Biomedical Engineering Reference
In-Depth Information
B
y
approach from easily available monomers as well. Among them, the
method of A
2
+ B
3
polymerization provides a facile approach to prepare HBPs.
Step growth has been widely used to synthesize hyperbranched poly(sulfone-
amine)s,
20-22
poly(amide-amine)s,
16,23
hyperbranched
and
hyperbranched
poly(ester-amine)s
24,25
with commercially available monomers.
d
n
4
y
3
n
g
|
1
5.2.1.2 Chain Growth
Self-condensing vinyl polymerization (SCVP) was first proposed by Frech´t et
al.
26,27
The synthetic method is based on the design of an inimer bearing both a
vinyl monomer and an initiating group. These monomers can accomplish
chain growth and step growth through propagation of double bonds and also
initiation of double bonds, thus leading to HBPs in a one-pot reaction.
Self-condensing ring-opening polymerization (SCROP) can be called ring-
opening multibranching polymerization (ROMBP) as well. As a matter of fact,
the mechanism of SCROP is similar to that of SCVP. A vinyl group is replaced
by a heterocyclic group as the monomer part of the inimer. Besides, irreversible
reactions are considered for SCVP, whereas reversible preconditions usually
occur for SCROP.
28,29
The SCROP approaches stem from the classical ring-
opening reaction mechanism (ROP), for instance polyesters.
Proton-transfer polymerization (PTP) was also proposed by Frech´t.
30
PTP
is featured with a proton transfer for the activation of the nucleophile used in
epoxide opening in each propagation step. The addition of a catalytic amount
of initiator such as OH
2
can initiate polymerization of the H-AB
2
monomer,
thus affording the reactive nucleophile, which is able to further react with
another monomer unit to afford a dimer. The latent reactivity now passes on
to one of the original epoxy groups.
Hybrids of different polymerization methods can give rise to a brand-new
HBP and meanwhile render itself unique structures and characteristics.
8
5.2.2 Functionalization of HBPs
Different synthetic approaches are suitable for different monomers, but these
methods basically play a role in constructing the backbones of HBPs, most of
which do not possess specific functions. With the motivation of biomedical
applications, the resulting HBPs require further functionalization by tethering
external functional components or alteration of the backbone structures from
the intrinsic chemical components. Benefiting from three-dimensional topo-
logical architecture and numerous terminal groups, HBPs are inclined to be
further modified compared with other polymeric materials. Functionalization
of HBPs generally involves terminal modification, hybrid modification, and
backbone
modification,
which
is
of
great
importance
to
their
further
applications, as shown in Figure 5.1.
