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.
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:
|P-M- + X+||(C)|
|P-M- + S/ X+||(D)|
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:
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.First Week.
|Tube Number||Toluene (ml)||Styrene (ml)||BuLi (mmol)|
|Figure 1: Reaction Summary|
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