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

Asymmetric Star Polymers Synthesis and Properties 83
Scheme 7
reaction was completed. The reaction sequence used for the synthesis of the
(PI) 2PS miktoarm star is given in Scheme 5.
The method takes advantage of the steric hindrance of the polystyryllithium
living end, which in combination with the excess silane used for the linking reaction,
reduces the possibility for the formation of the coupled byproduct. The
reaction sequence was monitored by SEC and the reaction products were characterized
by MO, differential laser refractometry and LALLS, revealing that well
defined polymers were prepared.
This method was further developed by Iatrou et al. [22]. All possible combinations
of A 2 B polymers with A and B being PS, PI or PB were prepared. A more sophisticated
and complicated high vacuum technique was used to ensure the formation
of well defined products. High degrees of molecular, structural and compositional
homogeneity were achieved by this technique, as was evidenced by the combination
of all the molecular and spectroscopic characterization data. In a more recent
study stars having deuterated PS arms, (PI) 2 (d-PS) were also prepared [23].
An A 2 B star having two PS arms and one poly(2-vinyl pyridine) (P2VP) arm,
(PS) 2 (P2VP) was prepared by Eisenberg et al. using a different approach [24].
Living PS chains were linked to dichloromethylsilane, CH 3 SiCl 2 H to produce the
two arms of the star. In another reactor living P2VP was reacted with allyl bromide.
A hydrosilylation addition of the Si-H group of the two-arm star to the vinyl
group of the end-functionalized P2VP was performed to produce the final
miktoarm star. The reaction sequence is outlined in Scheme 6.
The miktoarm stars were characterized by medium polydispersities
(M w/M n=I=1.33–1.50) probably due to incomplete hydrosilylation. It is characteristic
that only small molecular weight P2VP arms were used to facilitate the
linking reaction. This is evidence of the limitations of the hydrosilylation reaction
for the preparation of miktoarm stars.
Anionic polymerization techniques and naphthalene chemistry were used by
Teyssié et al. to prepare A 2B miktoarm stars, where A is poly(ethylene oxide)
(PEO) and B is PS, PI, poly(a-methyl styrene) or poly(tert-butyl styrene) [25].
The reaction sequence is shown in Scheme 7.