ANIONIC POLYMERIZATION OF STYRENE

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Objective

The butyl lithium induced anionic polymerization of styrene is used to produce a "living" polymer. The "living nature" of the polymerization is demonstrated, and the influence of monomer and initiator concentration of the degree of polymerization is investigated.

Introduction

Anionic polymerization takes place with monomers possessing electron-withdrawing groups such as nitrile, carboxyl, phenyl, and vinyl. These polymerizations are initiated by nucleophilic addition to the double bond of the monomer. This nucleophile can be an ion such as hydroxide, alkoxide, cyanide or a carbanion. The carbanion can originate from an organo-metallic species such as an alkyl lithium or Grignard reagent. An alternative means of initiation is electron transfer which occurs when alkali metals or similar species are the initiators.

In this experiment styrene will be polymerized anionically by using Bu^-Li⁺ as the initiator.

Termination usually will not occur in the absence of impurities. The coupling and disproportionation terminations for free radical polymerizations are not possible with anionic polymerizations due to coulombic repulsion of two negatively charged species. Termination involves a proton transfer from another species such as from solvent, monomer, polymer, water etc. But, there is no inherent termination steps in highly pure system. Non terminated polymeric species are referred to as "living" polymers.

A "living" polymer does not grow indefinitely, nor does its molecular weight increase beyond certain limits. If the supply of monomer is depleted then the growth is suspended. The chain end is still active. If more monomer is added to the system, then the polymer will continue to propagate.

In ionic polymerizations a counterion is always present. Anionic polymerizations in solution may have association with the counterion in the following ways:

where these represent an intimate ion pair, a solvent (S) separated ion pair, a pair of free ions, and a solvated ion, respectively. The more free the anionic end is, the more reactive it will be toward the monomer. Thus, solvent effects can be very important. It has been reported that the order of reactivity noted in an alkyllithium initiated polymerization was butadiene > isoprene > styrene when a hydrocarbon solvent was employed, and that a solvent change to tetrahydrofuran gave styrene> butadiene > isoprene.

The molecular weight of polymers produced anionically can be easily predicted from the amount of starting materials used. Consider that each initiating molecule initiates one chain, that all initiations occur essentially simultaneously, and that the anions produced compete equally for the entire weight of the monomer. For an initiator such as butyl lithium which initiates a polymer growing from one end, the number average degree of polymerization,n (where n = Mn/Mw of repeat unit), if given by

where [M] and [C] are the initial molar concentrations of the monomer and catalyst, respectively.

If the initiation is by sodium naphthalene or some other material to some other material producing a dianion, the Xn will by given by

For a radically initiated polymerization, the molecular weight is directly dependent on the monomer concentration and inversely proportional to the square root of the initiator concentration:

Experimental

The importance of keeping these reactions dry and oxygen free cannot by overemphasized; even the pouring of monomers and solvents in air can introduce enough impurities to cause the reaction to fail.

1. Remove four large test tubes from the drying oven and flame them by holding them with forceps or a test tube holder and heating them strongly in the flame of a Bunsen or Fisher burner (be careful not to melt the glass). Heat the entire tube and then reheat the lower part.
2. As soon as the top area of the tubes has cooled sufficiently to permit handling, the mouth of each tube is fitted with a serum stopper. The tube are numbered 1-4 with a ball-point pen on the stopper. The tubes are fitted with a long gas delivery needle and a short needle to serve as a gas escape valve. Dry, oxygen free nitrogen is flushed through the tubes for five minutes.
3. While keeping the nitrogen flowing and at room temperature, 30 ml of dried toluene is injected into each of the tubes, followed by 3 ml of purified styrene in three of them and 1.5 ml in the fourth one. Continue N₂ purge for 5 min.
4. The gas inlet and outlet needles are removed and an amount of butyl lithium (BuLi) solution equivalent to 0.28 mmol of BuLi is injected into each of tubes 1, 2, and 4, and an amount equivalent to 0.14 mmol of BuLi is injected into tube 3. The contents are swirled after the addition.

Tube Number	Toluene (ml)	Styrene (ml)	BuLi (mmol)
1	30	3	0.28
2	30	3	0.28
3	30	3	0.14
4	30	1.5	0.28
Figure 1: Reaction Summary

5. After 30 minutes of polymerization, 3 ml of styrene is injected into tube 1 and swirled.
6. After 1 hour reaction time, 10 ml of methanol are injected into each of the tubes, the contents are swirled.
7. Pour the contents of the tubes into 200 ml of stirred methanol.
8. Decant off the supernatant and vacuum filter the polymer. (Store the polymer for the second week.)

1. Prepare a 1g/100 ml solution of polymer in reagent grade toluene. Filter the solution in a fine porosity filter to remove any particulates.
2. Use a Ubbelhode viscometer and a constant temperature bath (25^oC) to measure the intrinsic viscosity. (Same procedure as the Dilute Solution Viscosity experiment.)
3. Repeat the above procedure for at least three other concentrations.

Calculations

1. Calculate the molecule weight of each of the polymers produced using the Mark- Houwink Equation. (K and a for styrene in toluene at 25^oC are 1.7x10^-4 dl/g and 0.69 respectively)
2. Calculate for the polymers produced.
3. Determine % yield.

Post Test

1. You are polymerizing styrene anionically. You start with 10 g of styrene and 1 ml of 1M solution if initiator; the total volume of the system including an inert solvent is 50 ml. What would the molecular weight and of the polymer be if the initiator were butyl lithium? sodium-naphthalene?
2. If an anionic polymerization were found in which the rate of propagation was much faster than the rate of initiation , what would be the effect on the MW distribution?
3. What would be an objection to using ethyl alcohol for a polymerization solvent in an anionic system?

References

1. Pearce, Eli M., Carl E. Wright, Binoy K. Bordoloi. Laboratory Experiments in Polymer Synthesis and Characterization. Educational Modules for Materials Science and Engineering Project, 1982.
2. Szwarc, M. "Living Polymers", Nature vol. 178, Nov. 24, 1956, pp. 1168-1169.
3. Billmeyer, F. W., Jr. Textbook of Polymer Science 2nd Edition, Interscience, New York, pp. 311-379.
4. Odian, G. Principles of Polymerization 2nd Edition, Wiley-Interscience, New York, pp. 372-379.