Chapter 7. Ionic Polymerization

Transcription

1 hapter 7. Ionic Polymerization 7.2. ationic Polymerization E = 2 E 2 electrophile carbocation ationic Initiators 1) Protonic acids: 2 S 4, 3 P 4 2) Lewis acids: All 3, BF 3, Til 4, Snl 4 (oinitiator) Proton donor or cation source: 2, alkyl chloride (Initiator) BF 3 2 BF 3 All 3 l All 4

2 3) Autoinitiation Lewis acid and 2 AlBr 3 AlBr 4 AlBr cation source 2 are the same 4) Ionizable compounds ( 6 5 ) 3 l ( 6 5 ) 3 l Stable carbocation l l nly useful with very reactive monomers such as vinyl ethers Tropylium chloride 5) I 2 I 2 In situ generation of I I 2 I 2 I I 2 I 2 I Ionpair formation

3 6) M A M A Electron transfer process N N2 N N2 Electron donor Electron acceptor adical cation

4 Mechanism, Kinetics, and eactivity in ationic Polymerization o Markovnikov s rule Addition of electrophile to monomer The more stable carbocation intermediate is formed. When an unsymmetrical alkene undergoes addition with ENu, then the electrophile, E, adds to the carbon of the alkene that has the greater number of hydrogen substituents, and the nucleophile, Nu, to the carbon of the alkene with the fewer number of hydrogen substituents" 2 o carbocation More stable major 1 o carbocation

5 o ate of addition to aliphatic monomers ( 3 ) 2 = > 3 = > = X X nly isobutylene provides the requisite carbocation stability for cationic polymerization X eactivity (parasubstituted) X: 3 > 3 > > l Electrondonating Electronwithdrawing rtho substituents retard the addition Steric hindrance Vinyl ether: particularly reactive ' ' ' Delocalized carbocation

6 o Propagation Two steps slow 2 fast 2 2 atedetermining πcomplex ovalent bond formation o Solvent effect As solvent polarity, i i : charged species is generated Polar solvent stabilizes the charged species

7 Degree of association between cationic chain end and anion (A ) A A A A covalent intimate ion pair = contact ion pair solventseparated ion pair solvated ions = free ions Small p The more intimate the association, the lower the p. Large p In poorly solvating solvents As solvent polarity, p more separation In solvating solvents (ether solvent) As solvent polarity, p less πcomplex formation Polar solvent stabilizes the initial state (monomer ion pair) at the expense of the transition complex (charge is dispersed over a larger volume).

8 slow 2 2 Polar solvent stabilizes the initial state harge is dispersed over a larger volume

9 o hain transfer reactions M = styrene, I = 2 S 4 1) With monomer 2 S 4 3 S 4 2) By ring alkylation Electrophilic substitution 2 S 4 3 S 4

10 3) By hydride abstraction from the chain to form a more stable ion 2 S 4 S 4 hain branching 4) With solvent by electrophilic substitution 2 S 4 3 S 4

11 o Termination reactions ombination of chain end with counterion (i.e., change from ionic to covalent bonding) F 3 F 3 3 Bl l 3 Bl2

12 o hain transfer to monomer is so common Proton trap intercepts proton before it transfers to monomer. Lower overall yield, higher mol. wt., lower polydispersity index 2 N 2 N Proton trap The bulky tbutyl groups prevent reaction with electrophiles larger than the proton.

13 o Telechelic polymers Inifer: initiator chain transfer agent l l Bl 3 l Bl 4 3 initiator 3 ocatalyst 3 3 l l l l l l chain transfer agent l l readily convertible to other functionalities

14 o Pseudocationic polymerization I = l 4, M = styrene, S = hydrocarbon solvent p much slower compared with most cationic processes. l 3 l 3 ovalently bonded perchlorate ester Monomer is inserted between bonds

15 o Living cationic polymerization M = isobutylene, I = tertiary ester Bl Bl Bl 3 ( 3 ) δ δ 3 Bl Very tightly bound But still active ion pair

16 M = vinyl ether, I = I 2 /I or I 2 /ZnI 2 I ZnI 2 I ZnI 2 Insertion of monomer into an activated I bond Low polydispersity, Block copolymers are formed upon addition of a second monomer

17 Kinetics i = k i [I] [M] p = k p [M ] [M] t = k t [M ] tr = k tr [M ] [M] Steadystate assumption i = t k i [I] [M] = k t [M ] or [ ] k k [ I][ M] = p p i k t 2 M = k i [ I][ M] k t

18 Table 7.2 ationic Propagation ate onstants, k p Monomer Solvent Temperature( ) Initiator k P (L/mol s) Styrene None 15 adiation 3.5 X 10 6 αmethylstyrene None 0 adiation 4 X 10 6 ibutyl vinyl ether None 30 adiation 3 X 10 5 ibutyl vinyl ether l Sbl 6 5 X 10 3 tbutyl vinyl ether l Sbl X 10 3 Methyl vinyl ether l Sbl X hloroethyl vinyl ether l Sbl 6 2 X 10 2

19 In the absence of any chain transfer [ M ][ M] k DP = p p ν = = = t k t [ M ] k p [ M] If transfer is the predominant mechanism controlling chain growth p p ν = = = tr k tr k [ M ][ M] kp [ M ][ M] k tr k DP = ationic Free radical [] [ ] 2 p I, M [] 1 I, [ M ] 2 t ν [ M ], independent of [I] ν p [] [ ] 1 I 2, M The difference arises from radical disproportionation and combination reactions chacteristic of free radical termination. Increasing [I] increases the probability of radical termination, which is not the case in cationic polymerization.

20 o Diene monomers ationic polymerization only for copolymer synthesis o Nonconjugated dienes ationic cyclopolymerization Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph

21 7.2.3 Stereochemistry of ationic Polymerization (1) Greater stereogularity at lower temperature (2) Degree of stereoregularity varies with initiator counterion (3) Degree and type of stereoregularity (isotactic or syndiotactic) vary with solvent polarity Ex tbu Isotactic in nonpolar solvents Syndiotactic in polar solvents In polar solvents : both carbocation chain end and counterion are strongly solvated free carbocation Syndiotactic placement In nonpolar solvents : strong association between carbocation chain end and counterion Isotactic placement

22 Isotactic poly(vinyl ether) Several models have been proposed 1) Sixmembered cyclic chain end Formed by coordination of the carbocation of the terminal carbon with the oxygen of the alkoxy group attached to the fifth carbon atom axial X bulkiest equatorial X equatorial X 2 X Attack of monomer at 1 from the backside causing inversion. Alkoxy group at 1 have the same configuration as that at original 3

23 2) Strong carbocation chain end counterion association Insert monomer: alkoxy groups are anti to one another X X X X As long as propagation proceeds faster than rotation about carboncarbon bonds in the chain, a regular repeating isotactic polymer results.

24 3) sp 2 hybridization of the terminal chain carbon atom back ' ' X ' Back ' front In nonpolar solvent X ' ' Front In polar solvent ' isotactic ' ' ' X syndiotactic ' ' X

25 The last model satisfactorily explains most of the experimental observations. Poly(αmethylstyrene) Due to two bulky groups on the 3rd carbon, backside attack is hindered. Syndiotactic placement is favored in both polar and nonpolar solvents. Although isotacticity as solvent polarity Poly(vinyl ether) Being less hindered, exhibit a more pronounced preference for isotactic placement in nonpolar solvents As T, electroregularity an be interpreted in terms of small difference in activation energy for front and backside attack as well as of the effect on conformational mobility of the growing chain. ounterionmonomer interactions are assumed to be negligible, since both may be considered electron rich

26 7.2.4 ationic opolymerization Table 7.3 eactivity ratios ounterion effect eactivity ratios vary with initiator type and solvent polarity. No apparent tendency for alternating copolymers to form. Block copolymers or homopolymer blends are more likely.

27 Table 7.3 ationic eactivity ratios Monomer 1 Monomer 2 oinitiator solvent Isobutylene Styrene Phlorostyrene 1,3 Butadiene 1,3Butadiene Isoprene yclopentadiene Styrene Styrene αmethylstyrene αmethylstyrene pmethylstyrene transβmethylstyrene cisβmethylstyrene transβmethylstyrene AlEtl 2 All 3 All 3 BF 3 Et 2 Snl 4 All 3 Til 4 Snl 4 Snl 4 Snl 4 Snl 4 Snl 4 3 l 3 l 3 l Ph 3 Etl 3 l Ph 3 Etl l4 l 2 l 4 /PhN 2 (1:1) l 4 /PhN 2 (1:1) Temperature ( ) r 1 r 2 Ethyl vinyl ether ibutyl vinyl ether BF 3 l hloroethyl vinyl ether αmethylstyrene BF 3 l

28 7.2.5 Isomerization 3 3 X 3 X 3 shift X 3 Monomer 3 3 More stable tertcarbocation 3 1,3addition polymer terpene rearrangement elieves strain in the bicyclic system zoneresistant elastomer upon copolymerization with isobutylene

29 7.3 Anionic Polymerization Anionic Initiators Initiation: nucleophilic addition to monomer Nu Nu where : N 2, N,, =, e withdrawing group resonance

30 o Initiators Weak base initiator For monomer with strong e withdrawing group Strong base initiator For monomer with phenyl or weak e withdrawing group 3 K 3 3 N 2 N 2 N 2 N N N N 2 N Super glue yanoacrylate adhesive

31 1. Initiators that react by addition of a negative ion I M IM rganometallic compounds of the alkali metals :e.g. butyllithium rganolithium compounds: low melting and soluble in inert organic solvents 2. Initiation by electron transfer 1) Free alkali metals In liquid ammonia, ether solvents 2) omplexes of alkali metals and unsaturated or aromatic compounds

32 Electron transfer D M D M Metal Electron donor Monomer Addition complexes of alkali metals and naphthalene or stilbene Na Na Na Ph Ph Ph Ph Na eaction with monomer D M D M Na Ph Ph 2 Ph Ph Ph Ph Na Na Na Ph Ph Ph Na Ph D Addition complexes (called ketyls) of alkali metals and nonenolizable ketones, such as benzophenone

33 7.3.2 Mechanism, Kinetics, and eactivity Li compounds ovalent carbonmetal bonds igher alkali metals More ionic bonds In polar solvents, free solvated ions Nu Nu addition of anion to monomer In nonpolar solvents, close association between ions πcomplex formation Li Li ' Initiation by e transfer ' πcomplex formation Na Na 2 Na Na Na ' Li

34 Potassium amideinitiated polymerization in liquid ammonia KN 2 K N 2 M 2 N M N 2 = k [ N ][ M] Propagation 2 N i i 2 slow hain termination: transfer to solvent 2 N M M p = k p [ M ][ M] M N N n 3 2 N M M n 2 = k M N tr tr [ ][ ] Steadystate assumption i = tr k i 3 ki [ N ][ M] = k [ M ][ N ] [ ] [ N2 ][ M] M = p n = M 2 k p k M i k tr [ N2 ][ M] [ N ] tr N 3 M M n1 ν = p tr ν = DP = k k tr tr [ N3 ] [ M ][ M] [ M ][ N ] k p 3 = k k tr [ M] [ N ] p 3

35 o hain transfer to monomer Initiation by the resultant vinyl anion Unsaturated end groups N N N N N o Termination mechanism in MMA polymerization Backbiting nucleophilic displacement of methoxide by the enolate anion chain end to form a cyclohexanone ring. N N N

36 o Living anionic polymerization e.g., M = styrene, I = naphthalenesodium, T = 78 o [ ] i >> p d M = kp[] I o[ M] dt where [I] o = initial concentration of initiator [ ] [ M] d M = k p [] I dt o ln [ M] [ M] No termination or chain transfer reactions ν = [ M] o [ M] [] I o o kp [ I] o dt = k [] I dt [ M] = [ M] e ν = p [ M] [] I o For simple anionic initiator (e.g., BuLi) ν = DP For electrontransfer initiators DP = 2ν Narrow molecular weight distribution o o M completely consumed o

37 Table 7.4 k p for polystyrene In poorly solvating hydrocarbon solvents Li 2 Li Li association 2 p Larger alkali metal cations Weaker coordination than Li p In a more polar solvating medium (e.g., ether) Li the most strongly solvated of the alkali metal cations Libased initiators the fastest p

38 Table 7.4 k p for polystyrene ounterion Solvent k p (L/mol s) Na Tetrahydrofuran 80 Na 1,2Dimethoxyethane 3600 Li Tetrahydrofuran 160 Li Benzene Li yclohexane (5100)X10 5

39 Solvating power 1,2dimethoxyethane > tetrafydrofuran > benzene > cyclohexane 3 3 eactivity of monomer N e withdrawing 3 > > > 3 resonance Methyl substitution on the αcarbon p N > N > 3 > 3 induction destabilization of the carbanion and steric interference with both chainend solvation and approach of monomer

40 7.3.3 Stereochemistry of Anionic Polymerization Polar solvents: favor syndiotactic placement Nonpolar solvents: favor isotactic placement o Sixmembered cyclic chain end Li Li 3 Isotactic placement as Solvent polarity Lithium Li Li 3 Less strongly coordinating higher alkali metal ions

41 o Ion pairing ounterionmonomer coordination isotactic In nonpolar solvents ounterionsolvent coordination syndiotactic In polar solvents Li 3 3 Li 3 3

42 o Diene Polymerization: isoprene, 1,3butadiene cis1,4 polymerization By Libased initiators In nonpolar solvents e.g., Almost entirely cis1,4 isoprene = synthetic natural rubber BuLi initiator in pentane or hexane 1) scis conformation by πcomplexation Li 3 Li 3 2) is configuration by forming a sixmembered ring transition state Li Li

43 is1,4poly(1,3butadiene) configuration Somewhat lower stereospecificity BD Favors strans conformation isoprene Favors scis conformation 3 3

44 7.3.4 Anionic copolymerization Table 7.5 Anionic eactivity atios Solvent effect r 1 r 2 M 1 = St M 2 = BD I = nbuli S = hexane T = 25 o 0.03 < 12.5 M 1 = St M 2 = BD I = nbuli S = TF T = 25 o 4.0 > 0.3 In homopolymerization, St is more reactive than BD Polystyryl anion exhibits preference towards BD in hexane Li Li Li Li 3 3 The role of counterion is effectively nullified in TF

45 Table 7.5 Anionic eactivity atios Monomer 1 Monomer 2 Initiator Solvent Temperature ( ) r 1 r 2 Styrene MMA Na nbuli N 3 None Butadiene nbuli None nbuli exane nbuli exane nbuli TF nbuli TF EtNa Benzene Isoprene nbuli yclohexane Acrylonitrile Li None Vinyl acetate Na N Butadiene Isoprene nbuli exane MMA Acrylonitrile NaN 2 Li N 3 None Vinyl acetate NaN 2 N

46 T effect r 1 r 2 M 1 = St M 2 = BD I = nbuli S = hexane T = 25 o T = 50 o S = TF T = 25 o T = 78 o Not to any appreciable extent in hexane Solvation is felt more strongly at lower T in TF

47 omogeneous and heterogeneous processes M 1 = St M 2 = MMA I = Na S = N 3 M 1 = St M 2 = MMA I = nbuli S = none r 1 = 0.12 r 2 = 6.4 r 1 = 0 r 2 = omogeneous Soluble nbuli only PMMA is formed MMA is more reactive than St eterogeneous No detectable styrene in polymer Insoluble metallic lithium or MeLi Block copolymer Initial St polymerization on the initiator surface Detachment Initiation of MMA by detached polystyryl anion

48 Nucleophilic displacement of aliphatic dihalides by the dianion Formed by electron transfer initiation Li Li l l X X X X

49 Block copolymers by living polymerization method Polystyreneblockpoly(methyl methacrylate) 3 n n m1 n 3 3 X If MMA was polymerized first, the copolymer would not form. Living PMMA is not basic enough to add to styrene

50 StBDSt (SBS) triblock polymer B initiator B combination BB B BBBBBBBBBB S SSSSSBBBBBBBBBBSSSSS 50,000~70,000 10,000~15,000 Starblock (radial) copolymer 4 Living block copolymer Sil 4 Si Major advantage: much lower viscosities than their linear counterparts

51 7.4 Group Transfer Polymerization (GTP) Living polymers at room temp or above Initiators : SiMe 3 compound Transfer to carbonyl oxygen 3 3 ester ( 3 ) 3 Sil ( 3 ) 3 Si 2 N Li 3 strong base Si( 3 ) anion ( 3 ) 3 Si N S Si( 3 ) 3 ArS Si( 3 ) 3 atalysts : Anion or Lewis acid F 2, N, N 3, 3 SiF 2 Donate electron to silyl group ZnX 2, 2 All, ( 2 Al) 2 withdraw electron from monomer

52 TABLE 7.6 ompounds used in Group Transfer Polymerization monomers initiators catalysts solvent Me Anionic Acetonitrile = 2 Me 2 = F 2 1,2Dichloroethane SiMe 3 N Dichloromethane N 3 N,NDimethylacetamide Me Me 3 SiF 2 N,NDimethylacetamide = 2 Me 3 Si 2 Me Lewis acid ZnX 2 =N 2 Me 3 SiN 2 All ( 2 Al) 2 Ethyl acetate Propylene carbonate Terahydrofuran Toluene =N SSiMe 3 Me =N ArSSiMe 3 =

53 3 3 initiator Si( 3 ) 3 3 GTP F MMA 2 catalyst ( 3 ) 3 Si F Si( 3 ) n Si( 3 ) 3

54 Difunctional initiator hain propagates from each end Block copolymer nce the monomer is consumed, a different monomer may be added.

55 Termination emoval of catalyst, protonation, alkylation 3 protonation 3 Si( 3 ) 3 alkylation BrSi( 3 ) 3

56 Mechanism Nu 3 Si( 3 ) 3 3 MMA 3 Nu Si( 3 ) 3 Nu Si( 3 ) membered ring transition state ypervalent silicon Propagating chain is completely covalent ypervalent silicon intermediate is formed by activation with the nucleophilic catalyst (Nu ) Silyl group is transferred to carbonyl group of the incoming monomer molecule via 8membered ring transition state

57 Where Lewis acid catalysts are used 3 3 LA 3 Nu 3 Si( 3 ) 3 3 MMA Si( 3 ) Si( 3 ) 3 LA 3 atalyst coordinates the carbonyl oxygen of monomer rendering the monomer more susceptible to nucleophilic attack by the initiator

58 Mechanism 2 Propagating species: enolate anions As in conventional anionic polymerization of acrylate monomers 3 Si( 3 ) 3 3 GTP and conventional anionic polymerizations are remarkably similar in terms of polymer tacticity and reactivity ratios in copolymerization. GTP of MMA exhibits the same backbiting chain terminating reaction. Silyl group undergoes intermolecular exchange, which is inconsistant with the proposed GTP mechanism

59 3 3 silyl ketene acetal 3 Si( 3 ) Enolate anion Si( 3 ) Enolate anion and silyl ketene acetal chain ends are in rapid equilibrium with a hypervalent silicon complex The silicon complex provides Low eqm conc of enolate anions for propagation and mechanism for maintaining living chain ends

60 f. o Termination mechanism in MMA polymerization Backbiting nucleophilic displacement of methoxide by the enolate anion chain end to form a cyclohexanone ring