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

Asymmetric Star Polymers Synthesis and Properties 85
Scheme 10
2.2.1.3
Synthesis of A 3 B Miktoarm Star Copolymers
The work on the A 2 B stars was expanded to the synthesis of miktoarm stars of
the type A 3 B, where A is PI and B is PS [28]. SiCl 4 was used as the linking agent.
Living PS chains were reacted with an excess of the linking agent for the preparation
of the trichlorosilane end-capped PS. After the evaporation of excess SiCl 4
the macromolecular linking agent was added to a slight excess of the living PI
chains to obtain the A 3 B star. The reaction sequence, given in Scheme 10, was
monitored by SEC. The products after extensive characterization were found to
have high degree of molecular and compositional homogeneity.
A similar procedure was followed for the synthesis of miktoarm stars of the
type B(A-b-B) 3 , A being PI and B PS [27].
A different approach but still in the frame of the chlorosilane method was
adopted by Tsiang for the synthesis of (A-b-B)B 3 miktoarm star copolymers,
where A is PS and B is PB [29]. Living PB chains were reacted with SiCl 4 in a molar
ratio 3:1, followed by the addition of the living diblock PS-b-PBLi. The key
step of the method is the succesfull synthesis of the (PB) 3SiCl intermediate product.
The reduced steric hindrance of the PBLi chain end poses questions about
the purity of this polymer, since several byproducts, such as (PB) 2SiCl 2, (PB) 4Si,
PBSiCl 3 can be formed in the first step of the synthesis. SEC analysis was performed
to monitor the reaction sequence.
It is obvious that the method developed by Tsiang is very demanding with regard
to the stoichiometry of the reagents. The byproducts are almost impossible
to separate.
2.2.1.4
Synthesis of A n B (n‡‡‡‡ 5) Miktoarm Star Copolymers
Miktoarm star copolymers of the type A 5 B were prepared [30] in a similar manner
to the A 2 B and A 3 B type stars. The reaction sequence is outlined in
Scheme 11.
Living PS chains were reacted with 1,2-bis(trichlorosilyl)ethane in a ratio
Li:Cl=1:6. Dropwise addition of the living polymer solution into the vigorously
stirred solution of the linking agent was performed to minimize the coupling
product. Under these conditions 15% of the coupled product was formed. The