Table 6. 1 Commercially Important Vinyl Polymers
Chapter 6. Free Radical Polymerization
6.1. Introduction
Table 6. 1 Commercially Important Vinyl Polymers
6. 2 Free Radical Initiators
Four types Peroxide s and hydroperoxide s Azo compounds Redox initiators
Photoinitiator s
6.2.1. Peroxides and Hydroperoxides ( ROOR )
( ROOH )
Thermal homolysis
Radical combination
Wastage reaction
∵ Cage effect
2 Ph
Ph Ph
(confining effect of solvent molecules)
Induced decomposition Wastage reaction
O Ph
Ph
C Ph + Ph
Ph
C Ph
Radical stability
CH
3 CH 3 O
Ph C O > CH C O > 3 Ph CO > CH 3 C O CH 3 CH 3 Unstable radical
Stable radical More wastage reaction Less wastage reaction
Half-lifes BPO (Benzoyl peroxide)
1.38 h at 145 C
CH 3
Ph C O O H CH 3 C O O C CH 3 CH 3
1.7 h at 130 o C 1.1 h at 85 C
No initiation of polymerization Initiation of polymerization
∵ No N in polymers
6.2.2. Azo Compounds
(CH 3 ) 2 C N N C(CH )
2 (CH 3 ) 2 C + N 3 2 2
AIBN (Azobisiso(butyronitrile))
Radical combination
(CH 3 ) 2 C C(CH 3 ) 2
2 (CH 3 ) 2 C
CN (CH 3 ) 2 C C N C(CH 3 ) 2
(CH 3 ) 2 C (CH 3 ) 2 C
6.2.3. Redox Initiators Production of free radicals by one–electron transfer reactions. Useful in low–temp polymerization and emulsion polymerization
OOH
3+
Ph 2+ C(CH ) Fe
3 2 + Fe Ph C(CH ) 2 + OH 3 +
2+
3+
+ Fe HO + OH + HOOH Fe
2-
2-
3 SOOSO
SO
3 2 O 3 4 + SO 4 + S 2 O 3
Radicals in aqueous phase
The major advantages of photoinitiation: The reaction is essentially independent of temp Polymerization may be conducted even at very low temperatures Better control of the polymerization reaction
The reaction can be stopped simply by removing the light source
6.2.5. Thermal Polymerization
Diels-Alder dimer Transfer of H atom to monomer
Molecule-induced homolysis
Rapid formation of radicals by reaction of nonradical species
6.2.6. Electrochemical Polymerization
RCH CH 2 + e RCH CH 2
RCH CH + e RCH CH
Radical ions : initiate free radical or ionic polymerization or both
Particularly useful for coating metal surfaces with polymer films
6. 3 Techniques of Free Radical Polymerization Techniques
Method Advantages Disadvantages
Bulk Simple Rxn exotherm difficult to control No contaminants added
High viscosity
Suspension Heat readily dispersed Washing and drying required
Low viscosity
Agglomeration may occur Obtained in granular form Contamination by stabilizer
Solution
Heat readily dispersed
Added cost of solvent
Low viscosity
Solvent difficult to remove Used directly as solution Possible chain transfer with solvent Possible environmental pollution
Emulsion
Heat readily dispersed
Contamination by emulsifier
Low viscosity
Chain transfer agents often needed
High MW obtainable
Washing and drying necessary
Used directly as emulsion. Works on tacky polymers
6. 4 Kinetics and Mechanism of Polymerization
Initiation
Formation of the initiator radical Addition of the initiator radical to monomer
Coupling or combination Termination
CH 2 CH HC H 2 C
CH 2 CH + HC H 2 C
YY
Disproportionation
CH CH + H 2 C H 2 C
Coupling almost exclusively at low temp Polystyrene
CH CH + HC H 2 C CH 2 CH HC 2 H 2 C
Disproportionation PMMA
∵ Steric repulsion
Electrostatic repulsion Availability of αH for H transfer PMMA 5; PSt 2
Initiation
Initiator
Propagation
M (x+1)
Termination
k tc
M (x+y)
Coupling
k td
M x + M y Disproportionation
Initiation rate (R i )
d [] M •
= 2 fk d [] I
dt
where [M•] = total conc of chain radicals [I] = molar conc of initiator
f = initiator efficiency = 0.3 ~ 0.8
Termination rate (R t )
d [] M •
= 2 k t [] M •
dt
Steady-state assumption
R i =R t
2 fk d [] I
2 fk d [] I = 2 k t [] M • [] M • =
Propagation rate (R p )
d [] M
fk d [] I
= k p [ ][ ] M • M = k p [] M
R p ∝ [][] I , M
dt
Average kinetic chain length ( ) ν
= Average # of monomer units polymerized per chain initiated
R p R p k p [ ][ ] M • M k p [] M k p [] M
R t 2 k [] M • 2 k t [] M •
2 ( fk t k d [] I ) 2
DP =
Disproportionation
2 ν Coupling
• Gel effect , Trommsdorff effect, Norris-Smith effect
Occurs in bulk or concentrated solution polymerizations
Chain mobility↓
[M•] ↑
Release of more heat
Autoacceleration
Explosion
Polymerization of MMA in benzene 50°C benzoylperoxide
Gel effect
• Chain transfer reactions
Transfer of reactivity from the growing polymer chain to another species
1) C.T. to Polymer Chain-end radical abstract a H atom from a chain
Chain branching
2) Backbiting : intramolecular chain transfer
3) C.T. to Initiator or Monomer
Important with monomers containing allylic hydrogen , such as propylene
Resonance-stabilized radical
Reactive radical
∴ High-mol-wt polypropylene cannot be prepared
CH 3 CH 3
CH 2 C + CH 2 C allylic hydrogen
COOCH 3 COOCH 3
Propagation
C.T. to monomer
COOCH 3 O 3
COOCH 3 COOCH 3
Resonance-stabilized radical Resonance-stabilized radical
4) C.T. to Solvent or Chain transfer agents CCl 4
Thiol (RSH)
Transfer reaction rate
R tr = k tr [ ][ ] M • T
where T = transfer agent Kinetic chain length
k p [] M
ν w/o transfer agent =
2 k t [] M •
k p [ ][ ] M • M
k p [] M
ν tr =
R t + R tr 2 k t [] M • + k tr [ ][ ] M • T 2 k t [] M • + ∑ k ∑ tr [] T
with transfer agent
1 2 k t [] M • + k tr [] T 1 ∑ k ∑ tr [] T 1 ∑ C T [] T
k p [] M
k p [] M ν
[] M
ν tr
k tr
= C T Chain transfer constant
1 1 C T [] ∑ T
= + ν tr ν
[] M
Table 6.5 C T for St and MMA Benzene
Strong C-H bond
Low C T
Toluene
Higher C T CHCl 3
Benzylic H
Resonance stabilization
CCl 4 CCl 3 •
CBr 4 CBr 3 • Weak S-H bond
Large C
RSH
When [T] is high ,k tr >> k p ⇒
Very-low-mol-wt polymer
Telomerization
Telomer
CH 3 CH 2 CH 2 CH 2 CH 2 -H
Cl
C Cl Cl
C Cl
Cl
C Cl
Cl
C Cl
Cl
Cl
Cl
Cl
ο Inhibitors Added to monomers to prevent polymerization
during shipment or storage Must be removed by distillation of monomer or extraction
Alkylated Phenol
OR
OH
R' . +
R'H +
OR'
R' . R
'R
R R'
Inhibitor
No inhibitor
% Polymerization More inhibitor
Time
Induction period
Induction periods are common even with purified monomer because of the presence of oxygen , which is, itself, an inhibitor .
ο Living free radical Polymerization No chain termination or chain transfer
o Atom Transfer Radical Polymerization (ATRP) Polymerization of styrene
Transfer of Halogen atom
. + Cu(II)(bpy)Cl CH 3 CHCl + Cu(I)(bpy)
CH 3 CH Ph
Ph
Preventing radical termination
H 2 C CH propagation reactions from occurring
Ph
CH 3 . CHCH 2 CH
CH 3 CHCH 2 CHCl + Cu(I)(bpy)
+ Cu(II)(bpy)Cl
bpy = bipyridyl = bpy = bipyridyl =
TEMPO (2,2,6,6- te tra m ethyl p iperidinyl-1- o xy) Too stale to initiate polymerization Promotes decomposition of BPO to
benzoyloxy radicals
Propagation Prevent termination
[] M o
DP =
[] I o
All the chains are initiated at about the same time . No chain termination or transfer reactions .
The chains all grow to approximately the same length . Polydispersity is low.
ο Emulsion Polymerization
Smith-Ewart kinetics
R p = k p [] M ⎜ ⎟
where [M] = concentration of monomer in the polymer particle
N= # of polymer particles
Rate of radical entry into the particle
Average kinetic chain length
k p [] M k p [] M k p [] M N
2 fk d [] I
DP = ν
( a s [] E )
where k = constant (0.37~0.53)
µ = rate of increase in the volume of a polymer particle
[E] = concentration of micellar emulsifier
a s = interfacial area occupied by an emulsifier molecule
in the micelles
N ∝ [][] I , E R p ∝ [][] I , E DP = ν ∝ [] I , [] E
6.5. Stereochemistry of Polymerization Steric and electrostatic interactions are responsible for stereochemistry
of free radical polymerization 1) Interaction between Terminal unit & Monomer
Mirror approach
Isotactic polymer
Nonmirror approach
Syndiotactic polymer
2) Interaction between Terminal unit & Penultimate unit
P: bulkiest group Syndiotactic
If terminal carbon has sp 2 planar structure,
Interaction between terminal unit & monomer is dominant factor
If terminal carbon has sp 3 hybridization,
Interaction between penultimate unit & terminal unit is dominant factor
Stereoregularity ↑ as T ↓
Free radical polymerization of MMA at T<0 o C
Crystalline polymer Syndiotactic
(Expect) As size of substituent groups ↑ , stereoregularity ↑
CH 3 CH 2 C CO
O CH 2
(Experiment) bulkier substituent Less syndiotactic than PMMA under the same conditions
6.6 Polymerization of Dienes
6.6.1. Isolated Dienes Cooperative addition of one double bond to the other of the monomer
Cyclic polymer
5- or 6-membered ring
Cyclopolymerization : Formation of a cyclic structural unit in a propagation step
Independent reaction
Crosslinked polymer
Cyclopolymerization of divinylformal
Head-to-tail mode head-to-tail mode Six-membered ring six-membered ring
OO
Head-to-head mode head-to-tail mode Five-membered ring
five-membered ring
Diallyl monomer Highly crosslinked polymers
Used for manufacturing
electrical or electronics items ( circuit boards , insulators ,
C television components , etc) preimpregnating glass cloth or fiber for
fiber-reinforced plastics
Diallyl phthalate
Used for applications requiring
good optical clarity
2 ( eyeware lenses , camera filters , panel covers , and the like)
Diethylene glycol bis( allyl carbonate )
6.6.2. Conjugated Dienes
H H H C CH
C CH CH 2 1,3-butadiene
trans CH 2 CH CH 2 CH 2 CH 2 H
Pendant vinyl groups
unsaturation in the chain
Isoprene (2-methyl-1,3-butadiene)
trans CH 2 CH CH 2 CH CH 2 CH 2 CH 2 H
trans-1,4-addition 1,2-addition
3,4-addition
cis-1,4-addition
Natural rubber
As T↑ , cis-1,4 ↑
As T↑ , cis-1,4 ↑
CH 2 Sn(n-C 4 H 10 )
3 ~ 100% at 60 C
Table 6.6 Structures of Free Radical-Initiated Diene Polymers
percent Polymerization
43 39 18 - Isoprene
12 77 2 9 Chloroprene
46 10 81-86
6.7 Monomer Reactivity
k p : -C 6 H 5 < -CN < -COOCH 3 < -Cl < -OCOCH 3
o Three factors in monomer reactivity
1) Resonance stabilization of free radicals : k p ↓
-C 6 H 5 > -CN > -COOCH 3 > -Cl > -OCOCH 3 Reactive monomer
Stable monomer Stable radical
Reactive radical Inverse relationship between monomer reactivity and k p
Radical reactivity is the major factor.
2) Polarization of double bond by substituents: k p ↓
Extreme polarization prevents free radical polymerization e.g.
CH 2 CH
CN
No radical polymerization
Cationic polymerization
Heterolytic fission Homolytic fission
3) Steric hindrance : k p ↓
CH CH CH CH
CH 2 CH > CH 2 C >
COOCH 3 MA
MMA
∵ Resonance stabilization of the radical by hyperconjugation and steric hindrance
Overlap of p-orbital with σ bonds Overlap of p-orbital with σ bonds
favorable
∆S <0: monomer → covalently bonded chain structure
unfavorable ∴ Degree of freedom (randomness) ↓ (1) The higher the resonance stabilization of monomer , l∆Hl ↓
(2) Steric strain in polymer , l∆Hl ↓ (3) Decrease in H-bonding or dipole interaction on polymerization , l∆Hl ↓
Resonance Stabilization
∆H
of monomer Steric strain Decrease in H-
bonding or dipole
Table 6.7 ∆H and ∆S of Polymerization
-△H
-△S
(kJ/mol) (J/mol) Acylonitrile
75 101 Methyl methacrylate
- Vinyl acetate
90 - Vinyl chloride
71 - 71 -
where
M x +M M (x+1) k dp = depropagation rate constant
k dp
k dp
k p [M]
k p [M] - k dp
R p =R dp
At T c
k p [ ][ ] M • M = k dp [] M •
Equilibrium constant
[] M •
[ ][ ] M • M e [] M e k dp
where [M] e = equilibrium monomer concentration ∆G = ∆G o + RT ln K
where
∆G o = ∆G in standard state ( pure monomer or 1-M solution monomer ,
pure polymer or 1-M repeating units of polymer) At equilibrium
ln [] M e =
∆ S + R ln [] M
or
RT c R RT c R
H ∆ RT
ce
TM
ln
Table 6.8 T c of Pure Liquid Monomers
[] M e
Methyl methacrylate
-△H = 35 kJ/mol T o
c = 66 C
Resonance stabilization Steric strain in polymer
6.8 Copolymerization
Reaction Rate
k 11
M 1 • +M 1 M
k 11 [M 1 •] [M 1 ]
k 12
M 1 • +M 2 M
k 12 [M 1 •] [M 2 ]
k 21
M 2 • +M 1 M
k 21 [M 2 •] [M 1 ]
k 22
M 2 • +M 2 M
k 22 [M 2 •] [M 2 ]
where k 11 and k 22 : self-propagation rate constant k 12 and k 21 : cross-propagation rate constant where k 11 and k 22 : self-propagation rate constant k 12 and k 21 : cross-propagation rate constant
d [] M 1
k 21 [ ][ M 1 M 2 • ]
= k 11 [ M 1 • ][ ] M 1 + k 21 [ M 2 • ][ ] M 1 [ M 1 • ] =
k 12 [] M 2
dt
d [] M 2
= k 12 [ M 1 • ][ ] M 2 + k 22 [ M 2 • ][ ] M 2
dt
o Instant composition of copolymer
d [] M 1 [] M 1 ⎛ k 11 [ M 1 • ] + k 21 [ M 2 • ] ⎞
d [] M 2 [] M 2 ⎝ k 12 [ M 1 • ] + k 22 [ M 2 • ] ⎠
o Monomer reactivity ratio
k 11 k 22 = r 1 = r 2
k 12 k 21
o Copolymer (composition) equation
d [] M 1 [] M 1 ⎛ r 1 [][] M 1 + M 2 ⎞
d [] M 2 [] M 2 ⎝ [][] M 1 + r 2 M 2 ⎠ d [] M 2 [] M 2 ⎝ [][] M 1 + r 2 M 2 ⎠
1 = mole fraction of M 1 in the feed
[][] M 1 + M 2
f 2 = mole fraction of M 2 in the feed
d [] M 1
F 1 = mole fraction of M 1 in the copolymer
d [][] M 1 + d M 2
F 2 = mole fraction of M 2 in the copolymer
F [] M 1 f d 1
[] M 1 [] M 1 ⎛ r 1 [][] M + M ⎞
d [] M 2 1 − F 1 [] M 2 f 2
d [] M 2 [] M 2 ⎝ [][] M 1 + r 2 M 2 ⎠
F 1 d [] M 1 f 1 [] M 1
F 2 d [] M
f 2 [] M
d [] M 1 [] M 1 ⎛ r 1 [][] M 1 + M 2 ⎞
d [] M 2 [] M 2 ⎝ [][] M 1 + r 2 M 2 ⎠
⎟⎟ ⎝ f + r 2 ⎠
fF + r 2 F = r 1 f + f = r
⎜⎜ r ⎟⎟ 1
Finemann and Rose Equation r 2
-r 1
1) r 1 =r 2 =1 No preference for homopolymerization or copolymerization
Random copolymer
F 1 =f 1 e.g.
M 1 = ethylene M 2 = vinyl acetate
r 1 = 0.97
r 2 = 1.02
2) r 1 =r 2 =0 Alternating copolymer
F 1 = 0.5 e.g.
M 1 = styrene
M 2 = maleic anhydride
r 1 = 0.041
r 2 = 0.01
3) 0<r 1 ,r 2 <1 More common
e.g.
M 2 = methyl methacrylate r 1 = 0.52
M 1 = styrene
r 2 = 0.46
Azeotropic copolymerization
1 1 ⎛ r 1 [][] M 1 + M 2 ⎞
[] M [] M
d [] M
[] M 2 ⎝ [][] M 1 + r 2 M 2 ⎠
d [] M 2 [] M
r 1 [][] M 1 + M 2 [] M 1 1 − r 2 1 − r 2
[][] M 1 + r 2 M 2 [] M 2 1 − r 1 2 − r 1 − r 2
4) 1 << r 1 and r 2 << 1
Essentially homopolymer e.g.
M 1 = styrene
M 2 = vinyl acetate
r 1 = 55
r 2 = 0.01
M 1 = styrene
M 2 = vinyl chloride
r 1 = 17
r 2 = 0.02
5) r 1 • r 2 =1
Ideal copolymerization
k 11 k 21
cf. r 1 = r 2 = 1 Real random copolymerization
k 12 k
The more r 2
1 and r 2 diverge , the less random
e.g. r 1 f 1 + 2 f 1 f 2 + r 2 f 2 M = MMA
M 2 = vinyl chloride
r 1 = 10
r 2 = 0.1
cf. Ideal solution
Table 6.9 Reactivity Ratios
M 1 M 2 r 1 r 2 Temperature(℃) Styrene
Methyl methacrylate
Vinyl acetate
Styrene
Maleic anhydride
Vinyl chloride
Methyl methacylate
Vinyl chloride
Methyl methacylate
Vinyl acetate
Methyl methacylate
Acrylonitrile
Methyl methacylate
Vinyl acetate
1 << r 1 and r 2 << 1
r 1 • r 2 =1
r 1 =10, r 2 = 0.1
r 1 =r 2 =0 0<r 1 ,r 2 <1
r =r =1
Azeotropic composition
Q-e Scheme (Alfrey-Price Treatment)
k 12 =P 1 Q 2 exp (- e 1 e 2 )
where P 1 = reactivity of radical M 1 •
Q 2 = reactivity of monomer M 2
e 1 = polarity of monomer M 1
e 2 = polarity of monomer M 2
k 11 P 1 Q 1 exp ( − e 1 e 1 ) ⎛ Q 1 ⎞
exp [ − e
k 12 P 1 Q 2 exp ( − e 1 e 2 ) ⎝ Q 2 ⎠
exp − e e − e
Styrene : standard
Q = 1.00
e= - 0.80
Resonance factor
Polarity factor
(+ steric factor) + : electron-withdrawing group
Table 6.10 Reactivity (Q) and Polarity (e) of Monomer
Monomer
e 1-vinylnaphthalene
1.94 -1.12 p- Nitro styrene
1.63 0.39 p- Methoxy styrene
1.36 -1.11 Styrene
1.00 -0.80 Methyl meth acrylate
0.74 0.40 Acrylonitrile
0.60 1.20 Methyl acrylate
0.42 0.60 Vinyl chloride
0.20 Vinyl acetate
Little tendency
Resonance -stabilization No resonance -stabilization
M 1 = Styrene M 2 = Vinyl acetate
r 1 = 55
r 2 = 0.01
Styrene Q = 1.00 e = - 0.80 Vinyl acetate Q = 0.026 e = - 0.22
r = 0.46
M 1 = Styrene M 2 = MMA
r = 0.52
Styrene Q = 1.00 e = - 0.80
e = 0.40 Each radical has about twice as much tendency to react with
MMA
Q = 0.74
its opposite monomer
∵ Polar effect
Copolymerization of styrene and maleic anhydride Strong tendency toward alternation (r 1 and r 2 = ~ 0)
1) Polar effects in the transition state
Electron transfer
Nonbonded resonance form
Electron transfer
Nonbonded resonance form
2) Formation of Charge-transfer complexes between comonomers Homopolymerization of charge-transfer complex
~ (DA) -
n D .. A + D .. A ~ (DA) n+1 D .. A
charge-transfer complex <Evidence>
(1) R p maximum when monomer composition ratio = 1:1 maximum conc. of donor-acceptor complex
(2) Alternation is independent of monomer feed ratios, and other reactive monomers included with the feed fail to react while alternating copolymer is forming
(3) R p is enhanced by addition of Lewis acids , which increase the acceptor properties of one of the monomers
(4) Chain transfer agents have little effect on the mol. wt. of the copolymer
Other compounds that undergo free radical-initiated copolymerization with vinyl monomers are CO and SO 2
CH 2 CH CO
CH 2 CH C
polyketones
SO CH 2
2 CH CH 2 CH SO 2
polysulfones
alternation