Anionic Polymerization 2024 Chemistry course

Anionic Polymerization 2024

1. Anionic Polymerization Prof . Cyrille Boyer

2. Introduction

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 10 6 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. 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 + → AM n+1 - + X + No termination step: AM n+1 - + AM m+1 - → No reaction

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. C 4 H 9 Li Also used other nucleophilic compounds Alkoxides, hydroxides, cyanides, amines and phosphines Macromolecules 2017, 50, 18, 6979-6997

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. 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. 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. 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. 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. Another example of electron transfer

12. Example of tertiary amine: Neutral nucleophile Initiators

13. 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

14. Choice of Initiator Ideally, pick an initiator with a similar pK a 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

15. pKa of conjugate acid of carbanion

16. 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:

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

18. 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, 4 th Ed. 2004 John Wiley & Sons; Hoboken, NJ; pp 200.

19. 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?

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

21. Monomer activities

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

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

24. Reactivities

25. Monomer Selection of initiator depends on monomer

26. Selection of initiator depends on monomer Initiator

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

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

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

30. Case of polar monomers

31. 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

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

33. Example of potential side reactions with methyl acrylate

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

35. 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

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

37. 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

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

39. Proposed mechanism

40. 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

41. 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

42. Polymerization of functional monomers

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

44. Synthesis of poly(functional methacrylate) Monomers

45. Synthesis of poly(functional methacrylate)

46. Termination reactions

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

48. 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.

49. 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 CO 2 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

50. Anionic polymerization kinetics

51. 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 + B A

52. Polymerization Kinetics Ln([M] /[M] t ) Time Ln([M] /[M] t ) = k p app × time → Constant concentration of active group Ln([M] /[M] t ) = Rp= k p 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(M /M) is linear

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

54. Ln(M /M) Time A C → Constant active groups (carbanion), i.e. Ln(M /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

55. 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 o C.

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 o C.  increased solvating power of the reaction medium result in increased fraction of free ions present relative to ion pairs.

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

58. Factors affecting k p app [M - ] is combination of [P - ] + [P - (C + )]  [M - ] = [P - ] + [P - (C + )] R p = k p - [P - ][M] + K p ± [P - (C + )][M] (1) R p = = k p app × [M - ] × [M] (2) Combined (1) and (2) k p app P - (C + ) ↔ P - + C + Dissociation constant: if [P - ]=[C + ]    

59. 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.

60. 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?

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 M n at full monomer conversion can be calculated by Assuming all the anionic initiators have been activated What is the evolution of Mn versus monomer conversion?

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

63. Molecular weight Monomer conversion Linear evolution of Mn versus monomer conversion → Prediction of the molecular weight versus monomer conversion M n = [M] /[Initiator] × 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 th e consequence for the dispersity (or polydispersity)?

64. → 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

65. 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

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

67. 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

68. 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?

69. 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

70. 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)

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

72. Set-up