Anionic Polymerization - ACS Publications - American Chemical Society

3 Anionic Polymerization Maurice Morton

Downloaded by CHINESE UNIV OF HONG KONG on March 11, 2016 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch003

Institute of Polymer Science, The University of Akron, Akron, OH 44325

Historical Review Special Features of the Anionic Mechanism Absence of Termination Processes Effect of Counterion (Initiator) Effect of Solvents and Reaction Conditions Synthesis Capabilities Block Copolymers Functional End-Group Polymers Initiation Processes in Anionic Polymerization Initiation by Electron Transfer Initiation by Nucleophilic Attack Mechanism and Kinetics of Homogeneous Anionic Polymerization Polar Media Nonpolar Media

The term "ionic polymerization" basically involves the chemistry of heterolytic cleavage of chemical bonds, as opposed to the homolytic reactions that characterize the well-known free-radical polymerization mechanism. Hence, essential and profound differences exist between these two mechanisms of polymerization. Although these differences are also found between radical and ionic mechanisms in ordinary reactions, they exert a much more drastic influence on the result, that is, the growth of a long chain molecule to macro dimensions. Thus, one would expect that the two mechanisms could lead to quite different results in most simple reactions, in terms of rate, yield, or mode of the reaction. In the case of polymerization, however, such differences, can, in fact, decide whether any high polymer is obtained at a l l . The differences between the homolytic and heterolytic mechanisms of polymerization are reflected in the influence of the following factors on the course of the reaction: 0097 6156/85/0285-0051 $06.00/0 © 1985 American Chemical Society

Tess and Poehlein; Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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The e f f e c t of monomer s t r u c t u r e on reactivity ( p o l y m e r i z a bility) and on presence of " s i d e r e a c t i o n s , " t h a t is, those r e a c t i o n s t h a t i n t e r f e r e w i t h c h a i n growth, f o r example, t e r m i n a t i o n , t r a n s f e r , and rearrangement. This factor i s relevant to both mechanisms. Type of i n i t i a t o r . This factor does not influence the propagation reaction i n the f r e e - r a d i c a l mechanism, but can have a profound effect on i o n i c propagation. Nature of the medium. A g a i n , the f r e e - r a d i c a l mechanism generally e x h i b i t s l i t t l e or no dependence of the propagation r e a c t i o n on the type of s o l v e n t or medium (J_), whereas i o n i c systems can show very large effects, not only on the k i n e t i c s but on the a c t u a l chain structure as w e l l .

Items 2 and 3 a r i s e from the f a c t t h a t both the " c o u n t e r i o n " and the medium i t s e l f can markedly affect the nature of the growing c h a i n end. Thus, the growing c h a i n end may assume v a r i o u s forms that depend on the extent of e l e c t r i c a l charge separation and range a l l the way from a polarized covalent (sigma) bond to a completely dissociated state of free ions. This c h a r a c t e r i s t i c presents the g r e a t e s t d i s t i n c t i o n between the mechanisms of f r e e - r a d i c a l and i o n i c polymerization. Hence, i n contrast to f r e e - r a d i c a l polymerization, the nature of the a c t i v e species i n i o n i c polymerization i s s t i l l surrounded by a great deal of mystery and ignorance, e s p e c i a l l y because i t may vary from one system to another. Furthermore, the e l u c i d a t i o n of the a c t i v e s p e c i e s i n such i o n i c systems i s made even more d i f f i c u l t by the known s e n s i t i v i t y of these systems to t r a c e s of impurities. Ionic polymerization has been subdivided into two broad categ o r i e s ; c a t i o n i c and a n i o n i c , on the b a s i s of the nature of the "charge" on the t i p of the growing chain. The above c l a s s i f i c a t i o n does not n e c e s s a r i l y r e f e r to the presence of free c a t i o n s or anions. In fact, i t would seem that, i n the majority of cases, the main s p e c i e s i n v o l v e d are " i o n - p a i r s " i n which a c o u n t e r i o n i s associated with the cation or anion. These i o n i c systems are quite complex and r e q u i r e i n t e n s i v e study and a c e r t a i n degree of s p e c i a l i z a t i o n ; therefore, they have generally been considered as separate f i e l d s of i n v e s t i g a t i o n . The " Z i e g l e r - N a t t a " systems (which may be heterogeneous or homogeneous) are not yet understood enough to be c l a s s i f i e d into a p a r t i c u l a r i o n i c system, although i n the m a j o r i t y of cases they are c o n s i d e r e d as c o o r d i n a t e d a n i o n i c p o l y m e r i z a t i o n s . For the sake of c l a r i t y and s i m p l i c i t y , t h i s d i s c u s s i o n w i l l be l i m i t e d to the s i m p l e r homogeneous a n i o n i c polymerization systems. F i n a l l y , t h i s chapter i s not a comprehensive review of the work done i n t h i s f i e l d , but rather a concise discussion of the present s t a t e of u n d e r s t a n d i n g . F u r t h e r e x p l o r a t i o n of a n i o n i c polymerization can be found i n two recent books (_2, 3), the former being an assembly of t o p i c a l papers presented at a recent symposium, and the l a t t e r being a more comprehensive treatment of t h i s subject.


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H i s t o r i c a l Review Because the mechanism of an a d d i t i o n p o l y m e r i z a t i o n r e a c t i o n i s defined by the character of the growing chain end, and because most p o l y m e r i z a t i o n s i n v o l v e v i n y l and a l l i e d compounds, we are concerned with the presence of carbon atoms that are free r a d i c a l s , carbenium ions, or carbanions. Such d e f i n i t i o n s made i t d i f f i c u l t to understand and c l a s s i f y the e a r l i e s t i n s t a n c e s of a n i o n i c p o l y m e r i z a t i o n , t h a t i s , the p o l y m e r i z a t i o n of butadiene and isoprene by sodium, p r o b a b l y f i r s t r e p o r t e d by H a r r i e s (_4) and Matthews and Strange (5). Although t h i s polymerization soon became a commercial process for synthetic rubber of the polybutadiene type and was s t u d i e d i n t e n s i v e l y by Z i e g l e r f o r many y e a r s , i t s significance as a carbanionic chain reaction was not recognized for a l o n g time because of the heterogeneous nature of the r e a c t i o n components. Thus, i t was known that m e t a l l i c sodium i s capable of adding two atoms e i t h e r 1,4 or 1,2 a c r o s s a conjugated 1,3-diene, and t h a t such d i - a d d u c t s can f u r t h e r add Na and RNa groups a c r o s s a d d i t i o n a l molecules of the diene, but t h i s reaction was considered a " s t e p - w i s e " p o l y m e r i z a t i o n without regard to the p r e v a i l i n g mechanism. E v e n t u a l l y s c i e n t i s t s recognized (ca. 1950) t h a t , because the t e r m i n a l carbon atom i n these diene "adducts" i s attached to an a l k a l i m e t a l , i t must have the nature of a carbanion. A c t u a l l y , the e a r l i e s t examples of a n i o n i c p o l y m e r i z a t i o n studied were the base-catalyzed polymerizations of ethylene oxide i n v o l v i n g a ring-opening chain reaction:

A RONa + CH -CH > ROCH CH ONa > > (1) Because the monomer was not a v i n y l compound and the a c t i v e chain end was an a l k o x i d e , t h i s reaction was not considered an important case of anionic polymerization. I r o n i c a l l y , t h i s reaction a c t u a l l y i s a very good example of the a n i o n i c mechanism and can be s a t i s f a c t o r i l y s t u d i e d because i t i s a homogeneous r e a c t i o n . In f a c t , i t was F l o r y (6) who f i r s t pointed out the unique consequences that a r i s e from such a polymerization i n which presumably the alkoxide chain end does not undergo any "side reactions," that i s , termination. Flory remarked that i n such a s i t u a t i o n i n which a l l the growing chains have equal access to the monomer, the chains w i l l tend to reach s i m i l a r lengths, that i s , the molecular weight of the polymer w i l l have the very narrow Poisson d i s t r i b u t i o n : 2

P

V n

=

1

+

2

1/P

2

2

(

2

)

N

where P i s the weight-average number of units per chain and P i s the number-average number of u n i t s per c h a i n . T h i s example was a p p a r e n t l y the f i r s t case of a " l i v i n g polymer" t h a t d i d not i n v o l v e a v i n y l monomer. A s i m i l a r case was l a t e r found i n the b a s e - c a t a l y z e d r i n g opening polymerization of c y c l i c siloxanes. In t h i s case, on the basis of k i n e t i c measurements, i t was shown (7, 8) that the a c t i v e s p e c i e s i s the free s i l a n o l a t e anion ( - S i - 0 ~ ) and no t r u e w


n


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t e r m i n a t i o n step e x i s t s whereby c h a i n s cease growing. However, a l t h o u g h i t was proven c o n c l u s i v e l y (8) that each i n i t i a t o r molecule produced a polymer chain, the Poisson d i s t r i b u t i o n was not a t t a i n e d because of the p r e v a l e n c e of s i l o x a n e bond interchange between the polymer and the growing s i l a n o l a t e ion. More r e c e n t l y , a narrow d i s t r i b u t i o n of c h a i n l e n g t h s was a t t a i n e d (9) i n these systems i f the b a s e - c a t a l y z e d s i l o x a n e bond interchange was suppressed (by u s i n g the much more r e a c t i v e c y c l i c t r i m e r and a weaker base). Although the unique features of the above polymerizations were r e c o g n i z e d , the same was d i f f i c u l t to do for v i n y l and diene monomers i n which the much more r e a c t i v e a n i o n , t h a t i s , a c a r banion, was i n v o l v e d . Thus, although some of the e a r l i e r studies, such as those of Robertson and Marion (10) on sodium polymerization of butadiene i n toluene, and Higginson and Wooding (11) on styrene polymerization by potassium amide i n l i q u i d ammonia, demonstrated the presence of an a n i o n i c mechanism, the absence of any t r u e termination step i n these i n v e s t i g a t i o n s was not recognized because of the presence of many transfer reactions. Much more was learned about the true nature of the carbanionic mechanism after more i n t e n s i v e i n v e s t i g a t i o n s t h a t f o l l o w e d two s p e c i a l developments. The f i r s t was the d i s c o v e r y of the s t e r e o s p e c i f i c p o l y m e r i z a t i o n of isoprene to the high c i s - 1 , 4 polymer (12) by l i t h i u m or i t s organic compounds. This polymerization v i r t u a l l y synthesized the structure of natural rubber. The other development was the d i s c o v e r y o f the n o n t e r m i n a t i n g ( " l i v i n g " ) nature of the p o l y m e r i z a t i o n of s t y r e n e by sodium naphthalene (13). Because both of these systems i n v o l v e homogeneous reactions, i t was possible to subject them to i n v e s t i g a t i o n with regard to k i n e t i c s and stoichiometry, and thus determine the main c h a r a c t e r i s t i c s of the anionic mechanism. These developments and p o s s i b i l i t i e s aroused an increased interest i n the f i e l d . Special Features of the Anionic Mechanism The s p e c i a l features of the anionic mechanism that d i s t i n g u i s h i t from the other mechanisms can be c l a s s i f i e d as f o l l o w s : Absence of T e r m i n a t i o n P r o c e s s e s . The p o s s i b i l i t y of h a v i n g carbanionic species that show a n e g l i g i b l e rate of termination i s now r e a l i z e d . In other words, just as the growing chain end i n the a l k o x i d e p o l y m e r i z a t i o n of e t h y l e n e oxide r e p r e s e n t s a " s t a b l e " s a l t of an a l k a l i metal and an a l c o h o l , the s t y r y l sodium c h a i n end, i n the p o l y m e r i z a t i o n of s t y r e n e by sodium naphthalene, r e p r e s e n t s a " s t a b l e " s a l t of sodium and a hydrocarbon. This r e l a t i o n s h i p was f i r s t noted i n the p a r t i c u l a r case of the sodium naphthalene systems i n which the o r g a n o m e t a l l i e s p e c i e s i s s t a b i l i z e d by a h i g h degree of s o l v a t i o n by an e t h e r , such as tetrahydrofuran, so that no observable side reactions e x i s t , that i s , t e r m i n a t i o n of c h a i n s , at l e a s t not w i t h i n the time s c a l e of the polymerization reaction. Such " l i v i n g " polymer systems, however, are not l i m i t e d to p o l y m e r i z a t i o n s i n s o l v a t i n g media, such as e t h e r s . Thus, the l i t h i u m - c a t a l y z e d polymerizations, which can lead to the synthesis of c i s - 1 , 4 - p o l y i s o p r e n e , a l s o demonstrate the v i r t u a l absence of


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termination (14). Because l i t h i u m polymerizations can be c a r r i e d out i n both ether and hydrocarbon s o l v e n t s , they can be used to demonstrate the r o l e of the solvent i n these reactions. In a l l of these cases, the absence of any noticeable termination process can lead to a very narrow d i s t r i b u t i o n of chain lengths, i n accordance w i t h Equation 2. However, a l t h o u g h necessary, t h i s c o n d i t i o n i s not s u f f i c i e n t to guarantee such a r e s u l t because the r a t e of i n i t i a t i o n of chains a l s o i s a factor. Hence, the question of the molecular weight and i t s d i s t r i b u t i o n w i l l be discussed together with the question of i n i t i a t i o n processes. Effect of Counterion ( I n i t i a t o r ) . As stated e a r l i e r , i n a l l i o n i c p o l y m e r i z a t i o n s a p o s s i b l e i n f l u e n c e exerted by the c o u n t e r i o n e x i s t s and originates from the i n i t i a t o r . In other words, u n l i k e the case of f r e e - r a d i c a l polymerization, the type of i n i t i a t o r used may a c t u a l l y affect the nature of the chain growth process. This r e l a t i o n s h i p i s s t r i k i n g l y demonstrated by the o r g a n o a l k a l i polymerization of dienes i n which the proportion of c i s - and transi t as w e l l as 1,2 u n i t s i n the c h a i n i s governed by the a l k a l i metal c o u n t e r i o n . T h i s r e s u l t was, of course, the b a s i s f o r the d i s c o v e r y of the s t e r e o s p e c i f i c p o l y m e r i z a t i o n of isoprene by l i t h i u m i n i t i a t o r s (12). Foster and Binder (15) showed the effect on the m i c r o s t r u c t u r e of both p o l y b u t a d i e n e and p o l y i s o p r e n e prepared by means of the f i v e w e l l - k n o w n a l k a l i m e t a l s , and demonstrated that the 1,4 t y p e o f a d d i t i o n d e c r e a s e s w i t h i n c r e a s i n g e l e c t r o p o s i t i v i t y o f the m e t a l . Hence, the microstructure of polyisoprene varies a l l the way from a very high 1,4 content w i t h l i t h i u m to a very h i g h 3,4 content w i t h rubidium or cesium. F o s t e r and Binder a l s o showed the a s i m i l a r e f f e c t on the structure of polybutadiene with regard to 1,4 versus 1,2 u n i t s , but the p r o p o r t i o n of the c i s - 1 , 4 isomer was, of c o u r s e , much lower. Effect of Solvents and Reaction Conditions. The term "solvent" i s customarily used rather l o o s e l y i n polymerization reactions because such "solvents" may refer either to the actual medium i n which the r e a c t i o n i s c a r r i e d out, or to t r a c e m a t e r i a l s present i n the medium. Hence, the term r e a l l y encompasses any component other than monomer and i n i t i a t o r . Thus, i n f r e e - r a d i c a l polymerization, the r o l e of the s o l v e n t i s l i m i t e d to " i n t e r f e r i n g " with the normal propagation r e a c t i o n , e i t h e r through c h a i n t r a n s f e r or even by t e r m i n a t i o n ( i n h i b i t i o n or r e t a r d a t i o n ) . E i t h e r of these e v e n t s can affect only the chain length or the o v e r a l l rate, or both. In c o n t r a s t , i n a n i o n i c systems i n which the s o l v e n t may not a c t u a l l y interrupt the propagation process, i t may play an a c t i v e r o l e i n c o n t r o l l i n g both the r a t e and mode of the c h a i n growth step. This c o n t r o l i s perhaps most d r a m a t i c a l l y i l l u s t r a t e d i n the case of the o r g a n o l i t h i u m p o l y m e r i z a t i o n s i n connection with two s p e c i f i c aspects: c h a i n m i c r o s t r u c t u r e o f p o l y d i e n e s and copolymerization of dienes and styrene. The dramatic e f f e c t of even t r a c e s of e t h e r s on the m i c r o s t r u c t u r e of p o l y b u t a d i e n e and p o l y i s o p r e n e i n o r g a n o l i t h i u m polymerizations i n hydrocarbon media was demonstrated very effect i v e l y by T o b o l s k y et a l . (16, 17) They showed t h a t h i g h l y s o l v a t i n g ethers, such as H^-furan, when present i n approximately



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s t o i c h i o m e t r i c p r o p o r t i o n to the amount of l i t h i u m , can cause a sharp decrease i n the 1,4 addition reaction i n favor of the 1,2 (or 3,4) r e a c t i o n . They suggested t h a t the e t h e r s , by c o o r d i n a t i n g w i t h ( s o l v a t i n g ) the l i t h i u m c a t i o n , cause a g r e a t e r charge s e p a r a t i o n and thereby mimick the b e h a v i o r of the more e l e c t r o p o s i t i v e , larger cations of the a l k a l i metal series, which are a l s o known to l e a d to h i g h s i d e - v i n y l s t r u c t u r e s i n the polymer. The e f f e c t of such s o l v e n t s on the m i c r o s t r u c t u r e i s i l l u s t r a t e d i n T a b l e I on the b a s i s of NMR a n a l y s i s (18, 19), which g i v e s more a c c u r a t e r e s u l t s than IR spectroscopy methods. However, the r e s u l t s are, i n general, s i m i l a r to those published previously (16, il). Table I a l s o shows that the i n i t i a t o r and monomer concentration can a l s o markedly a f f e c t the c h a i n s t r u c t u r e i n nonpolar media (these factors apparently have no effect i n the presence of polar s o l v e n t s ) . Thus, the h i g h e s t c i s - 1 , 4 content i n the case of both polybutadiene and polyisoprene i s obtained i n the absence of any solvents and at very low i n i t i a t o r l e v e l s . The e q u a l l y dramatic effect of polar solvents on the copolymerization behavior of dienes with styrene i s i l l u s t r a t e d i n Table II. The r values show that the r e l a t i v e rate uf entry of the diene and styrene monomers i s apparently c o n t r o l l e d very c l o s e l y by the nature of the c a r b o n - l i t h i u m bond. Thus, i n hydrocarbons the preference i s very strong for the dienes, whereas, i n the presence of a highly s o l v a t i n g medium such as H^-furan, the exact reverse i s t r u e . S o l v e n t s of i n t e r m e d i a t e p o l a r i t y show a l e s s e r e f f e c t . A p p a r e n t l y , the e f f e c t of the s o l v e n t i n i n f l u e n c i n g the charge s e p a r a t i o n at the c a r b o n - l i t h i u m bond profoundly i n f l u e n c e s the k i n e t i c s of the copolymerization. Synthesis C a p a b i l i t i e s The " l i v i n g " nature of polymer c h a i n s i n homogeneous a n i o n i c polymerization leads to novel p o s s i b i l i t i e s of chemical synthesis t h a t are not a v a i l a b l e i n p o l y m e r i z a t i o n systems i n which the growing c h a i n has a s h o r t l i f e t i m e . The three p o s s i b i l i t i e s o f f e r e d by the " l i v i n g " ends of these polymer c h a i n s are A. attainment of a very narrow m o l e c u l a r weight d i s t r i b u t i o n , much narrower than previously accomplished by fractionation; B. format i o n of b l o c k copolymers by s e q u e n t i a l a d d i t i o n of d i f f e r e n t monomers; and C. formation of polymers having functional end groups by a d d i t i o n of an a p p r o p r i a t e reagent at the c o n c l u s i o n of the polymerization. In regard to Item A, the f a c t o r s c o n t r o l l i n g the m o l e c u l a r weight d i s t r i b u t i o n are d i s c u s s e d more f u l l y i n a l a t e r s e c t i o n under the t o p i c of i n i t i a t i o n r e a c t i o n s . Furthermore, a compreh e n s i v e r e v i e w of accomplishments i n t h i s area can be found i n a very recent book (_3). Hence, t h i s d i s c u s s i o n w i l l be l i m i t e d to Items B and C. B l o c k Copolymers. In p r i n c i p l e , a b l o c k copolymer h a v i n g any number of b l o c k s can be prepared by s e q u e n t i a l a d d i t i o n of d i f ferent monomers. However, i n practice, t h i s preparation may have some l i m i t a t i o n s imposed by the i n c l u s i o n , with the added monomers,


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Table I .


Chain Microstructure of Organolithium Polydienes

Initiator Cone. (M)

Solvent Downloaded by CHINESE UNIV OF HONG KONG on March 11, 2016 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch003

57

Microstructure (mole %) cis-1,4 trans-1,4 3,4 1,2

Polyisoprene (18, 19.) 4

Benzene n-Hexane n-Hexane None None H -furan (20)

3xl0~ 1x10"^ 1x10-^ 3x10"^ 8x10"; 3x10~

69 70 86 77 96

25 25 11 18 0 26

52 68 56 36 86 39 7

36 28 37 62 9 52 10

4

4

6 5 3 5 4 66

Polybutadiene (18, 19, 21) 8x10" 6 1x10' 5 2x10 5 3x10 -2 7x10" •6 3x10" -3 2x10" -4

Benzene Cyclohexane n-Hexane n-Hexane None None H/-furan

Table I I .

12 4 7 8 5 9 85

Copolymerization of Styrene and Dienes i n Organol i t h i u m Systems (25 °C)

Monomer (M )

Solvent

2

r

l

r

2

Butadiene (22)

None Benzene Triethylamine Diethyl ether H^-furan

0.04 0.04 0.5 0.4 4.0

11.2 10.8 3.5 1.7 0.3

Isoprene

Benzene (23) Triethylamine (24) H -furan (25)

0.26 1.0 9.0

10.6 0.8 0.1

4



58


of a d v e n t i t i o u s i m p u r i t i e s c a p a b l e of t e r m i n a t i n g some of the a c t i v e chain ends. These anionic systems, therefore, offer unique o p p o r t u n i t i e s f o r the s y n t h e s i s of b l o c k copolymers of known architecture, i n regard to both composition and chain length. The s p e c i a l f e a t u r e of such polymers i s that each b l o c k can, i n p r i n c i p l e , have the P o i s s o n d i s t r i b u t i o n (Equation 2) of c h a i n l e n g t h s , that i s , approach v i r t u a l m o n o d i s p e r s i t y and thus add a new dimension to the synthesis of "tailor-made" polymers. A s t r i k i n g example of the unusual p r o p e r t i e s of such w e l l c h a r a c t e r i z e d b l o c k copolymers i s o f f e r e d by the "ABA" types of polymers, known as " t h e r m o p l a s t i c elastomers." In these e l a s t o mers, the A b l o c k s c o n s i s t of p o l y s t y r e n e ( m o l e c u l a r weight approximately 10,000-15,000), and the B block i s a polydiene, such as polybutadiene or polyisoprene ( m o l e c u l a r weight a p p r o x i m a t e l y 50,000-75,000). These e l a s t o m e r s can be s y n t h e s i z e d by the s e q u e n t i a l a d d i t i o n of the two monomers to an o r g a n o l i t h i u m i n i t i a t o r (26, 27) i n hydrocarbon s o l v e n t s t h a t y i e l d rubbery p o l y d i e n e s of high 1,4 c o n t e n t . Because these e l a s t o m e r s are "pure" b l o c k copolymers, the homopolymer blocks are incompatible and tend to separate i n t o two phases. The net e f f e c t i s t h a t the p o l y s t y r e n e , which i s the l e s s e r component, s e p a r a t e s as a d i s p e r s e d phase w i t h i n the p o l y d i e n e medium. In view of the r e s t r i c t i o n s imposed by the chemical bonds between the blocks, the two phases do not separate on a macro s c a l e , but form a very f i n e l y d i v i d e d ( a p p r o x i m a t e l y 200 A) d i s p e r s i o n of p o l y s t y r e n e i n the p o l y d i e n e . The r e g u l a r i t y of such a d i s p e r s i o n of p o l y s t y r e n e spheres i n a p o l y i s o p r e n e m a t r i x i s shown i n the t r a n s m i s s i o n e l e c t r o n microphotograph of F i g u r e 1. A l l of these phase s e p a r a t i o n s can o n l y occur from s o l u t i o n s or m e l t s . At ambient temperatures, then, the polystyrene dispersion i s i n a glassy state i n which each p o l y s t y r e n e p a r t i c l e a c t s as a v i r t u a l l y r i g i d juncture for a large number (approximately 200) of polydiene chain ends and thus c r e a t e s an e l a s t i c network. However, because the network j u n c t i o n s are i n the form of t h e r m o p l a s t i c p o l y s t y r e n e p a r t i c l e s , the network i s r e v e r s i b l y destroyed and reformed on heating and c o o l i n g . These ABA b l o c k copolymers thus owe t h e i r p r o p e r t i e s to the i n c o m p a t i b i l i t y of the i n d i v i d u a l b l o c k s and to the unusual morphology of the r e s u l t i n g m a t e r i a l . They are an e x c e l l e n t example of the s p e c i a l f e a t u r e s t h a t a r i s e from b l o c k and g r a f t polymers i n which different homopolymer chains are l i n k e d together chemically. Their mechanical properties can a l s o be quite unusual. Figure 2 shows t y p i c a l s t r e s s - s t r a i n curves of a series of styrenei s o p r e n e - s t y r e n e b l o c k copolymers. The e f f e c t of the amount of dispersed polystyrene i s obvious, as i s the unusually high t e n s i l e strength of these elastomers, e s p e c i a l l y i n view of t h e i r amorphous nature. These high values of t e n s i l e strength have been ascribed (28, 29) to the " f i l l e r " effect of the polystyrene p a r t i c l e s and to t h e i r a b i l i t y to y i e l d at high stress and absorb the s t r a i n energy that would otherwise cause rupture.



3.

MORTON


ULTRA-THIN

SECTION-LINEAR

59

SIS-I

Figure 1.

Transmission e l e c t r o n micrograph of a styrene-isoprenestyrene t r i b l o c k copolymer.

Figure 2.

Tensile properties of styrene-isoprene-styrene t r i b l o c k copolymers.


60


Functional End-Group Polymers. The formation of various functional groups on the " l i v i n g " ends of anionic polymer chains i s based on the well-known reaction of organometallie compounds with various reagents. Thus, o r g a n o a l k a l i compounds can react as i l l u s t r a t e d i n the f o l l o w i n g t y p i c a l reactions: RM + C0

> RC00M

2

H

RM + C H ^ 2 2


x

(3)

> R-CH -CH OM 2

2

(4)

0

R

f

I RM + R'CHO

> R-C-OM

(5)

I H where M r e p r e s e n t s an a l k a l i m e t a l . The r e s u l t i n g p r o d u c t s , on h y d r o l y s i s , y i e l d c a r b o x y l i c acids, a l c o h o l s , etc. The i n t e r e s t i n such polymers w i t h f u n c t i o n a l end groups centers mainly on the d i f u n c t i o n a l v a r i e t y because t h i s variety can offer i n t e r e s t i n g p o s s i b i l i t i e s of various l i n k i n g reactions, such as c h a i n e x t e n s i o n or network f o r m a t i o n , which are important i n l i q u i d polymer t e c h n o l o g y . To s y n t h e s i z e such d i f u n c t i o n a l polymers i t i s necessary f i r s t to have a d i - a n i o n i c polymer chain, such as i s formed from i n i t i a t i o n reactions i n v o l v i n g an e l e c t r o n transfer process as w i l l be discussed l a t e r . However, d i - a n i o n i c s p e c i e s are g e n e r a l l y i n s o l u b l e i n i n e r t s o l v e n t s such as hydrocarbons because of the tendency of o r g a n o m e t a l 1 i c s to a s s o c i a t e i n such s o l v e n t s . Hence, s o l v a t i n g s o l v e n t s such as ethers are generally required to o b t a i n homogeneous systems, and the presence of such e t h e r s , as mentioned e a r l i e r , i s a s e r i o u s o b s t a c l e to the s y n t h e s i s of rubbery, high 1 , 4 - p o l y d i e n e s . T h i s s i t u a t i o n has posed a serious problem i n the development of l i q u i d polymer technology for the polydienes. Recently, however, aromatic e t h e r s and t e r t i a r y amines were used as s o l v e n t s t o form homogeneous s o l u t i o n s of o r g a n o d i l i t h i u m i n i t i a t o r s and polymers v/ithout e x e r t i n g any major e f f e c t on the m i c r o s t r u c t u r e of polydienes (30-32). Hence, these ethers and amines can be used i n the s y n t h e s i s of rubbery, d i f u n c t i o n a l p o l y d i e n e s h a v i n g a very narrow molecular weight d i s t r i b u t i o n (33). A recent p u b l i c a t i o n (34) describes how d i f u n c t i o n a l polyisoprenes made i n t h i s way were end-linked to form r e l a t i v e l y uniform e l a s t i c networks t h a t exhibited superior properties. I n i t i a t i o n Processes i n Anionic Polymerization The a n i o n i c p o l y m e r i z a t i o n systems based on the a l k a l i m e t a l s i n v o l v e o n l y two processes, i n i t i a t i o n and p r o p a g a t i o n , because termination can be avoided, i f desired. The i n i t i a t i o n processes f a l l i n t o two broad c l a s s e s : electron transfer and n u c l e o p h i l i c attack.


3.

MORTON


61

I n i t i a t i o n by E l e c t r o n T r a n s f e r . T h i s mechanism of i n i t i a t i o n operates i n polymerizations by a l k a l i metals or t h e i r complexes and was best elucidated i n the case of the sodium naphthalene complex (13), which was used to form the well-known " l i v i n g " polymers. In these complexes, the naphthalene i s a r a d i c a l anion (35) formed by t r a n s f e r of an e l e c t r o n from sodium i n the presence of a h i g h l y s o l v a t i n g solvent such as H^-furan:


~Na

+ fU-furan

Na +

Greenish

+

(H^-furan)

(6)

blue

This process thus has the effect of d i s s o l v i n g sodium metal to form a b r i g h t g r e e n i s h - b l u e s o l u t i o n . T h i s s o l u t i o n can a c t u a l l y be considered as a s o l u t i o n of sodium i n i t s m e t a l l i c state. In the presence of a monomer, such as s t y r e n e , a r a p i d t r a n s f e r of electrons occurs from the naphthalene: CH =CH 2

(H -furan) 2

Greenish

blue

+ CH =CH" 2

Na

+

(H -furan) 4

(7)

Red followed by coupling of the s t y r y l radical-anions, Na

CH =CH" 2

+

+

thus:

CH-CH -CH -CH Na 2

+

2

(8) Na

6

6

pzn. Hence, the f i n a l r e s u l t i s the i n i t i a t i o n of a di-anionic growing chain. This i n i t i a t i o n process i s quite fast, r e l a t i v e to the propagation process, so that a l l the growing chains are formed during a s h o r t - t i m e i n t e r v a l and have equal o p p o r t u n i t y to grow. T h i s condition produces a narrow molecular weight d i s t r i b u t i o n . These reactions with styrene are so rapid that the experimental technique i n v o l v e s dropwise addition of the monomer to the sodium naphthalene s o l u t i o n at low temperature (-78 °C). Hence, the f i r s t few drops convert a l l the sodium naphthalene into s t y r y l disodium. Further a d d i t i o n s then r e s u l t i n c h a i n growth t h a t i s governed m a i n l y by mixing c o n d i t i o n s . Reactions 7 and 8 are somewhat o v e r s i m p l i f i e d presentations. The t r a n s f e r of a second e l e c t r o n from the naphthalene to the styrene i s a l s o possible and produces a monostyrene di-anion rather than the distyrene di-anion shown i n Reaction 8. In addition, the p o s s i b i l i t y of coupling of naphthalene and styrene radical-anions exists. However, the o n l y s p e c i e s ever i s o l a t e d (e.g., by h y d r o l y s i s to 1,4-dipheny1 butane) from t h i s type of i n i t i a t i o n process has been r e l a t e d to the d i s t y r e n e s p e c i e s of R e a c t i o n 8.


62


This r e s u l t can probably be ascribed to the fact that the styrene r a d i c a l anions are the most r e a c t i v e ( l e a s t stable) of the species. As s t a t e d e a r l i e r , the e l e c t r o n - t r a n s f e r processes i n these sodium naphthalene systems are a c t u a l l y the homogeneous analogs of the reactions of monomers with the metals. Thus, the i n i t i a t i o n of butadiene by sodium may be written as Na + CH =CH-CH=CH Downloaded by CHINESE UNIV OF HONG KONG on March 11, 2016 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch003

2

2

> CH -CH=CH-CH Na 2

+

(9)

2

+

rr _ ^ - ^ N a CH -CH=CH-CH -CH -CH=CH-CH CH -CII=CH-CH Na . NaX^ rr — Na CH -CH=CII-CH Na 2

2

2

2

2

2

Na

+

(10)

+

2

2

+

+

(11)

Reaction 10 represents coupling of the r a d i c a l anions, and Reaction 11 r e p r e s e n t s a second e l e c t r o n - t r a n s f e r s t e p . R e a c t i o n 11 does occur i n the case of sodium and butadiene (10). This r e s u l t i s not surprising because an e l e c t r o n transfer step such as Reaction 9 can o n l y occur at the metal s u r f a c e and cannot g i v e r i s e to a high concentration of r a d i c a l anions i n the homogeneous phase (as i n the case of the naphthalene complex). Hence, these r a d i c a l anions are quite l i k e l y to remove a second e l e c t r o n from the sodium when they are i n the v i c i n i t y of the metal surface. The above reactions of butadiene are a l l shown as 1,4 for the sake of s i m p l i c i t y , although a s u b s t a n t i a l p r o p o r t i o n of 1,2 a d d i t i o n s c o u l d occur as w e l l . These additions depend on the nature of the medium. A heterogeneous i n i t i a t i o n reaction, such as shown i n Reactions 9 - 1 1 , w o u l d be e x p e c t e d t o be q u i t e s l o w r e l a t i v e t o t h e homogeneous propagation steps that f o l l o w . Hence, t h i s s i t u a t i o n would not l e a d to the c o n d i t i o n s necessary f o r the P o i s s o n d i s t r i b u t i o n of c h a i n l e n g t h s . T h i s aspect of the heterogeneous i n i t i a t i o n p r o b a b l y obscured the absence of chain termination i n these systems that were the b a s i s of the w e l l - k n o w n sodium p o l y b u t a d i e n e s y n t h e t i c rubbers. The phenomenon of the " l i v i n g " c h a i n end i n these p o l y m e r i z a t i o n s was r e c o g n i z e d o n l y w i t h the advent of the homogeneous i n i t i a t i o n systems of the sodium naphthalene type. I n i t i a t i o n by N u c l e o p h i l i c A t t a c k . In c o n t r a s t to the a l k a l i metals, the a l k a l i organometal l i e s can i n i t i a t e polymerization by a d i r e c t n u c l e o p h i l i c attack on the w electrons of the monomer. This polymerization can happen with both s o l u b l e and i n s o l u b l e i n i t i a t o r s , but the l a t t e r w i l l l e a d to a very slow r e a c t i o n . The homogeneous i n i t i a t i o n systems of t h i s type are best exemplified by the o r g a n o l i t h i u m compounds, l a r g e l y a Iky 1 1 i t h i u m , which are s o l u b l e i n a v a r i e t y of s o l v e n t s , i n c l u d i n g the hydrocarbons. However, even i n these homogeneous systems, the r a t e of the i n i t i a t i o n reaction i s g r e a t l y affected by the nature of the medium and i s much more r a p i d i n the presence of s o l v a t i n g media, f o r example, e t h e r s , than i n hydrocarbons. The k i n e t i c s of these i n i t i a t i o n reactions are a l s o influenced by the structure of the i n i t i a t o r . For example, s e c - b u t y l l i t h i u m i s a much more a c t i v e i n i t i a t o r than n - b u t y l l i t h i u m i n hydrocarbon s o l v e n t s and can lead


3.

MORTON


63

to polymers of narrow m o l e c u l a r weight d i s t r i b u t i o n . When the i n i t i a t i o n step i s not s u f f i c i e n t l y f a s t , however, then the molecular weight d i s t r i b u t i o n may be considerably broadened. This i s often the case i n hydrocarbon media (14) d e s p i t e the complete absence of any termination. Some of the aspects of the i n i t i a t i o n reaction w i l l be further explored i n the f o l l o w i n g discussion of the k i n e t i c s and mechanisms of these polymerizations.


Mechanism and K i n e t i c s of Homogeneous Anionic Polymerization The o v e r a l l p o l y m e r i z a t i o n r e a c t i o n i n a homogeneous medium i n v o l v i n g , for example, an organolithium i n i t i a t o r , may be written as f o l l o w s : RLi

+ M

RM - L i + M

> RMLi > RM

j + 1

(12) Li

(13)

where M represents the monomer. Reaction 12 represents i n i t i a t i o n and Reaction 13 represents the propagation step. The determination of the i n d i v i d u a l r a t e c o n s t a n t s of the two r e a c t i o n s from p o l y m e r i z a t i o n r a t e measurements poses a r a t h e r i n t r a c t a b l e mathematical problem (36). Fortunately, however, i t i s possible to measure these rate constants separately. The rate constant of the i n i t i a t i o n s t e p can be measured from t h e i n i t i a l r a t e of disappearance of i n i t i a t o r , and that of the propagation step can be measured from the r a t e of disappearance of monomer a f t e r a l l the i n i t i a t o r has r e a c t e d . These techniques have been a p p l i e d to a number of such anionic polymerizations. As might be expected, most attention has been given to the propagation reaction because i t i s the actual c h a i n - b u i l d i n g process, but some i n i t i a t i o n studies have a l s o been c a r r i e d out. Here a g a i n the nature of the medium has been found to e x e r t a profound e f f e c t on the k i n e t i c s and mechanism; therefore, the two classes of s o l v e n t s , s o l v a t i n g and nonsolvating, should be treated separately. P o l a r Media. The p o l a r s o l v e n t s , which g e n e r a l l y act as c o o r dinating ligands to s o l v a t e the metal counterion, encompass such compounds as ethers and amines. A number of k i n e t i c studies of the propagation r e a c t i o n have been c a r r i e d out ( 3 7 - 4 0 ) for s e v e r a l monomers and i n i t i a t o r s i n t h i s type of s o l v e n t , for example, H ^ furan and dimethoxyethane. In a l l cases, the r e a c t i o n r a t e was found to be f i r s t order with respect to monomer, i n accordance with Reaction 13. However, the rate dependence on the number of growing chains (RM-Li) was found to vary anywhere from h a l f order to f i r s t order. Hence, Reaction 13 may not describe the a c t u a l mechanism i n t h i s respect. On the basis of k i n e t i c and e l e c t r o l y t i c measurements (35-38), the actual a c t i v e species i n these s o l v a t i n g media may be twofold, that i s , ion pairs and dissociated anions. In fact, more than one type of i o n p a i r i s p o s s i b l e ( f o r example, c o n t a c t , s o l v e n t separated, etc.). However, for the sake of s i m p l i c i t y , the actual propagation steps are best represented as f o l l o w s :


64


RMjLi ^ M

kp

RM

J + 1

>RMj + L i M

Li^

e-

+

kp

>RM]

(14)

+1

+L i

+


+

where R M J L i r e p r e s e n t s i o n - p a i r s p e c i e s , RM~ r e p r e s e n t s f r e e a n i o n s , and k+ and kl" are the r e s p e c t i v e propagation r a t e constants. Thus, the o v e r a l l rate can be expressed as

+

Rp = k p [ R M j i ] [M] + k~[RMj] [M]

(15)

L

Taking into account the e q u i l i b r i u m constant K , and assuming that i t i s s m a l l ( l i t t l e d i s s o c i a t i o n ) , we obtain e

Rp/[M] = kp[RMjLi] + kp K * [RMjLip

(16)

e

where [ R H L i ] represents the t o t a l concentration of both types of growing c h a i n s ( t o t a l i n i t i a t o r ) . Equation 16 can then be rearranged to a convenient l i n e a r form: R

±

P

[M]

[RMjLi]

2

kpfRMjLip"

I

-

+ k K p

(17)

e

Equation 17 can then be p l o t t e d as a l i n e a r function of rate versus i n i t i a t o r concentration to y i e l d k~ and k ~ K . Furthermore, i f K i s determined, for example, from c o n d u c t i v i t y measurements, then the a b s o l u t e v a l u e of k~ i s a l s o a v a i l a b l e . A number of such measurements have been taken and y i e l d r a t e constant v a l u e s f o r various monomers (mainly styrenes and dienes) and v a r i o u s c o u n t er i o n s and s o l v e n t s (_3). In g e n e r a l these data i n d i c a t e t h a t , although the free anions are only present i n very s m a l l proportion ( K M 0 ) , they are responsible for most of the chain propagation because t h e i r r a t e c o n s t a n t s (k ^ 1 0 * - 1 0 M ~ s e c ~ ) are s e v e r a l o r d e r s g r e a t e r than those of tne i o n p a i r s ( k ^ l 0 M * " s e c ~ ) . Hence, R e a c t i o n 14 seems to r e p r e s e n t an adequate p i c t u r e of the anionic mechanism i n these systems. e

e

- 6

e

Anionic Polymerization - ACS Publications - American Chemical Society

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