Alkali metal initiators

The discovery that lithium and its alkyls produce a highly cis-1,4 polyisoprene in hydrocarbon solvents (103) has led to a renewed interest in metal and metal alkyl initiated polymerization. About the same time Szwarc (109) postulated an electron transfer mechanism for the initiation of polymerization by sodium naphthalene in ether solvents. This was extended to lithium metal catalysis by Tobolsky (80) and Overberger (83) and subsequently generalized to cover all alkali metal initiation, e" + M M (1) ... 

The use of alkali metals for anionic polymerization of diene monomers is primarily of historical interest. A patent disclosure issued in 1911 (16) detailed the use of metallic sodium to polymerize isoprene and other dienes. Independendy and simultaneously, the use of sodium metal to polymerize butadiene, isoprene, and 2,3-dimethyl-1,3-butadiene was described (17). Interest in alkali metal-initiated polymerization of 1,3-dienes culminated in the discovery (18) at Firestone Tire and Rubber Co. that polymerization of neat isoprene with lithium dispersion produced high t /s- 1,4-polyisoprene, similar in structure and properties to Hevea natural rubber (see Elastomers, synthetic-polyisoprene Rubber, natural). 

With regard to the heterogeneity or homogeneity of these systems one may make the following generalizations 1. The alkali metal initiated polymerizations are heterogeneous, at least with respect to initiation, by virtue of the insolubility of the alkali metals in the various solvents. 2. In polar solvents the alkyl alkali metal systems appear to represent a soluble class of initiator systems. 

HI. Mechanism of Anionic Polymerizations A. Alkali-Metal Initiated Polymerizations... 

Although the alkali metal initiated polymerizations of dienes have been known since 1911 (25, 40), it appears that it was not until 1928 that Schlenk and Bergman (57) first proposed a mechanism for the process. These authors suggested that a monosodium adduct was formed which they wrote as... 

Table V. Alkali Metal Initiators Used in This Work...

This initiation process is similar to alkali metal initiation in this respect. The monomer in these systems often has a lower electron affinity than the polycyclic... 

If R is alkyl, commonly the nucleophile and/or the R group will bear a nitro group so that the single-electron transfer is favorable. If R is aryl, initiation can be by singleelectron transfer from an electrode or an alkali metal. Initiation by light, hv, is also possible. The SrnI process can be stopped with radical inhibitors. Systems that would not have reacted by an SnI or Sn2 process may react by an SrnI mechanism since the R-L bond is considerably weakened in the radical anion compared to the neutral. 

This initiation process is thus similar to alkali metal initiation in (a). That this reaction occurs is shown by electron spin resonance measurements, which indicate the complete disappearance of radicals in the system immediately after the addition of monomer. The monomer in these systems often has a lower electron affinity than the polycyclic hydrocarbon, but dimerization of the monomeric radical anion [Eq. (8.15)] drives the equilibrium of reaction (8.14) to the right. Dimerization of radical centers is highly favored by their high concentrations, typically 10 -10 M and the large rate con-stants (10 -10 L/mol-s) for radical coupling. (Note that the dimerization occurs to form the styryl dicarbanion instead of CH2CH0CH0CH2 , since the former is much more stable.) The styryl dianions are colored red (the same as styryl monocarbanions formed via initiators such as n-butyllithium). Anionic propagation occurs at both carbanion ends of the styryl dianion ... 

Alkali Metals Initiation by direct attack on the alkali metal involves transfer of the loosely held electron from a Group lA metal atom to the monomer. A radical ion (i.e., a species having both ionic and radical centers) is formed ... 

The initiation process is thus similar to alkali metal initiation described earlier [cf. Eq. (8.10)]. The dimerization of radical anions is highly probable because of their high concentrations, typically jlO -10" M, and the large rate constants (10 -10 L/mol-s) for radical coupling (Odian, 1991). Anionic propagation takes place by monomer addition at both carbanion ends of the styryl dianion ... 

Polybutadiene. Unlike cis- and trans-1,4-polybutadiene, high vinyl 1,2-polybutadiene has a chiral center which can exist in one of three different stere-ochemically related forms. The material can either be atactic, leading to an amorphous elastomer, or it can be isotactic or syndiotactic, both of which are crystalline. The elastomeric amorphous form has found utility in tire tread applications (271) and although certain molybdenum (272) coordination catalysts can produce this material, commercialization has focused on anionic alkali metal initiators modified with Lewis bases. Of the two crystalline forms, isotactic 1,2-polybutadiene with a melting temperature of 126° C is the most elusive isomer. A few chromium systems based on soluble salts and aluminum alkyls have been reported to give 45% of the isotactic polymer in a mixture of the atactic isomer (273,274). 

Anionic polymerization of 1,3-butadiene allows for a wide range of heterotactic vinyl enchainment levels. The structures obtained in hydrocarbon solvents with alkali metal initiators are shown in Table 9. Lithium polybutadiene maintains the lowest vinyl, highest 1,4 content possible via anionic polsrmerization. The microstructure produced via a free-radical emulsion polymerization is provided as a reference. 

Table 9. Microstructure of Alkali Metal Initiated Polybutadienes ...

A wide variety of typical anionic initiators is described for the polymerization of PMVK. Grignard reagents are used as well as complexes formed of alkylaluminum or alkylzinc compounds with alkali metal alkyls (so called -ate complexes ). Alkali metal initiators and alkoxides are also described. Some examples are given in Table 5. 

The replacement in the above systems of the alkali metal initiators by onium salts, typically tetraalkylammonium or tetraalkylphosphonium halides, drastically improves the synthesis of polyethers. For instance, poly(POx)s of controlled molar masses, up to 150000gmor, ° are obtained in very short reaction times using tetrabutylammonium chloride and bromide as initiating species in the presence of a slight excess of... 

Alkali metals initiate anionic polymerization indirectly by an electron transfer process ... 

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