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

Asymmetric Star Polymers Synthesis and Properties 89
Using a method similar to that of Xie and Xia, Young et al. prepared (PS) 2 (PI) 2
miktoarm stars [36]. Living PS chains were reacted with SiCl 4 in a molar ratio
2:1 for the formation of the two-arm product. Subsequent addition of living PI
chains led to the formation of the miktoarm star.
A 2 B 2 stars, A being PI and B PB were also prepared by two different methods
[37]. The first method involved the end-capping reaction of living PI chains with
2–3 units of styrene in order to increase the steric hindrance of the active chain
end, followed by titration with SiCl 4 and finally reaction with an excess of PBLi.
According to the second method living PI chains were reacted with SiCl 4 in a
molar ratio 2:1 at –40 °C. This low temperature route was performed in order to
reduce the reactivity of the living chain end, thus avoiding the formation of macromolecular
linking agents with different functionalities. Subsequent addition
of excess PILi led to the formation of the A 2B 2 miktoarm stars.
Stars of the type A 2B 2 were also prepared by the method developed by Quirk
et al. In this case A was PS and B PI or PB [38, 39]. Stars of the type A 2(B-b-A) 2,
where A was PS and B PB were also synthesized by this method. The disadvantages
of the method have already been mentioned. In order to overcome these
problems the reaction of the living PS chains with the divinyl compound were
monitored by SEC and UV spectroscopy, by observing the increase in absorbance
of the diphenyl alkyllithium species at 438 nm. It is obvious that the method
is very demanding experimentally and a lot of effort has to be exercised for
the preparation of well defined products.
2.2.1.6
Synthesis of A n B n (n>2) Miktoarm Star Copolymers
Multiarm miktoarm stars have been prepared by a variety of methods. Model
miktoarm stars, called Vergina star copolymers, bearing 8 PS and 8 PI branches,
PS 8 PI 8 were synthesized using chlorosilane chemistry [40]. A silane with 16 Si-
Cl bonds Si[CH 2 CH 2 Si(CH 3 )(CH 2 CH 2 Si(CH 3 )Cl 2 ) 2 ] 4 was used as linking agent.
Living PS chains were reacted with the linking agent in a molar ratio 8:1 for the
preparation of the eight-arm star. Even a slight excess of PSLi (~5%) can be used
without the incorporation of more than eight arms due to the steric hindrance
of the already attached chain per Si atom. A small excess of PILi was finally added
to prepare the desired product.
The most widely used method for the preparation of miktoarm stars of the
type A n B n is the DVB method, which has already been mentioned. The polymers
prepared by this method have PS as A arms and PtBuMA, PtBuA, PBuMA, PEO
or P2VP as B arms [41–43]. SEC was used to monitor the reaction steps and the
molecular characterization data showed that the products were not of the same
degree of homogeneity as those prepared by the chlorosilane method, due to the
disadvantages inherent of the method.
Structures of the same type have also been prepared by cationic polymerization
techniques, as can be seen in Scheme 16. Vinyl ethers having isobutyl-, acetoxy
ethyl-, and malonate ethyl- pendant groups have been used. Hydrolysis of