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

Asymmetric Star Polymers Synthesis and Properties 77
Scheme 2
arms by fractionation, the final products and the different kind of arms, which
were isolated before the coupling reaction, were characterized by membrane osmometry
(MO) and static light scattering (SLS), revealing that well defined star
polymers were prepared.
Using the same procedure Pennisi and Fetters prepared a series of asymmetric
polybutadiene (PB) stars in which the third arm was of variable molecular
weight [2]. It was found more efficient to add the living PB solution to the methyltrichlorosilane
linking agent in order to reduce the formation of the coupled
byproduct. Similar characterization techniques were also employed in this case.
Asymmetric polyisoprene (PI) three-arm stars with variable length of the
third arm were synthesized using the same method [8]. The reaction of the living
PI chains with excess methyltrichlorosilane was performed at 5 °C. This low
temperature was selected in an effort to minimize the coupled byproduct. Nevertheless
the reduced steric hindrance of the PILi chain end in association with
the low molecular weight of the polydienes used (M n =5500 and 1100) led to the
formation of an appreciable amount of the coupled byproduct, which was later
separated by fractionation, with the excess of the last coupled arm, using a solvent-precipitant
system. Pure products were finally isolated as evidenced by the
molecular characterization techniques used (SEC, MO, SLS).
Asymmetric PS stars of the type (PS A ) n (PS B ) n were also prepared by the divinylbenzene
(DVB) method [9]. Living PS chains, prepared by sec-BuLi initiation, were
reacted with a small amount of DVB producing star homopolymers. The DVB core
of the stars contains active anions which, if no accidental deactivation occurs, are
equal to the number of the arms that have been linked to this core. These active
sites are available for the polymerization of an additional quantity of monomer.
Consequently further addition of styrene produced asymmetric star polymers