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Chain-growth Polymerization
by Dr. Walaiporn Prissanaroon-Ouajai
Dept. of Industrial Chemistry KMUTNB
411317 Polymer Chemistry (updated 2/2552)411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
2
Chain-growth polymerization Ø formation of polymers via chain reaction
Key factors for chain-growth polymerization
Ø monomersØ initiator (to break π-bond)
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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Monomers for chain-growth polymerization
Aldehydeor ketone
Alkene(olefins & vinyl monomers)
except
Acetylene
H2C C C CH2
H XH2C C C CH2
H H
H2C C C CH2
H Cl
HC CH
Ring-opening polymerization
Diene
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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Mechanisms of chain-growth polymerization1. Initiation
2. Propagation
3. Termination (Dependent on type of active center)
Propagating chain(polymer chain with active center)
Addition polymerization
Active center = +
Active species (initiator fragment with active center, can be +, - or radical)
Active center = radical
Active center = -
PolymerChain transferring agent Dead chain
(polymer chain without active center)
Active center
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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Summary Mechanisms of polymerization for polyethylene
R R
Degree of polymerization (DP, Xn) = number of monomer unit
in a polymer chain
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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Characteristics of Chain-growth Polymerization1. การเพิ่มของ MW จะเปนไปอยาง
รวดเร็วในชวงแรกของปฏิกิริยา แตเมื่อเวลาผานไป MW จะเปล่ียนแปลงนอย
2. MW α time แต จํานวนโมเลกุล α time
3. ปฏิกิริยาจะเกิดเร็วมาก (10-1-10-6s) และ DP จะเพิ่มขึ้นอยางรวดเร็วในชวงตนปฏิกิริยา (High MW at low conversion)
4. Monomer จะคอยๆ ลดลงอยางชาๆ5. ไมมี by-product
time
MWTime to reach 106 MWPS ~ 7.6s , PMMA ~1.5s, PVC ~0.13s
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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Some comparisons between Step-growth and Chain-growth polymerizations Step-growth Chain-growth
6. Mn of polymer High Mn at high conversion(Mn α time)
High Mn at low conversion(Mn α time but n α time)
2. [M] with time Immediately disappeared
1. Monomer type Contain at least 2 functionalities Contain unsaturated bond
Gradually decrease
3. Reactivity Reactivity of functional end group is independent on size of polymer
Reactivity of active centre decreases With longer polymer chain
5. Mixture composition during reaction
Dimer, oligomer, polymer and trace monomer (< 1%)
Monomer and polymer with high Mn
time
Mnor
X n
Chain
Step
Xn Xn
Wt. fr
actio
n
Wt. fr
actio
n
“PER”
Step Chain
4. Rate Growth of chains is usually slow (minutes to days)
Chain growth is usually very rapid (<sec)
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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Free Radical PolymerizationMonomer for free radical polymerization
Most alkenes
where R = neutral group or some e-withdrawing character
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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Initiator for free radical polymerization
sometimes called "catalysts"
Ø a source of free radicalsØ radicals must be produced at an acceptable rate at convenient
temperaturesØ have the required solubility behaviorØ transfer their activity to monomers efficientlyØ be amenable to analysis, preparation, purification
Requirements for an initiator
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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1. Organic peroxides or hydroperoxides
Cumyl hydroperoxide
2. Azo compounds
Examples of free radical initiation reactions
Benzoyl peroxide (BPO)
2,2'-Azobisisobutyronitrile (AIBN)
Ø Low dissociation energy of the O-O bondØ But reagents are unstable.
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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3. Redox systems
4. Electromagnetic radiation
Redox initiator = Initiator + Reducing agent
hydrogenperoxide
persulfate
Ø Soluble in water (can also work in organic solvents)
Ø Low dissociation energy then can proceed at relative low Temp → reduce side effect
Ø photochemical initiation involves the direct excitation of the monomer or photolytic fragmentation of initiators
Ø photochemical initiators include a wider variety of compounds411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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Free Radical Initiator Efficiency
Ø reactive species can undergo as alternatives to adding to monomers to commence the formation of polymer
Ø two radicals are trapped together in a solvent (cage) resulting in “direct recombination”
2-cyanopropyl radicals from AIBN acetoxy radicals from acetyl peroxide
benzoyloxy radicals from BPO
Solvent Cage
Reduce free radical efficiency
(the efficiency with which these radicals initiate polymerization)
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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Reactions between the initiator radical and the solvent
For example, carbon tetrachloride is quite reactive towards radicals because of the resonance stabilization of the solvent radical produced.
Ø These species are less reactive than the initiator radicalsØ These species can be recombined with the initiator radicals
Reduce free radical efficiency
Terminate polymerization via chain transfer reaction
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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Ø f depends on the conditions of the polymerization, including the solvent.Ø In many experimental situations, f = 0.3-0.8.
Free Radical Initiator Efficiency (f)radicals incorporated into polymerradicals formed by initiatorf =
Note: f should be monitored for each system studied.Evaluation of initiator efficiency1. Direct method - End-group analysis
Limitation: difficult in addition polymers (very higher MW than condensation polymers)
R Rn
2. Indirect method – Reaction with scavengers
diphenylpicrylhydrazyl radicals411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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1. InitiationMechanism of Free radical Polymerization
Step1: Dissociation of initiator
Step2: Reaction of radical with 1st monomer
In case of asymmetry monomer Ex.
There are 2 possible ways for the reaction of radical to 1st monomer
Part I is higher possibility (low Ea)and radical can resonance with X group
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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2. Propagation
Again! for
there are 2 possible ways for the reaction of propagating chain to next monomer
head-to-tail configuration, H-T
head-to-head configuration, H-H
headtail
Part I (H-T) is higher possibility and more stable411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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3. TerminationØ two propagating chains are deactivated, resulting in dead polymer
Two principal modes of termination
Little monomer is left
Low efficiency of active centresin long propagating chains
Reasons for termination
1) Combination or Coupling (connect two active centers)
2) Disproportionation (transfer an atom (normally H) from one propagating chain to another)
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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Ø produce one polymer chain with single "head-to-head" linkage
Ø a polymer chain contains two initiator fragments (R) per molecule
Ø higher average MW
Comparison of termination by coupling and disproportionationCoupling Disproportionation
Ø produce two polymer chains Ø one polymer chain contains double bond
and another contains only single bond Ø each polymer chain contains one initiator
fragments (R)Ø lower average MW
Note: - Since the disproportionation requires bond breaking, Etd > Etc- Coupling occurs at lower temperature.
Examples: At 60 °C polyacrylonitrile → 100% couplingpoly(vinyl acetate) → 100% disproportionationPS and PMMA → both processes
H
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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Kinetics of Free radical Polymerization1. InitiationStep1: Dissociation of initiator
Step2: Reaction of radical with 1st monomer
where kd = Rate constant for dissociation of initiator
where ka = Rate constant for formation of active center
If f = free-radical efficiency
“Rate determining step”
1/2 Rate of radical formation = Rate of initiator dissociationFrom differential rate law
(for most initiators Ex. Peroxide, azo)
Ri = d[ R. ] = 2 f kd [I] dt
+ 1 d[R.] = - d[I] = kd[I]2 dt dt
Rate of initiation (Ri) = Rate of radical formation
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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kd and activation energies (Ed) for some initiator decomposition reactions.
Data from J. C. Masson
Effect of temperature on Ri
k = Ae (-E*/RT)
ln k = ln A – (E*/RT)ln kd1 = – E* 1 - 1
kd2 RT T1 T2
Arrhenius equation Evaluation of kd at different temperature
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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+ 1 d[R.] = - d[I] = kd[I]02 dt dt
Evaluation of kd
d [I] = - kd dt[I]0
d [I] = - kd dt[I]0∫ ∫
t=0
t=t
t=0
t=t
ln [I] = - kd t[I]0
where [I] = concentration of initiator at t = t[I]0 = concentration of initiator at t = 0
time
ln [I][I]0
Slope = -kd
Assume f = 1
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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2. Propagation
where kp = Rate constant for propagation
Assumption: kp is a constant independent of the size of the growing chain (same kp for every propagation steps)
3. Termination
where ktc = Rate constant for termination by couplingktd = Rate constant for termination by disproportionation
Rp = d[RMn. ] = kp[RMn-1
.][M] = kp[RMn.][M]
dt
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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3. Termination
where ktc = Rate constant for termination by couplingktd = Rate constant for termination by disproportionation
where kt = ktc + ktd
From differential rate law
- 1 d[RMn.] = kt[RMn
.]22 dt
Rt = d[RMn. ] = 2 kt[RMn
.]2
dt
Rate of termination (Rt) = Rate of RMn. reduction
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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Rp = kp[RMn.][M]0
In propagation step
Stationary state radical concentration [RMn.]
At the beginning of polymerization Ri >> Rt
After a period of time
Ri = Rt
Lots of RMn. are formed in Initiation step whereas
lots of RMn. are disappeared in termination step
Total radical concentration [RMn.] becomes constant stationary state
2 f kd[I] = 2 kt[RMn.]2
[RMn.] = [f kd [I] ]
kt
1/2
∆[RMn.] = 0
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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Rp = kp[RMn.][M] [RMn
.] = [f kd [I] ]kt
1/2but
therefore Rp = kp f kd [I] [M]kt
1/21/2
Overall Rate of Polymerization (Rpol ) α Rp
Rpol = K [I]0 [M]01/2Initial rate of polymerization
a) Effect of [I] on Rpol ; Rpol α [I]1/2
b) Effect of [M] on Rpol ;- if free radicals have very high efficiency (f→1) and do not depend on [M]
Rpol α [M]- if free radicals have low efficiency (f→1) and depend on [M]
Rpol α [M]3/2
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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Rpol = K [I] [M]1/2
log Rpol = log K + 1/2 log [I] + log[M]
(b) [AIBN] in MMA (l) [BPO] in styrene (n)[BPO] in MMA (p)at constant [M]
Log-log plots of Rp versus concentration which confirm the kinetic order.
(a) [MMA] varied at constant [I]
a) Data from T. Sugimura and Y. Minoura, J. Polym. Sci. A-1, 2735 (1966)b) Data from P. J. Flory, Principles of Polymer Chemistry, copyright 1953 by Cornell University,
Slope = 1
Slope = 1/2
a) log Rpol = (log K+1/2 log [I]) + log[M]b) log Rpol = (log K+log[M]) + 1/2 log [I])
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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Rate constants at 60 °C and activation energies for some propagation and termination reactions
Data from R. Korus and K. F. O’Driscoll
overall values
kp/(kt)1/2 = polymerizability (or ability of monomer to be polymerized)
Rp = kp f kd [I] [M]kt
1/21/2
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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Rp = kp f kd [I] [M]kt
1/21/2
Rp = -d[M] = kp f kd [I] [M]dt kt
1/2 1/2
d[M] = - kp f kd [I] dt[M] kt
1/2 1/2
d[M] = - kp f kd [I] dt[M] kt
1/2 1/2∫
t=0
t=t ∫t=0
t=t
ln [M] = - kp f kd [I]0 t[M]0 kt
1/2 1/2
where [M]0 and [I]0 = [M] and [I] at t = 0
Evaluation of [M] at any time
[RMn.] = Ri
2 kt
1/2
Ri = Rt = 2 kt[RMn.]2
At stationary state
Rp = kp Ri [M]2kt
1/2
Rp = kp (Ri) [M](2kt)1/2
1/2
Evaluation of Rp when Ri is known
Rpol α (Ri)1/2
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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Mean kinetic chain length : ν
ν = number of monomers added into active centers = - d[M]/dt = Rpnumber of active centers - d[I]/dt Ri
number of monomer moleculespolymerized per chain initiated
At stationary-state condition, Ri = Rt ν = Rp =Rt
Number-average degree of polymerization (Xn)
1. Coupling
2. Disproportionation
Xn = 2 ν
Xn = ν
Assume f = 1 and no chain transfer reactions
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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Summary: Effect of [M], [I] and their natures on Rpol and ν
• In the same system of M and I- high Rpol and high MW polymer result from high [M]- high Rpol and low MW polymer result from high [I]
• kp/(kt)1/2 (polymerizability) tells the ability of monomer to be polymerizedAt 60oC kp/(kt)1/2 for MMA = 0.678, kp/(kt)1/2 for styrene = 0.0213
ν of PMMA > ν of PS (32 times) when same I (same kd), [I] and [M] are used
• Initial Rpol and ν can be evaluated when [I]0 and [M]0 are given
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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Average Radical lifetime, τØ Average time of radical exists in polymerizationØ Average time elapsing between formation and termination of active centers
τ = concentration of active center = [RMn.]
rate of loss of active centers Rt
= [RMn.] = 1
2kt [RMn.]2 2kt [RMn
.]
but Rp = kp[RMn. ][M] or [RMn
.] = Rpkp[M]
kp f kd [I] [M]kt
1/21/2
Ri = 2 f kd [I]
Ø Evaluate kp by measuring τ and Rp with known [M]τ = kp[M]2 kt Rp
Ø τ depends on nature of I (Kd) and [I], not [M]
Ø Evaluate kt by measuring τ and Ri
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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Example The polymerization of ethylene at 130 °C and 1500 atm was studied using different concentrations of the initiator, 1-t-butylazo-1-phenoxycyclohexane. The rate of initiation was measured directly and radical lifetime were determined using the rotating sector method. The following results were obtained, Evaluate kt.
(data from T. Takahashi and P. Ehrlich, Polym. Prepr., Am.Chem. Soc. Polym. Chem. Div. 22, 203 (1981)).
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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“Trommsdorff effect”
Auto-acceleration Rpol α [M]0
Rpol = kp f kd [I] [M]kt
1/21/2
Reprinted from G. V. Schulz and G. Harborth, Makromol. Chem. 1, 106 (1948).
Acceleration of the polymerization rate for different [MMA]0 in benzene at 50 oC
“Gel effect”
At low [M]0Effect of [M]0 on conversion → 1st order
(indica
teR po
l)
At high [M]0 (> 40%)Effect of [M]0 on conversion > 1st order
high [M]0 → high initial Rpol→ high viscosity of medium → Difficult to terminate(kt decreases)
Large increase in both Rpol and ν
At low conversion
Note: [M] = 100% → Bulk polymerization (no solvent)
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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Problem of auto-acceleration
generally, ∆H = 10 - 30 kcal/molMost of free radical polymerizations are “exothermic reaction”
Solving1. stop reaction before gel effect2. reduce medium viscosity by
adding solvent
Large Rpol → large released heatØ Explosion if poor venting systemØ High MWD
Mole fraction of i-mers as a function of Xi for termination by combination for various values of p.
p = %conversion
Xi
Mole
fracti
on
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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1. Transfer to monomerDifferent types of CTR (depend on chain transferring agent)
2. Transfer to initiator
3. Transfer to solvent
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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Mean kinetic chain length : ν in the presence of CTR
Assume f = 1
Terminations includeCouplingDisproportionationCTR
νtr= Rp = RpΣRt Rt + Rtr, M + Rtr, I + Rtr, S
ktr
tr
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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“Mayo-Walling Equation”
1. Coupling + CRT
2. Disproportionation + CRTXn = 2 νtr
Xn = νtr
Evaluation of Xn in the presence of CRT
Reversetr
tr
where (ν)o = ν without chain transferνtr = ν with chain transfer
CM , CI , CS = Rate constants for chain transfer to monomer, initiator,and solvent, respectively
1 = 1 + ∑Ctr x [CT agent]νtr νo [M]
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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Evaluation of chain transfer constants
1 = 1 + CS x [S]νtr νo [M]
Assume: CRT to M and I are ignored
1ν
X 105
[S][M]
Effect of CTR to solvent for PS at 100 oC.
Data from R. A. Gregg and F. R. Mayo, Discuss. Faraday Soc., 2, 328 (1947).
1ν0
Slope = Cs
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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ØEasy to processØDesirable for particular applications such as lubricants or plasticizers
Controlling Xn of polymer by CTR
CTR reduces MW →
solvent or CT agent is chosen and its concentration selected to produce the desired value of ν
1 = 1 + CS x [S]νtr νo [M]
1 = 1 + ∑Ctr x [TR agent]νtr νo [M]
Mercaptans (R-SH) have particularly large Ctr for many common monomersand are especially useful for molecular weight regulation. Ex. At 60°C, styrene has Ctr for C4H9-SH = 21 (107 times > Ctr for C6H6 at 60oC)
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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Chain transfer to polymer1) Inter-molecular chain transfer
Polymer side chain branching(Graft copolymer)
Monomer
M-M-M-M-M
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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graft copolymers have polymeric side chains which differ in the nature of the repeat unit from the backbone.
Graft copolymerization
polybutadiene PS radical
Ex. Butadiene-styrene copolymer (SBS) =High impact PS (HIPS)
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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2) Intra-molecular chain transfer
“Back-biting”
Ø Long chain branching can occur at high pressure to produce LDPE.
Short chain branching (normally ethyl or butyl group)
0.941 g/cm3, low degree of branching
0.910–0.940 g/cm3, high degree of chain branching
0.915–0.925 g/cm3, significant numbers of short branches (higher tensile strength and higher impact than LDPE).
Common types of PE
411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)
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Suppressing polymerization
1. Inhibition:
2. Retardation:
Commercial monomers are required to prevent their premature polymerizationduring storage by adding either “retarders” or “inhibitors”
depending on degree of protectionblocks polymerization completely until it is removed
slows down polymerization process by competing for radicalsLess protection efficiency
Hydroquinone
Nitrobenzene411317 by Dr. Walaiporn Prissanaroon-Ouajai (IC-KMUTNB)