Chap 11. Non-Radical Addition Polymerization

1. Chap 11. Non-Radical Addition Polymerization Anionic Polymerization -the growing chain end bears a negative charge The mechanism of anionic polymerization is a kind of repetitive conjugate addition reaction . (the "Michael reaction" in organic chemistry) Cationic Polymerization -the growing chain end bears a positive charge The mechanism of cationic polymerization is a kind of repetitive alkylation reaction.

2. Anionic Polymerization General Scheme

3. Anionic Polymerization Styrene Polymerization

4. Anionic Polymerization Characteristics of an Ideal Anionic Polymerization • The rate of polymerization for methacrylates and styrenes is high even at -78 oC. This is partly for the high concentration of the anion centers. • The degree of polymerization • K=1 or 2 depending on initiator used. Negative centers repel one another and thus termination by recombination is not possible. An ideal polymerization is “living”, which does not terminate until a terminator is added. Initiation is normally very fast relative to propagation and all chains grow simultaneously. This leads to polymers with low polydispersity or monodispersity. Theoretically:

5. Anionic Polymerization Initiation by Electron Transfer • Polymerization mostly done in THF and not nonpolar solvents like cyclohexane or benzene for the solubility the complex in THF. • The degree of polymerization is given by

6. Anionic Polymerization Initiation by Nucleophilic Attack • Polymerization can be done in both polar and nonpolar solvents. • The degree of polymerization is given by

7. Anionic Polymerization Initiation by Living Polymer But not Because the starting anion has to be a stronger Lewis base than the resulting anion.

8. Anionic Polymerization Propagation • Kp can vary by orders of magnitude. • The polydispersity remains low because the rate of inter-conversion between the different forms is much faster than that of polymerization.

9. Anionic Polymerization Termination

10. Anionic Polymerization • (1) proton donorH2O or ethanol • Strong baseis not enough for initiation. • (2) • Ctr,s=10-3 (small chain transfer constant) • EtOH high MW product • ethoxide no longer living. • H2O Ctr,s=10 • H2O low MW polymer • No living polymer Strong baseis not enough for initiation.

11. Anionic Polymerization • (3) Termination can occurred by hydride elimination without impurities. • a) • b) anionic species(active center) react with chain endsto form inactive • allylic anion. 1,3 diphenylallyl anion very unreactive, highly resonance stabilized

12. Anionic Polymerization • Termination of polar monomer • In this case, althoughinitiator oractive center attackmonomer, that means • non-polymerization.

13. Anionic Polymerization • Backbiting orintramolecular rexn • 4) Hugginson-Wooding System • J.Chem. Soc. 1952 • Polymerization of styrene conducted in liq. NH3 at bp -33C • (1) reaction rate ↑ as [I] and [M]2 • I=K+NH2- rate ↑ as [NH2-] ↑but as [K+] ↓ • (2) MW[K+] and [NH2-] • (3) Polymer is formed withoutunsaturation. Cyclic trimer at the end of chain

14. Anionic Polymerization Initiation step Dissociation of initiator If [K+] , thenRi 

15. Anionic Polymerization Propagation Termination Occurs by chain transfer Overall Rate using Steady state assumption. (RiRt). Rtr=ktr,s[M-][NH3+] Add KCl Rp slow down [K+]=[NH2-]

16. Activation energy for Xn Anionic Polymerization In dehydrate state, Chain transfer constant for solvent

17. Anionic Polymerization In Flory If there is no termination rxn, we can obtain narrow MW distribution. 5) Base Initiated Polymerization - a powerful nucleophile is required as initiator  

18. Anionic Polymerization 6) Practical Comments purity import! If we use metal as an initiator, propagation rate is fast. 7) Propagation Kinetics propagationis not so fast compared to radical polymerization About manyliving polymers [M:-] = [I] [M] = is about 10-9 to 10-7 molar [M:-] = 10-3 to 10-2 molar kp for free radical case is 5103 l/molesec Kp : depends on solventand counter ion Counter ion and active center can be separated by changing solvent then rxn rate increase conc. of anion = conc. of initiaor

19. Anionic Polymerization • (1) Evaluatation of Individual Propagation Rate Constants • Propagation rate constant for free ion and ion pair. • [P-]:conc. of free ion • [P-(C+)]: conc. of ion pair

20. Anionic Polymerization * How to measure kp, kp, K ? t

21. Anionic Polymerization A salt that must be soluble in THF with common ion to gegen ion is added to reaction mixture. At high added salt conc. Conc. of added salt is [CZ] [C+][CZ] Conc. of living and conc. of free ion Now Then able to get kp-, kp,K from two graphs.

22. Anionic Polymerization Effect of gegen ion on Anionic Polymerization of Styrene - Why kp- is the same value? ; kp- is much more larger than kp Thus we can say that reactivity of free ion is much greater than that of ion pairs. - In the case of dioxane? ;In dioxane which is not tend to be solvating it has reverse tendency compared to the case of THF. Solvation is not important in dioxane. Cs is too big that there is no difference. Explanation is that there is not so solvating power of Cs

23. Anionic Polymerization Li+ genenion in aromatic hydrocarbon Look at difference. Unassociated species Let’s say we are using BuLi initiator. solvation as well as  is important! Although 1,2 diethoxyethane reduce , kp varies 1~1000 fold because of highly solvating ether. Reactivity of free ion < Reacitivity ofion pair In aromatic hydrocarbon, unassociated species dominate rate. Depend on unassociated species in very low conc. Covalent character

24. Anionic Polymerization Evidence — viscosity measurement before and after term we find that living polymer is associated after termination, viscosity drops. Because initiators and ion pairs are reduced, Polymerization rexn in Aliphatic HC is lower than inaromatic HC.

25. Anionic Polymerization Lenz P.437 Table 13-9 Effect of solvent and gengenion on Copolymerization of Styrene and isoprene at 25C Generallysodium is more ionic than lithium

26. Cationic Polymerization The growing chain bears a positive charge. The active sites are either carbenium ions or oxonium ions. Electron donating groups are needed as the R groups because these can stabilize the propagating species by resonance. Ex)

27. Cationic Initiators Proton acids with unreactive counterions Lewis acid + other reactive compound: * To use Lewis acid effectively as initiators, use coinitiator. cationogen

28. Cationic Polymerization Typical Initiator Systems Co-initiator Initiator SnCl4 H2O AlCl3 HCl H2SO4 H2SO4 More acidic initiators are most effective in initiating polymerization Order of reactivity AlCl3 > AlRCl2 > AlR2Cl >AlR3 HCl > CH3COOH > C6H5NO2 > > H2O >> CH3OH > CH3COCH3 Ex)

29. Cationic Polymerization • Termination • Kinetics Problem : temination reactions occur randomly. [ * ] can control rxn

30. Cationic Polymerization

31. Chain Transfer Reactions - Cationic vinyl polymerization is plagued by numerous side reactions, most of which lead to chain transfer Ex) • Difficult to achieve high MW • because each initiator • (*initiator can give rise to many separate • chains because of chain transfer) • These side reactions can be minimized • But ! not eliminated by running the reaction • at low temperature

32. Cationic Polymerization • 1) Ring opening polymerization • (1) Mechanism • carbon type polymzn. • Example of ROR • :cyclic amides, sulfides, acetals, esters, lactam, alkanes, … • (2) Polymerizability • - unstable ring or ring which be cyclized easily are very reactive • * 3,4 and 7-11 membered ring is the reactive ring • 5,6 membered rings are stable and polymerize slowly, • but, possible to polymerize **3-membered ring most easily polymerize

33. ( C H ) + 2 4 + O O ( C H ) O 2 4 ( C H ) - 2 4 A Cationic Polymerization • (3) polymerization of THF(Polytetrahydrofuran) H2O which is role of cocatalyst increase polyerization rate It is possible to occur living polymerization, but occur termination ortransfer together.

34. Cationic Polymerization Initiation • (4)Kinetics ex) styrene, stannic-chloride-H2O System [SnCl4OH-]H+ Propagation – can have a low activation energy and be rapid or Simple propagation reaction Overall rate of polymerization may actually increase w/ decreasing temperature, means that termination has a high activation energy.