New Vistas in Anionic Polymerization - ACS Publications

6 New Vistas in Anionic Polymerization MICHAEL SZWARC Department of Chemistry, State University College of Forestry at Syracuse University, Syracuse 10, Ν. Y.

Anionic polymerization carried out under suitable conditions results in the formation of living poly Downloaded by CORNELL UNIV on July 24, 2016 | http://pubs.acs.org Publication Date: January 1, 1962 | doi: 10.1021/ba-1962-0034.ch006

mers—i.e. species which may grow further, if a suitable monomer is present in the system.

This

characteristic feature of living polymers, which arises from the elimination of all the termination steps, permits the following:

preparation of block

polymers, polymers possessing two terminal func tional

groups,

monodispersed

polymers,

etc.;

studies of the thermodynamics of the propagation step—i.e. determination of

∆F,∆H,and ∆S of the

propagation for a high molecular weight polymer and for oligomers; determination of the absolute rate constants of homopropagation polymerization.

and of co-

A review of methods and re-

sults is given.

m nionic polymerization may be carried out under conditions preventing termina tion and the resulting polymers retain their ability to grow. W e shall refer to such species as " l i v i n g " polymers in contradistinction to those w h i c h are terminated and are known as " d e a d " polymers. T h e lack of termination has many important ramifications: it provides interesting syntheses, permits investigation of the thermodynamics of polymerization processes, and greatly simplifies studies of polymerization kinetics. T h e synthetic opportunities arising from the existence of living polymers have been discussed i n previous publications ( J 9 , 20, 21), hence i n this article only a brief summary of the subject is given. Three important facets may be explored: preparation of block and graft polymers; preparation of polymers w i t h the desirable functional end groups; and preparation of narrow molecular weight polymers. T h e living ends of a suitable polymer may initiate polymerization of another monomer, and thus lead to the synthesis of block polymers free of homopolymers. F o r example, one prepares living polystyrene then adds pure methyl methacrylate to its solution and produces i n this w a y a block polymer of styrene and methyl methacrylate (22). Actually, it is possible to produce living polymers w i t h two active ends w h i c h can form a block polymer containing three segments—ABA. If the living end of A initiates polymerization of B, and vice versa, one can 96 PLATZER; POLYMERIZATION AND POLYCONDENSATION PROCESSES Advances in Chemistry; American Chemical Society: Washington, DC, 1962.

SZWARC

Anionic Polymerization

97

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produce a whole spectrum of polymer molecules all having the same composition and molecular weight, but differing i n the distribution of monomers along the chain, e.g.

L e v y and Schlick (9) applied this technique to the system polystyrene-polyisoprene and produced polymers containing 3, 5, 7, and 9 blocks respectively. T h e reactive end of a living polymer P attacks a suitable group on a dead polymer P and grafts P on P as shown b y Schreiber ( 1 5 ) , who grafted living polystyrene on dead poly (methyl methacrylate). Such a procedure may lead to cross linking or to the formation of a loop, if both ends of the living polymer are active. Other block polymers prepared by this technique are: polymers of styrene and ethylene oxide (14), polymers of styrene and dimethylsiloxanes (13), and polymers of styrene and vinylpyridine (16). The presence of living ends permits the addition of desirable functional end groups to the polymer molecule; carboxylation introduces carboxyl groups; addition of ethylene oxide produces hydroxyl groups, etc. Polymers possessing two living ends are transformed i n this way into bifunctional macromolecules w h i c h may be used for further synthetic work. Polystyrene terminated by carboxyl groups may be condensed with nylon terminated by amino groups to form a block polymer of polystyrene and nylon. Whenever the initiation of polymerization is fast and termination is eliminated, monodispersed polymers may be formed by slow addition of monomer to a w e l l stirred solution of low molecular weight living polymers. This technique, sug gested by the author, was developed by M c C o r m i c k and Brewer (12), B y water and Worsfold (17), Wenger (27), and others. Polymers of the narrowest mo lecular weight distribution were actually produced by this method. Other synthetic possibilities provided by the living polymer technique may permit syntheses of star-shaped polymers, uniform networks of cross-linked poly mers, regular branched polymers, etc. A

J}

A

B

Thermodynamic Studies T h e thermodynamic studies of living polymers stem from the fact that these species retain their ability to grow by adding further monomer molecules and, therefore, in accordance with the principle of microscopic reversibility, they should also degrade into lower polymers and monomer. It follows that a solution of living polymer must come to equilibrium w i t h its own monomer. If k denotes the rate constant of propagation and k the rate constant of depropagation, then the follow ing equation describes the state of equilibrium: p

d

CO

«0

00

«0+1

PLATZER; POLYMERIZATION AND POLYCONDENSATION PROCESSES Advances in Chemistry; American Chemical Society: Washington, DC, 1962.

ADVANCES IN CHEMISTRY SERIES

98

Here F * denotes a living η-mer, [M] the equilibrium concentration of the mono mer, and n the m i n i m u m size of a living polymer. P * represents therefore, the living η-mer w h i c h may grow, but w h i c h cannot degrade spontaneously. e

0

o

00

00

F o r h i g h molecular weight polymers, V] Ρ* ~ V, * a n d , hence i n such a system [M] « k /k = Kf , where K denotes the equilibrium constant of the propagation step. This equilibrium constant, like any thermodynamic entity, is independent of the reaction mechanism. T h e polymerization of living polymers proceeds b y an anionic mechanism and the equilibrium constant, K , determined from the equilibrium concentration of the monomer, is derived from studies of an anionic system. Nevertheless, this value of K applies equally w e l l to radical or carbonium ion polymerizations and more generally to any polymerization of a monomer, provided the structure of the polymer does not change w i t h the type of reaction involved. Since — R T l n K gives the free energy change of the propaga tion step, and d l n K / d T leads to the respective AH of propagation, all the thermo dynamic functions pertaining to the propagation step are determined b y this simple technique. J h e studies of M c C o r m i c k ( I I ) and of Worsf old and B y water (31, 32) illustrate applications of this method to such systems as a-methylstyrenepoly-( α-methylstyrene) a n d styrene-polystyrene. T h e results obtained for the system a-methylstyrene-poly( α-methylstyrene) are shown i n F i g u r e 1. p

e

d

p

1

e

e


e

e

e

Ô

TETRAMER.

HIGH POLYMER.

•3

I 3.0

I 3.5

I 4.0

io y τ

ο

* OUR. RESULTS

•

* WORSFOLD-BYWATER

Δ

s M( CORMICK

I 4.5

i

I 5.0

Figure J . Equilibria between a-methylstyrene polymers and oligomers and monomelic a-methylstyrene PLATZER; POLYMERIZATION AND POLYCONDENSATION PROCESSES Advances in Chemistry; American Chemical Society: Washington, DC, 1962.

SZWARC

99


E q u i l i b r i u m between a h i g h molecular weight l i v i n g polymer and its monomer exhibits some features w h i c h do not apply to a similar equilibrium pertaining to a l o w molecular weight l i v i n g polymer. A s stated above, any thermodynamic conclusion remains v a l i d whatever path is chosen to perform the investigated change. I n the reaction P + M > P , we may break the chain of the η-mer somewhere i n the middle, insert the monomer unit and link it to the fragments, thus rebuilding the polymer molecule w h i c h w o u l d now contain η + 1 units instead of n. If the chain is sufficiently long, this process should not be affected by any changes taking place at its ends. Hence, the increase i n free energy, AF, due to chain enlargement is the same whether radicals, ions, com plexes or stable and unreactive moieties form the chain ends. However, if the chain is short, this argument no longer applies and the respective Δ Ρ may depend on the size of the polymer molcule as w e l l as on the nature of its end.


n

n + 1

T h e equilibria between short, living polymers and their monomer were recently investigated i n our laboratory (25, 26). T h e starting material was a solution of a well-defined living oligomer P*\, w h i c h could further add monomer units, but d i d not degrade. Increasing amounts of monomer were added to this solution, the system brought to e q u i l i b r i u m at the desired temperature, a n d the growing ends terminated by adding a drop of water. T h e e q u i l i b r i u m concentra tion of the monomer [M] was then determined as a function of the variable [ M ] (concentration of the initially added monomer) for a constant [ P * ]. A plot of [M] vs. [ M ] , ( F i g u r e 2) illustrates the results (25, 26) obtained for the sys tem l i v i n g α-methylstyrene " t e t r a m e r - a - m e t h y l s t y r e n e monomer at 0 ° C . T h e experimental curve passes through the origin, proving that the a-methylstyrene tetramer grows, but does not degrade—i.e. it is not a mixture of dimers, trimers, tetramers, etc. w i t h a P = 4—but a well-defined chemical species. Evidence, w h i c h is presented later, indicates the structure of this tetramer to be e

0

e

0

n

-C(Ph) (CH ).CH .CH£C(Ph)(CH ) .C(Ph)(CH ) . C H . C H 3

2

3

3

2

2

C-(Ph)(CH ). 3

T h e system resulting from addition of monomer to l i v i n g oligomer, P * , is represented by the following set of equations: o

Λ-Μ

*dt

P*

Ρΐθ+2

^«0+1

Ρη* 2

+

?±

Ρ* 3

Κ

PnV;

+Μ

τ±

Ρ*

Ρ*

0+

M

K

+ 1

η

n0

η0+2

0+

/wy

+ ί + 1

Such a system must fulfill the conditions given below: Ç

il = [F * ] n

[F* ] + 2[P V ] + 3[Ρ*^] + 1

n

2

+

0

initia

i

. . . . = [M], ~

[M]

e

Let us now assume that K = K \ — K i = Then, the investigated system is uniquely determined by K , [P*Jinitial, [M] , and [M] . In fact, as shown by Tobolsky (23), the following equation relates the variables listed above: m

m+

nQ+

m

([M]

0

-

[AfJ.)/[/?,]

Q

INITIAL

= Κ ΊΜ].·(1 Λ0

e

-

K .[M] )~i n0

e

Hence, each point of the curve shown i n Figure 2 determines K , since each one refers to a particular set of values for [M) , [M) , and [P*Jinitial (the latter being, of course, a m

Q

e



100


0.1

-0.5 M

0 f

1.0 moles/ liter

1-5

Figure 2. Equilibrium between living a-methylstyrene tetramer and a-methylstyrene monomer constant). If our assumption about the constancy of K were correct, then the cal culated K should be independent of [M ] . The plot of K as a function of [M ] , for the a M S - t e t r a m e r - a M S monomer system at 0° C . is shown i n Figure 3. ( M S is used for methylstyrene, S for styrene.) It shows that K^+fs are not constant; however, extrapolating the curve to zero concentration of M gives the true value of K . Having determined K , we assume in turn that all the remaining K (j 9^ 0) are constant and equal to K i which, of course, must be different from K . Now, the relation between [Af] , [M] , [P*Jinitiai, K , and K^i can be derived, (25) and from it, K is calculated for each pair of values of [M] and [M] , inserting for [P*J initial and K their constant values. Thus, Κ„ +ι is found to be a function of [M] and, if its value is not constant, the correct K +i may again be derived by extrapolation to [M] = 0. The procedure is repeated, and by inserting the values of K and i t ^ + i we find the value of / Γ , and so on. W e investigated two systems b y this method, namely, the living a - m e t h y l styrene tetramer + monomer a n d the l i v i n g a-methylstyrene dimer + monomer. Both the tetramer and dimer represent those oligomers w h i c h may grow, but not degrade. This means that the present tetramer is different from a tetramer ob tained from the dimer. (Further discussion of this point is given below.) T h e results lead to the equilibrium constants for the systems: m+j

m

m

0

0

0

m

m

m+j

no+

0

no

e

m

no+i

0

e

m

ϋ

0

m

0

m

ηο+2

Dimer -j- Monomer

Κι

c o . Such an assymptotic value of [M] is equal to the reconclusion. It should be emphasized that the same values were obtained for K from studies of both dimer and tetramer systems. It seems that the equilibrium constant for the system e

0

0

e

œ

Pentamer + Monomer

30,000 Styrene" 2-Vinylpyridine 1200 α-Methylstyrene " Styrene 2.5 α-Methylstyrene ~ α-Methylstyrene 1000 />-Methylstyrene ~ Styrene 300 />-Methylstyrene " />-Methylstyrene very low 2-Vinylpyridine ~ Styrene 4500 2-Vinylpyridine ~ 2-Vinylpyridine +

living poly ( p-methylstyrene ) and polystyrene but an enormous change is observed for 2-vinylpyridine~~. T h e lower value of k i n the addition of α-methylstyrene to living poly styrene reflects the effect of polarity and also the steric strain w h i c h is greater i n this reaction than i n the addition of styrene to living polystyrene. This steric strain is still greater i n the addition of α-methylstyrene to living poly ( a - m e t h y l styrene) (compare k w i t h fc ). However, it is interesting to notice AB

aMeBt

M e S

SfS


SZWARC

109


that for this pair of monomers the decrease i n the propagation rate constant amounts to a factor of 25; whereas a factor of about a million is found for the respective propagation equilibrium constants. This shows that the strain i n the final state—i.e. i n poly(a-methylstyrene) is much greater than the strain i n the transition state describing the addition of α-methylstyrene to poly (α-methylstyrene). This point was emphasized earlier by Alfrey. T h e described technique for determining k i n anionic copolymerization is limited to those reactions for w h i c h k is not much smaller than k —i.e. if the reaction AB

A

B

BB

+ Β

A-

ΛΒ-

is not m u c h slower than the reaction


AB-

+ Β

ABB-

If this condition is not fulfilled, another technique has to be applied. Such a technique was developed i n our laboratory, and is useful i n determining k /k where k is the propagation rate constant and k that of initiation b y an anion. It was pointed out (19) that if a polymerization is described by the equations p

p

iy

{

Initiator + M IM + M IMn-j- M

IM,

ki

—> IM ,

k

2

—

/A/

p

*

N + 1

P

and termination is eliminated in such a process, then the initiator is never quan titatively used i n the reaction. Denoting by / the fraction of consumed initiator, and by M and I the total amounts of monomer and initiator introduced i n the system, one finds that the following equation relates them: t o t a l

t o t a l

^toui/Zfui

= (V*0[ln(l

-/]

+/

In a copolymerization involving a slow step, one considers k

AB

^B,B

a s

k > the concentration of ^ A P

responds to [ M ]

t o t a l

.

as

-

In this way, k /k BB

AB

[/] tai» to

2

as fe and t

^ the added monomer Β cor

was determined for systems such as

CH -G(Ph) - + Styrene 2

a n (

k,

A B

to be (24) ^,850. Since k for styrene homopolymerization is 600 liters per mole second, k is calculated to be ~^1.5 liters per mole second. BB

AB

Acknowledgment T h e author acknowledges the help given by his colleagues and students, and in particular he thanks M . L e v y and J . S m i d whose enthusiasm and hard work made these investigations a success. This work was supported by the National Science Foundation (Grant N o . G14393) and the Quartermaster Corps (Grant No. DA-19-129-QM-1297). Literature Cited (1) (2) (3) (4) (5)

Allen, G., Gee, G., Stretch, C., J. Polymer Sci. 48, 189 ( 1960 ). Allen, G., Gee, G., Stretch, C., Polymer 2, 151 ( 1961 ). Brody, H., Richards, D., Szwarc, M., Chem. Ind. ( London) 45, 1473 ( 1958 ). Dainton, F., private communication. Frank, C. F., et al, J. Org. Chem. 26, 307 ( 1961 ). PLATZER; POLYMERIZATION AND POLYCONDENSATION PROCESSES Advances in Chemistry; American Chemical Society: Washington, DC, 1962.


110


(6) Geacintov, C., Smid, J. Szwarc, M., J. Am. Chem. Soc. 83, 1253 ( 1961 ); 84, in press. (7) Lee, C. L., Smid, J . , Szwarc, M., Ibid., 83, 2961 ( 1961 ). (8) Lee, C. L., Smid, J . , Szwarc, M., J. Phys. Chem. in press. (9) Levy, Schlick, S., Ibid., 64, 883 ( 1960 ). (10) Levy, M., Feld, M., Szwarc, M., Trans. Faraday Soc. i n press. (11) McCormick, H. W . , J. PolymerSci.25, 488 ( 1957 ). (12) Ibid., 36, 341 ( 1959 ); 41, 329 ( 1959 ). (13) Morton M . , Rembaum, Α., Bostick, Ε. E . , Ibid., 32, 530 ( 1958 ). (14) Richards, D . H., Szwarc. M., Trans. Faraday Soc. 55, 1644 ( 1959 ). (15) Schreiber, H., Makromol. Chem. 36, 86 ( 1959 ). (16) Sigwalt, P., Fontanille, M., Compt. rend, 251, 2947 ( 1960 ). (17) Siriani, R. F . , Worsfold, D . J . , Bywater, S., Trans. Faraday Soc. 55, 2124 ( 1959 ). (18) Spach, G . , Levy. M., Szwarc, M., Ibid., in press. (19) Szwarc, M., Makromol. Chem. 35, 132 ( 1960 ). (20) Szwarc, M., Nature 178, 1168 ( 1956 ). (21) Szwarc, M., Levy, M., Milkovitch, R., J. Am. Chem. Soc. 78, 2656 ( 1956 ). (22) Szwarc, M., Rembaum, Α., J. Polymer Sci., 22, 189 ( 1956 ). (23) Tobolsky, Α. V . , Ibid., 25, 220 ( 1957 ); 31, 126 ( 1958 ). (24) Ureta, E . , Levy, M., Szwarc, M., unpublished results. (25) Vrancken, Α., Smid, J . , Szwarc, M., J. Am. Chem. Soc. 83, 2772 ( 1961 ). (26) Vrancken, Α., Smid, J . , Szwarc, M., Trans. Farady Soc. i n press. (27) Wenger, F . , Makromol. Chem. 34, 143 ( 1960 ). (28) Worsfold, D . J . , Bywater, S., Can. J. Chem. 36, 1141 ( 1958 ). (29) Ibid.,38,1891 ( 1960 ). (30) Worsfold, D . J . , Bywater, S., J. Chem. Soc. 1960, 5234. (31) Worsfold, D . J . , Bywater, S., J. Polymer Sci. 26, 299 ( 1957 ). (32) Worsfold, D . J . , Bywater, S., in press. RECEIVED December 4, 1961.


New Vistas in Anionic Polymerization - ACS Publications

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