Homogeneous Stereoselective Anionic Polymerizations

The varions faetors influencing the stereoregnlarity, when the propagating chain end is a carbanion, are conveniently highlighted in a study of the polymerization of methyl methacrylate by organolithium catalysts. 

The propagating chain end in an anionic reaction initiated by a reagent such as n-butyl hthium can be thought of as existing in one of the following states, analogous to carbonium ion formation. 

The extent of the separation will depend on the polarity of the reaction medium and in nonpolar-hydrocarbon solvents, such as toluene, covalent molecules, or contact 

The elfects of solvent and temperature arc manifest in the polymerization of methyl methacrylate with w-butyl hthium at 243 K in a series of mixed solvents prepared from tolume and dimethoxyethane (DME). The NMR spectra of the products indicate the compositions in Table 6.2 and reveal that a predominantly isotactic material is produced in a low-polarity medium, but that this becomes highly syndiotactic as the solvating powCT of the medium increases. 

An additional point emerges from this highCT syndiotactic contexts are obtained when the Lewis-base strength of the solvait increases, and this factor probably accounts for the efficiency of the ether in this system. Whrai the catalyst is 9-lluQrraiyl hthium, the reaction of methyl methacrylate at 195 K in tolume leads to isotactic polymer, whereas a change of solvent to tetrahydrofuran results in a syndiotactic product 

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