The document describes anionic polymerization, a technique developed by M. Szwarc in 1956, known for its precise molecular weight control and ability to synthesize tailored polymer architectures. Key processes include initiation using carbanion initiators and propagation without termination, allowing for the production of block and functional copolymers. Factors like initiator choice, solvent effects, and the nature of the monomers used are critical for optimizing the polymerization process and achieving desired polymer properties.

1. Anionic Polymerization
Prof. Cyrille Boyer

2. 2
Introduction

3. 3
Anionic Polymerisation
• Michael Szwarc reported in the year 1956 the first anionic polymerisation
https://onlinelibrary.wiley.com/doi/full/10.1002/macp.201700217
Lowest dispersity of all synthetic methods
known, described by a Poisson distribution.
Excellent molecular weight control by the
ratio of monomer and initiator, [M]/[I]
High and even extremely high molecular
weights exceeding 106 g mol–1 can be
achieved.
Complete chain end functionalization is
possible, enabling the synthesis of
AB‐diblock, ABC‐triblock and even
(AB)n‐multiblock copolymers as well as a
broad range of precisely end‐functionalized
polymers. M. Szwarc, Nature 1956, 178, 1168.

4. 4
Anionic polymerization
• Two main steps:
→ As free radical polymerization, there are initiation and propagation steps
Initiation step:
Dissociation of anionic initiator: A-X ↔ A- + X+
Addition to monomer: A- + M + X+ → AM- + X+
Propagation step: AM- + n × M + X+ → AMn+1
- + X+
No termination step: AMn+1
- + AMm+1
- → No reaction

5. 5
Anionic Polymerization – Initiation Step
• Initiating species is a carbanion and the counterion is typically a metallic
species
– Usually organometallic compounds
• n-butyl, sec-butyl, di-phenyl ethyl
– Li, Na, K, or Cs
– Alkyl lithium compounds are the most used, i.e. C4H9Li
– Also used other nucleophilic compounds
• Alkoxides, hydroxides, cyanides, amines and phosphines
Macromolecules 2017, 50, 18, 6979-6997

6. 6
Alkyl lithium Initiators
• The most extensively used and robust systems
– Highly soluble in a wide range of hydrocarbons.
– The heavier the metal, the less soluble the initiator tends to be.
• Alkyl and aryl alkali metal initiators are highly soluble in ethers
– However, very reactive with ether
Why alkyl lithium is very reactive in ether solvent?
– Lower temperatures are required or less reactive species
• Benzyl potassium, cumyl cesium, diphenylmethyl lithium
• Large resonance stability

7. 7
• Nucleophilic addition (monomer addition):
• Propagation
• The extensive use of alkyl lithium initiators is due to their solubility in hydrocarbon
solvents.
• Alkyls or aryls of the heavier alkali metals are poorly soluble in hydrocarbons, a
consequence of their more ionic nature.
Alkyl lithium Initiators

8. 8
Initiation and propagation
What is the consequence on the consumption of the initiator during the
polymerisation? Can you plot [initiator] versus time (qualitatively)?
How this compares with radical polymerisation? Can you plot [Initiator]
versus time (qualitatively)?
→ Initiation step is usually very fast and much faster than the propagation in anionic
polymerisation

9. 9
Initiation (by electron transfer)
• Szwarc and coworker have studied the interesting and useful polymerizations
initiated by aromatic radical-anions such as sodium naphthalene
• The naphthalene anion–radical (which is colored greenish-blue) transfers an
electron to a monomer such as styrene to form the styryl radical–anion

10. 10
Initiation (by electron transfer)
The styryl dianions so-formed are colored red (the same as styryl monocarbanions
formed via initiators such as n-butyllithium). Anionic propagation occurs at both
carbanion ends of the styryl dianion
The styryl radical–anion is shown as a resonance hybrid of the forms wherein the anion
and radical centers are alternately on the a- and b-carbon atoms. The styryl radical–
anion dimerizes to form the dicarbanion

11. 11

12. 12
Another example of electron transfer

13. 13
• Example of tertiary amine:
Neutral nucleophile Initiators

14. 14
Metal-free anionic polymerisation
In the relatively few anionic polymerizations initiated by neutral nucleophile, such as
tertiary amines or phosphines the proposed propagating species is a zwitterion

15. 15
Choice of Initiator
• Ideally, pick an initiator with a similar pKa value to the monomer (see the
next slide for an example of pKa values)
– Ensures the reactivities are relatively the same
– Initiator with a lower reactivity, then polymerisation is slow or non-
existent
– Initiator too high, then side reactions are favoured (see example,
methacrylate)
• Must be reactive enough to attack monomer, i.e. stronger nucleophile (more
aggressive)
• Ethylene, dienes, and styrenes
– Alkyl lithium compounds
• Acrylates and methacrylates
– 1,1-diphenylhexyl lithium, cumyl cesium or potassium

16. 16
pKa of conjugate acid of carbanion

17. 17
Choice of initiators
• If monomer substituent Y is strongly e- withdrawing;
→ then activated monomer is relatively stable
→ relatively weaker nucleophiles can initiate it
ex: epoxy: ethoxyanion initiate ring polymerization with variety of initiators
• If substituent Y is weakly e- withdrawing:
→ need stronger nucleophile to initiate it:

18. 18
Propagation step (monomer type)
Anionic polymerization takes place with monomers possessing electron-withdrawing
groups such as nitrile, carbonyl, phenyl, and vinyl.

19. 19
Type of monomers
Monomer Anionic
Ethylene 
1-Alkyl olefins 
1,1 Dialkyl olefins 
1,3-Dienes 
Styrenes 
Halogenated olefins 
Vinyl esters 
(Meth)acrylates 
(Meth)acrylonitrile 
(Meth)acrylamide 
Vinyl ethers 
N-Vinyl carbazole 
N-Vinyl pyrrolidone 
Aldehydes, ketones 
Odian, G. Principles of Polymerization, 4th Ed. 2004 John Wiley & Sons;
Hoboken, NJ; pp 200.

20. 20
Choice of the monomer
• Vinyl monomers need to support carbanion
Y can be a range of electron-withdrawing groups
→ How withdrawing impacts monomer reactivity?
• Monomer should have no protic or acidic hydrogens
Why?

21. 21
Consider potential side reactions
• No electrophilic groups
• There are some exceptions: certain groups are electrophilic but less reactive to
carbanion of interest:

22. 22
Monomer activities

23. 23
Reactivity of the monomer can be estimated
by pKa value of conjugate acid

24. 24
Other monomers (no vinyl monomers)
• Cyanoacrylate
• Isocyanate: R-N=C=O
• N-carboxyanhydrides

25. 25
Reactivities

26. 26
Monomer
Selection of initiator depends on monomer

27. 27
Selection of initiator depends on monomer
Initiator

28. 28
However, do not forget to consider potential side reactions
Selection of initiator depends on monomer

29. 29
Selective living anionic polymerization
Why do we see selectivity if DPHLi is used as
initiator?

30. 30
Selective living anionic polymerization
What do you expect if nBuLi is used as initiator?

31. 31
Case of polar monomers

32. 32
Polar monomers (methacrylates)
• Polar monomers, such as methyl (meth)acrylate, methyl vinyl ketone, and
acrylonitrile, are more reactive than styrene and 1,3-dienes because the polar
substituent stabilizes the carbanion propagating center by resonance interaction
to form the enolate anion.
• Several different nucleophilic substitution reactions have been observed in the
polymerization of methyl methacrylate. Attack of initiator on monomer converts
the active alkyllithium to the less active alkoxide initiator

33. 33
Polar monomer – side reaction
• Intramolecular backbiting for (meth)acrylate

34. 34
Example of potential side reactions with
methyl acrylate

35. 35
Effect on the initiator system on the tacticity
Atactic polymers
Syndiotactic polymers
Isotactic polymers

36. 36
Control of Tacticity – Changing Initiators or
Solvent
• Polymerization of MMA – Effect of solvent
Using 1,1-diphenylhexyl lithium as initiator
– In toluene yields highly isotactic polymer
– In THF yields syndiotactic polymer

37. 37
PMMA – effect of solvent and adding Lewis acid
Nikos Hadjichristidis and Akira Hirao, Anionic Polymerization, Springer (book)

38. 38
Uraneck, C.A. (1971), Influence of temperature on microstructure of anionic‐initiated
polybutadiene. J. Polym. Sci. A‐1 Polym. Chem., 9: 2273-2281.
doi:10.1002/pol.1971.150090814
Polymerization of butadiene in cyclohexane
Effect on the cis/trans- configuration with
temperature

39. 39
Effect on the cis/trans- configuration with
temperature

40. 40
• Proposed mechanism

41. 41
Addition of small amount of THF (polar
solvent )
Uraneck, C.A. (1971), Influence of temperature on microstructure of anionic‐initiated
polybutadiene. J. Polym. Sci. A‐1 Polym. Chem., 9: 2273-2281.
doi:10.1002/pol.1971.150090814

42. 42
Effect on the cis/trans- configuration-
Changing Initiator types
Example of Polymerization of isoprene
Using butyl lithium initiator results in >96% 1,4-cis microstructure (similar to
natural rubber)
Using butyl sodium or potassium results in more than 50% 1,4-trans
microstructure

43. 43
Polymerization of functional monomers

44. 44
Strategies to introduce functionality
Macromolecules 2014, 47, 6, 1883-1905

45. 45
Synthesis of poly(functional methacrylate)
Monomers

46. 46
Synthesis of poly(functional methacrylate)

47. 47
Termination reactions

48. 48
Termination (Spontaneous)
• Living polymers do not live forever. In the absence of terminating agents, the
concentration of carbanion centers decays with time.

49. 49
Termination
• Water: Most anionic (as well as cationic) polymerizations are carried out in an
inert atmosphere with rigorously cleaned reagents and glassware since trace
impurities (including moisture) lead to termination.
• The hydroxide ion is usually not sufficiently nucleophilic to reinitiate
polymerization and the kinetic chain is broken.
• Water has an especially negative effect on polymerization, since it is an active
chain-transfer agent.
• Oxygen and carbon dioxide from the atmosphere add to propagating carbanions
to form peroxide and carboxyl anions. These are normally not reactive enough to
continue propagation.

50. 50
End-Capping of Polymer Chains
• Adding carbon dioxide at the end of anionic polymerization of
styrene can result in carboxylic acid formation
• In case of styrene, adding CO2 can result in 70% carboxylic acid
formation due to side reactions
Solution: By adding 1,1-diphenylethylene first (reducing the
reactivity of carbanion), >98% carboxylic acid formation results
• Reaction with ethylene oxide results in an alcohol functionality

51. 51
Anionic polymerization kinetics

52. Anionic Polymerization
• Four distinct types of ion pairs
– Covalent species (I): chemical bond between ion species
– Contact ion pair (II): covalent bond broken, but virtually no charge separation
– Solvent-separated ion pair (III): solvent molecules separate the charges, but
they are still close
– Free ion pairs (IV): highly solvated, both species free to diffuse through the
system
B-A+ B- ||A+ B- + A+
BA

53. 53
Polymerization Kinetics
Ln([M]
0
/[M]
t
)
Time
Ln([M]0/[M]t) = kp
app × time → Constant concentration of active group
Ln([M]0/[M]t) = Rp= kp
app× [M-] × [M]
[M-] corresponds to active species, where [M] is the total concentration of all types of
living anionic propagating centers (free ions and ion pairs).
→ Most of anionic polymerization: Constant active groups (carbanion), i.e. Ln(M0/M) is
linear

54. 54
Ln(M
0
/M)
Time
A
C
B
Polymerisation Kinetics
What mean B and C?

55. 55
Ln(M
0
/M)
Time
A
C
→ Constant active groups (carbanion), i.e.
Ln(M0/M) is linear (A)
→ Decrease of active species (C) –
carbanion is consumed by a side reaction
→ Increase of active species (B), very
specific condition, such as slow addition of
initiator
B
Polymerisation Kinetics

56. 56
Polymerization Kinetics
Effect of solvent
Why such difference between solvents?
Table shows the pronounced effect of solvent in the polymerization of styrene by
sodium naphthalene at 25 oC.

57. 57
Polymerization Kinetics
Effect of solvent
Why such difference between solvents?
Table shows the pronounced effect of solvent in the polymerization of styrene by
sodium naphthalene at 25oC.
 increased solvating power of the reaction medium result in increased
fraction of free ions present relative to ion pairs.

58. Anionic Polymerization
B-A+ B- ||A+ B- + A+
BA
Polarity of the solvent

59. 59
Factors affecting kp
app
• [M-] is combination of [P-] + [P-(C+)]  [M-] = [P-] + [P-(C+)]
• Rp = kp
-[P-][M] + Kp
±[P-(C+)][M] (1)
• Rp = = kp
app× [M-] × [M] (2)
Combined (1) and (2)
• kp
app=
[kp
−[P−]+ Kp
±[P−(C+)]
[M−]
• P-(C+) ↔ P- + C+ Dissociation constant: 𝐾 =
𝑃
−
[𝐶
+
]
[𝑃
−
𝐶
+
]
,
• if [P-]=[C+] 𝐾 =
𝑃
−
2
[𝑃
−
𝐶
+
]
 𝑃
−
= √(𝐾 × 𝑃
−
𝐶
+
)

60. 60
• Effect of counterion on Anionic Polymerization of Styrene
Polymerization Kinetics
Table shows that the dissociation constant for the ion pair decreases in going from
lithium to cesium as the counterion in THF.
The order of increasing K is the order of increasing solvation of the counterion.
In dioxane: Solvation is not important in dioxane. The ion pair with the highest
reactivity is that with the weakest bond between the carbanion center and counterion.
The bond strength decreases, reactivity increases with increasing size of counterion.

61. 61
Anionic Polymerization – Evolution of Mn
versus monomer conversion
• Because termination only occurs when small molecules are added
(or leaked in), the system never dies
– Polymerization continues til monomer is used up
– Alkyl and aryl lithium compounds are stable for several days
What is the expected molecular weight at full monomer
conversion? Can you write the equation to link Mn,
monomer concentration, initiator concentration?

62. 62
Anionic Polymerization – Evolution of Mn
versus monomer conversion
• Because termination only occurs when small molecules are added (or
leaked in), the system never dies
– Polymerization continues til monomer is used up
– Alkyl and aryl lithium compounds are stable for several days
• Mn at full monomer conversion can be calculated by
Assuming all the anionic initiators have been activated
 
  init
mono
n MW
MW
I
M
M 


What is the evolution of Mn versus monomer conversion?

63. 63
Molecular
weight
Monomer conversion
• Linear evolution of Mn versus monomer
conversion
→ Prediction of the molecular weight versus
monomer conversion
Mn = ([M]0/[Initiator]0) × α × MW (Monomer)
+ MW (Initiator)
What is the condition for this equation to be
valid?
Anionic Polymerization – Evolution of Mn
versus monomer conversion

64. 64
Molecular
weight
Monomer conversion
• Linear evolution of Mn versus monomer
conversion
→ Prediction of the molecular weight versus
monomer conversion
Mn = [M]0/[Initiator]0 × α × MW (Monomer) +
MW (Initiator)
Anionic Polymerization – Evolution of Mn
versus monomer conversion
Condition: In anionic polymerisation, the initiation is fast (all chains are activated at the
beginning of the polymerization) in comparison to the rate of chain propagation.
What is the consequence for the dispersity (or polydispersity)?

65. 65
→ Narrow dispersity (or
polydispersity) if fast addition is
achieved
Living polymerization Đ ~ 1.005-
1.20
In contrast higher dispersity will be
achieved if a slow activation
Non-living polymerization Đ ~ 1.5 -
2.0
Anionic Polymerization – Evolution of Đ
versus monomer conversion

66. 66
Complex architectures – block copolymers
• Preparation of block copolymer, i.e., addition of monomers at the end of
the polymerization, the polymer chain can be chain-extended
→ Stability of the anionic species (if conditions are maintained), i.e. active growing
chains remain constant

67. 67
Synthesis of Block Copolymers
• Need to consider the reactivity of monomers
– Styrene and dienes have similar reactivities
– Methacrylates will NEVER initiate styrene
Why?

68. 68
Synthesising Block Copolymers
• Need to consider reactivity of monomers
– Styrene and dienes have similar reactivities
– Methacrylates will NEVER initiate styrene
– pKa of toluene ~ 43
– pKa of ethyl acetate ~ 30

69. 69
Synthesis of Block Copolymers (Case of
(meth)acrylate)
• Need to consider reactivity of monomers
– Styrene and dienes have similar reactivities
– Methacrylates will NEVER initiate styrene
• Styrene can initiate methacrylates under certain conditions but side
reactions
Why do side reactions occur when MMA is initiated by PS?
How can these side reactions be minimized or eliminated?

70. 70
Synthesizing Block Copolymers
• Need to consider reactivity of monomers
– Styrene and dienes have similar reactivities
– Methacrylates will NEVER initiate styrene
• Styrene CAN initiate methacrylates under certain conditions,
but side reactions can occur
• Need to convert the chain end with 1,1-diphenylethylene
CH2 CH
-
Li
+
CH2 CH CH2 C
-
Li
+
C
H2
CH2 CH CH2 CH2 C
-
CH3
O O
CH3
Li
+
C
H2
O
O
CH3
C
H3

71. 71
Telechelics and Thermoplastic Elastomers
• Difunctional initiators
– Easiest way to synthesize A-B-A triblock copolymers
• Hard-soft-hard segments
– Styrene is the hard block – high glass transition
temperature
– Butadiene is the soft block – low glass transition
temperature (below room temperature)

72. 72
Complex architectures – star polymers
Macromolecules 2014, 47, 6, 1883–1905

73. 73
Set-up