Carbanion pump

SCBs play an important role in the formation of other block copolymers. For example, the relatively less nucleophilic poly(ethylene oxide) oxyanion cannot initiate the polymerization of styrene, which needs a more nucleophilic alkyllithium initiator. To enable the synthesis of multi-block copolymers from various combinations of monomers by anionic mechanisms, it is important to modify the reactivity of the growing anionic chain end of each polymer so as to attack the co-monomer. There have only been a few reports on the polymerization of styrene initiated by an oxyanion (see <2001MM4384> and references cited). Thus, there exists a need for a transitional species that is capable of converting oxyanions into carbanions. In 2000, Kawakami and co-workers came up with the concept of the carbanion pump , in which the ring-strain energy of the SCB is harnessed to convert an oxyanion into a carbanion (Scheme 13) <2000MI527>. 

The carbanion pump method has been successfully applied for the preparation of different block copolymers including poly(ethylene oxide)-block-polystyrene, poly(ethylene oxide)-block-polystyrene-block-poly(ethylene oxide), poly(ethylene oxide)-block-poly(methyl methacrylate), poly(ethylene oxide)-block-poly(methylmethacry-late)-block-poly(ethylene oxide) (shown in Scheme 14), and poly(ferrocenyldimethylsilane)-block-(methyl methacrylate) <2004MI856, 2004MM1720, 2006MI(928)292>. 

We proposed the concept of a carbanion pump, where a silacyclobutane having high ring distortional energy was used to convert an oxyanion into a carbanion, which can further initiate polymerization of styrene as shown in Scheme 4.5. 

To improve the efficiency, 1,1-diphenylethylene was used to trap the initially formed carbanion from potassium terf-butoxide (BuOK) and dialkylsilacyclobutane. When a twofold excess of dimethylsilacyclobutane was added over 90 min, the carbanion pump efficiency reached almost 88%. Diphenylsilacyclobutane gave almost quantitative efficiency, which could be used to synthesize block copolymer from the propagating end of poly(ethylene oxide) to methyl methacrylate to give polymers with a narrow molecular weight distribution. Such a system was also used by other researchers successfully. ... 

Scheme 4.5. Concept of carbanion pump to convert oxyanion into carbanion by the aid of ring opening of silacyclobutane ring by oxyanion through the formation of silicon-oxygen bond.

The ability of MSCB to undergo ring opening with the subsequent formation of C-Li, C-Na, and C-K bonds was used to generate carbanions (i.e., SCB as a carbanion pump) via their reaction with 1,1 -diphenylethylene with the aim of preparing various block copolymers [54-56]. This seems to be the most interesting polymerization application of alkali-metal catalysis of MSCBs. With the use of a fourfold... 

Such a silacyclobutane carbanion pump system was used to synthesize various di- and triblock copolymers. 

In [57], the efficiency of the carbanion pump system, which is the tert-butoxy anion - 1,1-dimethyl-1-silacyclobutane-1,1-diphenylethylene - was compared with that of the system in which a-methylstyrene was used instead of 1,1 -diphenylethylene. 

Hyun J-Y, Kawakami Y (2004) Silacyclobutane as Carbanion pump in anionic polymerization. III. Synthesis of di- and tri-block copolymer by diphenylsilacyclobutane-potassium tert-butoxide system. Polym J 36 856-865... 

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