Biomedical Engineering Reference
In-Depth Information
2.2.5
One-Block Collapse of Diblock or Triblock
Copolymers
A unique polymer chain composed by more than one block can be
designed to undergo selective collapse in only one of the blocks. If the
starting chain is a diblock copolymer of type A-B, this results in the
formation of nanoparticle-coil copolymers or molecular “tadpoles”.
This kind of structure is especially interesting from a biological
point of view. Naturally occurring polymers such as peptides and
proteins, often incorporate diff erent architectures or conformations
(i.e. globular, linear) into the same macromolecule, enabling the
formation of discrete and unique nanoenvironments for catalysis and
self-assembly [47, 48]. Recent developments in polymer therapeutics
have seen the launch of several highly successful drugs with mixed
globular-linear architecture, in which a globular protein is attached
to a synthetic linear polymer such as poly(ethylene glycol) [49]. In
these systems the synthetic linear-globular assembly showed vast
performance enhancements over the natural globular protein on its
own.
The first preparation and isolation of diblock tadpole molecules
was published by Tao and Liu in 1997 [50]. They used a polystyrene-
block -poly(2-cinnamoylethyl methacrylate) precursor, and they
forced the intramolecular collapse by photo-cross-linking of the
cinnamoyl units. However, their method only allowed the formation
of tadpoles in very low yields. The process involved the tedious
separation of the tadpoles from the nanospheres that were also
formed in the reaction, using size-exclusion chromatography (SEC).
Hawker and co-workers described the synthesis of nanoparticle-
coil structures from linear diblock copolymers of poly(ethylene
glycol)- block -poly[styrene- random -(4-vinylbenzocyclobutene)]
(Scheme 2.5) [29, 51]. The collapse was carried out by the slow
addition of the precursor linear copolymer to a benzyl ether
solution at 250 ° C. It is worth noticing that the linear block is water
soluble, although the entire molecule was not in this case, probably
due to the short dimension of the PEG fragment. A similar family
of nanoparticle-coil copolymers were prepared by Pyun and co-
workers having a poly( n- butyl acrylate) coil attached to the same
styrenic nanoparticle [52].
 
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