1. 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.

2. 6. 1 Introduction Type of polymerization.
Polymer: poly  many mer  many parts
(Greek) “Macromolecules”
Monomer: mono  single mer  single part
Polymerization

3. 6. 1 Introduction Type of polymerization.
Classified by polymerization mechanism:
(1) step-reaction or step-growth polymerization
(2) chain-reaction or chain-growth polymerization
(3) ring-opening polymerization (step- or chain-growth)

4. 6. 1 Introduction N =N0(1-p) Type of polymerization.
at 98% conversion:
Type of polymerization.
Carothers
p : conversion ratio N0 : initial number of molecules
N : the total number of molecules at a given time

5. 6. 1 Introduction The formation of polyvinyl monomer. CH2 = CHX
A. Type of polymerization.
polymerization
Addition polymerization
Condensation polymerization
Free-radical polymerization
Ionic polymerization
Complex coordination polymerization
The formation of polyvinyl monomer.
CH2 = CHX

6. 6. 1 Introduction Free-radical polymerization
Free radical are independently-existing species that have unpaired electron. Normally they are highly reactive with short life time.
Free radical polymerization’s are chain polymerization’s in which each polymer molecules grows by addition of monomer to a terminal free-radical reactive site known as active center.
After each addition the free radical is transferred to the chain end.
Chain polymerization is characterized by three distinct stages, Initiation, propagation and termination.

7. B. Commercialized free-radical polymerization.

8. 6.2 Free Radical Initiators.
peroxide and hydroperoxide
azo compounds
redox initiator
photoinitiator
 6.2.1 Peroxides and Hydroperoxides
A. Benzoyl 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.

9. 6.2.1 Peroxides and Hydroperoxides
(6.5)
d. Other peroxides.
Diacetyl peroxide        Di-t-butyl peroxide
  Diacetyl peroxide      Di-t-butyl peroxide
                          (half-life:10hours at 120℃)

10. 6.2.1 Peroxides and Hydroperoxides
e. Promoters : Inducing initiation at lower temperature.
(6.9) + - +
(6.10) +

11. 6.2.1 Peroxides and Hydroperoxides
B. Hydroperoxide
a. Thermal decomposition hydroperoxide
 b. Cumyl hydroperoxide.

12. 6.2.2 Azo Compounds. A. α,α'-Azobis(isobutyronitrile) (AIBN).   
A. α,α'-Azobis(isobutyronitrile) (AIBN).
a. Decomposition of AIBN.
b. Half-life of isobutyronitrile radical : 1.3 hours at 80℃.

13. 6.2.2 Azo Compounds. B. Side reaction : Cage effect. b. Ketenimine
a. Tetramethylsuccinonitrile
b. Ketenimine                

14. 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.

15. 6.2.4 Photoinitiator A. Peroxide and Azo compound.
Photolysis and thermalysis.
B. Photolabile initiator.

16. · · 6.2.5 Thermal Polymerization.
A. Polymerization without initiators.
a. Dimer formation by Diels-Alder reaction.
The Diels-Alder reaction is an organic chemical reaction (specifically, a cycloaddition) between a conjugated diene and a substituted alkene, commonly termed the dienophile, to form a substituted cyclohexene system.
Otto Paul Hermann Diels and Kurt Alder first documented the novel reaction in 1928 for which they were awarded the Nobel Prize in Chemistry in 1950 for their work on the eponymous reaction. 10
b. Radical formation from dimer. 11 12

17. 6.2.6 Electrochemical Polymerization.
A. Polymerization of electrolysis.
  a. Cathode reaction :
electron transfer to monomer molecule to form radical anion (6.22)
b. Anode reaction :
electron transfer to anode to form radical cation (6.23) 
B. useful for Coating metal surfaces with polymer films.

18.  6.3 Techniques of Free Radical Polymerization.

19. 6.3 Techniques of Free Radical Polymerization.
 6.3.2 bulk .
Neat monomer (and initiator)
Simplest formulation and equipment
Most difficult in control, when polymerization is very exothermic
Common problems: (1) heat transfer (2) increase in viscosity
If polymer is insoluble in monomer  polymer precipitate
(1) viscosity would remain similar
(2) occlusion of radicals (within the polymer droplet) is unavoidable
Radical occlusion  autoacceleration  crosslinked polymer nodules
(i.e. popcorn polymerization)
The crosslinked nodules (light weight and large volume) may cause
fouling or fracture of the polymerization apparatus
Commercial uses
Casting formulations
Low MW polymers for adhesives, plasticizers, tackifiers,
and lubricant additives

20. 6.3.2 Suspension. Suspension polymerization
Disperse monomer droplets in a noncompatible liquid (e.g. H2O)
Polymerize the monomer by an initiator (soluble in the monomer)
Stabilize the dispersion with a stabilizer (e.g. poly(vinyl alcohol) or
methyl cellulose)
Isolate granular bead products by filtration or spray drying
Heat transfer is efficient and reaction is easily controlled
Similar to bulk polymerization in kinetics and mechanism
Not applicable for tacky polymers (e.g. elastomers)
due to the tendency of agglomeration
Commercial uses
For making granular polymers, e.g. PS, PVC, PMMA

21. 6.3.3 Solution. Use monomer solution Heat transfer is very efficient
MW may be severely limited by chain transfer reaction
(probably caused by the solvent molecule or its impurities)
Solvent residues  difficult to remove completely
Environmental concerns  organic solvent waste
Use supercritical CO2 as a polymerization solvent 
nontoxic, inexpensive, easily removed and recycled

22. 6.3.4 Emulsion. Developed in 1920s at Goodyear Tire and Rubber Co.
Widely used for large-scale preparation
For water-based (latex) paints or adhesives
Emulsion  oil-in-water. Inverse emulsion  water-in-oil (less stable)

23. 6.3.4 Emulsion. Emulsification
Add emulsifying agent (e.g., soap or detergent) in an aqueous solution
 to form micelles
Monomers enter and swell the micelles
Initiation
Radicals (redox type) are generated in the aqueous phase and diffuse into micelles
Propagation
Polymerization propagated within micelles
More monomers enter micelles to support the polymerization
Termination
Termination by radical combination when a new radical enters the
micelle
Results
Extremely high MW are obtainable, but often too high to be useful
Chain transfer reagents are often added to control the MW
Also suitable for tacky polymers (∵ small particles are relative stable and can resist agglomeration)

24. 6.3.4 Emulsion. TABLE 6.3. Typical Emulsion Polymerization Recipesa
Ingredients, Conditions
Ingredients (parts by weight)
Water
Butadiene
Styrene
Ethyl acrylate
2-Chloroethyl vinyl ether
p-Divinylbenzene
Soap
Potassium persulfate
1-Dodecanethiol
Sodium pyrophosphate
Conditions
Time
Temperature
Yield
aRecipes from Cooper.23
bSodium lauryl sulfate.
190 70 30 - 5
0.3
0.5
12hr
50oC
65%
Styrene-Buradiene
Copolymer
Polyacrylate
Latex
133 93 2 3b 1
0.7
8hr
60oC
100%

25. 6.4 Kinetic and Mechanism of polymerization.
A. Mechanism of free-radical polymerization.
a. Initiation.   
1) Decomposition.
     Initiator → 2R
2) Addition.
(6.25) .

26. 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)

27. 6.4 Kinetic and Mechanism of polymerization.
 c. Termination.
(Combination, disproportionation, primary raidical termination )
 1) Combination.
(6.27)
Polystyrene radical (combination (77 %) + disproportionation (23 %)).
(6.29)

28. 6.4 Kinetic and Mechanism of polymerization.
 2) Disproportionation (79 %).
(6.28)
Poly(methyl methacrylate) radical.
(6.30)
3) Promary Radical Temination
combines with initiator radicals
 at high initiator levels under very high viscosity
(6.31)

29. 6.4 Kinetic and Mechanism of polymerization.
- Acrylonitrile : Combination virtually exclusively at 60℃.
- Poly(vinyl acetate) : Disproportionation (100 %).
Factors favor disproportionation 
Steric repulsion
Availability of -H for H-transfer
Electrostatic repulsion induced by the polar group (e.g., ester)
polyacrylonitrile  stable R• → chain coupling
poly(vinyl acetate)  unstable R• → chain disproportionation 

30. B. Kinetic of free radical polymerization.

31. B. Kinetic of free radical polymerization.
a. Assumption.
1) The rates of initiation, propagation, and termination are all different.
  2) Independent of chain length. (but 동일한 속도로 진행)
  3) Negligible end group.
  4) At steady state, constant radical concentration (- d[M.]/dt = 0).
    (steady state assumption)
 b. Initiation (Ri)
              
f : Initiator efficiency.
           
kd : Decomposition rate constant.
  [I] : molar concentration of initiator.
 [M ·] : molar concentration of radical.
Ri = dt
-d[M ·]
= 2fkd[I]
f =
radicals formed from initiator
radicals that initiate a polymer chain
f : initiator efficiency ( )

32. B. Kinetic of free radical polymerization.
 c. Termination rate ( Rt )
     
        
 d. Propagation rate ( Rp )
    
   Steady state assumption.
Rt= dt
-d[M ·]
= 2kt[M ·]2
kt = ktc+ ktd
Ri=Rt
Formation and destruction of radicals occur at the same rates
(assuming the majority of polymers are formed during the steady state)
[M·] remains constant  Ri = Rt
Rp = dt
-d[M]
= kp[M][M ·]
= kp[M]
[M ·]= 2

33. B. Kinetic of free radical polymerization.
e. Average kinetic chain length ( ) 
(Rt = Ri at steady state)
 Disproportionation :
   
Combination :
DP = 
DP = 2

34. B. Kinetic of free radical polymerization (Deviation).
Kinetics deviate from normal conditions
As viscosity becomes very high 
(bulk or concentrated solution polymerization)
when polymer precipitates
when chain transfer reaction occurs
Kinetics deviation in bulk or concentrated solution polymerization
As viscosity become very high
 Chain mobility reduce
 Termination rate decrease (higher MW)
 Polymerization rate autoaccelerate
 Reaction exotherm increase
 Gel forms
Gel effect / Trommsdorff effect / Norris-smith effect

35. B. Kinetic of free radical polymerization (Deviation).
 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.

36. B. Kinetic of free radical polymerization (Deviation).
Kinetics deviate from normal conditions
As viscosity becomes very high 
(bulk or concentrated solution polymerization)
when polymer precipitates
when chain transfer reaction occurs
Chain transfer —
terminate the growing chain by generating a new radical species
low MW / broad MWD / chain branching

37. C. Chain transfer reactions :
Growing radicals move to other parts
   by hydrogen abstracting.
   Lowering average kinetic chain length.
* Growing radicals move to other polymer chain.
(6.32) (
highly branched (mostly contain 3 or 4 C in length)
* Backbiting self polymer chain.
branched (mostly contain 5 or 6 membered cyclic transition states)
(6.33)
 LDPE : branching polymer.

38. C. Chain transfer reactions
 a. Moving to initiators or monomers.
(6.34)
(6.35)
Chain transfer to monomer (6.38)  in the presence of allylic H
Polypropylene cannot be prepared by free radical polymerization 

39. C. Chain transfer reactions
Methyl methacrylate
Competition reactions:
(A) Chain transfer to monomer  allylic radical
(B) Chain addition to monomer  3 and resonance-stabilized radical  A B
 Chain addition is favored (because B create a more stable radical)
 PMMA can be prepared via radial polymerization
 b. Moving to solvent.
Polystyrene made in CCl4 contains Cl due to Cl transfer (6.36)(6.37)
(6.36)
(6.37)

40. C. Chain transfer reactions (cf. 182 p)
c. Moving to chain transfer agent.
(6.39)
tr 1 =  +
[M ]
         
  Ct : Chain transfer constant.
   [T] : Concentration of chain transfer agent.
d. Telomerization : At high concentration of transfer agent,
ktr>kp.
    Low-molecular-weight polymers are obtained.
              (Telomer)

41. Alkylated phenol Very good stabilizer for monomer
Inhibit free radical polymerization via chain transfer reactions
Facilitate monomer shipment and storage
When monomers contain inhibitors
Remove inhibitors (by distillation or extraction)
Add extra initiator (to consume the inhibitor)
Exclude O2 (which is also a radical inhibitor)
Usually exhibit an induction period

42. D. Living free radical polymerization : Atom transfer polymerization.
Living polymerization / living polymers
Absence of chain termination or chain transfer
Chain ends remain active all the time
Add more monomer  increase MW
Add different monomer  form block copolymer
First reported for an anionic polymerization system (1950s)
Low polydispersity (as low as 1.05)
Living polymerization by atom transfer polymerization
(for radical polymerization)

43. D. Living free radical polymerization : Atom transfer polymerization.
  a. Copper(I) bypyridyl(bpy) complex: 
Polymerization of styrene
with 1-chloro-1-phenylethane and Cu(I)(bpy) complex

44. D. Living free radical polymerization : Atom transfer polymerization.
 b. TEMPO (18) : 2,2,6,6-tetramethylpiperidinyl-1-oxy.
Polymerize styrene with benzoyl peroxide plus TEMPO
(2,2,6,6-tetramethylpiperidinyl-1-oxy)
TEMPO is too stable to initiate the polymerization
TEMPO can promote the decomposition of peroxide,
forming the initial radical
TEMPO combines reversibly with chain ends,
keeping them alive (6.45)
TEMPO

45. 6.5 Stereochemistry of polymerization.
A. General consideration.
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.
Consideration factors:
interaction between chain end and monomer
influence by the configuration of penultimate repeating unit
끝에서 2번째에 있는

46. 6.5 Stereochemistry of polymerization.
Approach of monomer
If rotation does not occur before monomer addition:
(1) mirror approach (6.46)  isotactic
(2) nonmirror approach (6.47)  syndiotactic
mirror approach
isotactic
nonmirror approach
syndiotactic

47. 6.5 Stereochemistry of polymerization.
Radical polymerization usually lack of stereoregularity, because:
radical center is sp2 (less crowded)
no counter ions
Syndiotactic PMMA
By radical polymerization below 0℃
First reported in 1958
Possible due to steric influence of penultimate unit (6.48)
(1) if the radical center assumes substantially a sp3 hybridization
(2) assumes rotation is slower than propagation

48. 6.5 Stereochemistry of polymerization.
If the radical center assumes a sp2 planar structure (6.47), then
(1) the interaction between monomer and chain end would dominate,
(2) and adapt a nonmirror arrangement

49. 6.6 Polymerization of Dienes
  6.6.1 Isolated Dienes
A. Crosslinked or cyclopolymerization.

50. 6.6 Polymerization of Dienes
Diallyl monomers
for making highly cross-linked thermosetting allyl resins
(or as a crosslinking agent)
Uses of monomer 21 
For electrical or electronic items
(circuit boards, insulators, television components, etc.)
(2) For fiber-reinforced plastics
(for pre-impregnating glass cloth or fiber)
Uses of monomer 22 
For applications requiring good optical clarity
eyeware lenses, camera filter, panel covers

51. 6.6.2 Conjugated Dienes. Can undergoes 1,2- and /or 1,4-addition 1,2-
cis-1,4
trans-1,4
Radical polymerization of 1,3-butadiene
1,2-addition (20%) ; 1,4-addition (80%) with majorly trans
increase T
 increase cis-1,4 structure
 but keep the same 1,2-/1,4- ratio (Table 6.6)

52. 6.6.2 Conjugated Dienes.

53. 6.7 Monomer Reactivity A. Thermodynamic feasibility.
a. ΔGp = ΔHp - TΔSp
     ΔGp : Gibbs free energy change of polymerization.
     ΔHp : Enthalpy change of polymerization.
     ΔSp : Entropy change of polymerization.
     ΔGp < 0 : favorable free energy of polymerization.
b. Values of ΔH and ΔS for several monomers.
c. Polypropylene and isobutylene :
   
  ΔG < 0 → unfavorable polymerization.
     because of kinetic feasibility

54. 6.7 Monomer Reactivity -H (kJ/mol) -S (J/mol) ΔGp = ΔHp - TΔSp
TABLE 6.7. Representative Enthalpies, H, and
Entropies, S, of Polymerizationa
-H
(kJ/mol)
-S
(J/mol)
ΔGp = ΔHp - TΔSp
Monomer
Acrylonitrileb
1,3-Butadiene
Ethylene
Isoprene
Methyl methacrylate
Propylene
Styrene
Tetrafluoroethylene
Vinyl acetate
Vinyl chloride 77 78
109 75 65 84 70
163 90 71
109 89
155
101
117
116
104 -
However, propylene or isobutylene (contain allylic H)
dose not undergo normal free radical polymerization
due to the propensity for chain transfer of the allylic H
aValues selected from Ivin.29c Polymerization temperature
25oC unless otherwise indicated.
b74.5oC.
c127oC.

55. 6.7 Monomer Reactivity Reactivity of monomers  Stability of monomer
Stability of monomer radical
(more important)
Styrene
Its vinyl group should be stable
due to conjugation resonance with the phenyl ring
Polymerization however still undergoes readily
 due to the formation of highly stable radical (33)

56. 6.7 Monomer Reactivity (1) In gas phase, addition to ethyl radicals 
acrylonitrile ＞ styrene ＞ vinyl acetate  radical stabilization effect
(2) In solution phase, polar effect make acrylonitrile very stable
(3) Incorporation of 14C into polymer (in solution)
(or monomer reactivity toward benzoyloxy radical)
styrene ＞ vinyl acetate ＞ acrylonitrile

57. 6.7 Monomer Reactivity Monomer stability aspect (ref. Table 6.4)
Vinyl acetate  very stable monomer
 but, high kp (2640)
Styrene  unstable monomer
 but, low kp (176) (due to resonance stabilization)
Acrylonitrile  although propagating radical is stabilized by –CN
 yet, high kp (1960)
Methyl methacrylate has much smaller kp (515) than methyl
acrylate (2090)
 due to steric hindrance and the hyperconjugation stabilization
effect of the intermediate radicals

58. 6.7 Monomer Reactivity Steric effects H2C=CHR, H2C=CR2, CHR=CHR
1,2-disubstituted monomers are most difficult to polymerize
(except F-containing monomers)
But, 1,2-disubstituted polymers are thermodynamically more stable
than 1,1-disubstituted polymers
 (∴ kinetic control)

59. 6.7 Monomer Reactivity Depropagation has larger S
∵ TS term increases with T
∴ T  increase; kdp  increase
At T = Tc
(i.e. ceiling temperature)
 Rp = Rdp

60. 6.8 Copolymerization. Self-propagation / cross-propagation
in steady-state, [M1] and [M2] remain constant
M1  M2 and M2  M1
proceed at equal rate
 k12 [M1][M2] = k21[M2][M1]
Rate of disappearance of M1 and M2

61. B. Kinetics of copolymerization.
b. 
c. let, (reactivity ratio)
  steady state assumption.
    
 d. solving   
: Copolymer equation or copolymer composition equation.
d[M1]/d[M2] : the molar ratio of the two monomers in the copolymer
[M1], [M2] : the initial molar concentration of monomers in the
            reaction mixture
and

62. B. Kinetics of copolymerization.
d[M1]/d[M2]: molar ratio of repeat units in copolymer
[M1]/[M2]: molar ratio of monomers in initial feed
r1 = k11/k12 ; r2 = k22/k21
r1  1 M1 tends to self-propagate
r1  1  copolymerization is preferred
Knowing r1 and r2  can project the desired d[M1]/d[M2]
(for conversion 10%)
f1 and f2: molar fractions of M1 and M2 in the initial feed
F1 and F2: molar fractions of M1 and M2 in the final copolymer

63. B. Kinetics of copolymerization.
The copolymer equation can be rewritten
Determination of reactivity ratios 
(1) Prepare copolymers using a range of monomer feed ratios
(2) Stop the polymerization at the early stages
(3) Determine the repeat unit ratios of the copolymers

64. B. Kinetics of copolymerization.

65. C. Significance of reactivity ratio (r1, r2).
 truly random copolymer
F1 = f1 (curve A)
ethylene (M1)  vinyl acetate (M2)
r1 = 0.97, r2 = 1.02
(2) r1 = r2 = 0
(i.e. k11= 0, k22= 0)
 truly alternating copolymer
F1 = 0.5 (curve B)
styrene (M1) maleic anhydride (M2)
r1 = and r2 = 0.01
(3) 0 < r1 < 1, 0 < r2 < 1
(more common)  curve C
styrene (M1)  methyl methacrylate (M2)
r1 = 0.52, r2 = 0.46 

66. C. Significance of reactivity ratio (r1, r2).
(i.e. k11= 0, k22= 0)
 truly alternating copolymer
F1 = 0.5 (curve B)
styrene (M1) maleic anhydride (M2)
r1 = and r2 = 0.01
e.g. 60 oC
St(r1=0.01)—maleic anhydride(r2=0)
F1～f1 plot

67. C. Significance of reactivity ratio (r1, r2).
Azeotropic polymerization
At cross point between curve C and the diagonal (i.e., f1 = F1)
No change in [M1]/[M2] and d[M1]/d[M2] during the polymerization ∴ ∴ ∵  ∴
[St]/[MMA] = (1-0.46)/( ) = 1.125

68. C. Significance of reactivity ratio (r1, r2).
Azeotropic polymerization
At cross point between curve C and the diagonal (i.e., f1 = F1)
No change in [M1]/[M2] and d[M1]/d[M2] during the polymerization
Azeotrope point
r1=0.6
r2=0.3
The azeotrope is important, particularly in industry, because the monomer and copolymer composition do not change with conversion, thus producing copolymers homogeneous in composition. Copolymerizations under the other conditions will change the instantaneous compositions along the composition curve. A
r1=0.5
r2=0.5
F1～f1 plot

69. C. Significance of reactivity ratio (r1, r2).
For r1 > 1 and r2 > 1  azeotropic polymerization can also
happen, in principle, but never been observed in free
radical polymerization
(4) r1 >> 1, r2 << 1  curve D
styrene (M1)  vinyl acetate (M2)
r1 = 55, r2 = 0.01
styrene (M1)  vinyl chloride (M2)
r1 = 17, r2 = 0.02
(5) when r1r2 = 1  often referred as ideal copolymerization
However, a random distribution of monomers only occurs
when r1 and r2 are both close to 1
Methyl methacrylate (M1) and vinyl acetate (M2)
r1 =10, r2 = 0.1 (r1r2=1)
 curve E (instead of curve A)

70. D. Alfrey-price Q-e scheme.
A semiempirical relationship
Express reactivity ratios in terms of characteristic constants of
each monomer
Independent of comonomer
r1 = k11/k12 ; r2 = k22/k21
Q1, Q2: measures of the reactivity of M1, M2
(related to resonance stabilization of monomer)
e1, e2: measures of the polarity of the monomers

71. D. Alfrey-price Q-e scheme.
Styrene (chosen as the standard): Q = 1.00, e = -0.80
Q increase with resonance stabilization
e is less negative with electron attracting substituent group

72. D. Reactivity ratios  stable unstable stable
Controlled by: steric / resonance / polar effects
Steric effects: 1,2-disubstituted ethylene is difficult to form homopolymer
Resonance effects: more stable intermediate easier to form
( less reactive in propagation)
Styrene / vinyl acetate (r1 = 55, r2 = 0.01)
 ∵ styryl radical is much more stable than ethyl acetate radical
(2) Styrene / methyl methacrylate (r1 = 0.52, r2 = 0.46)
 ∵ styryl radical and methyl methacrylate radicals are similarly stable
Polar effects: styrene / methyl methacrylate
the e-withdrawing methyl methacrylate or e-donating styrene
 increase the tendency of adding to the opposite member 
stable
unstable
stable

73. E. Charge transfer complex polymerization (alternating copolymer).
Theories: (1) polar effects in the TS (2) charge-transfer interactions
Maleic anhydride / styrene (r1  0 , r2  0)
maleyl radical
Polar effects 
involve resonance-stabilized
transition states
styryl radical

74. Regarding to formation of charge-transfer complexes
shows characteristic UV max of the complex
acts as a homopolymerization of the complex
propagating chain end is a donor-acceptor (D-A) complex
(not a conventional radical, nor an ionic species)
Evidence favoring D-A complex
Reach maximum rate at 1:1 monomer feed
Alternation occur at all feed ratio
Fail to incorporate other added monomer
Rate enhanced by Lewis acids (by increasing acceptor property)
5. MW is hardly affected by any chain transfer agents

75. Regarding to formation of charge-transfer complexes
Other examples 
polyketones
polysulfones
Both reactions tend toward alternation copolymerization
SO2 and alkenes is known to form 1:1 complexes
Possibly helped by polar effects in TS or charge transfer complex