Chapter 7. Ionic polymerization 7.1 Introduction 7.2 Cationic polymerization 7.3 Anionic polymerization 7.4 Group transfer polymerization 7.1 Introduction Presence of counterions (= gegenions) Influence of counterions • Solvation effect more complex than free radical polymerizations but more versatile ex) counterion Cl CH3 CH3 C C Cl CH3 CH3 + BCl3 Cl CH3 CH3 C C CH3 CH3 + BCl-4 7.1 Introduction O Application : in ring-opening polymerizations of cyclic ethers O O , O lactams NH , O lactones C O O and in the polymerization of aldehydes ketones RCH (CH2)m , O RCR' Commercial processes (Table 7.1) far fewer in number reflect a much narrower choice of monomers monomers must contain substituent groups capable of stabilizing carbocations or carbanions the necessity for solution polymerzation TABLE 7.1. Commercially Important Polymers Prepared by Ionic Polymerization Polymer or Copolymer Cationica Polyisobutylene and polybuteneb (low and high molecular weight) Isobutylene-isoprene copolymerc (“butyl rubber”) Isobutylene-cyclopentadiene copolymer Hydrocarbond and polyterpene resins Coumarone-indene resinse Poly(vinyl ether)s Anionicf cis-1,4-Polybutadiene cis-1,4-Polisoprene Styrene-butadiene rubber (SBR)g Styrene-butadiene block and star copolymers ABA block copolymers (A= styrene, B=butadiene or isoprene) polycyanoacrylateh aAlCl 3 and BF3 b”Polybutenes” Major Uses Adhesives, sealants, insulating oils, lubricating oil and grease additives, moisture barriers Inner tubes, engine mounts and springs, chemical tank linings, protective clothing, hoses, gaskets, electrical insulation Ozone-resistant rubber Inks, varnishes, paints, adhesives, sealants Flooring, coatings, adhesives Polymer modifiers, tackifiers, adhesives Tires Tires, footware, adhesives, coated fabrics Tire treads, belting, hose, shoe soles, flooring, coated fabrics Flooring, shoe soles, artificial leather, wire and cable insulation Thermoplastic elastomers Adhesives most frequently used coinitiators. are copolymers based on C4 alkenes and lesser amounts of propylene and C5 and higher alkenes from refinery streams. cTerpolymers of isobutylene, isoprene, and divinylbenzene are also used in sealant and adhesive formulations. dAliphatic and aromatic refinery products. eCoumarone (benzofuran) and indene (benzocyclopentadiene) are products of coal tar. fn-Butyllithium most common initiator. gContains higher cis content than SBR prepared by free radical polymerization. hMonomer polymerized by adventitious water. 7.2 Cationic polymerization 7.2.1 Cationic initiators 7.2.2 Mechanism, kinetics, and reactivity in cationic polymerization 7.2.3 Stereochemistry of cationic polymerization 7.2.4.Cationic copolymerization 7.2.5 Isomerization in cationic polymerization 7.2.1 Cationic Initiators The propagating species : carbocation Initiation E+ + CH2 Initiator + (7.1) ECH2CR2 CR2 mineral acid : H2SO4, H3PO4 lewis acid : AlCl3, BF3, TiCl4, SnCl4 Coinitiator - + BF3 + H2O HOBF3 H AlCl3 + RCl AlCl4-R+ 7.2.1 Cationic Initiators Very active Lewis acid autoionization - + AlBr4 AlBr2 2AlBr3 Other initiators (C6H5)3CCl (C6H5)3C (7.5) + Cl + Cl (7.6) Cl CR2 + HI ICH I2 + CH2 CR2 ICH2CIR2 + ICH2CR2I - (7.7) (7.8) 7.2.1 Cationic Initiators Other initiators M+A M + + A - (7.9) CH CH2 + CH CH2 N N + RNO2 · + · -2 RNO (7.10) 7.2.2 Mechanism, Kinetics, and Reactivity in Cationic Polymerization A. Carbocationic Initiation. addition of the electrophilic species – the more stable carbocation (Markovnikov’s rule) intermediate is formed. H3C H3C C CH2 H3C + + C HCl CH3 H3C Stability of carbocation (CH3)2C CH2 > CH3CH CH2 > CH2 CH2 7.2.2 Mechanism, Kinetics, and Reactivity in Cationic Polymerization A. Carbocationic Initiation. For a series of para-substituted styrenes, the reactivity for substituent group CH2 CH X OCH3 > CH3 > H > Cl (Because of steric hindrance) X Vinyl ethers CH2 CHOR R'+ R'CH2 + CH OR R'CH2 + CH OR (7.11) 7.2.2 Mechanism, Kinetics, and Reactivity in Cationic Polymerization B. Propagation Step Two Step ① -complex of chain end and approaching monomer ② formation of covalent bond ① + + CH2 CR2 ② slow CR2 + CH2 fast + CH2CR2 (7.12) 7.2.2 Mechanism, Kinetics, and Reactivity in Cationic Polymerization C. Influences polymerization rate ① Solvent polarity (polarity favors the initiation step) ② Degree of association between the cationic chain end and counterion (A-) A Covalent + A Intimate ion pair + A - Solvent-separated ion pair + + ASolvated ions (7.13) 7.2.2 Mechanism, Kinetics, and Reactivity in Cationic Polymerization D. Chain transfer reaction 1. With monomer : + CH2CH HSO 4 + CH2 + CH CH + CH3CH HSO4 CH (7.14) 2. By ring alkylation CH2 CH CH2 + - CH HSO 4 + CH2 CH (7.15) CH2 + CH3 + - CH HSO 4 7.2.2 Mechanism, Kinetics, and Reactivity in Cationic Polymerization D. Chain transfer reaction 3. By hydride abstraction from the chain to form a more stable ion : - + - CH2CH HSO 4 + CH2CH2 + CH2CHCH2 + HSO 4 CH2CCH2 (7.16) 4. With solvent-for example, benzene-by electrophilic substitution : + CH2CH HSO 4 + + CH2 CH (7.17) CH2CH + - + CH3CH HSO 4 7.2.2 Mechanism, Kinetics, and Reactivity in Cationic Polymerization E. Termination reaction Termination are the combination of chain end with counterion. ① styrene + CF3COOH O CH2CH+ - OCCF3 O CH2CHOCCF3 (7.18) ② Isobutylene + BCl3/H2o CH3 CH2C + CH3 CH3 BCl3OH- CH2C Cl + BCl2OH CH3 (7.19) 7.2.2 Mechanism, Kinetics, and Reactivity in Cationic Polymerization F. Proton trap It intercepts the proton before it transfers to monomer. A:proton trap·· B:monomer·· A The result lower overall yield higher molecular weight lower polydispersity index. + CH2CR2 + B CH CR2 + N (7.20) N + H 7.2.2 Mechanism, Kinetics, and Reactivity in Cationic Polymerization G. Telechelic Polymer inifer Cl (bifunctional compound) CH3 CH3 C C Cl CH3 CH3 CH3 Cl + BCl3 CH3 + Cl C CH3 CH3 CH3 CH3 CH3 C C Cl CH3 CH3 CH2C + CH3 + Cl CH3 C C CH3 CH3 CH3 C CH3 CH3 + Cl C CH2 CH3 Cl + (7.21) BCl-4 CH3 CH3 C C CH2C CH3 CH3 CH3 CH3 C C Cl CH3 CH3 CH3 CH2C Cl + + CH3 CH3 CH3 CH3 C C C C Cl CH3 CH3 CH3 CH3 CH3 + (7.22) CH3 (7.23) 7.2.2 Mechanism, Kinetics, and Reactivity in Cationic Polymerization H. Pseudocationic Polymerization CH2CH OClO3 + CH2 Ph CH CH2CHCH2CH Ph The propagating chain end is a covalently bonded perchlorate ester The reaction proceeds at a much slower compared with most cationic processes. Ph Ph OClO3 (7.24) 7.2.2 Mechanism, Kinetics, and Reactivity in Cationic Polymerization I. To prepare living polymers under cationic conditions. (Termination or chain transfer reaction 없이 중합반응이 종결되는 예) ex1) Tertiary ester + BCl3 / Isobutylene polymerization ① formation of tertiary carbocation-initiating species. O O R3C+ R3COCCH3 + BCl3 BCl3 (7.27) - OCCH3 ② Polymerization to yield polyisobutylene terminated : appearance of a very tightly bound – but still active – ion pair CH2 C(CH3)2 CH3 R3C CH2C CH3 CH3 CH2C CH3 O … – OCCH3 BCl3 (7.26) 7.2.2 Mechanism, Kinetics, and Reactivity in Cationic Polymerization I. To prepare living polymers under cationic conditions. (Termination or chain transfer reaction 없이 중합반응이 종결되는 예) ex2) I2 / HI or I2 / ZnI2 : vinyl ether propagation CH2 OR CH CH2 I ZnI2 CH CH2 OR CH CH2 OR I ZnI2 (7.27) CH OR Living Polymer Termination이 전체적으로 멈춘 곳에서 chain end가 여전히 active 한 성질을 가지고 있는 polymer Monomer 첨가 시 분자량이 증가하며 starting monomer와 다를 경우 block copolymer형성 매우 길고 anionic polymerization에 많이 이용 대부분의 living polymer는 낮은 온도에서 합성 Living polymer란 용어는 정지 반응이 일어나지 않는 이온 중합에 이용 7.2.2 Mechanism, Kinetics, and Reactivity in Cationic Polymerization J. Kinetics Expression of general initiation, propagation, termination, and transfer rates Ri ki [ I ][ M ] [I ] R p k p [ M ][ M ] : molar concentration of initiation [ M ] : molar concentration of monomer [ M ] : molar concentration of Rt kt [ M ] Rtr ktr [ M ][ M ] cationic chain end As with free radical polymerization approximation to a steady state for the growing chain end. thus Ri Rt ki [ I ][ M ] kt [ M ] or [M ] ki [ I ][ M ] kt 7.2.2 Mechanism, Kinetics, and Reactivity in Cationic Polymerization TABLE 7.2. Representative Cationic propagation Rate Constants, Monomer Styrene -Methylstyrene i-Butyl vinyl ether i-Butyl vinyl ether i-Butyl vinyl ether Methyl vinyl ether 2-Chloroethyl vinyl ether aData RP Solvent Temperature (oC) Initiator None None None CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 15 0 30 0 0 0 0 Radiation Radiation Radiation C7H7+SbCl6C7H7+SbCl6C7H7+SbCl6C7H7+SbCl6- from Ledwith and Sherrington.19 a kp (L/mol s) 3.5 106 4 106 3 105 5 103 3.5 103 1.4 102 2 102 [ M ] ki [ I ][ M ] kt [ M ] [ I ] Kinetics, and Reactivity in Cationic Polymerization 7.2.2 Mechanism, Ri Rt ki [ I ][ M ] [ M ] ] R p, one obtains Substituting for [ M[ M] in ki [ I ][ M ] kt [ M ] kt [M ] 2 k [ I ][ M ] k k [ I ][ M ] [R Mi ] Rt Rip p i kt k ki [ I ][ M ] kt [ M2 ] t k p ki [ I ][ M ] R p k p [ M ][ M ] k p [ M ] In the absence of any kchain [ IDP ][transfer, M] R[ M i p ] = DP (the kinetic chain lengthk=t ) Rt kt [ M ] kt k t R k [ M ][ M ] k [ M] ] k p 2 R k [ M ][ M p p p p p k k [ I ][ M ] R DPpi DP p Rkt Rktrt [ M k]tr [ M ][ Mkt ] ktr t RRp kkp [[M ][ M kkp [ M ] ] M ][ M ] p p the DP p chain growth, If transfer is predominant controlling DP R mechanism ][ M ] ktr k Rtrt ktr [kM [ M ] t t DP Rp Rtr k p [ M ][ M ] ktr [ M ][ M ] kp ktr 7.2.2 Mechanism, Kinetics, and Reactivity in Cationic Polymerization K. Difference between free radical and cationic processes. [I ] [I ] [I ] [ I[]M ] [M ] [M ] [M [ M] ] [M ] Ri ] Rt [M [M ] cationic Ri free Rt radical process ][tM ] kt [process M ] Riki[ I R Ri Rt propagation rate proportional root ki [ I ][ M ]to kthe ]k [ I ][ t [ Msquare ki[[M I ][ ]M dependence ]ki [ Ik][t [MM] ] M ]of kt [ M ]first-order i (Rp) initiator concentration kt k [ I ][ M ] k [ I ][ M ]2 k [ I ][ M ] [M ] i i i [ M ] [ M ] k k [ I ][ M ] kt -d[M]fk d [ I ] p i fk d [ I ] R -d[M] k k p t =k t kp[M] tk t kp[M] Rp = dtkRkp [=I ][ 2 k dt t M] p i 2 2 k k [ I ][ M ] k k [ I ][ M ] R p i p i p R pinitiator k p [ M ][ M ] k p [ DP () dependent of initiator concentration independent of Rp Rp kt DP k kt Rtt kt [ M ] k concentration R p kkpp[M] [ M ][ M ] k p [ M ] kp[M] kp[M][M·] = = = DP R p k p [ M ][ M ] kR [ M ] k [ M ][ M R k [ M ][ M ] ] k pk 2 p pp pp 2kt[M·] 2kt[M·] Rt2 ( fkktkt [d M ] k [ IDP ] DP t DP k [ M ][ M ] ktr R k [ M ] k R k [ M ] tr tr t t t t R p k p [ M ][ M ] kt p DP R k [ M ][ M ] k R k [ M ][ M ] k p p p p p Rtr ktr[ M][DP M ] ktr DP Rtr ktr [ M ][ M ] kRtrtr ktr [ M ][ M ] k 7.2.2 Mechanism, Kinetics, and Reactivity in Cationic Polymerization L. Nonconjugation diene – Cationic cyclopolymerization + R C6H5 R C6H5 + C6H5 C6H5 monomer + R C6H5 C6H5 R C6H5 C6H5 (7.28) 7.2.3 Stereochemistry of Cationic Polymerization Cationic Polymerization ex) vinyl ether lead to stereoregular structures. R - methylstyrene O CH CH2 CH2 C CH3 Vinyl ether observation resulting (1) greater stereoregularity is achieved at lower temperatures (2) the degree of stereoregularity can vary with initiator (3) the degree and type of stereoregularity (isotactic or syndiotactic) vary with solvent polarity. 7.2.3 Stereochemistry of Cationic Polymerization Solvent effect EX) t-butyl vinyl ether forms isotactic polymer in nonpolar solvents. forms mainly syndiotactic polymer in polar solvents. ( cationic chain end and the counterion are associated ) 7.2.3 Stereochemistry of Cationic Polymerization Solvent effect In polar solvents both ions 1) be strongly solvated 2) the chain end – exist as a free carbocation surrounded by solvent molecules In nonpolar solvents 1) association between carbocation chain end and counterion would be strong 2) counterion could influence the course of steric control. Models proposed for vinyl ether polymerization OR H 5 6 2 4 CH2 5 CH CH 3 2 2 1+ X C C C RO H RO H RO H C CH2 C C H H 4 CH2 1 C 2 (7.29) CH2 OR R O + X OR H CH2 6 3 CH2 C H H C CH2 OR R O + XROCH (7.30) CH2 CH2 C RO CH2 H RO C CH2 H RO CH2CH+ X C H OR - OR H CH2 CH2 C RO C + H RO RO CH2 CHOR CH2 C RO C C X C + H RO H H - H (7.31) OR C+ CH2 C CH2 H H CH2 X - X CH2 - CH2 C C H RO CH2 RO H C H RO CH2 H RO C+ X - H front side CH2 + XC C + ' R' R R R CH2 back (isotactic) CH2 CH2 C R back side R' C ' R R R CH2 + XC R' ' H R C H C front R (syndiotactic) - CH2 + X C C C R R R' R' R' R CH2 CH2 X + CH2 CH2 CH2 C C C ' ' ' R R R R R R (7.32) (7.33) 7.2.4 Cationic Copolymerization A. Copolymerization equation - the situation is complication by counterion effects. B. Reactivity ratios vary with initiator type and solvent polarity. C. Temperature – unpredictable effect D. Steric effects (Table 7.3) E. commercial cationic copolymers – butyl rubber (prepared from isobutylene and isoprene.) protective clothing tire inner tubes TABLE 7.3. Representative Cationic Reactivity Rations (r)a Monomer 1 Isobutylene Styrene p-Chlorostyrene Ethyl vinyl ether 2-Chloroethyl vinyl ether aData r1 r2 -100 -103 -103 -78 0 -92 -78 0 -78 0 43 115 2.5 0.60 1.60 9.02 1.2 0.05 0.33 1.80 0 0 0.4 4.5 1.17 1.99 5.5 2.90 1.74 1.10 Monomer 2 1,3-Butadiene 1,3-Butadiene Isoprene Cyclopentadiene Styrene Styrene -Methylstyrene -Methylstyrene p-Methylstyrene trans--Methylstyrene cis--Methylstyrene trans--Methylstyrene i-Butyl vinyl ether -Methylstyrene AlEtCl2 AlCl3 AlCl3 BF3·OEt2 SnCl4 AlCl3 TiCl4 SnCl4 SnCl4 SnCl4 CH3Cl CH3Cl CH3Cl PhCH3 EtCl CH3Cl PhCH3 EtCl CCl4 CH2Cl2 SnCl4 CCl4/PhNO2(1:1) 0 1.0 0.32 SnCl4 CCl4/PhNO2(1:1) 0 0.74 0.32 BF3 CH2Cl2 -78 1.30 0.92 BF3 CH2Cl2 -23 6.02 0.42 from Kennedy and Marechal.5 bEt = C H , Ph = phenyl. 2 5 Solventb Temperature (oC) Coinitiatorb 7.2.5 Isomerization in Cationic Polymerization CH2 CH CH3 CH CH3 CH3 H:shift CH CH CH3 + X XCH2 CH3 XCH2 CH2 C + CH3 + CH3 monomer CH2 CH2 C CH3 R R R (7.34) + (7.35) + 7.3 Anionic Polymerization 7.3.1 Anionic initiators 7.3.2 Mechanism, kinetics, and reactivity in anionic polymerization 7.3.3 Stereochemistry of anionic polymerization 7.3.4 Anionic copolymerization 7.3.1 Anionic Initiators Propagating chain - carbanion Nu- + CH2 CHR NuCH2 CH- (7.36) R Monomers having substituent group – stabilizing a carbanion resonance or induction Examples – nitro, cyano, carboxyl, vinyl, and phenyl. 7.3.1 Anionic Initiators The strength of the base necessary to initiate polymerization depends in large measure on monomer structure CH3 CH2 CH3 KHCO3 C CH2 NO2 high reactivity C CN (7.37) NO2 CN CH2 C CN H2O CH2 C (7.38) CN cyanoacrylate adhesives 7.3.1 Anionic Initiators Two basic types that react by addition of a negative ion that undergo electron transfer. ① The most common initiators that react by addition of a negative ion simple organometallic compounds of the alkali metals For example : butyllithium Character of organolithium compounds - low melting - soluble in inert organic solvents. Organometallic compounds of the higher alkali metals - more ionic character - generally insoluble 7.3.1 Anionic Initiators ② Electron transfer (charge transfer) by free alkali metal : solutions in liquid ammonia or ether solvents suspensions in inert solvents by addition complex of alkali metal and unsaturated or aromatic compounds. Electron transfer processes (involving metal donor D· , monomer M) D +M _ + D + M O Na+ Na + Na + Ph CH CH Ph CH Ph] Na + [Ph CH O _ D _ + M Na + PhCPh D + M 2 PhCPh Na+ O PhCPh Na+ Na O ONa Ph2C CPh2 7.3.2 Mechanism, kinetics, and reactivity in anionic polymerization A. Mechanism을 변화시킬 수 있는 요인 a. solvent polarity R + R- Me R-Me+ metal R- + Me+ solvent separated solvated ion ion pair ion pair Degree of association of ion counterion의 역할 polar solvent : solvated ion 우세 - X + CH2 CH XCH2 R non polar solvent : 이온들간의 association우세 CH R - complex형성 CHR' R Li + CH2 CH R RCH2CHLi Li CH2 R' R' 7.3.2 Mechanism, kinetics, and reactivity in anionic polymerization b. Type of cation (counterion) c. Temperature B. The rate of initiation - initiator 와 monomer의 structure에 의존 C. Initiation by electron transfer dianion 생성 Na + CH2 2 CH2 C R CH CH2 CH Na+ (7.46) R R Na+ Na+CHCH2CH2CHNa+ R R (7.47) 7.3.2 Mechanism, kinetics, and reactivity in anionic polymerization D. Kinetic KNH2 NH2- + M K+ + H2N NH2M- DP Because the second step is slow relative to the first, Ri ki [ NH2 ][ M ] Chain termination is known to result primarily by transfer to solvent: H2N(M)n- + NH3 H2N(M)nMH + NH2- Rate expressions for propagation and transfer may be written in the conventional way: Rp k p [M ][ M ] Rtr ktr [ M ][ NH 3 ] 7.3.2 Mechanism, kinetics, and reactivity in anionic polymerization D. Kinetic Assuming a steady state whereby Ri Rtr ki [ NH2 ][ M ] ktr [M ][ NH3 ] and k [ NH 2 ][ M ] [M ] i ktr [ NH 3 ] Substituting in Rp we obtain Rp The average kinetic chain length, Rp Rtr k p ki [ NH 2 ][ M ]2 ktr [ NH3 ] is expressed as k p [ M ][ M ] ktr [ M ][ NH 3 ] k p [M ] ktr [ NH 3 ] 7.3.2 Mechanism, kinetics, and reactivity in anionic polymerization E. Other types of transfer reactions CH2CH + CH2 CN CH2 + CH2 CH CN C O C CH2 CN C CN CH3 C CN CN C CH2 CH2CH2 + CH2 CH CH2CH CN CN CH3 CH2 C H2 C CO2CH3 C CH2 H3C CH3 CH2 _ + OCH3 O OCH3 C O CO2CH3 OCH3 H3C CH2 CO2CH3 CH3 7.3.2 Mechanism, kinetics, and reactivity in anionic polymerization F. In Ri R p Living Polymerization When impurities are rigorously excluded living anionic polymers can be made When the polymerization temperature is kept low d[M ] k p [ I ]o [ M ] dt all chains begin to grow simultaneously. [ M ] [ M ]o e k [ I ]t No termination, no chain transfer reaction. [ M ]o [ M ] [ I ]o as monomer is completely consumed. [ M ]o DP [ I ]o electron transfer initiators DP 2 7.3.2 Mechanism, kinetics, and reactivity in anionic polymerization G. Important factor in propagation rate. a. Association between counterion and terminal carbanion Li 2 CH2CHLi R CH2CH R CHCH2 Li (7.54) R TABLE 7.4. Representative Anionic Propagation Rate Constants, kp, for Polystyrenea Counterion Na+ Na+ Li+ Li+ Li+ Solvent kp (L/mol s)b Tetrahydrofuran 80 aData from Morton.30 1,2-Dimethoxyethane 3600 Bat 25oC unless otherwise noted. Tetrahydrofuran 160 cVariable temperature. Benzene 10-3-10-1c Cyclohexane (5-100)10-5c 7.3.2 Mechanism, kinetics, and reactivity in anionic polymerization G. Important factor in propagation rate. b. Monomer structure inductive destabilization of the carbanion CH3 CH2 CHCN > CH2 CCO2CH3 > CH2 CHC6H5 > CH2 CH steric effect CH3 CH3 CH2 CHCN > CH2 CCN CH > CH2 CHCO2CH3 > CH2 CCO2CH3 CH2 7.3.3 Stereochemistry of anionic polymerization A. Stereochemical of nondiene vinyl monomer With soluble anionic initiators (homogeneous conditions) at low temperatures, polar solvents favor syndiotactic placement nonpolar solvents favor isotactic placement. (stereochemistry depends in large measure on the degree of association with counterion, as it does in cationic polymerization) C CH3O C CH2 CH3 - O CH3 C CH3 C O Li+ C OCH3 CH2 CH O _ CH3 CH3 C O Li + C OCH3 (7.55) 7.3.3 Stereochemistry of anionic polymerization A. Stereochemical of nondiene vinyl monomer CH3 C CH2 OH3C C O _ C O Li + CH2 CH3 C C OCH3 OCH3 CH2 CH2 C C O CO 2 CH 3 CH3 CO C 2 CH 3 CH3 C (7.56) CH3 C O _ OCH3 Li + CH3 level of isotactic placement decreases – level of solvent isotactic placement decreases – as the polarity is increased as solvent is polarity is increased orthe as lithium replaced with the less strongly coordinating higher alkali metal ions. or as lithium is replaced with the less strongly coordinating higher alkali metal ions. 7.3.3 Stereochemistry of anionic polymerization A. Stereochemical of nondiene vinyl monomer Effect of solvent R R O R R C OR H RO OR C C O H O R H RO Li C OR H C O O Li O C R O O RO (a) (b) SCHEME 7.1. (a) Isotactic approach of methyl methacrylate in a nonpolar solvent(b) Syndiotactic approach of methyl methacrylate in tetrahydrofuran.(Circles represent backbone or incipient backbone carbons: R=methyl. Backbone hydrogens omitted.) 7.3.3 Stereochemistry of anionic polymerization B. Stereochemical of Dienes CH3 H2C C H C CH2 isoprene H2C CH H C 1,3-butadiene catalyst, solvent의 영향 Li-based initiator/nonpolar solvents cis-1,4 polymer의 생성이 증가 ex) Isoprene/BuLi/pentane or hexane cis-1,4 polyisoprene CH2 7.3.3 Stereochemistry of anionic polymerization formation of cis-polyisoprene – lithium’s ability s-cis comformation by pi complexation – hold isoprene CH2 CH3 CH2Li + CH2 C CH CH2Li CH2 CH2 C H (7.57) C CH3 forming a six-membered ring transition state – “lock” the isoprene into a cis-configuration + Li - CH2 C C H CH3 CH2 + CH2 CH C CH2 CH3 (7.58) CH2 CH2 steric effect CH2 H CH3 CH2 H C C C CH3 Li C C CH3 C CH2 CH2 C H CH2 CH CH3 (7.59) CH2 7.3.4 Anionic Copolymerization Complicating factors of counterion. ① solvating polar of the solvent Table 7.5 ② temperature effect ③ electron transfer initiator 사용 free radical polymerization Anionic polymerization Li+CHCH2CH2CHLi+ + Cl R Cl X X competition CHCH2CH2CH R X X (7.60) ④ contrasts between homogeneous and heterogeneous polymerization systems. relatively few reactivity ratios TABLE 7.5. Representative Anionic Reactivity Ratios (r)a Monomer 1 Monomer 2 Initiatorb Styrene Methyl methacrylate Na n-BuLi Butadiene n-BuLi n-BuLi n-BuLi n-BuLi n-BuLi EtNa Isoprene n-BuLi Acrylonitrile RLi Vinyl acetate Na Butadiene Isoprene n-BuLi Methyl methacrylate Acrylonitrile NaNH2 RLi Vinyl acetate NaNH2 aData from Morton.30 bBu=butyl, Et=ethyl, R=alkyl. cTHF=tetrahydrofuran. dTemperature cot specified in some instances. eNo detectable styrene in polymer. Solventc Temperatured r1 (oC) 0.12 6.4 0.04 0.03 0.04 4.0 11.0 0.96 0.046 0.12 0.01 3.38 0.25 0.34 3.2 11.2 12.5 11.8 0.3 0.4 1.6 16.6 12.5 0.01 0.47 7.9 6.7 0.4 NH3 None None Hexane Hexane THF THF Benzene Cyclohexane None NH3 Hexane NH3 None NH3 e 25 25 50 25 -78 40 50 r2 e 7.3.4 Anionic Copolymerization formation of block copolymers by the living polymer method. CH3 - n CH2 CH C6H5 R: - CH2CH: CH2CH C6H5 C6H5 n-1 (7.61) CH3 CH2CH C6H5 n CCO2CH3 m CH2 CH2C CH3 - CH2C: CO2CH3 m-1 CO2C 7.3.4 Anionic Copolymerization Commercial block copolymers ABA triblock polymers – Greatest commercial success ex) styrene-butadiene-styrene B initiator B:- combination -:BB:- B - :BBBBBBBBBB:- S (7.62) - - :SSSSSBBBBBBBBBBSSSSS: star-block (radial) – much lower melt viscosities, even at very high molecular weights ex) silicon tetrachloride 4 - : + SiCl4 Si (7.63) 7.4 Group Transfer Polymerization (GTP) (In the 1980s a new method for polymerizing acrylic-type monomers) GTP의 특성 ① Anionic polymerization에서 흔히 사용되는 monomer를 사용 Living polymer로 전환 ② Propagating chain Covalent character ③ Organosilicon이 개시제로 사용 R C C R OSiR3 (R CH3 OR + CH2 C CH2 RO2C CO2CH3 CH3 C CH2C C C OSiR3 OCH3 R CH3) (7.64) CH3 n R - HF2 R CO2CH3 RO2C CH3 C CH2C CH3 CH2C C OSiR3 living polyme r Organosilicon에서 SiR3가 transfer되어 중합을 형성(GTP) R CO2CH3 n OCH3 TABLE 7.6. Representative Compounds Used in Group Transfer Polymerization Monomersa CH2 CHCO2R Initiatorsa Me2C C OMe OSiMe3 Me CH2 CH2 CH2 CCO2R Me3SiCH2CO2Me CHCONR2 Me3SiCN CHCN RSSiMe3 Me CH2 CCN ArSSiMe3 O CH2 CHCR aR=alkyl, Ar=aryl, Me=methyl, X=halogen. b0.1 mol% relative to initiator. c10-20 mol% relative to monomer. dPreferred with Lewis acid catalysts. Catalystsa Solvents Anionicb HF2 - - CN N3 Me3SiF2 Lewis acid ZnX2 R2AlCl (R2Al)2O c Acetonitrile 1,2-Dichloroethaned Dichloromethaned N,N-Dimethylacetamide N,N-Dimethylformamide Ethyl acetate Propylene carbonate Tetrahydrofuran Toluened 7.4 Group Transfer Polymerization (GTP) * Synthesis of initiator R2CHCO2R R'2N-Li+ R2CCO2R R2C C O - R3SiCl CH2SSiMe3 + CH2 CHCO2R ZnI2 OSiR3 사슬의 양끝에서 성장 CO2R CH2SSiMe3 C OR OR 두 개의 작용기를 갖는 개시제 사용 R2C OSiMe3 CH2S CH2CH CH2CH C OR CH2S CH2CH CH2CH C OR CO2R OSiMe3 (7.65) 7.4 Group Transfer Polymerization (GTP) Speciality ① Once the monomer is consumed, a different monomer may be added ② chain can be terminated by removal of catalyst. ③ chain can be terminated by removal by protonation or alkylation. CH3 CH3OH CH3 CH2C C CH2CH OSiR3 CO2CH3 OCH3 CH3 C6H5CH2Br CH2CCH2C6H5 CO2CH3 (7.66) (7.67) 7.4 Group Transfer Polymerization (GTP) GPT mecahanism R C OR C R Nu OSiR3 - - R C C R O CH2 C CH3 R R C C CH2 OR O C CH3 OSiR3 C OCH3 Nu OR SiR3 O C OCH3 (7.68) 7.4 Group Transfer Polymerization (GTP) Chain transfer of GPT CH2 C C CH3 - O OSiR3 OCH3 + CH2 C C CH3 CH3O CH2 CH2 O Si (R3) O C C CH3 (7.69) C C OCH3 CH3O CH3