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

76 N. Hadjichristidis, S. Pispas, M. Pitsikalis, H. Iatrou, C. Vlahos
2
Synthesis
2.1
Stars with Molecular Weight Asymmetry
Three-arm polystyrene (PS) stars having two arms of equal molecular weight,
and a third one with molecular weight either half or twice that of the identical
arms, were prepared by Pennisi and Fetters [2]. Their approach involves the reaction
of living PS chains with a ten-fold excess of methyltrichlorosilane for the
preparation of the methyldichlorosilane end-capped PS. The addition of the
linking agent's solution to the dilute living polymer solution in benzene, under
vigorous stirring proved to be efficient for the preparation of the desired product.
No coupled byproduct, i.e., the two-arm “star” with a remaining Si-Cl bond
was detected following this procedure.
The excess silane was removed after freeze-drying the end-capped PS under
dynamic high vacuum and then heating the resulting porous material at 50 °C
for at least 72 h. Purified benzene was re-introduced into the reaction vessel to
dissolve the polymer. The methyldichlorosilane end-capped PS acted as a macromolecular
coupling agent when it was added to a solution containing a small
excess of living PS chains having molecular weight half or twice that of the endcapped
PS. It is well known [3–5] that PSLi cannot undergo complete reaction
with methyltrichlorosilane due to the steric hindrance of the polystyryllithium
anions. Therefore the living chains were end-capped with a small amount of
butadiene. The reduced steric hindrance of the butadienyllithium chain ends facilitates
the completion of the reaction with the methyldichlorosilane-capped PS.
The complete linking reaction of low molecular weight PS (