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

Synthesis of Branched Polymers by Cationic Polymerization 5
– ABC, ABCD ... etc -type star corresponds to a star with 3, 4 ... etc different
branches
Depending on the target structure and on the availability of initiators and
linkers, three main methods can be applied for the synthesis: core-first techniques,
core-last techniques, and mixed techniques.
In the first case, the arms are grown together from a single core which can be
either a microgel with an average number of potentially active sites or a well-defined
multifunctional initiator. However, to our knowledge, although there is no
specific limitation, cationic polymerization involving a microgel multifunctional
initiator has not been reported. Functionalization of the free end of the
branches can also be performed by quenching with a functional terminator.
In the second case, first the arms are synthesized separately and then linked
together using either a well-defined multifunctional terminator or a difunctional
monomer leading to a cross-linked core. The free end of the branches may
contain functional groups by using a functional initiator for the preparation of
the arms.
Both techniques generally lead to A n or (AB) n stars with branches of identical
nature and similar composition and length.
Although in anionic polymerization sequential coupling reactions with methyl
trichlorosilane or tetrachlorosilane have been used to obtain ABC or ABCD
heteroarm stars with three or four different branches respectively, such technique
is not available in cationic polymerization due to the lack of suitable coupling
agents. To prepare stars with different branches, most methods employ
mixed techniques. The first one is derived from the microgel core method applied
in three sequential steps: first stage polymerization to give a linear (co)polymer,
linking via a divinyl monomer, second stage polymerization initiated by
the active sites incorporated in the microgel core. The second method is based
on the use of a living coupling agent which is a non-homopolymerizable multivinylic
monomer. Upon addition of the living arms to the double bonds, new active
species arise that can be used to initiate a second stage polymerization leading
to new branches. To date, only one example could be found using living cationic
polymerization.
2.1
Synthesis Using a Difunctional Monomer as a Linker (Cross-Linked Core)
In cationic polymerization, this technique has been used only as a core-last technique.
It is based on the ability of a linear living polymer chain to act as a macroinitiator
for a second monomer. When the second monomer is a divinyl compound,
pendant vinyl groups are incorporated in the second block leading to
cross-linking reactions which may occur during and after formation of the second
block. These reactions provide multi-branched structures where the arms
are linked together to a compact microgel core of the divinyl second monomer.
This method is particularly suited to prepare stars with many arms. The average