight changes little throughout the reaction.the reaction.
4. 4. Long reaction times give high yields but affect Long reaction times give high yields but affect molecular weight little.molecular weight little.
5. 5. Reaction mixture contains only monomer, high Reaction mixture contains only monomer, high polymer, and about 10polymer, and about 10-8-8 part of growing chains. part of growing chains.
04/07/23 3
The Chemistry of Free Radical PolymerizationThe Chemistry of Free Radical Polymerization
D. Chain Transfer (sometimes)D. Chain Transfer (sometimes) – An atom is transferred – An atom is transferred to the growing chain, terminating the chain growth to the growing chain, terminating the chain growth and starting a new chain.and starting a new chain.
PPxx RR PPxx ++ RRHH++
PPxx ++ PPyy
HHPPxx PPyy
++
Chain Transfer to Chain Transfer Agent:Chain Transfer to Chain Transfer Agent:
Chain Transfer to Polymer:Chain Transfer to Polymer:
Chain Transfer to Monomer:Chain Transfer to Monomer:
PPxx. . + H+ H22C=CH-(C=O)ORC=CH-(C=O)OR
Causes BranchingCauses Branching
04/07/23 8
E. Inhibition and RetardationE. Inhibition and Retardation – a retarder is a substance – a retarder is a substance that can react with a radical to form products incapable that can react with a radical to form products incapable of reacting with monomer. An of reacting with monomer. An inhibitor inhibitor is a retarder is a retarder which completely stops or “inhibits” polymerization.which completely stops or “inhibits” polymerization.
2. Monomers that are susceptible to free radical addition2. Monomers that are susceptible to free radical addition
Hydroperoxides can generate radicals by Hydroperoxides can generate radicals by “induced “induced decomposition”decomposition” from growing polymer chains: from growing polymer chains:
PP + + HH OO OO RR
PHPH + + OO OO RR RR OOOO22
R-OO-OO-RR-OO-OO-R 2 RO2 RO + + OO22
What effect does this have on the polymerization process?What effect does this have on the polymerization process?
Acting as a chain-transfer agent, it reduces thedegree of polymerization and molecular mass.
04/07/23 18
5) Redox Initiator Systems5) Redox Initiator Systems
benzophenonebenzophenone excited stateexcited state
CC
RR
RR
HH NN
RR
RRPhPh PhPh
OHOH++ CC
RR
RR
NN
RR
RR
PhotosensitizerPhotosensitizer
CoinitiatorCoinitiator
04/07/23 22
55. Propagation. Propagation - - “Keeping the thing going!”“Keeping the thing going!”
A. The addition of monomer to an active center (free radical)A. The addition of monomer to an active center (free radical) to generate a to generate a newnew active center. active center.
RR CCHH22
CHCH22
XX
XX RR CCHH22
HHCC
XX
CCHH22
CHCH
XX
XX XX
etc.etc. etc.etc.RR CC
HH22
HHCC
XX
CCHH22
CHCH
XXnn
(continued)(continued)
04/07/23 23
Examples:Examples:
RR CCHH22
CHCH22
PhPh
PhPh RR CCHH22
HHCC
PhPh
CCHH22
CHCH
PhPhnn
RR CCHH22
CCHH22
CHCH
CC
OO
CHCH 33
OO
OOCHCH 33
OO
RR CCHH22
CCHH22
HHCC
CC
OO
CHCH 33
OO
CCHH22
CHCH
CC
OO
CHCH 33
OO
PolystyrenePolystyrene
PolymethylPolymethylAcrylateAcrylate
04/07/23 24
B. Configuration in Chain-Growth PolymerizationB. Configuration in Chain-Growth Polymerization
For large X, For large X, -substitution-substitution is sterically favoredis sterically favored
4) Radical Stability4) Radical Stability
33o o > 2> 2o o > 1> 1oo
04/07/23 29
5 ) “Bottom Line”5 ) “Bottom Line”
Resonance and steric hinderance considerations lead to the Resonance and steric hinderance considerations lead to the conclusion that conclusion that -substitution-substitution (head-to-tail)(head-to-tail) is strongly is strongly preferredpreferred in chain-growth polymerization. in chain-growth polymerization.
6. Termination -6. Termination - “Stopping the thing!”“Stopping the thing!”
A. Coupling (most common)A. Coupling (most common)
Px CH2
C
H
X
+ PyCH2
C
X
H
PPyyCCHH 22
CC
XX
HH
PPxx CCHH 22
CC
HH
XX- occurs head-to-head- occurs head-to-head- produces two initiator fragments (end-groups) produces two initiator fragments (end-groups) per chain.per chain.
04/07/23 31
B. DisproportionationB. Disproportionation
InIn MMxxCCHH
CCHH
HH HH++ InInMMyyCC
HHCCHH
HHHH
HH33CC CHCH22 MMyy InInCHCH22CHCHInIn MMxx ++
- Produce one initiator fragment (end-group) per chain- Produce one initiator fragment (end-group) per chain
- Production of saturated chain and 1 unsaturated chain - Production of saturated chain and 1 unsaturated chain per terminationper termination
04/07/23 32
C. Factors affecting the type of termination that will take C. Factors affecting the type of termination that will take place.place.
1) Steric factors -1) Steric factors - large, bulky groups attached directly large, bulky groups attached directly to the active center will hinder couplingto the active center will hinder coupling
2) Availability of labile 2) Availability of labile -hydrogens-hydrogens
3) Examples – 3) Examples – PS and PMMAPS and PMMA
One might assume electrostatic repulsion in this case.BUT, how about electrostatic attraction from thenitrogen to the carbon? Also, steric hindrance islimited.
At 60oC, this terminates almost exclusively bycoupling!
04/07/23 36
D. Primary Radical TerminationD. Primary Radical Termination
So molecular mass can be controlled using chain-transferSo molecular mass can be controlled using chain-transferagents, hydroperoxide initiators, OR agents, hydroperoxide initiators, OR higher levels ofhigher levels ofinitiator!initiator!
04/07/23 37
7. Chain-Transfer -7. Chain-Transfer - “Rerouting the thing!”“Rerouting the thing!”
A. Definition – The transfer of reactivity from theThe transfer of reactivity from thegrowing polymer chain to another species. Angrowing polymer chain to another species. Anatom is transferred to the growing chain,atom is transferred to the growing chain,terminating the chain and starting a new one.terminating the chain and starting a new one.
Chain-transfer occurs so readily that propylene won’t polymerizeChain-transfer occurs so readily that propylene won’t polymerizewith free radicals.with free radicals.
Definition – The transfer of reactivity from theThe transfer of reactivity from thegrowing polymer chain to another species. Angrowing polymer chain to another species. Anatom is transferred to the growing chain,atom is transferred to the growing chain,terminating the chain and starting a new one.terminating the chain and starting a new one.
8.8. Inhibition and Retardation -Inhibition and Retardation - “Preventing the thing“Preventing the thingor slowing it down!”or slowing it down!”
Definition – Compounds that slow down or stop poly-Compounds that slow down or stop poly-merization by forming radicals that are either too merization by forming radicals that are either too stable or too sterically hindered to initiate poly-stable or too sterically hindered to initiate poly-merization merization OROR they prefer coupling (termination) they prefer coupling (termination)reactions to initiation reactions.reactions to initiation reactions.
~~~P~~~PXX – CH – CH22-CH-CH.. + O= + O= =O =O
para-Benzoquinonepara-Benzoquinone
~~~P~~~PXX – CH – CH22-CH-CH22-O- -O-O- -O..Will NotWill Not
Kinetics of Free Radical PolymerizationKinetics of Free Radical Polymerization
1. Initiation1. Initiation
II 2 R 2 R.. Radical GenerationRadical Generationkkdd
RR.. + M + M M M11.. InitiationInitiation
kkii
Assuming that kAssuming that kii >>k >>kdd and accounting for the fact that two and accounting for the fact that two
Radicals are formed during every initiator decomposition,Radicals are formed during every initiator decomposition,The rate of initiation, RThe rate of initiation, Rii, is given by:, is given by:
RRii = = d[Md[Mii]] = 2fk = 2fkdd[[II]]
dtdt
f = efficiency of the initiator and is usually 0.3< f >0.8f = efficiency of the initiator and is usually 0.3< f >0.8
We assume that theWe assume that thereactivity of the growingreactivity of the growingchain is independent of thechain is independent of thelength of the chain.length of the chain.
04/07/23 46
3. Termination3. Termination
MMxx.. + + .. M Myy M Mxx-M-Myy ((Combination)Combination)
MMxx.. + + .. M Myy M Mxx + M + Myy ((Disproportionation)Disproportionation)
kktctc
kktdtd
Since two radicals are consumed in every termination, Since two radicals are consumed in every termination, then:then:
RRtt = 2k = 2kt t [M [M ..]]22
4. Steady State Assumption4. Steady State Assumption
Very early in the polymerization, the concentration of Very early in the polymerization, the concentration of radicals becomes constant because radicals becomes constant because RRii = R = Rtt
2fk2fkdd[[II] = 2k] = 2kt t [M [M ..]]22
04/07/23 47
2fk2fkd d [[II] = 2k] = 2kt t [M [M ..]]22
Solve this equation for [MSolve this equation for [M ..]:]:
[M[M ..] = (fk] = (fkd d [I]/k[I]/ktt))1/21/2
Substituting this into the propagation expressionSubstituting this into the propagation expression::
RRpp = k = kpp[M[M ..][M] = k][M] = kpp [M](fk [M](fkd d [I]/k[I]/ktt))1/21/2
Since the rate of propagation, RSince the rate of propagation, Rpp, is essentially the, is essentially the
rate of polymerization, the rate of polymerization israte of polymerization, the rate of polymerization isproportional to proportional to [I][I]1/21/2 and and [M][M]..
04/07/23 48
5. Kinetic Chain Length, 5. Kinetic Chain Length,
Definition – The average number of monomer unitsThe average number of monomer unitspolymerized per chain initiated. This is equal to thepolymerized per chain initiated. This is equal to theRate of polymerization per rate of initiation:Rate of polymerization per rate of initiation:
RRpp/R/Rii = R = Rpp/R/Rtt under steady state conditions.under steady state conditions.
kkpp[M][M[M][M..]] = =
kkpp[M][M]
2k2ktt[M[M..]]22
2k2ktt[M[M..]] = __= __kkpp[M][M]______
2(f k2(f kttkkdd[I])[I])1/21/2
will will decreasedecrease with increases in with increases ininitiator concentration orinitiator concentration orefficiency.efficiency.
DP = DP = if termination is exclusively by disproportionationif termination is exclusively by disproportionation..
DP = 2DP = 2if termination is exclusively by coupling.if termination is exclusively by coupling.
04/07/23 49
6. When Chain-transfer is Involved6. When Chain-transfer is Involved
When chain-transfer in involved, the kinetic chainWhen chain-transfer in involved, the kinetic chainlength must be redefined.length must be redefined.
CT agentCT agent No EffectNo Effect DecreasesDecreasesInhibitorInhibitor Decreases (stops!)Decreases (stops!) DecreasesDecreasesCT to PolyCT to Poly No EffectNo Effect IncreasesIncreasesTemperatureTemperature IncreasesIncreases DecreasesDecreases
04/07/23 51
Thermodynamics of Free Radical PolymerizationThermodynamics of Free Radical Polymerization
GGpp = = HHpp - T - TSSpp
HHpp is favorable for all polymerizations and is favorable for all polymerizations and SSpp
is not! However, at normal temperatures, is not! However, at normal temperatures, HHpp
more than compensates for the negative more than compensates for the negative SSpp term. term.
The The Ceiling TemperatureCeiling Temperature, T, Tcc, is the temperature above, is the temperature above
which the polymer “depolymerizes”.which the polymer “depolymerizes”.
At TAt Tcc , , GGpp= 0. = 0. HHpp - T - Tcc SSpp = 0 = 0
HHpp = T = Tcc SSp p TTcc = = HHpp/ / SSpp
04/07/23 52
Thiol-ene Polymerization: A Brief IntroductionThiol-ene Polymerization: A Brief Introduction
hiols (mercaptans) can react with any “-ene”; anyhiols (mercaptans) can react with any “-ene”; anydouble bond. After all, they ARE chain-transferdouble bond. After all, they ARE chain-transferagents!agents!
They serve as a “bridge” between step-growthThey serve as a “bridge” between step-growthand chain-growth polymerization processes becauseand chain-growth polymerization processes becausethey use free radicals in a step-growth polymerizationthey use free radicals in a step-growth polymerizationprocess.process.
If either thiol or ‘ene’ is only monofunctional, noIf either thiol or ‘ene’ is only monofunctional, nopolymerizations will take place. The thiol will servepolymerizations will take place. The thiol will serveas a chain-transfer agent and a standard free radicalas a chain-transfer agent and a standard free radicalpolymerization of the ‘ene’ will take place. If the polymerization of the ‘ene’ will take place. If the If the mole ratio of thiol to ‘ene’ is close to one, noIf the mole ratio of thiol to ‘ene’ is close to one, noEffective polymerization will take place.Effective polymerization will take place.
If both are difunctional and in stoichiometricIf both are difunctional and in stoichiometricbalance, a linear polymer will form.balance, a linear polymer will form.
In order to get a crosslinked thiol-ene polymer, theIn order to get a crosslinked thiol-ene polymer, thethiol must be at least trifunctional.thiol must be at least trifunctional.
04/07/23 54
The process begins with a hydrogen abstraction fromThe process begins with a hydrogen abstraction fromthe thiol – a very rapid process – to form a ‘thiyl’the thiol – a very rapid process – to form a ‘thiyl’radical:radical:
(HS)(HS)22-R-SH + -R-SH + .. In In (HS) (HS)22-R-S -R-S .. + H-In + H-In
