142 Advances in Polymer Science Editorial Board: A. Abe. A.-C ...

Asymmetric Star Polymers Synthesis and Properties 81
2.2.1.1.3
Anionic Polymerization Followed by Chlorosilane Coupling
The method is based on consecutive reactions of living, anionically prepared
polymer chains with multifunctional chlorosilane compounds, which act as
linking agents. Miktoarm star copolymers, terpolymers and quaterpolymers of
the type A 2 B, A 3 B, A 5 B, A 8 B 8 , (AB) 2 B, (AB) 3 B, A 2 B 2 , ABC and ABCD have been
prepared by this method. Using the suitable chlorosilane and the appropriate reaction
sequence it is rather easy to predetermine the structure of the final product.
The synthesis sequence can be monitored by SEC and all the arms and the
intermediate and final products can be characterized, thus providing unambiguous
proof for the formation of the desired products. The disadvantage of this
method is that it is time consuming compared with the other ones. Nevertheless
this limitation is a small price to pay given the potential of this method for generating
true model compounds of a wide variety of macromolecular architectures.
2.2.1.1.4
Living Cationic Polymerization Method
The recent development of living cationic polymerization systems has opened
the way to the preparation of rather well defined star homopolymers and miktoarm
star polymers [19 and see the chapter in this volume]. Divinyl ether compounds
were used as linking agents in a manner similar to the DVB method for
anionic polymerization. Typically the method involves the reaction of living polymer
chains with a small amount of the divinyl compound. A star polymer is
formed carrying at the core active sites capable of initiating the polymerization
of a new monomer. Consequently a miktoarm star copolymer of the type A n B n
is produced.
Several experimental parameters influence the value of n making it difficult
to have precise control over the structure [20]. The value of n increases by decreasing
the length of A arms, by increasing the feed ratio, i.e., the molar ratio of
the divinyl ether to the living ends and by increasing the concentration of the living
ends. The structure of the monomers and the linking agent plays an important
role in determining the quality of the produced stars. Monomers having
bulky groups lead to the formation of a large amount of low molecular weight
polymers at the beginning of the reaction sequence, thus leading to mixed products,
which are difficult to separate. Divinyl ethers having long and rigid spacers
between the two vinyl groups proved to be more efficient coupling agents. Due
to the analogy with the DVB method this approach is characterized by similar
disadvantages. Nevertheless the possibility of using monomers that cannot be
polymerized anionically makes the method attractive and susceptible to several
applications.