Chap 10. Non-Radical Addition Polymerization

1. Chap 10. Non-Radical Addition Polymerization Anionic Polymerization -the growing chain end bears a negativecharge 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 positivecharge 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 • 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: • 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.

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. Initiation by Living Polymer Anionic Polymerization 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 the 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 ends to form inactive • allylic anion. 1,3 diphenylallyl anion is very unreactive, highly resonance stabilized

12. Anionic Polymerization • Termination of polar monomer • In this case, although the initiator oractive center attacks themonomer, that results the • non-polymerization.

13. Anionic Polymerization • Backbiting orintramolecular reaction • 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+] , then Ri 

15. Anionic Polymerization Propagation Termination Occurs by chain transfer Overall Rate using Steady state assumption. (RiRt). Rtr=ktr,s[M-][NH3+] If KCl is added Rpdecreases [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, the narrow MW distribution can be obtained. 5) Base Initiated Polymerization - a strong nucleophile is required as the initiator  

18. Anionic Polymerization 6) Practical Comments purity import! If we use metal as an initiator, the propagation rate is fast. 7) Propagation Kinetics Comparing to the radical polymerization, the propagation doesn’t occur too fast For most of the living 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 the solvent then reaction rate increases conc. of anion = conc. of initiator

19. Anionic Polymerization • (1) Evaluation 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. The salt was added at high conc. Conc. of the added salt is [CZ] [C+][CZ] the conc. of living and the conc. of free ion Hence Then able to get kp-, kp,K from the 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 tend not to be solvated, it has reverse tendency comparing to the case of THF. Solvation is not important in dioxane. Cs is too high and there is no difference.

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

24. Anionic Polymerization Evidence — the 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 reaction 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 the co-initiator. cationogen

28. Cationic Polymerization Typical Initiator Systems Co-initiator Initiator SnCl4 H2O AlCl3 HCl H2SO4 H2SO4 More acidic initiators are the most effective in initiating polymerization HCl > CH3COOH > C6H5NO2 > > H2O >> CH3OH > CH3COCH3 Order of reactivity AlCl3 > AlRCl2 > AlR2Cl >AlR3 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, • which lead to chain transfer mostly. Ex) • Difficult to achieve high MW • (*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 the ring which cannot be cyclized easily are very reactive • * 3,4 and 7-11 membered ring is the most reactive ring • 5,6 membered rings are stable and polymerize slowly, • but, it still possible to be polymerized. **3-membered ring is the most easiest to be polymerized

33. ( C H ) + 2 4 + O O ( C H ) O 2 4 ( C H ) - 2 4 A Cationic Polymerization • (3) polymerization of THF(Polytetrahydrofuran) if H2O exist in the co-catalyst, the polymerization rate increases. If the living polymerization is possible to occur, thus the termination ortransfer also could be occurred.

34. Cationic Polymerization Initiation • (4)Kinetics ex) styrene, stannic-chloride-H2O System [SnCl4OH-]H+ Propagation – can has a low activation energy and can be polymerized rapidly or Simple propagation reaction Thetotal rate of polymerization may actually increases by decreasing the temperature, which means that the termination has a high activation energy.