Chapter 6. Free Radical Polymerization

Table 6. 1 Commercially Important Vinyl Polymers

6.1. Introduction

6. 2 Free Radical InitiatorsFour types

Peroxides and hydroperoxides

Azo compounds

Redox initiators

Photoinitiators

6.2.1. Peroxides and Hydroperoxides( ROOR ) ( ROOH )

Thermal homolysis

C O O C

O

Ph

O

Ph∆

C O

O

Ph2

Decomposition

C O

O

Ph Ph + CO2

Radical combination ∵ Cage effect(confining effect of solvent molecules)

Wastage reactionPh Ph Ph2

C O

O

Ph Ph C O

O

Ph Ph+

Induced decomposition

C O O C

O

Ph

O

Ph Ph C O O C

O

Ph

O

Ph

Ph

+

O C

O

PhC O

O

Ph Ph +

Wastage reaction

Radical stability

Ph C

CH3

CH3

O CH3 C

CH3

CH3

O C O

O

Ph C O

O

CH3> > >

Unstable radicalMore wastage reactionStable radical

Less wastage reaction

Half-lifesBPO (Benzoyl peroxide)

CH3 C

CH3

CH3

O O C

CH3

CH3

CH3C O O C

O

Ph

O

Ph

1.4 h at 85oC1.38 h at 145oC

Ph C

CH3

CH3

O O HC O O C

O

CH3

O

CH3

1.7 h at 130oC 1.1 h at 85oC

Promoters

C O O C

O

Ph

O

Ph

CH3

Ph NCH3

+

+CH3

Ph NCH3

O C

O

Ph+ O

O

Ph C-

CH3

Ph NCH3

O C

O

Ph++

No initiation of polymerization∵ No N in polymers

Initiation of polymerization

6.2.2. Azo Compounds

C(CH3)2N N

CNCN

(CH3)2C

CN

(CH3)2C2 + N2

AIBN (Azobisiso(butyronitrile))

Radical combinationCN

C(CH3)2

CN

(CH3)2CCN

(CH3)2C2

C N C(CH3)2

CN(CH3)2C

(CH3)2C

C N

(CH3)2C

C N

6.2.3. Redox Initiators

Production of free radicals by one–electron transfer reactions.

Useful in low–temp polymerization and emulsion polymerization

C(CH3)2

OOH

Ph Fe2+

O

Ph C(CH3)2 OH Fe3++ ++

-

HOOH Fe2+ + OH Fe3++ +-HO

O3SOOSO3 S2O32- SO4

2-+ + +-- - -SO4 S2O3

Radicals in aqueous phase

6.2.4. Photoinitiators

RSSRhν

2 RS

C

O

CH Ph

OH

Ph

O

CPh

OH

PhCH+hν

C

O

PhC

O

Ph CO

Phhν

2

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 reactionThe reaction can be stopped simply by removing the light source

6.2.5. Thermal Polymerization

PhH

Ph

+2

∆

CHCH2

CHCH2

CHCH3

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 CH2 RCH CH2+ e-_

RCH CH2RCH CH2 + e-+

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 controlNo contaminants added High viscosity

Suspension Heat readily dispersed Washing and drying requiredLow viscosity Agglomeration may occurObtained in granular form Contamination by stabilizer

Solution Heat readily dispersed Added cost of solventLow viscosity Solvent difficult to removeUsed 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 PolymerizationFormation of the initiator radicalAddition of the initiator radical to monomer

Initiation

Initiator RR CHCH2

Y

+ R CHCH2

YPropagation

CHCH2

Y

CHCH2

Y

+ CHCH2

Y

CHCH2

Y

TerminationCHCH2

Y

HC H2C

Y

+

Coupling or combination

Disproportionation

CHCH2

Y

HC H2C

Y

CHCH

Y

H2C H2C

Y

+

Coupling almost exclusively at low temp PolystyreneCHCH2 HC H2C+ CHCH2 HC H2C

CCH2

C

+

O

OCH3

CH3

C H2C

CO

OCH3

CH3

CCH

C

+

O

OCH3

CH3

HC H2C

CO

OCH3

CH3

Disproportionation PMMA

∵ Steric repulsionElectrostatic repulsionAvailability of α H for H transfer PMMA 5; PSt 2

Initiator Rkd

R + Mki

M1

Initiation

Propagation+ M

kpM2M1

+ Mkp

M3M2

+ Mkp

M(x+1)Mx

Termination

+ Myktc

M(x+y)Mx Coupling

+ Myktd

Mx + MyMx Disproportionation

Initiation rate (Ri)[ ] [ ]Ifk2dtMdR di =•

=

where[M•] = total conc of chain radicals[I] = molar conc of initiatorf = initiator efficiency = 0.3 ~ 0.8

Termination rate (Rt)[ ] [ ]2tt Mk2dtMdR •=•

−=

Steady-state assumption Ri = Rt

[ ] [ ]2td Mk2Ifk2 •= [ ] [ ]t

d

kIfkM =•

[ ] [ ][ ] [ ] [ ]t

dppp k

IfkMkMMkdtMdR =•=−=

Propagation rate (Rp)

[ ] [ ]M ,IRp ∝

Average kinetic chain length ( )ν

= Average # of monomer units polymerized per chain initiated

[ ][ ][ ]

[ ][ ]

[ ][ ]( )2

1

dt

p

t

p2

t

p

t

p

i

p

Ikfk2

MkMk2Mk

Mk2MMk

RR

RR

=•

=•

•===ν

ν=DP

ν2 Coupling

Disproportionation

• Gel effect, Trommsdorff effect, Norris-Smith effect

Occurs in bulk or concentrated solution polymerizations

η↑ Chain mobility↓ [M•] ↑Rt↓

Rp↑ Release of more heat Ri↑ Rp↑

ExplosionAutoacceleration

Polymerization of MMA in benzene 50°C benzoylperoxide

Gel effect

• Chain transfer reactions

Transfer of reactivity from the growing polymer chainto another species

Chain-end radical abstract a H atom from a chainChain branching

1) C.T. to Polymer

2) Backbiting : intramolecular chain transfer

3) C.T. to Initiator or Monomer

Important with monomers containing allylic hydrogen, such as propylene

Reactive radical

Resonance-stabilized radical

∴ High-mol-wt polypropylene cannot be preparedby free radical polymerization

CH2 C

CH3

COOCH3

CH2 C

CH3

COOCH3

+ allylic hydrogen

C.T. to monomerPropagation

CH2 CH

CH3

COOCH3

CH2 C

CH2

COOCH3

+

CH2 C

CH2

COOCH3

Resonance-stabilized radical

CH2 C

CH3

COOCH3

CH2 C

CH3

C

O

OCH3

CH2 C

CH3

COOCH3

CH2 C

CH3

C

O

OCH3

Resonance-stabilized radical

∴ High-mol-wt PMMA can be prepared by free radical polymerization

4) C.T. to Solvent or Chain transfer agents

CCl4

Thiol (RSH)

Transfer reaction rate

[ ][ ]TMkR trtr •=

where T = transfer agent

Kinetic chain length[ ][ ]•=νMk2Mk

t

p

[ ][ ][ ] [ ][ ]

[ ][ ] [ ]∑∑ +•

=•+•

•=

+=ν

TkMk2Mk

TMkMk2MMk

RRR

trt

p

tr2

t

p

trt

ptr

w/o transfer agent

with transfer agent[ ] [ ]

[ ][ ]

[ ][ ]

[ ]MTC1

MkTk1

MkTkMk21 T

p

tr

p

trt

tr

∑∑∑ +ν

=+ν

=+•

=ν

Tp

tr Ckk

= Chain transfer constant

[ ][ ]M

TC11 T

tr

∑+ν

=ν

Table 6.5 CT for St and MMA

Strong C-H bondBenzene Low CT

Toluene Benzylic H Higher CT

Weak S-H bond Large CT

CCl3•Resonance stabilization

CHCl3

CCl4 CT↑

CBr3•CBr4

RSH

Very-low-mol-wt polymerTelomerization

When [T] is high, ktr >> kp ⇒Telomer

CH3 CH2 CH2 CH2 CH2

- H

C Cl

Cl

Cl C Cl

Cl

Cl C Cl

Cl

Cl C Cl

Cl

Cl

ο Inhibitors

Added to monomers to prevent polymerizationduring shipment or storageMust be removed by distillation of monomer or extraction

Alkylated Phenol

R' . +

OHRR

R

R'H +RR

R

RR

R

ORR

R

RR RR

. O O

.

R.

R' . OR'RR

R

RR

R

RR

R

ORR RR

O

+ +'R

R R'

% Polymerization

Time0

No inhibitor

Inhibitor

More inhibitor

Induction period

Induction periods are common even with purified monomerbecause of the presence of oxygen, which is, itself, an inhibitor.

o Atom Transfer Radical Polymerization (ATRP)

ο Living free radical PolymerizationNo chain termination or chain transfer

CH3CHCl

Ph

+ Cu(I)(bpy) CH3CHPh

. + Cu(II)(bpy)Cl

H2C CHPh

CH3CHCH2CHCl

Ph Ph

+ Cu(I)(bpy) CH3CHCH2CH

Ph Ph

.+ Cu(II)(bpy)Cl

bpy = bipyridyl =

N N

Polymerization of styreneTransfer of Halogen atom

propagationPreventing radical terminationreactions from occurring

o Addition of a Stable Free Radical

CH2CHPh

. + N

H3CCH3

H3C CH3

O. CH2CHPh

N

H3C CH3

H3C CH3

O

TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxy)Too stale to initiate polymerizationPromotes decomposition of BPO to benzoyloxy radicals

Propagation Prevent termination[ ][ ]o

o

IMDP =

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[ ] ⎟

⎠⎞

⎜⎝⎛=

2NMkR pp

where [M] = concentration of monomer in the polymer particleN = # of polymer particles

Rate of radical entry into the particle

[ ] [ ] [ ][ ]Ifk2NMk

NR

MkMk

d

p

i

pp ==ρ

=ν

Average kinetic chain length

ν=DP

NRi=ρ

[ ]( ) 6.0s

4.0i EaRkN ⎟⎠

⎞⎜⎝

⎛µ

=

k = constant (0.37~0.53)µ = rate of increase in the volume of a polymer particle[E] = concentration of micellar emulsifieras = interfacial area occupied by an emulsifier molecule

in the micelles

where

[ ] [ ] 6.04.0 E ,IN∝ [ ] [ ] 6.06.0 E ,IDP −∝ν=[ ] [ ] 6.04.0p E ,IR ∝

6.5. Stereochemistry of Polymerization

Steric and electrostatic interactions are responsible for stereochemistry of free radical polymerization

1) Interaction between Terminal unit & MonomerMirror approach

CH2C

CH2C

Y

X

Y X .

C CYX

HH

CH2C

CH2C

Y X

CH2

Y XC

Y

X

.

etc

Isotactic polymer

Nonmirror approach

CH2C

CH2C

X

Y

Y X .

C CYX

HH

CH2C

CH2C

Y X

CH2

X YC

Y

X

.

etc

Syndiotactic polymer

2) Interaction between Terminal unit & Penultimate unit

C

C

P CH3

CO2CH3

H

HC

CO2CH3

CH3

CH2 C

CH3

CO2CH3

C

C

P CH3

CO2CH3

H

HC

CO2CH3

CH3

CH2 C

CH3

CO2CH3

P: bulkiest group Syndiotactic

If terminal carbon has sp2 planar structure, Interaction between terminal unit & monomer is dominant factor

If terminal carbon has sp3 hybridization, Interaction between penultimate unit & terminal unit is dominant factor

Stereoregularity ↑ as T↓

Free radical polymerization of MMA at T < 0 oCCrystalline polymerSyndiotactic

(Expect) As size of substituent groups ↑, stereoregularity ↑

(Experiment) bulkier substituent

Less syndiotactic than PMMA under the same conditions

CH2 CCH3

C OO CH2

6.6 Polymerization of Dienes6.6.1. Isolated Dienes

Crosslinked polymer

Cooperative addition of one double bond to the other of the monomer

Independent reaction

Cyclic polymer

5- or 6-membered ring

Cyclopolymerization:

Formation of a cyclic structural unit in a propagation step

Cyclopolymerization of divinylformal

.R +O O O O

R

O O

R

O O

R..

O O

R

O O

R.

head-to-tail mode six-membered ring

head-to-tail mode five-membered ring

Head-to-tail mode

Head-to-head mode

Six-membered ring

Five-membered ring

Highly crosslinked polymersDiallyl monomer

C

C

O

O

O

O

Used for manufacturing electrical or electronics items (circuit boards, insulators, television components, etc)preimpregnating glass cloth or fiber for fiber-reinforced plastics

Diallyl phthalate

OO

O

O

2

Used for applications requiring good optical clarity(eyeware lenses, camera filters, panel covers, and the like)

Diethylene glycol bis(allyl carbonate)

6.6.2. Conjugated Dienes

H2C CHHC CH2 RH2C CH2

HC CH2

.RH2C

HC CH CH2

..R

CH2CHCHCH2

CH2C C

CH2

H H

CH2C C

H

H CH2

Pendant vinyl groups unsaturation in the chain

cis trans

1,3-butadiene

1,4-addition1,2-addition

80%20%

CH2 C CHCH3

CH2

CH2CHCHCH2

CH3CH2CH

CCH2

CH3

CH3

CH2C C

CH2

H H

CH2C C

H

H CH2

cis trans

1,2-addition 3,4-addition cis-1,4-addition trans-1,4-addition

Isoprene (2-methyl-1,3-butadiene)

Natural rubber (Hevea) Predominant

10%As T↑ , cis-1,4 ↑

trans-1,4s-trans

s-cis As T↑ , cis-1,4 ↑cis-1,4

CH2

C

CHCH2

Sn(n-C4H10)3

cis-1,4s-cis

~ 100% at 60oC

Table 6.6 Structures of Free Radical-Initiated Diene Polymers

percent

MonomerPolymerization Temperature(℃) cis-1,4 trans-1,4 1,2 3,4

Butadiene -2020

100233

6222843

77585139

17202118

----

Isoprene -20-550

100257

17182312

9082726677

55552

45569

Chloroprene -4646

100

51013

9481-86

71

12

2.4

0.31

2.4

kp:

6.7 Monomer Reactivity-C6H5 -CN -COOCH3 -Cl< < < -OCOCH3<

o Three factors in monomer reactivity

1) Resonance stabilization of free radicals: kp↓

-C6H5 -CN -COOCH3 -OCOCH3-Cl> > > >

Reactive monomer Stable monomerReactive radicalStable radical

Inverse relationship between monomer reactivity and kp

Radical reactivity is the major factor.

R +

R R R R

Reactive monomer Stable radical

2) Polarization of double bond by substituents: kp↓

Extreme polarization prevents free radical polymerization

e.g.

CH2 CCN

CN CH2 CH

OCH3No radical polymerization

C C C CC C

Homolytic fissionHeterolytic fission

Anionic Cationic polymerization

3) Steric hindrance: kp↓

CH2 CH

X

CH2 CY

X CH CH CH CH

YX> > >

kp: MMA < MA

CH2 C

COOCH3

CH H

H CH2 C

COOCH3

H

MAMMA

∵ Resonance stabilization of the radical by hyperconjugation and steric hindrance

Overlap of p-orbital with σ bonds

o Free energy of polymerization∆Gp = ∆Hp - T∆Sp

∆H < 0 : exothermic, π-bond in monomer → σ-bond in polymer

∆S < 0 : monomer→ covalently bonded chain structure∴ Degree of freedom (randomness) ↓

favorable

unfavorable

(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 ↓

Monomer Polymerenergyπ-bond σ-bond

Resonance Stabilizationof monomer ∆H Steric strain

Decrease in H-bonding or dipole interaction

Table 6.7 ∆H and ∆S of Polymerization

Monomer-△H

(kJ/mol)-△S

(J/mol)Acylonitrile 77 1091,3-Butadiene 78 89Ethylene 109 155Isoprene 75 101Methyl methacrylate 65 117Propylene 84 116Styrene 70 104Tetrafluoroethylene 163 -Vinyl acetate 90 -Vinyl chloride 71 -

o Polymerization-depolymerization Equilibrium

Mx + M M(x+1)

kp

kdp

wherekdp = depropagation rate constant

Tc T

kdp

kp [M]

kp [M] - kdp

Ceiling temperature

Rp = Rdp

At Tc[ ][ ] [ ]•=• MkMMk dpp

Equilibrium constant[ ]

[ ][ ] [ ] dp

p

ee kk

M1

MMMK ==••

=

where [M]e = equilibrium monomer concentration

∆G = ∆Go + RT ln Kwhere ∆Go = ∆G in standard state

(pure monomer or 1-M solution monomer, pure polymer or 1-M repeating units of polymer)

At equilibrium∆G = 0

∆Go = ∆Ho - Tc∆So = - RTc ln K = RTc ln [M]e

[ ]eo

o

c MlnRSHT

+∆∆

= [ ]RS

RTHMln

o

c

o

e∆

−∆

=or

RSo∆

− [ ]RS

RTHMln

o

c

o

e∆

−∆

=

RHo∆[ ]eMln

1

cT1

Table 6.8 Tc of Pure Liquid Monomers

Monomer Tc(℃)

1,3-Butadiene 585Ethylene 610Isobutylene 175Isoprene 466Methyl methacrylate 198α-Methylstyrene 66Styrene 395Tetrafluoroethylene 1100

[ ]eo

o

c MlnRSHT

+∆∆

=

CH2 C

CH3

CH2 C

CH3

-△H = 35 kJ/mol

Tc = 66 oC

Resonance stabilization Steric strain in polymer

6.8 Copolymerization

Reaction Ratek11

M1• M1•+ M k11 [M1•] [M1]1

k12M1• M2• k12 [M1•] [M2]+ M2

k21M2• M1•+ M k21 [M2•] [M1]1

k22M2• M2• k22 [M2•] [M2]+ M2

where k11 and k22 : self-propagation rate constant

k12 and k21 : cross-propagation rate constant

o Steady-state assumption for [M1•] and [M2•]k12 [M1•] [M2] = k21 [M2•] [M1]

o Rate of consumption of M1 and M2

[ ] [ ][ ] [ ][ ]122111111 M M kM M k

dtMd

•+•=−

[ ] [ ][ ] [ ][ ]222221122 M M kM M k

dtMd

•+•=−

[ ][ ]

o Instant composition of copolymer[ ][ ]

[ ] [ ][ ] [ ]⎟⎟⎠

⎞⎜⎜⎝

⎛•+••+•

=222112

221111

2

1

2

1

M kM kM kM k

MM

MdMd

[ ] [ ][ ][ ]2 12

21211 Mk

M M kM •=•

o Monomer reactivity ratio

112

11 rkk

= 221

22 rkk

=

o Copolymer (composition) equation[ ][ ]

[ ][ ]

[ ] [ ][ ] [ ]⎟⎟⎠

⎞⎜⎜⎝

⎛++

=221

211

2

1

2

1

MrMMMr

MM

MdMd

o Instantaneous composition of feed and polymer[ ]

[ ] [ ]21

121 MM

Mf1f+

=−=f1 = mole fraction of M1 in the feed

f2 = mole fraction of M2 in the feed[ ]

[ ] [ ]21

121 MdMd

MdF1F+

=−=F1 = mole fraction of M1 in the copolymer

F2 = mole fraction of M2 in the copolymer

[ ][ ] 1

1

2

1

F1F

MdMd

−=

[ ][ ] 2

1

2

1

ff

MM

=[ ][ ]

[ ][ ]

[ ] [ ][ ] [ ]⎟⎟⎠

⎞⎜⎜⎝

⎛++

=221

211

2

1

2

1

MrMMMr

MM

MdMd

⎟⎟⎠

⎞⎜⎜⎝

⎛+

+=−=

−

211

221

1

2

11

1

ffrfrf

ff1

F1

FF1

⎟⎟⎠

⎞⎜⎜⎝

⎛++

=− 221

211

2

1

1

1

frfffr

ff

F1F

212

11

22221

211

211

221

1

2

1 fffrfrff2fr1

ffrfrf

ff

F1

+++

=+⎟⎟⎠

⎞⎜⎜⎝

⎛+

+=

22221

211

212

111 frff2fr

fffrF++

+=

[ ][ ]2

1

2

1

MdMd

FFF ==

[ ][ ]2

1

2

1

MM

fff ==

[ ][ ]

[ ][ ]

[ ] [ ][ ] [ ]⎟⎟⎠

⎞⎜⎜⎝

⎛++

=221

211

2

1

2

1

MrMMMr

MM

MdMd

2

21

2

1

rfffr

rf1frfF

++

=⎟⎟⎠

⎞⎜⎜⎝

⎛++

=

ffrFrfF 212 +=+ ( )

1

2

2 rFfr

FF1f

⎟⎟⎠

⎞⎜⎜⎝

⎛−=

−

Finemann and Rose Equation

( )F

F1f −

1

- r1

r2

Ff 20

22221

211

212

111 frff2fr

fffrF++

+=

1) r1 = r2 = 1

No preference for homopolymerization or copolymerizationRandom copolymer

F1 = f1

e.g. M1 = ethylene M2 = vinyl acetater1 = 0.97 r2 = 1.02

2) r1 = r2 = 0

Alternating copolymer

F1 = 0.5

e.g. M1 = styrener1 = 0.041

M2 = maleic anhydrider2 = 0.01

3) 0 < r1, r2 < 1

More common

e.g. M1 = styrener1 = 0.52

M2 = methyl methacrylater2 = 0.46

Azeotropic copolymerization

[ ][ ]

[ ][ ]2

1

2

1

MM

MdMd

=[ ][ ]

[ ][ ]

[ ] [ ][ ] [ ]⎟⎟⎠

⎞⎜⎜⎝

⎛++

=221

211

2

1

2

1

MrMMMr

MM

MdMd

[ ] [ ][ ] [ ] 1

MrMMMr

221

211 =++ [ ]

[ ] 1

2

2

1

r1r1

MM

−−

=21

21 rr2

r1f−−

−=

4) 1 << r1 and r2 << 1

e.g. M1 = styrene M2 = vinyl acetater1 = 55 r2 = 0.01M1 = styrene M2 = vinyl chlorider1 = 17 r2 = 0.02

Essentially homopolymer

5) r1• r2 = 1

e.g. M1 = MMAr1 = 10 r2 = 0.1

M2 = vinyl chloride

Ideal copolymerizationr1= r2 = 1 Real random copolymerization

The more r1 and r2 diverge, the less random2

22212

11

212

111 frff2fr

fffrF++

+=

( )( ) 211

112

211

211112

22112

12

1

2112

12

12

1

221

211

212

111 ffr

frffr

ffrfrfffr2fr

ffrfr

rfff2fr

fffrF+

=++

=++

+=

++

+=

2o2

o1

1

o2

o1

1

o22

o11

o11

21

1

XPPX

PPX

PXPXPX

PPP

+=

+=

+Ideal solution

cf.

cf.

22

21

12

11

kk

kk

=

Table 6.9 Reactivity Ratios

M1 M2 r1 r2 Temperature(℃)

Styrene Methyl methacrylate 0.52 0.46 60

Styrene Acrylonitrile 0.40 0.04 60

Styrene Vinyl acetate 55 0.01 60

Styrene Maleic anhydride 0.041 0.01 60

Styrene Vinyl chloride 17 0.02 60

Styrene 1,3-Butadiene 0.58 1.35 50

Styrene Isoprene 0.54 1.92 80

Methyl methacylate Vinyl chloride 10 0.1 68

Methyl methacylate Vinyl acetate 20 0.015 60

Methyl methacylate Acrylonitrile 1.20 0.15 60

Methyl methacylate 1,3-Butadiene 0.25 0.75 90

Ethylene Tetrafluoroethylene 0.38 0.1 25

Ethylene Acrylonitrile 0 7 20

Ethylene Vinyl acetate 0.97 1.02 130

r1 = r2 = 1

r1 = r2 = 0

0 < r1, r2 < 1

1 << r1 and r2 << 1

r1• r2 = 1r1 =10, r2 = 0.1

Azeotropic composition21

21 rr2

r1f−−

−=

Q-e Scheme (Alfrey-Price Treatment)

k12 = P1Q2 exp (- e1e2)

P1 = reactivity of radical M1•Q2 = reactivity of monomer M2

e1 = polarity of monomer M1

e2 = polarity of monomer M2

where

( )( ) ( )[ ]211

2

1

2121

1111

12

111 eeeexp

QQ

eeexpQPeeexpQP

kkr −−⎟⎟

⎠

⎞⎜⎜⎝

⎛=

−−

==

( )[ ]1221

22 eeeexp

QQr −−⎟⎟

⎠

⎞⎜⎜⎝

⎛=

Styrene: standardQ = 1.00 e = - 0.80

Resonance factor ( + steric factor)

Polarity factor+ : electron-withdrawing group- : electron-donating group

Table 6.10 Reactivity (Q) and Polarity (e) of Monomer

Monomer Q e1-vinylnaphthalene 1.94 -1.12p-Nitrostyrene 1.63 0.39p-Methoxystyrene 1.36 -1.11Styrene 1.00 -0.80Methyl methacrylate 0.74 0.40Acrylonitrile 0.60 1.20Methyl acrylate 0.42 0.60Vinyl chloride 0.044 0.20Vinyl acetate 0.026 -0.22

CH2 CH + CH2 CH

O C CH3

O

CH2 CH

O C CH3

OLittle tendency

No resonance-stabilizationResonance-stabilization

M1 = Styrene M2 = Vinyl acetate r1 = 55 r2 = 0.01Styrene Q = 1.00 e = - 0.80 Vinyl acetate Q = 0.026 e = - 0.22

M1 = Styrene M2 = MMAStyrene Q = 1.00 e = - 0.80

r1 = 0.52 r2 = 0.46

MMA e = 0.40Q = 0.74

H3

CH2 C

C

CH3

O

OCH3

+ CH2 C

CH3

C

O

OC

CH2 CH

Each radical has about twice as much tendency to react with its opposite monomer

∵Polar effect

Copolymerization of styrene and maleic anhydride

Strong tendency toward alternation (r1 and r2 = ~ 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) Rp 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 reactwhile alternating copolymer is forming

(3) Rp 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 SO2

CH2 CH

R

CH2 CH

R

CO

SO2

CH2 CH

R

C

O

CH2 CH

R

SO2

polyketones

polysulfones

alternation