Biomedical Engineering Reference
In-Depth Information
As a special case, there are also some reports on the use of diblock
copolymers for the fabrication of nanoparticle-coil copolymers or
molecular “tadpoles”. The strategy consists of functionalising just one
of the blocks with reactive groups, so that when the intramolecular
cross-linking is forced this block collapses into a globule, whereas
the other block remains as a coil (Scheme 2.1d). In this case, the
intramolecular collapse could be performed using any of the three
strategies previously mentioned.
2.2.2
Homo-Functional Chain Collapse
The synthesis of discrete nanoparticles using a strategy that involves
the intramolecular coupling of single polymer chains was proposed
as early as in the 1980s [24, 25]. Davankov and co-workers published
the synthesis of single-chain polystyrene nanoparticles using this
strategy in 1996 [26]. In this work, linear polystyrene chains ( M w
= 330 kDa) functionalised with chloromethylene groups were self-
crosslinked by means of a SnCl 4 catalyst, obtaining hyper-crosslinked
“nanosponges” of about 17 nm (in solution).
A few years later, Mecerreyes and co-workers presented the
synthesis of unimolecular particles from copolymers based on (i)
aliphatic polyesters (caprolactone-co- ε -caprolactone or L,L-lactide),
(ii) poly(methyl methacrylate) and (iii) polystyrene [27]. The
strategy that they used involved the synthesis of polymer backbones
with pendant acrylate functionalities. Then, such macromolecules
were intramolecularly crosslinked in ultradiluted solution using a
radical initiator such as 2,2-azo-bis-isobutyronitrile (AIBN).
Thayumanavan and co-workers used a similar strategy to
synthesise amine-functionalised polystyrene nanoparticles by
AIBN induced radical cross-linking of styrene groups [28]. They
demonstrated that the free pending amino groups were available for
further functionalisation, and they proposed that the system could
be useful for applications such as drug delivery.
One of the drawbacks of the strategies mentioned so far was
that in order to prevent the competing and statistically favoured
intermolecular cross-linking, the reactions needed to be carried
out in ultra-dilute conditions (ca. 10 -5 -10 -6 M). This precludes the
synthesis of such nanoparticles on a useful (multigram) scale. A
very clever strategy to overcome this problem was introduced by
Hawker and co-workers in 2002 [29]. The method consists of the
 
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