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

78 N. Hadjichristidis, S. Pispas, M. Pitsikalis, H. Iatrou, C. Vlahos
having n branches with molecular weight A and n branches with molecular weight
B. A small quantity of THF was used to accelerate the second polymerization step.
The last method for the synthesis of asymmetric stars suffers from the disadvantages
that characterizes the DVB method: the broad molecular weight distributions,
compared to stars prepared by chlorosilane chemistry, molecular heterogeneity,
since n is an average value, absence of complete control over the final
product etc. More details will be given in Sect. 2.2.1.1. SEC analysis revealed the
existence of high molecular weight species. This was attributed to the formation
of linked stars. These structures can be produced when active anionic living
arms react with other DVB-linked cores. It is evident from the above that the
products are not as pure as those produced by suitable chlorosilane chemistry.
Asymmetric three-arm PS stars, possessing chains of different molecular
weights were also prepared by Quirk and Yoo [10] using 1,4-bis(1-phenylethenyl)benzene
(PDDPE) as the linking agent. It was observed that the addition reaction
of polystyryllithium with PDDPE in THF leads primarily to the formation
of the monoadduct product, due to the ability of the negative charge to be delocalized
into the phenyl rings and the remaining vinyl group. The formation of this
product was then followed by the addition of the second polystyryllithium chain
in order to obtain the coupled product. The efficiency of the coupling reaction depends
on the control of the stoichiometry between the reactants. Finally the addition
of styrene in the presence of THF to promote the crossover reaction leads
to the formation of the asymmetric PS stars, as shown in Scheme 2. Unreacted
monoadduct product and PS B homopolymer (the second arm) were also observed
in the SEC trace of the final product due to incomplete linking reactions.
2.2
Stars with Chemical Symmetry
2.2.1
Miktoarm Stars
Star polymers of chemically different arms are usually called miktoarm stars.
Although there are several individual methods for the synthesis of miktoarm
stars four general methodologies have been developed. Three of them are based
on anionic polymerization and the fourth on cationic polymerization. In all of
them the use of appropriate linking agents is necessary.
2.2.1.1
General Strategies and Methods
2.2.1.1.1
Anionic Polymerization Method with Divinylbenzene (DVB)
The synthesis of miktoarm stars by the DVB method is a three step procedure.
The first step involves the preparation of the living arm by anionic polymeriza-