ANIONIC RING-OPENING POLYMERIZATION

The ring-opening polymerization of oxiranes, thiiranes, and thietanes can be initiated by both cationic and anionic methods, but there are some heterocyclic compounds such as lactones and lactams that are more suited to the anionic technique.

RLi H₂C CHR₁ RCH₂ C Li R₁ H +

nH₂C CHR₁ RCH₂C Li +

R₁ H

R CH₂ CHR₁ CH_{n 2}C Li R1

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Ionic Polymerization 115

Polyethylene oxide is readily prepared by the reaction of ethylene oxide with the potassium salt of an alcohol, and the chain is terminated by a transfer reaction to excess alcohol present.

Lactones can be used to prepare polyesters, but the ring size is an important consideration, e.g., the five-membered ring γ-butyrolactone will not polymerize, whereas the six-membered ring δ-valerolactone reacts.

Certain lactams will undergo ring-opening polymerizations to give polyamides, and nylon-6 can be prepared from the water-catalyzed reaction of caprolactam.

FIGURE 4.2 Behavior of the experimental propagation rate constant kp as a function of the concentration [E] of “living ends” for various salts of living polystyrene in tetrahydrofuran at 298 K. (From Szwarc, M., Carbanions, Living Polymers and Electron Transfer Processes, Interscience Publishers, 1968. With permission.)

0 0 1 2 3 4 5 6 7

100 200 300 400

([E]/mol dm^–3)^–1/2 10–3 kp/dm3 mol–1 s–1

Li⁺ Na⁺

K⁺

Rb⁺

Cs⁺

O K OR ROCH₂CH₂O K

CH₂CH₂O CH₂CH₂O K CH₂CH₂OH+K OR _n

+

ROH

monomer

C O

O

(CH₂)₄ C O

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116 Polymers: Chemistry and Physics of Modern Materials

Care must be taken with this reaction, and an alternative procedure is to use a two-component catalyst system by reacting the lactam with a base to produce an activated monomer. This then reacts with a promoter such as the acyl lactam, which initiates the ring-opening growth of the linear polymer. For a series of cyclic lactams, the reaction rates are a function of ring size and are in the order of 8- > 7- > 11- >> 5- or 6-membered rings. These are important commercial processes, and nylon-4 is also prepared using this type of reaction.

PROBLEMS

1. Write down the first two steps of the cationic polymerization of vinyl methyl ether initiated with HF.

2. NMR analysis of the product from the low-temperature cationic polymer-ization of 4-methyl-1-pentene showed that both isobutyl and methyl side groups were present. Explain this finding.

3. The structures of the polymers resulting from the cationic polymerization of 4-methyl-1-pentene with an AlCl₃ catalyst in C₂H₅Cl solvent at –78, –50, and +5°C were investigated by ¹³C nuclear magnetic resonance spec-troscopy (Mizuno and Kawachi, 1992). The main product of the reaction is structurally similar to an ethylene isobutylene alternating copolymer.

a. Propose a mechanism for the cationic polymerization.

b. Identify other possible products of this reaction.

4. The kinetics of the living polymerization of isobutylene using 5-tert-butyl-1,3-bis(2-chloro-2-propyl)benzene (t-Bu-m-DCC)/TiCl₄ initiating system in 2,4-dimethyl pyridine were determined by gravimetrically monitored polymer yield as a function of reaction time (Storey and Choate, 1997).

For this reaction to be classified as “living,” how would you expect:

a. ln [Mo]/[M] to vary with time?

b. M_n to vary with degree of conversion?

A set of apparent rate constant values, k_app, were obtained from ln [M_o]/[M]

vs. time plots, and a set of kapp values for different t-Bu-m-DCC concentra-tions (all other parameters being constant) were collected. From these, es-timate the kinetic order of the reaction with respect to [t-Bu-m-DCC].

[t-Bu-m-DCC] ×××× 10³/M k_app×××× 10⁴/s^–1

0.40 3.4

0.60 5.2

0.80 7.0

1.10 9.6

1.30 12

(CH₂)₅ C N _n O H H₂C

H₂C H₂C

C N C

C H H₂ O

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Ionic Polymerization 117

5. The polymerization of isobutylene (IB) has been carried out using a 1,4-bis(1-chloro-1-methylethyl)benzene (DiCumCl)/BCl₃ initiating system in chloroform at 75°C. An incremental monomer addition method was used, and data are summarized below, where W_p is the weight of polymer and M_n is the number average molecular weight. If [DiCumCl] = 4.6 × 10^–3 mol l^–1 and V_o = 25 ml, establish whether the system shows characteristics of a living polymerization.

6. From the definition of number average, derive an expression for M_n as a function of the kinetic chain length, v, assuming a Poisson distribution.

In doing so, remember that:

and

though does not depend on i.

Because:

demonstrate that

7. One of the first anionic reactions to be studied was the polymerization of styrene in liquid ammonia (Higginson and Wooding, 1952). This type of anionic polymerization consists of a two-step initiation process:

W_p (g) M_n ×××× 10^–3 (g mol^–1)

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118 Polymers: Chemistry and Physics of Modern Materials

involving the dissociation of the potassium amide into the constituent ions followed by addition of the anion to M to generate an active chain carrier.

By considering that this latter step is slow relative to the first and thus rate determining, and that chain termination occurs by transfer to solvent:

derive an expression for the rate of polymerization and kinetic chain length.

Given that the activation energy for the transfer process is larger than that for propagation and that the overall activation energy is +38 kJ mol^–1, predict how chain length and reaction rate will vary with increasing temperature.

8. In the anionic polymerization of styrene in liquid ammonia catalyzed by KNH₂ (Higginson and Wooding, 1952) considered in the previous prob-lem, it was found that plots of 1/[styrene] vs. time were linear. What is the order of the reaction with respect to [styrene]? Is this consistent with the derivation in Problem 7?

9. Design a synthetic route for the preparation of poly(styrene-block-meth-ylmethacrylate).

10. Suggest three possible routes to prepare poly(styrene-block-butadiene-block-styrene) by living anionic polymerization.

11. Consider the anionic polymerization of isoprene with butyllithium. What molar concentration of initiator would be required to obtain a sample with molecular weight 136,000 g mol^–1 if the isoprene concentration is 2 M?

What is the expected polydispersity of this sample?

12. In anionic polymerization, which of these two monomers is more reactive:

methyl methacrylate or methacrylonitrile?

REFERENCES

Bhattacharyya, D.N., Lee, C.L., Smid, J., and Szwarc, M., J. Phys. Chem., 69, 612, 1965.

Bywater, S., Polymerization initiated by lithium and its compounds, Adv. Polym. Sci., 4, 66 1965.

Bywater, S. and Worsfold, D.J., J. Phys. Chem., 70, 162 1966.

Higginson, W.C.E. and Wooding, N.S., J. Chem. Soc., 760, 1952.

Kennedy, J.P. and Lenger, A.W., Recent advances in cationic polymerization, Adv. Polym.

Sci., 3, 508, 1965.

Mizuno, A. and Kawachi, H., Polymer, 33, 57, 1992.

Storey, R.F. and Choate, K.R., Macromolecules, 30, 4799, 1997.

Szwarc, M., Carbanions, Living Polymers and Electron Transfer Processes, Interscience Publishers, 1968.

KNH K :NH

:NH M H N—M

2 2

2 ¹ 2

→ +

+  →

+ −

−

⋅⋅

k −

H N M M2 n NH3 H N M MH NH2 n

( )

^{−+  →k}1

( )

^{+ −2}

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Ionic Polymerization 119

BIBLIOGRAPHY

Allen, G. and Bevington, J.C., Eds., Comprehensive Polymer Science, Vol. 3, Pergamon Press, 1989.

Goethals, E.J., Ed., Cationic Polymerization and Related Processes, Academic Press, 1984.

Hogen-Esch, T.E. and Smid, J., Eds., Recent Advances in Anionic Polymerization, Elsevier Science Publishing, 1987.

Hsieh, H.L. and Quirk, R.P., Recent Anionic Polymerization: Principles and Practical Appli-cations, Marcel Dekker, 1996.

Ivin, K.J. and Saegusa, T., Eds., Ring Opening Polymerization, Elsevier Applied Science Publishers, 1984.

Kennedy, J.P. and Marechal, E., Carbocationic Polymerization, Krieger Publishing Company, 1990.

Lenz, R.W., Organic Chemistry of Synthetic High Polymers, Interscience Publishers, 1967, chap. 13, 14.

Morton, M., Anionic Polymerization: Principles and Practice, Academic Press, 1983.

Plesch, P.H., The Chemistry of Cationic Polymerization, Pergamon Press, 1963.

Puskas, J.E. and Kaszas, G., Carbocationic polymerization, in Encyclopedia of Polymer Science and Technology, John Wiley and Sons, 2003.

Quirk, R.P., Anionic polymerization, in Encyclopedia of Polymer Science and Technology, John Wiley and Sons, 2002.

Smith, D.A., Addition Polymers, Butterworths, 1968, chap. 3.

Szwarc, M., Carbanions, Living Polymers and Electron Transfer Processes, Interscience Publishers, 1968.

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In document 9813_C000.fm Page i Friday, June 15, 2007 10:45 AM (pagina 133-140)