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

Asymmetric Star Polymers Synthesis and Properties 79
Scheme 3
tion using a suitable initiator. The living precursor then reacts in the second step
with a small amount of DVB, leading to the formation of a star molecule bearing
within its core a number of active sites, which is theoretically equal to the
number of the A arms of the star polymer. Subsequent addition of another monomer,
in the third step, results in the growth of B arms of the miktoarm stars,
since the active star, prepared at the second step, acts as a multifunctional initiator
for the polymerization of the second monomer.
The growing B arms have anionic sites at their outer ends thus providing the
possibility of reacting with electrophilic compounds or other monomers towards
the preparation of end-functionalized stars or star-block copolymers.
This method can be carried out in inert atmosphere, avoiding the use of the
highly demanding and time consuming vacuum technique. It was first reported
by Okay and Funke [11] and by Eschwey and Burchard [12] and developed by
Rempp and collaborators [13–16]. Scheme 3 illustrates the DVB method.
Despite the advantages mentioned above the DVB method is characterized by
several disadvantages, the foremost being the architectural limitations. Only
stars of the type A n B n can be prepared and even in this case there is no absolute
control of the number of arms, n. In fact, n is an average value and is influenced
by several parameters. Specifically, n is increased by decreasing molecular
weight of the precursor A and by increasing the molar ratio of the DVB to living
chains. Another major problem is that a fraction of the living chains A are not
incorporated into the star structure due to accidental deactivation, the high molecular
weight of the chains (steric reasons) or the low molar ratio of DVB to living
chains. The unreacted living arms A can act as initiators after the addition of
the second monomer. Another disadvantage is that the B arms cannot be isolated
and characterized independently. Finally, reaction of the living ends with the
remaining double bonds of the DVB nodule can lead to the formation of loops
(intramolecular reaction) or networks (intermolecular reaction). From the
above, it is clear that the miktoarm stars prepared by this method are characterized
by rather poor molecular and compositional homogeneity.