Basicity of Initiators and Monomers

According to Section 18.1.2.2, the starting step of an anionic polymerization can occur via addition of an anion onto a monomer, formation of a zwitterion, or electron transfer to a monomer. Whether a specific initiator will start polymerization in a given monomer depends primarily on the basicity of the monomer and initiator. Monomers with strong accepting groups need only weak bases as initiators. In olefin derivatives CH2=CHR, therefore, the tendency toward anionic polymerization falls in the sequence of the substituents R  

2-Nitropropylene can even be initiated to polymerize with the weak base KHCO3, while vinylidene cyanide only needs the very weak Lewis bases water, alcohol, or ketone. Methyl methacrylate, on the other hand, is more difficult to induce to partake in anionic polymerization than acrylonitrile, because of the presence of the electron-donating CH3 groups (see Table 18-1). 

In general, the higher the pK of an initiator, the more readily a monomer can be induced to polymerize anionically (Table 18-1). In alkali-metal ketene, for example, dimers—the alcoholate anions similar to the triphenylcarbinol anion—are the real initiators (see below). According to Table 18-1, pK values of 19-20 are to be expected for these anions (cf. the values for the alkali compounds of acetophenone and triphenylcarbinol). Accordingly, these initiators should be able to polymerize acrylonitrile but not styrene, as is indeed the case. 

The irregularities in Table 18-1 also show, however, that inducibility does not depend on basicity alone. Sodium fluorenyl (pK = 31) does not, for example, initiate the polymerization of methyl methacrylate and styrene 

Initiating sodium compound Monomers in ether at 20 C Monomers in NH3 at — 33 C 

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