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

Asymmetric Star Polymers Synthesis and Properties 93
2.2.1.7
Synthesis of ABC Miktoarm Star Terpolymers
Miktoarm star terpolymers of the type ABC have been prepared by several
methods. Iatrou and Hadjichristidis reported the synthesis of a miktoarm star
consisting of PS, PI and PB branches radiating from the star center [51]. This was
achieved using the chlorosilane method and the step by step linking of the different
branches to the trichloromethylsilane, which was the linking agent, as
shown in Scheme 19.
A solution of living PI chains was added to a large excess of the silane, followed,
after the evaporation of the excess trichloromethylsilane, by the slow stoichiometric
addition (titration) of the living PS chains, exactly as was described
in the case of the A 2B 2 star copolymers. The formation of the desired product,
(PS)(PI)(CH 3)SiCl was monitored by SEC taking samples from the reactor during
the titration process. The ABC star was finally prepared by the addition of a
slight excess of PBLi.
The order of linking of the different branches to the silane plays an essential
role, since the less sterically hindered chain end, namely PBLi, has to be added
at the end of the procedure and the most sterically hindered chain end, namely
the PSLi has to be added at the titration step in order to prevent the complete reaction
with the macromolecular difunctional linking agent. The final products
were characterized by low molecular weight distributions and high structural,
compositional and molecular homogeneity indicating that this step by step addition
of the different branches provides excellent control during the synthetic
procedure.
Using the same route asymmetric AA'B miktoarm stars were also prepared [52].
These are stars having two chemically identical A arms but of different molecular
weights. In other words the B chain is not grafted at the middle of the A chain as in
the case of the symmetric A 2 B stars. A was deuterated PS and B PI in that case.
The macromonomer method was used by Fujimoto et al. for the preparation
of (PS)(PDMS)(PtBuMA) stars [53], as described in Scheme 20. The lithium salt
of the p-(dimethylhydroxy)silyl-a-phenyl styrene was synthesized and used as
initiator for the polymerization of hexamethylcyclotrisiloxane (D 3 ). Living PS
chains were reacted with the end double bond of the macromonomer, followed
by the anionic polymerization of the t-BuMA.
Scheme 19