Hanyang Univ. Spring 2008 Chap 10. Non-Radical Addition Polymerization Anionic Polymerization -the growing chain end bears a negative charge The mechanism of cationic polymerization is a kind of repetitive alkylation reaction. 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

Hanyang Univ. Spring 2008 General Scheme Anionic Polymerization

Hanyang Univ. Spring 2008 Styrene Polymerization Anionic Polymerization

Hanyang Univ. Spring 2008 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 o C. This is partly for the high concentration of the anion centers. The degree of polymerization K=1 or 2 depending on initiator used.

Hanyang Univ. Spring 2008 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 Anionic Polymerization

Hanyang Univ. Spring 2008 Initiation by Nucleophilic Attack Polymerization can be done in both polar and nonpolar solvents. The degree of polymerization is given by Anionic Polymerization

Hanyang Univ. Spring 2008 Initiation by Living Polymer Because the starting anion has to be a stronger Lewis base than the resulting anion. But not Anionic Polymerization

Hanyang Univ. Spring 2008 Propagation K p 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. Anionic Polymerization

Hanyang Univ. Spring 2008 Termination Anionic Polymerization

Hanyang Univ. Spring 2008 Anionic Polymerization (1) proton donor H 2 O or ethanol Strong base is not enough for initiation. (2) C tr,s =10 -3 (small chain transfer constant) EtOH high MW product ethoxide no longer living. H 2 O C tr,s =10 H 2 O low MW polymer No living polymer Strong base is not enough for initiation.

Hanyang Univ. Spring 2008 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 1,3 diphenylallyl anion is very unreactive, highly resonance stabilized

Hanyang Univ. Spring 2008 Anionic Polymerization Termination of polar monomer In this case, although the initiator or active center attacks the monomer, that results the non-polymerization.

Hanyang Univ. Spring 2008 Anionic Polymerization Backbiting or intramolecular reaction 4) Hugginson-Wooding System J.Chem. Soc Polymerization of styrene conducted in liq. NH3 at bp -33  C (1) reaction rate ↑ as [I] and [M] 2 I=K+NH 2 - rate ↑ as [NH 2 - ] ↑ but as [K + ] ↓ (2) MW  [K + ] and [NH 2 - ] (3) Polymer is formed without unsaturation. Cyclic trimer at the end of chain

Hanyang Univ. Spring 2008 Anionic Polymerization  If [K + ], then R i  Dissociation of initiator Initiation step 

Hanyang Univ. Spring 2008 Propagation Anionic Polymerization Termination Occurs by chain transfer Rtr=ktr,s[M-][NH3+] Overall Rate using Steady state assumption. (R i  R t ). If KCl is added  R p decreases [K + ]=[NH 2 - ]

Hanyang Univ. Spring 2008 Anionic Polymerization In dehydrate state, Chain transfer constant for solvent Activation energy for X n

Hanyang Univ. Spring 2008 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  

Hanyang Univ. Spring 2008 Anionic Polymerization 6) Practical Comments If we use metal as an initiator, the propagation rate is fast. purity import! 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 to molar [M: - ] = to molar k p for free radical case is 5  10 3 l/mole  sec K p : depends on solvent and counter ion Counter ion and active center can be separated by changing the solvent then reaction rate increases conc. of anion = conc. of initiator

Hanyang Univ. Spring 2008 (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 Anionic Polymerization

Hanyang Univ. Spring 2008 Anionic Polymerization * How to measure kp , kp, K ? t

Hanyang Univ. Spring 2008 Anionic Polymerization the conc. of living and the conc. of free ion A salt that must be soluble in THF with common ion to gegen ion is added to reaction mixture. [C + ]  [CZ] The salt was added at high conc. Conc. of the added salt is [CZ] Hence Then able to get k p -, k p , K from the two graphs.

Hanyang Univ. Spring 2008 Anionic Polymerization Effect of gegen ion on Anionic Polymerization of Styrene THFDioxane kpkp K  10 7 kp-kp- kpkp Li  Na K+K+ 60~ Rb+Rb+ 50~ CS+CS 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.

Hanyang Univ. Spring 2008 Anionic Polymerization Look at difference. Unassociated species Li + genenion in aromatic hydrocarbon Let’s say we are using the BuLi initiator. solvation as well as  is important! Although, the 1,2 diethoxyethane reduce the , k p varies 1~1000 fold because of highly solvating ether. Reactivity of free ion < Reacitivity of ion pair In aromatic hydrocarbon, unassociated species dominate rate. Depends on the unassociated species in very low conc. Covalent character

Hanyang Univ. Spring 2008 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.

Hanyang Univ. Spring 2008 Effect of solvent and gengenion on Copolymerization of Styrene and isoprene at 25  C Solvent % Styrene in copolymer Na + counter ionLi + counter ion Nonsolvent6615 Benzene6615 Triethyl ether7759 Ethyl ether7568 THF(highly saturating solvent) 80 Lenz P.437 Table 13-9 Anionic Polymerization Generally sodium is more ionic than lithium

Hanyang Univ. Spring 2008 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)

Hanyang Univ. Spring 2008 Cationic Initiators Proton acids with unreactive counterions Lewis acid + other reactive compound: * To use Lewis acid effectively as initiators, use the co-initiator. cationogen

Hanyang Univ. Spring 2008 Cationic Polymerization Typical Initiator Systems Co-initiator Initiator SnCl 4 H 2 O AlCl 3 HCl H 2 SO 4 H 2 SO 4 Order of reactivity AlCl 3 > AlRCl 2 > AlR 2 Cl >AlR 3  HCl > CH 3 COOH > C 6 H 5 NO 2 > > H 2 O >> CH 3 OH > CH 3 COCH 3 Ex) More acidic initiators are the most effective in initiating polymerization

Hanyang Univ. Spring 2008 Termination Kinetics Cationic Polymerization Problem : temination reactions occur randomly. [ * ] can control rxn

Hanyang Univ. Spring 2008 Cationic Polymerization

Hanyang Univ. Spring 2008 Chain Transfer Reactions -Cationic vinyl polymerization is plagued by numerous side reactions, which lead to chain transfer mostly. 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 Ex)

Hanyang Univ. Spring 2008 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

Hanyang Univ. Spring 2008 Cationic Polymerization (3) polymerization of THF(Polytetrahydrofuran) if H 2 O exist in the co-catalyst, the polymerization rate increases. If the living polymerization is possible to occur, thus the termination or transfer also could be occurred. O(CH 2 ) 4 O + - A + O (CH 2 ) 4 (CH 2 ) 4

Hanyang Univ. Spring 2008 Cationic Polymerization (4)Kinetics Initiation ex) styrene, stannic-chloride-H 2 O System [SnCl 4 OH - ]H + Propagation – can has a low activation energy and can be polymerized rapidly or Simple propagation reaction The total rate of polymerization may actually increases by decreasing the temperature, which means that the termination has a high activation energy.