Initiations by Alfin Catalysts
These are catalysts that are formed by combining an alkyl sodium with sodium alkoxide and with an alkali metal halide [184–189]. A typical, effective catalyst for polymerization of dienes consists of allyl sodium, sodium isopropoxide, and sodium chloride. The preparation of such a catalyst is carried out by combining amyl chloride with sodium and then reacting the product with isopropyl alcohol. After that, propylene is bubbled through the reaction mixture to convert amyl sodium to allyl sodium. The reactions can be summarized as follows
Di isopropyl ether may be used in place of isopropyl alcohol. In that case, the reaction does not require addition of propylene because the olefin forms in situ [189]. These catalysts are particularly effective in polymerizations of some conjugated dienes to very high molecular weight products. Styrene can also be polymerized by these catalysts. Polymers of the dienes that form are high in trans 1,4 repeat units. Butadiene is polymerized by these catalysts much more rapidly than is isoprene. On the other hand, 2,3-dimethylbutadiene fails to polymerize.
There is no general agreement about the mechanism of these polymerizations. Both anionic and free-radical mechanisms were proposed [184-189] as the most probable reaction paths. The role of sodium chloride is not clear in this mixture, though it was shown that it is essential [184–189]. It may act, perhaps, as a support for the catalyst and may be a part of some sort of lattice involving both sodium alkoxide and allyl sodium. The anionic mechanism is pictured as follows [184-189]:
The free-radical mechanism was suggested due to high predominance of trans-1,4 placement in polymerization of butadiene [189]. According to that mechanism a complex of sodium isopropoxide and allyl sodium forms first:
The monomer adsorbs on the surface and is visualized as displacing the allyl anion from the complex to form an ion pair first and then, through an electron rearrangement, a radical [189]:
The polymerization is assumed to go on nominis point via free-radical mechanism until a combination with an allyl radical takes place. Because the allyl radical is bound to the catalyst surface, combination does not take place readily and high molecular weights are attained.
