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

Asymmetric Star Polymers Synthesis and Properties 91
the esters led to the preparation of amphiphilic miktoarm star copolymers [44–
46]. The products were mainly characterized by SEC and NMR spectroscopy,
whereas only limited molecular characterization data were given in these studies.
Ishizu and Kuwahara have developed a macromonomer technique for the
synthesis of miktoarm stars of the type A n B n . PS and PI macromonomers having
end vinyl groups were prepared by the coupling reaction of the corresponding
living anions with p-chloromethylstyrene. Anionic copolymerization of the PS
and PI macromonomers was performed in benzene solutions using n-BuLi as initiator.
The products can be considered miktoarm stars of the type A n B m , as evidenced
by their solution and solid state properties [47]. The reaction series is
given in Scheme 17.
It was found that the reactivity ratios of the copolymerization system greatly
influences the number of the arms of the star polymer.
Diblock macromonomers having central vinyl groups were used for the synthesis
of (PS) n(PtBuMA) n miktoarm stars [48, 49]. The macromonomers were
prepared by sequential anionic polymerization of styrene, 1,4-divinyl benzene
(DVB) and tert-butyl methacrylate. The DVB monomer was left to react with the
living PS chains for short times (~5 min) so that a few DVB units can be incorporated
at the end of the PS chains and the formation of PS stars can be avoided.
Free radical polymerization in solution and in bulk using AIBN as initiator, tetramethylthiuram
as a photosensitizer and ethylene glycol dimethacrylate as a
cross linking agent was carried out for the synthesis of the miktoarm stars. A
similar experiment was performed using PS-b-P2VP diblocks having central
isoprene units [49].
A cyclophosphazene derivative was used as a linking agent to produce miktoarm
stars consisting of PS and Nylon-6 branches [50], according to Scheme 18.
The linking agent was prepared by reacting the hexachlorocyclotriphosphazene
with 4-hydroxy benzoic acid ethyl ester and subsequent hydrolysis with
NaOH. The acid groups thus prepared were transformed into acid chloride by
treatment with SOCl 2 . PS chains were linked to this linking agent by two methods.
The first method involved the addition of anionically living PS chains to the
linking agent. For the second method radical polymerization of styrene in the
presence of 2-aminoethanethiol was performed producing amine terminated
PS. These end-capped polymers were reacted with the linking agent. In both cases
the coupling was not complete. Hydrolysis of the remaining acid chloride
groups and titration of the resulting acid groups showed that less than 2 groups
remained unreacted. These acid groups were used for the ring opening polymerization
of e-caprolactam (e-CL) giving rise to the formation of miktoarm
stars. It is obvious that there is poor control over the coupling reaction of the PS
chains to the linking agent. The molecular weight of the PS arm affects the degree
of displacement of the acid chloride groups. The higher the molecular
weight of the PS the lower the number of arms incorporated at the star's center.
These results in combination with the use of ring opening and radical polymerization
in one of the possible routes leads to products with broad molecular
weight distributions and poor control over the final structure.