Abstract |
Stimuli–sensitive or “smart” polymers have been on the focal point of many research
groups, as exceptionally unique candidates for applications in rapidly burgeoning
research fields, including biotechnology and nanomedicine, electronic devices, sensors
and actuators and others. The present PhD thesis focuses on the synthesis of novel,
stimuli–degradable polymers, polymer networks and polymer–drug conjugates and
investigates their potential use as drug delivery devices.
After a general introduction in Chapter 1, the second Chapter of this thesis focuses on
the investigation of thioketal and thioacetal bonds as a new family of photolabile
linkages. Linear main–chain poly(thioketals) were synthesized via a polycondensation
reaction based on an acetal exchange mechanism. The chemical and macromolecular
characteristics of the synthesized polymers were characterized by SEC and 1H NMR
and ATR–FTIR spectroscopies. Next, the main chain photodegradation mechanism of
the polymers upon UV irradiation (λ = 254 nm, 0.063 mW cm–1) was examined. It was
found that the photolysis process proceeded in a chain–length dependent fashion,
leading to the production of dithiol and ketone as the main photoproducts. Next, a facile
chemical platform for the synthesis of photodegradable and thermo–reversible, model
hydrogels comprising poly(ethylene glycol) (PEG) as the elastic chains and a
dithioacetal at the cross–links will be presented. Hydrogels were synthesized via an
acid–catalyzed step–growth reaction of a difunctional PEG–thiol macromer with a
wisely selected aromatic dialdehyde molecule. Introduction of the photosensitive
dithioacetal bonds rendered the hydrogels photodegradable to obtain the initial
comonomers as the main photoproducts, which endowed the material with thermo–
reversible properties. The viscoelastic properties of the water swollen gels, their
photodegradation under UV exposure at λ = 254 nm with very low intensity (0.063 mW
cm–1), and the reversible formation of the hydrogel upon heating were investigated by
dynamic shear rheology. Mechanistic insights into the photodegradation mechanism of
the hydrogels were gained by 1H NMR spectroscopy kinetic studies on a model, small
molecule thioacetal compound. Finally, the photo–induced release of loaded cargo from
the synthesized hydrogels was investigated.
In the third Chapter of this thesis, we are focused on the investigation of light–sensitive
poly(acylhydrazones). A novel water soluble, light– and acid–cleavable, main–chain 6
poly(acylhydrazone) copolymer was synthesized via a step–growth reaction of a
dihydrazide monomer, adipic acid dihydrazide (AA), with a dibenzaldehyde modified
poly(ethylene glycol) (PEGAld) affording a hydrophilic PEGAld–adipic acid (PEGAld–
AA) copolymer. Photodegradation studies revealed a backbone photolysis process
attributed to the cleavage of the labile acylhydrazone bonds along the polymer chain.
The water–soluble poly(acylhydrazone) was conjugated with a hydrophobic anticancer
drug, doxorubicin (DOX), affording an amphiphilic polymeric prodrug which formed
spherical nanostructures in water. The synergistic effect of light–mediated degradation
and acid–induced hydrolysis of the acylhydrazone bonds along the polymer chains, as
well as the release kinetics of DOX from the polymeric prodrug were investigated.
In Chapter 4, encouraged by our findings regarding the light–sensitivity of the
acylhydrazone bonds, the synthesis, characterization and photochemical properties of
different main–chain photodegradable poly(acylhydrazones) with regularly inserted
photocleavable groups, with photo–sensitivity ranging from the UV to the visible light
range was investigated. Poly(acylhydrazones) were synthesized using a PEG
acylhydrazide macromonomer of Mn = 4000 g mol‒1 or 1500 g mol‒1, and judiciously
selected aromatic dialdehydes or diketones as the comonomers. The dialdehyde
monomers used in this study were terepthaldehyde (TPA) and dibenzaldehyde
tetraphenyl ethylene (TPEAld), whereas the diketone monomers were 1,4–
diacetylbenzene (DCB) and anthraquinone (ANQ). In addition, the synthesis of
photodegradable alternating poly(acylhydrazone) multiblock copolymers comprising
hydrophilic PEG blocks and hydrophobic PDMS blocks was pursued via the step–
growth reaction of dialdehyde–terminated PDMS with diacylhydrazide–terminated
PEG. The effect of the carbonyl group, aldehyde vs ketone, on the polymerization
kinetics, the self–assembly as well as the photodegradation kinetics of the copolymers
was investigated. Moreover, the influence of the chromophore on the self–assembly,
the photophysical properties and the photodegradation of the poly(acylhydrazones) was
examined. Mechanistic insights into the photodegradation mechanism of the PEG4kHy–
TPE copolymer were gained via 1H NMR spectroscopy, whereas, the aggregation
induced emission (AIE) properties, the photo–triggered drug release and the in vitro
cytotoxicity of PEG4kHyd–TPE copolymer nanoparticles in water were investigated,
rendering them attractive theranostic agents.
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Finally, in Chapter 5, a hybrid (organic–inorganic) mRNA delivery system comprising
polymer coated superparamagnetic iron oxide nanoparticles (SPIONs) is presented. The
SPIONs were coated with modified natural polymers, namely oxidized dextran and
quaternized chitosan, to bearing aldehyde and cationic quaternary ammonium groups,
respectively, on their surface. The physicochemical and morphological characteristics
of the hybrid particles were determined by ATR–FTIR spectroscopy, TGA analysis,
DLS, SEM and TEM, whereas their biocompatibility in the presence and absence of a
magnetic field was tested on T49D breast cancer cells. Owning to the presence of the
cationic (quaternized chitosan) or aldehyde (oxidized dextran) groups on the particle
surface, mRNA chains were bound via electrostatic interactions or covalent (imine)
bonds, respectively. The binding and transfection efficiency of mRNA was examined
using of a Green Fluorescent Protein–mRNA (GFP–mRNA) construct in preliminary
in vitro studies.
Chapter 6 summarizes the main Conclusions of the work carried out in this thesis and
provides some future perspectives and the open questions derived from the study.
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