Ionic Polymerization: Anionic and Cationic Polymerization 2009.10.26

Radical versus Ionic Chain Polymerizations General Considerations of Polymerizability Whether a particular monomer can be converted to polymer depends on both thermodynamic and kinetic considerations. Polymerization is possible only if the free-energy difference △ G between monomer and polymer is negative. (Thermodynamic feasiblity) The ability to carry out a thermodynamically feasible polymerization depends on its kinetic feasibility—on whether the process proceeds at a reasonable rate under a proposed set of reaction conditions.

Carbon–carbon double bond undergoes polymerization by both radical and ionic initiators. The difference arises because the p-bond of a vinyl monomer can respond appropriately to the initiator species by either homolytic or heterolytic bond breakage: Carbonyl group Carbon–carbon double bond Whether a vinyl monomer polymerizes by radical, anionic, or cationic initiators depends 有許多的聚合方法可以做成高分子 on the inductive and resonance characteristics of the substituent(s) present. The effect of the substituent manifests itself by its alteration of the electron-cloud density on the double bond and its ability to stabilize the possible radical, anion, or cation formed. Electrondonating substituents such as alkoxy, alkyl, alkenyl, and phenyl increase the electron density on the carbon–carbon double bond and facilitate its bonding to a cationic species. The carbon–carbon double bond in vinyl monomers and the carbon–oxygen double bond in aldehydes and ketones are the two main types of linkages that undergo chain polymerization.

Comparison of Free Radical and Ionic Polymerizations “The only difference is the "flow" of the electrons during propagation” a double bond equals a single bond plus two more electrons Free radical vinyl polymerization Anionic vinyl polymerization Contrary to the high selectivity shown in cationic and anionic polymerization, radical initiators bring about the polymerization of almost any carbon–carbon double bond. Radical species are neutral and do not have stringent requirements for attacking the p-bond or for the stabilization of the propagating radical species. Resonance stabilization of the propagating radical occurs with almost all substituents, for example Whether a vinyl monomer polymerizes by radical, anionic, or cationic initiators depends on the inductive(誘導的) and resonance characteristics of the substituent(s) present. The effect of the substituent manifests(顯示 表露) itself by its alteration(改變 變化) of the electron-cloud density on the double bond and its ability to stabilize the possible radical, anion, or cation formed. 不論單體聚合成高分子是利用radical 陰離子 陽離子起始劑, 都必須考慮到雙鍵取代基的誘導或共振效應. 也就是說側鏈取代基會改變雙鍵的電子雲密度 或是 穩定自由基 The ability to carry out a thermodynamically feasible polymerization depends on its kinetic feasibility—on whether the process proceeds at a reasonable rate under a proposed set of reaction conditions. Thus, whereas the polymerization of a wide variety of unsaturated monomers is thermodynamically feasible, very specific reaction conditions are often required to achieve kinetic feasibility in order to accomplish a particular polymerization. Cationic vinyl polymerization

The intermediate species include the tight or contact ion pair (II) (also referred to as the intimate ion pair) and the solvent-separated or loose ion pair (III). The intimate ion pair has a counter- or gegenion of opposite charge close to the propagating center (unseparated by solvent). the solvent-separated ion pair involves ions that are partially separated by solvent molecules. The propagating cationic chain end has a negative counterion. For an anionic polymerization the charges in species II-IV are reversed; that is, B carries the negative charge and A the positive charge. There is a propagating anionic chain end with a positive counterion. Alternate terms used for free ion and ion pair are unpaired ion and paired ion, respectively.

Propagating species stablized by resonance Electron withdrawing groups (ester, cyano) or groups with double bonds (phenyl, vinyl) are needed as the R groups because these can stabilize the propagating species by resonance. Electrondonating substituents such as alkoxy(烷氧基), alkyl, alkenyl, and phenyl increase the electron density on the carbon–carbon double bond and facilitate its bonding to a cationic species.

Resonance stabilization of the propagating radical occurs with almost all substituents 1. 2. 3. Compare with: Stabilization of the propagating carbanion occurs by delocalization of the negative charge over the a-carbon and the nitrogen of the nitrile group. Acrylonitrile polymerization (Anionic) Styrene polymerization (Cationc)

Monomer Requirements (anionic polymerization) 1.In general, for vinyl monomers, need monomer that supports a stable carbanion. 2.Monomer should have no protic or acidic hydrogens

3. contains no electrophilic groups (carbanion is a very strong nucleophile ) 4. Carbanion generated must be able to attack its own monomer .

Choosing Initiator (for anionic polymerization) Must be strong enough to initiate monomer, i.e. stronger nucleophile (more aggressive)

Reactivity trends in monomers (in anionic polymerization) 單體在陰離子聚合的相對反應性 Oxyanion氧離子

Solvent Characteristics

Important Solvent Effects in Anionic Polymerization 1. Association Effects : Low dielectric (nonpolar) solvent are poor environments for ions; Possible to form micelle-like aggregates. See P433 A complication for polymerizations initiated by organolithium compunds in nonpolar solvents, such as cyclohexane, n-hexane, and benzene, is association (aggregation) of the various organolithium compounds because of the small size of lithium and its possessing more low-energy orbitals than electrons. Each lithium associates and participates in bonding with two or more organic moieties. The phenomenon is important since the associated species are generally unreactive and it is only the unassociated species that are active in initiation and propagation. There is an equilibrium between the dormant associated species and the reactive unassociated species.

2. Degrees of dissociation of counterion and chain (happens much more frequently )

Effects of Solvent Polarity on Polymerization Solvent effects in anionic polymerization

Effect of Counterion on Anionic Polymerization of Styrene The decrease in K has a very significant effect on the overall polymerization, since there is a very significant change in the concentration of the highly reactive free ions. No detectable dissociation fo pair to free ions in dioxane! (K值對整體反應速率的影響)The decrease in K has a very significant effect on the overall polymerization, since there is a very significant change in the concentration of the highly reactive free ions. Thus the free-ion concentration for polystyryl cesium (K ?0:02 107) is less than 10% that of polystyryl lithium (K ?107). The majority of the propagation is carried by free ions; only about 10% of the observed reaction rate is due to ion pairs. It is worth mentioning that K values independently measured by conductivity are in excellent agreement with those obtained from the kinetic measurements. contact ion pair Kp± = rate constant for all ion pairs K = dissociation constant Kp- = rate constant for free ion solvent-separated or loose ion pair (III).

Kinetics of Anionic Polymerization In protic media: Termination in ionic polymerization ⇒ only by transfer or side rxn ⇒ no coupling/disproportionation IF transfer leads to anion that is reactive enough to initiate monomer ⇒ transfer IF transfer leads to anion that won’t initiate monomer ⇒ termination 胺基鈉

Living Anionic Polymerizations No transfer No termination events due to other side reactions that might occur due to impurities ⇒ need solvent with no protic groups (aprotic) ⇒ eliminate H2O to get dry solvent O2 CO2 Other reactive species like NH2, anything that can snatch H off “lifetime” of propagating anion can be very long (∼ hours) 3. Need system with very rapid Ri >> Rp fast initiation start polymerization at same time to get highly controlled MW + polydispersity All chains start at the same time and finish the monomer off ∼ monodisperse MW distribution Quantitative and instantaneous dissociation of initiator:Liv-ing polymers 若起始劑在聚合之前完全分解,則動力學將是特別簡單              GA → G+ + A-              G+ + A- + M → G+ + AM-     成長中之陰離子,可能是free ion(AM-),ion pair(AM-G+),或兩者同時存在.為了簡化,只考慮     free ion,則成長反應可寫成            A-M-   +   M  →   A-M-M-            A-M-M-  +  M  →   A-M2-M-            A-Mn-1-M- + M  →  A-Mn-M-     陽離子G+總是在陰離子附近,至於兩者相距多遠,則視溶劑之介電常數而定. Rate of polymerization    rp = -d[M]/dt = Kp[A-][M]           where [A-] is the total concentration of anions of all degree of polymerization   [A-] = constant =  [GA]O    rp = -d[M]/dt = Kp[GA]0[M]         積分可得 [M] = [M]O exp(-Kp[GA]0t) 可經由實驗測[M] as a function of t 再繪圖 ln [M]/[M]0  vs  t ,便可求得 kp

Synthesis of a Diblock Copolymer want to make polystyrene–b–poly(methyl methacrylate) PS–b–PMMA

Short list of monomers, in increasing electrophilicity

Another useful initiator Electron transfer Transfers an electron to a monomer.

Synthesis of a Triblock Copolymer

Styrene-Butadiene Rubber (SBR)

Comparison of Cationic and Anionic Polymerizations Some differences between cationic and anionic polymerization 1. Rates are faster for cationic (1.or more orders of magnitude faster than anionic or free radical) is very reactive, difficult to control and stabilize → more transfer occurs → more side reactions → more difficult to form “living” systems → hard to make polymers with low PDI or block copolymers 2. Living cationic only possible for a specific subset of monomers 3. Most industrial cationic processes are not living – recent developments are improving this

Kinetic Steps for Cationic Polymerization Initiation: Use Acids Most important in industry!! A Lewis acid is a chemical compound, A, that can accept a pair of electrons from a Lewis base, B, that acts as an electron-pair donor, forming an adduct, AB. P.374 Protonic (Brønsted) acids initiate cationic polymerization by protonation of the olefin. The method depends on the use of an acid that is strong enough to produce a resonable concentration of the protonated species but the anion of the acid should not be highly nucleophilic; otherwise it will terminate the protonated olefin by combination (i.e., by covalent bond formation) The requirement for the anion not to be excessively nucleophilic generally limits the utility of most strong acids as cationic initiators. Hydrogen halides are ineffective as initiators of cationic polymerization because of the highly nucleophilic character of halide ions. One forms only the 1:1 addition product of alkene and hydrogen halide under most conditions. Hydrogen iodide shows some tendency to initiate polymerization of the most reactive monomers, such as vinyl ethers and N-vinylcarbazole (Sec. 5-2g-3). Other strong acids with less nucleophilic anions, such as perchloric, sulfuric, phosphoric, fluoro- and chlorosulfonic, methanesulfonic, and trifluoromethanesulfonic (triflic) acids, initiate polymerization, but the polymer molecular weights rarely exceed a few thousand. 5-2a-2 Initiation by Lewis acids almost always requires and/or proceeds much faster in the presence of either a proton donor (protogen) such as water, hydrogen halide, alcohol, and carboxylic acid, or a carbocation donor (cationogen) such as an alkyl halide (e.g., t-butyl chloride and triphenylmethyl chloride), ester, ether, or anhydride. Thus, dry isobutylene is unaffected by dry boron trifluoride but polymerization occurs immediately when trace amounts of water are added

Monomer Requirements (cationic polymerization) Typical Cationic Monomers: Butyl rubber—also known as polyisobutylene and PIB 1. alkenes, isobutenes 2.Vinyl ehters 4.Styrene 3.Vinyl acetates

Propagation Rearrangements can occur, especially if a more stable carbocation can be formed (e.g. tertiary carbocation) (most common for 1-alkenes, α olefins) This occurs via intramolecular hydride (H-) shifts Usually slow: If Rp ≤ rearrangement rate, will get rearranged product If R p > rearrangement rate, will get random copolymer

β-Proton Transfer (Kinetic Chain Maintained): It occurs readily because much of the positive charge of the cationic propagating center resides not on carbon, but on the β-hydrogens because of hyperconjugation. Proton transfer to monomer: involves transfer of a β-proton to monomer with the formation of terminal unsaturation in the polymer B) Hydride ion transfer from monomer C) Proton transfer to counterion (“spontaneous termination”)

Termination and Transfer (Several Possibilities) A) Termination with counterion: kills propagating cation, kinetic chain (kt) (i) Combination (ii) Anion Splitting B) Transfer or termination to impurity or solvent

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