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