Types of Vinyl Polymerization

Method Advantages Disadvantages

Bulk (Neat) Simple equipmentRapid reactionPure polymer isolated

Heat buildupGel effectBranched or crosslinked product

Solution Good mixingReady for application

Lower mol. Wt.Low Rpoly

Solvent Recovery

Suspension(Pearl)

Low viscosityDirect bead formation

Removal of additives

Emulsion High Rpoly

Low TemperaturesHigh Mol. Wt.High surface area latex

Removal of additivesCoagulation neededLatex stability

Inverse Emulsion Water in oil latex formedInversion promotes dissolution in water

Fate of Initiator Radicals

• Radical reactionsRecombination in solvent cage

R +

X

R

Xki

•Chain initiation, Ri = 2 f kd [I]

•Efficiency factor, f = 0.1 - 0.9

Reaction with polymer radicals (kt)--primary termination

Reaction with initiator (MIH)

Hydrogen abstraction from polymer chains (chain transfer to polymer)

Reaction with solvent or inhibitor

Radical Initiators

• Azo Initiators

CH3

H3C

CN

NN

CH3

CN

H3C

azobisisobutyronitrile, AIBN

CN

NN

CN1,1'-azobis(1-cyclohexanenitrile)

H3C

CN

NN

CH2CH2COOH

CN

H3CHOOCCH2CH2-

4,4'-azobis(4-cyanovaleric acid)

Decomposition of Azo Initiators

• 2- bond cleavage to liberate nitrogen

CH3

H3C

CN

NN

CH3

CN

H3C heat

or light

CH3 CCH3

CN

2 + N2

Cage Recombination ---Side reaction- irreversible coupling of succinonitrile radicals, efficiency decreases at high conversion

CH3

H3C

C

N

CH3

C

CH3

NC

CH3

C

N

H3CC

CH3

C

N

H3C CH3

CH3

C

N

N

C

CH3CH3

Peroxy Initiators

• High temperature initiators

C OO

HC O

OR

C OO

R

O

Hydroperoxides Dialkyl Peroxides Peresters

Td = 155-175 C 100-135 C 110-130 C

C OO

O

R

ODiacyl Peroxides

SO

OSO O

O

OO

O

Persulfates

Td = 35-80 C 50-90 C

Moderate temperature initiators

Peroxy Initiators

• Low temperature initiators, 35-60 C

CO

O

O

RO

O O

Peroxycarbonates

2 CO

O

O

OO

OO

O

O

Di-t-butyl peroxylate, DBPOX

O2 + 2 CO2

3-bond cleavage process?

-cleavage to carbon centered radical

O + CH3

Redox Initiation

0-5 C in water

SO

OSO O

O

OO

O

+ Fe++ SOO

O

O

OS O

O

O

+Fe+++

+

0-5 C in organic/aqueous phase

OOH

cumyl hydroperoxide

Fe+++O

+ OH +Fe+++

Decomposition of Peroxy Initiators

• 1-bond cleavage process

If R = aryl, acyl radical initiates = alkyl, CO2 lost before initiation occurs

R OO

O

R

O

heatR O

O

2

- CO2

RR O

O

R

Reaction of benzoyloxy radicals with styrene

PhCO2-CH2 CH

80%

Sty

Ph OO

O

Ph

O

heat2 PhCO2

PhCO2

Ph

-CO2

1%

HC CH2PhCO2

6%Sty

CH

PhCO2

14%

Sty

H2C

Chain Transfer

• Hydrogen transfer to growing polymer chain

R

X

P CH X

H+ R S

H R

X

P CH X

HH

+ R S

ktr

R S +

XR S

Xka

kp

•Reinitiation of growing chain using transferred radical

Effect of Chain Transfer on Rp and DP

Relative rate constants

Type of effect Effect on Rp

Effect on DP

kp.>> ktr

ka ~ kp

Normal None Decrease

kp<< ktr

ka ~ kp

Telomerization None Large decrease

kp>> ktr

ka < kp

Retardation Decrease Decrease

kp<< ktr

ka << kp

Inhibition Large decrease

Large decrease

Control by Chain Transfer• Chain transfer depends upon nature and

concentration of chain transfer agent.

1DP

=1

DPo+

[SH][M]Ctr

Where Ctr is the chain transfer constant that includes the rate constants for hydrogen abstraction and re-initiation of a new chain

Ctr is specific for a given monomer at a given temperature

Common Chain Transfer Agents

Transfer agent Styrene,

Ctr x 104

Vinyl Acetate

Ctr x 104

Toluene 0.125 21.6

Di-n-butyldisulfide

24 10,000

Carbon tetrabromide

22,000 390,000

n-butyl mercaptan

210,000 480,000

Additional Chain Transfer Processes

• Chain transfer to monomer, Ctr x 104

– Ethylene, 0.4- 4.0; Styrene, 0.3-0.6

Chain transfer to polymer--branchingPolyethyleneVinyl acetateVinyl chloride

Vinyl acetate, 1.75-2.8Vinyl chloride, 10.8-16Allyl systems, 50-100

Transfer to Polymer

• Polyethylene branching

HH

M

M

ktr

ka

HH H

MH

H

M

Long branches

Short branches

Inhibition of Radical Polymerization• Must stop oxygen- and carbon centered radicals

Carbon centered radicals stopped by addition

•Oxygen centered radicals stopped by hydrogen transfer

RH + O2 R +

HOORH + R

HOO

HOOH+

Radicals generated by auto oxidation

R + O2 ROO

ROO + RH ROOH + R

ROOH + OHRO May be metal catalyzed

Critical Inhibitor Properties

An inhibitor should not add to, abstract from or otherwise reach with monomer or solvent

Inhibitors should not undergo self reaction or unimolecular decomposition

Inhibitors must react rapidly with the propagating and/or initiator derived radicals to terminate polymer chains

Trapping Oxygen Centered Radicals

OH

BHT

O

+ ROHRO

OO

OR

RO

Trapping carbon centered radicals

• Carbon centered radicals stopped by addition to oxygen or carbon

O O

Benzoquinone

OR

OR.

O OR

O O

R HR O O

R

R H+

HO O

R

Tautomerize

Typical InhibitorsOH OH

OCH3

Monomethylhydroquinone, MEHQ

OH

3,5-ditert-butyl catechol

OH

OHHQ

O O

Benzoquinone

O O

Cl Cl

ClClChloranil

O2 FeCl3 CuCl2 S

Stable Radical Inhibitors

N N

O2N

NO2

O2N

Diphenylpicrylhydrazyl, DPPH

N

O

TEMPO

O

O

Galvanoxyl

NN

NN

Triphenylverdazyl