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

Asymmetric Star Polymers Synthesis and Properties 99
Scheme 25
with each of these arms being a diblock copolymer of isoprene and styrene. Two
of these blocks are connected to the star's center with their PS end, while the other
two blocks with their PI end. Consequently the asymmetry is due to the different
topology of the arms.
The synthetic procedure for the synthesis of the inverse starblock copolymers
is given in Scheme 25. Diblock arms (I) having the living end at the PS chain end
were prepared by anionic polymerization with sequential addition of monomers.
In order to accelerate the crossover reaction from the PILi to the PSLi
chain end a small quantity of THF was added prior the addition of styrene. The
living diblock (I) solution was added dropwise to a stoichiometric amount of
SiCl 4 until two arms are linked to the silane. This step was monitored by SEC and
is similar to a titration process. The end point of the titration was determined by
the appearance of a small quantity (~1%) of trimer in the SEC trace. The diblock
(I) was selected over the diblock (II) due to the increased steric hindrance of the
styryl anion over the isoprenyl anion, which makes easier the control of the incorporation
of only two arms into the silane.
The difunctional macromolecular linking agent was then reacted with a small
excess of the living diblock (II) for the preparation of the inverse star block copolymers.
In order to facilitate the reaction with the macromolecular linking
agent the living diblock was end-capped with 3–4 units of butadiene.
Extensive molecular characterization studies of the arms and the final products
by MO and SLS and compositional characterization studies by 1 H NMR,
UV-SEC and differential refractometry lead to the conclusion that the copolymers
are characterized by a high degree of molecular and compositional homogeneity.