Chapter 6. Free Radical Polymerization

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Chapter 6. Free Radical Polymerization
6.1 Introduction
6.2 Free Radical Initiators.
6.3 Techniques of Free Radical Polymerization.
6.4 Kinetic and Mechanism of polymerization.
6.5 Stereochemistry of polymerization.
6.6 Polymerization of Dienes
6.7 Monomer Reactivity
6.8 Copolymerization.
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 6. 1 Introduction
A. Type of polymerization.

• Free-radical polymerization
• Ionic polymerization
• Complex coordination polymerization

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B. Commercialized free-radical polymerization.
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6.2 Free Radical Initiators.
 6.2.1 Peroxides and Hydroperoxides A.
Benzoly peroxide and other peroxides   a.
Thermal decomposition of BPO.  
b. Half-life of benzoyloxy radical 30 min at
100? c. Cage effect confining effect of
solvent molecules.
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 6.2.1 Peroxides and Hydroperoxides
d. Other peroxides.
Diacetyl peroxide        Di-t-butyl
peroxide   Diacetyl peroxide     
Di-t-butyl peroxide                          
(half-life10hours at 120?)
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 6.2.1 Peroxides and Hydroperoxides
e. Promoters Inducing initiation at lower
temperature.
(6.9)

-
(6.10)


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 6.2.1 Peroxides and Hydroperoxides
B. Hydroperoxide   a. Thermal decomposition
hydroperoxide
 b. Cumyl hydroperoxide.
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6.2.2 Azo Compounds.
   A. a,a'-Azobis(isobutyronitrile) (AIBN).
  a. Decomposition of AIBN.
b. Half-life of isobutyronitrile radical 1.3
hours at 80?.
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6.2.2 Azo Compounds.
B. Side reaction Cage effect.   a.
Tetramethylsuccinonitrile
b. Ketenimine                
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6.2.3 Redox Initiators.
  A. One electron transfer reaction.  
a. Making free radical by one electron transfer
by redox reaction.   b. Low-temperature
reaction.  c. Emulsion polymerization.
                                                 
                                                  
                   
B. Example of redox system.
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6.2.4 Photoinitiator
A. Peroxide and Azo compound.  
Photolysis and thermalysis.
B. Photolabile initiator.
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6.2.5 Thermal Polymerization.
A. Polymerization without initiators.   a.
Dimer formation by Diels-Alder reation.
b. Radical formation from dimer.


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6.2.6 Electrochemical Polymerization.
A. Polymerization of electrolysis.
  a. Cathode reaction
electron transfer to monomer ion forming radical
anion (6.22) b. Anode reaction
electron transfer to anode forming
radical cation (6.23)  B. Coating metal
surfaces with polymers.
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 6.3 Techniques of Free Radical Polymerization.
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 6.3 Techniques of Free Radical Polymerization.
 6.3.1 Bulk A. Reactor charges.   a.
Monomer.   b. Initiator (soluble in
monomer).   B. Problems.   a. Heat
transfer.  b. Viscosity.   c.
Auto-acceleration.
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 6.3.2 Suspension.
A. Reactor charges.  a. Monomer.   b.
Initiator (soluble in monomer).   c. Water or
other liquid.   d. Stabilizer Poly(vinyl
alcohol), CMC B. Vigorously stirring to keep
suspension.
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6.3.3 Solution.
A. Reactor charges.   a. Monomer (soluble in
solvent).   b. Initiator (soluble in
solvent).   c. Solvent. B. Refluxing
solution.
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 6.3.4 Emulsion.
A. Reactor charges.   a. Monomer.  b. Redox
initiator  c. Soap or emulsifier.  d. Water.
 e. Others (cf. Table 6.3). B. Polymerization
in swollen micelle.   Latex products.
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 6.3.4 Emulsion.
TABLE 6.3. Typical Emulsion Polymerization
Recipesa
Styrene-Buradiene Copolymer
Polyacrylate Latex
Ingredients, Conditions Ingredients (parts by
weight) Water Butadiene Styrene Ethyl
acrylate 2-Chloroethyl vinyl ether
p-Divinylbenzene Soap Potassium persulfate
1-Dodecanethiol Sodium pyrophosphate Conditi
ons Time Temperature Yield
133 - - 93 5 2 3b 1 - 0.7 8hr 60oC ?100
190 70 30 - - - 5 0.3 0.5 - 12hr 50oC 65
aRecipes from Cooper.23 bSodium lauryl sulfate.
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6.4 Kinetic and Mechanism of polymerization.
A. Mechanism of free-radical polymerization.  
a. Initiation.    1) Decomposition.
     Initiator ? 2R    2)
Addition.
(6.25)
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6.4 Kinetic and Mechanism of polymerization.
b. Propagation.
(6.26)
 1) Head-to-tail orientation predominant
reaction.       Steric and electronic effects.
   2) Examples of not exclusively head-to-tail
orientation.
(13-17 of head to head)   (5-6 of head to
head)   (19 of head to head)
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6.4 Kinetic and Mechanism of polymerization.
 c. Termination.    1) Combination.
(6.27)
Polystyrene radical.
(6.29)
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6.4 Kinetic and Mechanism of polymerization.
 2) Disproportionation.
Poly(methyl methacrylate) radical.
? Repulsion of ester group.     ? Easy alpha
hydrogen abstraction. 3) Acrylonitrile
Combination virtually exclusively at 60?. 4)
Poly(vinyl acetate) Disproportionation.
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B. Kinetic of free radical polymerization.
  a. Assumption.   1) The rates of
initiation, propagation, and termination are all
different.   2) Independent of chain length.
  3) Negligible end group.   4) At steady
state, constant radical concentration.
    (steady state assumption)
 b. Initiation (Ri)                f
Initiator efficiency.             kd
Decomposition rate constant.   I molar
concentration of initiator.  M molar
concentration of radical.
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B. Kinetic of free radical polymerization.
 c. Termination rate ( Rt )               
 d. Propagation rate ( Rp )         Steady
state assumption.
kt ktc ktd
RiRt
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B. Kinetic of free radical polymerization.
e. Average kinetic chain length ( )
?
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B. Kinetic of free radical polymerization.
 f. Gel effect Trommsdorff effect, Norris-smith
effect.    1) Difficult termination reaction
because of viscosity.    2) Ease propagation
reaction because monomer size is small,   
  even though high viscosity.    3)
Autoacceleration by exotherm of propagation
reaction.    4) To obtain extraordinary high
molecular weight polymer like gel.
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C. Chain transfer reactions Growing radicals
move to other parts
   by hydrogen abstracting.    Lowering average
kinetic chain length.   a. Growing radicals move
to other polymer chain.
b. Backbiting self polymer chain.
(6.33)
 LDPE branching polymer.
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C. Chain transfer reactions
 c. Moving to initiators or monomers.
(6.34)
(6.35)
 d. Moving to solvent.
(6.36)
(6.37)
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C. Chain transfer reactions
e. Moving to chain transfer agent.
(6.39)
            Ct
Chain transfer constant.   
 T Concentration of chain transfer
agent.   f. Telomerization At high
concentration of transfer agent, ktrgtkp.    
Low-molecular-weight
polymers are obtained.             
 (Telomer)
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D. Leaving free radical polymerization Atom
transfer polymerization.
  a. Copper(I) bypyridyl(bpy) complex 
(6.42)
(6.43)
(6.44)
 b. TEMPO (18) 2,2,6,6-tetramethylpiperidinyl-1-
oxy.
(6.45)
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6.4 Kinetic and Mechanism of polymerization.
 c. Synthesis of block copolymers like anionic
polymerization.  d. Monodisperse polymerization
(PI1.05). E. Kinetics of Emulsion
polymerization.   a.           
N the number of particles.   b.   
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6.5 Stereochemistry of polymerization.
A. General consideration.   a. Stereoregular
polymerization Ionic and complex coordination  
   polymerization.    1) Terminal ion pair
counter ion.    2) Terminal complex active site.
   3) Low temperature.  b. Stereo-irregular
polymerization Free-radical polymerization.
   1) No stereoregulating radical terminal
group.    2) Somewhat higher temperature.
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6.5 Stereochemistry of polymerization.
B. Factors influencing stereochemistry in
free-radical polymerization.  a. Interaction
between the terminal chain carbon and the
   approaching monomer molecule.
C. Stereoregular free-radical polymerization of
PMMA.   (syndiotatic PMMA)   a. Polymerization
temperature below 0?.   b.
(6.48)
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6.5 Stereochemistry of polymerization.
 c. Terminal carbon sp2( planar )
    Penultimate repeating unit Bulky ester
group.   d. Poly(2,4,6-triphenylbenzylmethacryla
te)
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1) Less syndiotatic than PMMA. 2) More polar
effect than steric effect.
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6.6 Polymerization of Dienes
  6.6.1 Isolated Dienes A. Crosslinked or
cyclopolymerization.
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6.6.2 Conjugated Dienes.
A. Structure of conjugated Diene monoer.
Isoprene
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B. a. 1,2-Addition Pendent vinyl group.
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 b. Stereochemistry isotactic, syndiotactic,
atactic.
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6.6.2 Conjugated Dienes.
C. 1,4-Addition Delocalized double bond   a.

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D. 3,4-Addition
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E. Polymerization reaction and temperature.
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6.6.2 Conjugated Dienes.
TABLE 6.6 Structure of Free Radical-Initiated
Diene Polymersa
Percent
polymerization Temperature (oC) -20
20 100 233 -20 -5 50 100 257 -46 46 100
Monomer Butadiene Isoprene Chloroprene
cis-1,4 trans-1,4 1,2 3,4
- - - - 4 5 5 6 9 0.3 1 2.4
6 22 28 43 1 7 18 23 12 5 10 13
77 58 51 39 90 82 72 66 77 94 81-86 71
17 20 21 18 5 5 5 5 2 1 2 2.4
aData from Cooper34 p. 275.
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6.6.2 Conjugated Dienes.
F. s-cis and s-trans
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6.7 Monomer Reactivity
A. Thermodynamic feasibility.   a. ?Gp ?Hp
- T?Sp      ?Gp Gibbs free energy change of
polymerization.      ?Hp Enthalpy change of
polymerization.      ?Sp Entropy change of
polymerization.      ?Gp lt 0 favorable free
energy of polymerization.   b. Values of ?H
and ?S for several monomers.   c.
Polypropylene and isobutylene       ?G lt 0
? unfavorable polymerization.      because of
kinetic feasibility
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6.7 Monomer Reactivity
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6.7 Monomer Reactivity
B. Factors of monomer reactivity in free radical
polymerization.   a. The stability of the
monomer toward addition of a free radical.   b.
The stability of the monomer radicals.
 c. Order of monomer reactivity.   
 Acrylonitrile gt Styrene gt Vinyl acetate.   d.
Order of benzoyloxy radical initiation.   
 Syrene gt Vinyl acetate gt Acrylonitrile  
  Benzoyloxy radical Ph14CO2
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6.7 Monomer Reactivity
C. The inverse relationship between monomer
stability and    polymerization rate.  a.
Vinyl acetate not Stable monomer but high rate
constant.  b. Steric and polar effects Not
clear-cut generalization.     Lower rate
constant of MMA than MA.   c. 1,2 disubstituted
monomer difficult to polymerize in free radical.
     Exception Tetrafluoroethylene.
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6.7 Monomer Reactivity
D. Ceiling temperature (Tc)   a.
 b. Definition of ceiling temperature.   ?Gp
0 equal forward and backword reactions.
  c. High Tc favorable polymerization.     
 Low Tc unfavorable polymerization.     
Exception a-methylstyrene (Tc66?).
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6.8 Copolymerization.
A. Mechanism of copolymerization.
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B. Kinetics of copolymerization.
 a.    b.    c. let,
(reactivity
ratio)     steady state assumption.     
 d. solving         Copolymer equation
or copolymer composition equation. dM1/dM2
the molar ratio of the two monomers in the
copolymer M1, M2 the initial molar
concentration of monomers in the
            reaction mixture
and
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C. Significance of reactivity ratio (r1, r2).
a. r1 r2 8 Homopolymer.   b. r1 r2
0 Alternating polymer.
 c. r1 r2 1 Copolymer composition depending
on feeding    monomers in
the reaction temperature.   d. r1 r2 1
Ideal copolymerization like ideal liquid
vaporization.   e. r1 r2 gt 1 Azotropic
copolymerization
(polymer composition not depending on feeding).
  f. Determination of r1, r2 Measure
copolymer composition by    NMR or other
method at low conversion ( lt10 )
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D. Alfrey-price Q-e scheme.
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E. Charge transfer complex polymerization(alternat
ing copolymer).   a. Styrene and maleic
anhydride(D-A complex).
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E. Charge transfer complex polymerization
(alternating copolymer).
b.
(6.57)
c.
(6.58)