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

96 N. Hadjichristidis, S. Pispas, M. Pitsikalis, H. Iatrou, C. Vlahos
The chlorosilane method was also used by Hadjichristidis et al. for the synthesis
of miktoarm stars having PS, PI and PMMA arms [55, 56]. The reaction
sequence is presented in Scheme 22. The monofunctional macromolecular
linking agent (PS)(PI)(CH 3 )SiCl was prepared using procedures similar to
those described above, followed by reaction with a dilute solution of a dilithium
initiator, formed by the reaction between 1,1-diphenylethylene (DPE) and Li.
This route was carried out to ensure that only one of the initiator's active centers
reacts with the linking agent. The remaining active center was used to polymerize
MMA in THF at –78 °C yielding the desired miktoarm star. According
to this procedure the PMMA branch cannot be isolated and cannot be characterized
independently. It was observed that during the synthesis of the difunctional
initiator a rather large amount (as high as ~30%) of monofunctional species
was also formed. These active species do not affect the synthesis of the ABC
star but they give PS-b-PI diblocks by reacting with the (PS)(PI)(CH 3)SiCl linking
agent, thus reducing the yield of the desired polymer and making its isolation
from the crude product more difficult. Nevertheless this was the first attempt
to combine the chlorosilane method with the polymerization of methacrylates.
A similar technique was employed for the synthesis of miktoarm stars having
PS, PEO, poly(e-caprolactone) (PCL) or PMMA branches [57]. A PS-b-PMMA
diblock copolymer possessing a central DPE derivative, bearing a protected hydroxyl
function was prepared. After deprotection and transformation of the hydroxyl
group to an alkoxide the anionic ring opening polymerization of the third
monomer (EO or e-CL) was initiated. Only limited characterization data were
given in this communication.
2.2.1.8
Synthesis of ABCD Miktoarm Star Quaterpolymers
Only one case concerning the synthesis of a miktoarm star quaterpolymer has
appeared in the literature. It consists of four different branches, namely PS, poly(4-methyl
styrene) (P4MeS), PI and PB [35]. The reaction sequence for the
preparation of this miktoarm star is presented in Scheme 23. The procedure was
similar to the one adopted for the synthesis of the ABC-type terpolymers by the
chlorosilane method. The characteristic of this method is that two of the arms
were incorporated to the linking agent by titration. Consequently the order of
addition plays an important role for the preparation of well defined products. PS
was chosen to react first with an excess of SiCl 4 , followed after the evaporation
of the excess silane, by the titration with the more sterically hindered P4MeS so
that only one arm can be incorporated in the star. The moderately hindered PILi
anion was then added by titration, followed by the addition of the fourth arm,
which is the least sterically hindered PBLi anion so that complete linking can be
achieved. The reaction sequence was monitored by SEC and these results in
combination with the molecular and spectroscopic characterization data
showed that well defined quaterpolymers were prepared.