Biomedical Engineering Reference
In-Depth Information
Cross-linking reactions readily occurred in the polymerizations of A
2
+
B
2
+B
3
monomers in aqueous emulsions (Scheme 4) [25, 26]. The size of
the cross-linked particles was dependant on the polymerization conditions.
Micrometer-sized particles were formed when the reaction was carried out
in a water-toluene-diisopropylamine mixture, whereas smaller nanoparticles
were formed when the reaction was conducted in an ultrasonic bath.
We tried to optimize the polycoupling conditions by varying such param-
eters as polymerization time, monomer concentration and monomer add-
ition mode, in an effort to control the polymer formation and to render the
polymers soluble and processable. The optimization worked well and our
A
2
+B
3
approach offered ready access to a soluble
hb
-PAE containing lumi-
nescent anthracene and fluorene chromophores (Scheme 5) [27]. Similarly,
soluble azo-functionalized polymers
hb
-P
13
and
hb
-P
15
were obtained from
the palladium-catalyzed polycoupling of triiodoarenes (
12
and
14
)withadi-
ethynylazobenzene (
11
) [28].
2.1.2
Mechanistic Considerations
The overall reactions for the formation of
hb
-PAEs via Pd-catalyzed coup-
ling are depicted in Scheme 6. As discussed above, two synthetic strategies
are currently employed. The first one utilizes AB
2
-type monomers, building
up hyperbranched architecture through repetitive coupling of the triple bond
with aryl halide. If no side reactions occur, this protocol allows only one sin-
gle internal cyclization of an aryl halide with the focal acetylene unit, thus
yielding
hb
-PAEs without any cross-link points. On the other hand, this in-
ternal ring closure leads to the formation of various polymeric species with
different propagation possibilities, which are the cause for an increased MWD
at higher conversions. Terminating the focal unit by core molecules can nicely
overcome this problem (cf., Scheme 1) or even reverse the trend: with an in-
crease in the conversion, the MWD becomes narrower [21, 22].
The second method separates the functional groups into two monomers,
which facilitates synthetic work and offers greater choices to monomeric
structure. In the first step, A
2
and B
3
monomers couple together to form an
AB
2
-type dimer that continues to react to form the hyperbranched architec-
ture (Scheme 6). This is the case, only if the molar ratio of A
2
to B
3
is 1 : 1
and the initiation is considerably faster than the propagation [29]. It becomes
immediately clear that the resultant structure is highly dependant on the type
of monomers and the polymerization conditions. For the latter, it has been
found that the mode of monomer addition plays a crucial role. Whereas the
addition of a B
3
monomer into a solution of A
2
yields insoluble polymer gel,
the opposite addition mode furnishes hyperbranched polymers with excellent
solubility [30].
