Self-Assembly of Synthetic and Biological Polymeric Systems of ...

a) O
b)
H 2C CH 2
O
H 2C CH
Figure 3.4. Oxyranes used in the synthesis of the block copolymers a) ethylene oxide and b)
styrene oxide.
Anionic polymerization proceeds via organometallic sites, carbanions (or oxanions) with
metallic counterions. Carbanions are nucleophiles; consequently, the monomers that can be
polymerized by anionic polymerization are those bearing an electro-attractive substituent on
the polymerizable double bound. Initiation of polymerization is accomplished by analogous
low molecular weight organometallic compounds (initiators). A wide variety of initiators has
been used so far in order to produce living polymers. Among them, the most widely used are
organolithiums. The main requirement for employment of an organometallic compound as an
anionic initiator is its rapid reaction with the monomer at the initiation step of the
polymerization reaction and, specifically, with a reaction rate larger than that of the
propagation step. This leads to the formation of polymers with narrow molecular weight
distributions because all active sites start polymerizing the monomer almost at the same time.
Propagation proceeds through nucleophilic attack of the carbanionic site onto a monomer
molecule with reformation of the first anionic active center. The situation is similar in the case
of the ring opening polymerization of cyclic monomers containing heteroatoms (oxyranes,
lactones, siloxanes, etc.). Under appropriate experimental conditions, due to the absence of
termination and chain transfer reactions, carbanions (or, in general, anionic sites) remain
active after complete consumption of monomers, giving the possibility of block copolymer
formation, in the simplest case, by introduction of a second monomer into the polymerization
mixture. However, a variety of different synthetic strategies have been reported for the
preparation of linear block copolymers by anionic polymerization.
As an example, in the polymerisation reaction of a monofunctional poly(oxyethylene) the
addition of ethylene oxide (E) is initiated by a monofunctional alcohol (ROH), part of which is in
the form of its alkali-metal salt (e.g., RO-K + .) Initiation is instantaneous, and the subsequent
reaction scheme is written:
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