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

88 N. Hadjichristidis, S. Pispas, M. Pitsikalis, H. Iatrou, C. Vlahos
1,2 PB block was thus prepared. Hydrosilylation chemistry was employed for the
incorporation of the -Si(CH 3 )Cl 2 or -Si(CH 3 )Cl groups to the 1,2-PB double
bonds. Subsequent addition of 1,4 PBLi or P2VPK leads to the formation of the
umbrella copolymers. The limited control exercized over the hydrosilylation reaction
means that the number of the arms cannot be accurately predicted. Roovers
and collaborators succeeded to prepare umbrella-star copolymers [(PS-u-
P2VP) n ] m . The synthesis is based on the reaction of (PB-1,2)-b-PSLi with chlorosilane
having 32 peripheral Si-Cl bonds followed by hydrosilylation of the PB
and reaction with P2VPLi.
2.2.1.5
Synthesis of A 2 B 2 Miktoarm Star Copolymers
The synthesis of miktoarm star copolymers of the type A 2B 2 was first reported
by Xie and Xia [34]. A chlorosilane method was employed to prepare PS 2PEO 2
stars according to Scheme 14. Living PS chains were reacted with SiCl 4 in a molar
ratio 2:1 leading to the formation of the two-arm product. The remaining Si-
Cl bonds can be used for the linking reaction of living PEO chains. The process
is facilitated by the increased steric hindrance of the living PS chain ends. It is,
indeed, very difficult to prepare the three- or four-arm PS stars. From this point
of view the control of the stoichiometry is not very important in this specific
case. Using CH 3SiCl 3 instead of SiCl 4 A 2B miktoarm stars were also prepared.
A different approach was followed by Iatrou and Hadjichristidis for the synthesis
of PS 2PB 2 miktoarm stars [35] according to Scheme 15. The first step involved
the reaction of living PS chains with excess SiCl 4, followed by the evaporation
of the excess silane in a similar manner as was described in the case of the
A 3 B stars. The second PS arm was incorporated by slow stoichiometric addition
(titration) of one living PS chain to each PSSiCl 3 . This procedure was monitored
by SEC taking samples from the reactor during the titration. The last step involved
the addition of a small excess of living PB chains for the preparation of
the (PS) 2 (PB) 2 miktoarm star.
Scheme 14
Scheme 15