5. AIE polymers via other RDRP
methods
Of the many types of RDRP, Cu(0)-RDRP and NMP were the other methods to
synthesize AIE polymers. Cu(0)-RDRP was first reported by Matyjaszewski,
and co-workers in 1997 when they discovered that the addition of
zerovalent copper metal powder into standard ATRP polymerizations of
styrene and (meth)acrylates dramatically improved the rate of reaction
by as much as 10-fold compared to without any addition of the powder via
simple electron transfer process to remove excess Cu(II) deactivator
species.[168] In 2006, Percec, Sahoo and
co-workers termed this unique polymerization technique as
Single-Electron Transfer Living Radical Polymerization
(SET-LRP),[169] which helped differentiated it
from the standard ATRP technique.
NMP was first reported and patented by Solomon, Rizzardo and Cacioli of
CSIRO Australia in 1986.[35] Similar to RAFT and
ATRP, NMP is a technique that bears resemblance to an ideal living
polymerization: (1) the ability to control desired polymer product
molecular weight with low dispersity (Ð ) values, (2) has
realistic industrial potential with simple implementation steps that
often requires only a single unimolecular initiator to produce the
desired product, and (3) no need for use of transition metal catalysts.
NMP uses alkoxyamine initiators which can undergo homolysis of the C-O
bond to yield a stable nitroxide radical that is characteristic of a
persistent radical, leading to favoured generation of one product over
all others.[170-173] Given the benefits Cu(0)-RDRP
and NMP can offer, they are considered suitable methods for synthesizing
AIE polymers.
In 2021, Haddleton, Zhang and co-workers synthesized cationic
glycopolymers structurally similar to poly(ionic liquids) (PILs) using
Cu(0)-RDRP technique.[174] Post polymerization
modifications were performed on the poly(4-vinyl pyridine) (P4VP) groups
with halogen-functionalized D-mannose and TPE units, thus imparting AIE
properties onto the resulting polymer. The resulting polymer can be
viewed as cationic glycopolymers which is a hybrid material possessing
both PIL and glycopolymer properties, which includes specific
carbohydrate-protein recognition and antibacterial activities in
bacteria such as Gram-positive Staphylococcus aureus and
Gram-negative Escherichia coli . The TPE units have the ability to
improve the interaction between PILs and bacteria surface
biomacromolecules, causing further aggregation of PILs and concentration
of TPE units in the bacteria leading to AIE fluorescence for
fluorescence imaging. The combination of AIE-active TPE units with
glycopolymers and PILs enables the tracking, killing and detection of
bacteria. For the synthesis of PILs, Cu(0)-RDRP was used to polymerize
4VP to obtain P4VP at high conversion of 94% at room temperature
conditions in DMSO:H2O solvent system (v/v = 1:1). Then,
quaternization reaction was employed to modify P4VP with organobromides
such as TPEBr, Mannose-Br, or 2-bromopropanol, yielding PILs of
P4VP-ManTPE and P4VP-BPTPE. The PILs adsorbs onto the bacteria surface
via electrostatic interactions between positively-charged pyridine rings
and negatively charged bacterial membranes, while the hydrophilic parts
may insert into the hydrophobic membrane parts to kill the bacteria. The
authors noted that cationic glycopolymers have better bactericidal
effects on Gram-positive bacteria than Gram-negative bacteria due to the
difference in cell membrane structures. Concanavalin A was used to show
that sugar-containing PILs can recognize proteins, leading to
aggregation and significant AIE effect. It is also worthwhile to note,
detection by fluorescence emission of bacteria became more sensitive
when bacteria concentration increases due to the AIE effect.
Similar to RAFT and ATRP, Cu(0)-RDRP need not be limited to the commonly
known AIE-active TPE molecule, other variations were explored by many
research groups, for example, Hudson and co-workers in 2018, prepared
three different acrylic monomers using organic semiconductors as motifs,
and employed them as electron-transporting (n-type)
materials.[175] Cu(0)-RDRP method was used to
prepare triazine-, oxadiazole-, and benzimidazole- containing polymers
from room-temperature reaction using Cu(0) wires to give low dispersity
(Ð = 1.14) and high conversions of up to 97%. The authors faced
a problem with benzimidazole-containing monomers due to the large
induction period prior to onset of polymerization attributed to slow
coordination of benzimidazole groups to CuBr2. Due to
the limited solubility of these hydrophobic monomers in polar solvents
such as DMSO, DMF and isopropanol, difficulties were faced when
selecting appropriate solvents. N -methyl-2-pyrrolidone andN,N -dimethylacetamide (DMAc) solvents were found to be effective
in dissolving the monomers and assisting in the catalysts activities.
Higher molecular weights of the resulting polymers were also
successfully achieved at a lower conversion percentage and broaderÐ , which could be due to poorer overall polymer solubility.
Nevertheless, Cu(0)-RDRP was successfully employed by the authors to
synthesize polymers with optical properties from challenging monomers
containing N -donor groups with low dispersity values (Ð =
1.14 – 1.39) and conversions higher than 92%. All polymers were found
to be thermally stable, as determined from only a single step
decomposition at 275 °C, making them ideal for processing into organic
devices such as organic light-emitting diodes
(OLEDs),[176] organic photovoltaics
(OPVs),[177] organic thin-film transistors
(OTFTs),[178] organic electrochemical transistors
(OECTs) and organic thermoelectric (OTE)
generators.[179]
AIE molecules can also include transition metals such as iridium (Ir)
which allows tuning of the color of fluorescence emitted. In 2019,
Hudson and co-workers synthesized a series of bottlebrush copolymers
(BBCPs) from red (IrPIQ-MM), green (IrPPY-MM), and blue (tBuODA-MM)
(RGB) luminescent macromonomers using a carbazole-based host, which was
then used to prepare multiblock organic fibres with similar structures
to nanoscale RGB pixels (Figure 9A ).[180]The different blocks were then combined to give di- and tri-block
luminescent BBCPs, which displays AIE effects between blocks as the
solvent polarity changes. The authors elaborated on solvent-responsive
luminescent encoded patterns by quantifying the changes in energy
transfer efficiency and interchromophore distance among the different
blocks after aggregation. White LED mimicking pentablock nanofibers were
then synthesized containing multiple discrete emission zones by
combining the different building blocks with charge-transporting
materials. Well-defined interfaces in BBCPs can be used to regulate
energy transfer between the segments. Förster resonance energy transfer
(FRET) were observed with significant color change when BBCPs aggregate.
Multicomponent nanofibers with increasing complexity can be prepared
using this method to conduct studies on optoelectronic interaction
between and within BBCPs.
An initiator containing a norbornene moiety and a carbazole-based
acrylic monomer (CzBA) were used to copolymerize with 8 wt% of
different luminescent material, yielding materials with tunable colors
via Cu(0)-RDRP. In this study, acrylic-based monomers containing
Ir(piq)2(acac), Ir(ppy)2(acac), (piq =
2-phenylisoquinoline, ppy = 2-phenylpyridine) (IrPIQ and IrPPY), and
donor-acceptor type monomer
4-(5-([1,1′-biphenyl]-4-yl)-1,3,4-oxadiazol-2-yl)-N,N -di-p -tolylaniline
(tBuODA) were employed to produce polymers that emit red, green, and
blue light respectively. Grafting through approach was used to
give tBuODA75-b -IrPPY75-BB,
tBuODA60-b -IrPIQ90-BB, and
IrPPY100-b -IrPIQ50-BB diblock
copolymers in a way that emits colors intermediate to both constituent
homopolymers. Well-defined two-color interface was observed upon
combining the chromophores with a BBCP controllable solvent polarity. As
the water fraction increases from 0 to 98% in the solution of
tBuODA75-b -IrPPY75-BB,
tBuODA60-b -IrPIQ90-BB,
phosphorescence was observed to increase significantly for the red and
green iridium fluorophores, while blue fluorescence decreases gradually.
Due to significant spectral overlap between tBuODA and IrPPY emission
profiles, energy transfer efficiency for
tBuODA75-b -IrPPY75-BB could not
be accurately determined. In addition, BBCPs can be used to prepare
nanofibers with multiple compartments. A linear “pentablock” nanofiber
mimicking a white OLED design with discrete RGB emissions was prepared,
possessing unique characteristics such as high energy efficiency and
diffuse lighting for use as a potential next-generation solid state
lighting.
In addition, Cu(0)-RDRP can be employed to synthesize polymers capable
of behaving as drug carriers as exemplified by Jia, Tang and co-workers
of which they synthesized a brush-like polymer with AIE features for
drug delivery and intracellular drug
tracking.[181] The study aims to improve the
loading capacity of drug carriers by using a brush-like polymer with
many functional groups capable of holding the target drug compound. By
combining TPE bromoisobutyrate (TPEBIB) and anticancer drug doxorubicin
(DOX),[182] it could potentially lead to a
compound capable of real-time monitoring of cell targeting, drug release
and cancer cell viability. TPEBIB was synthesized and used as an
initiator in the copolymerization of poly(ethylene glycol) acrylate
(PEGA) and hydrazine (Hyd) monomers via Cu(0)-RDRP, and subsequently
conjugating DOX to the centre carrier through the hydrazone bonds to
form the complex carrier TPE-PEGA-Hyd-DOX smart prodrug containing
approximately 11 wt% DOX.[181] DOX is released in
a controlled manner when exposed to cancer cells due to hydrazine bond
cleavage in acidic conditions due to improved cellular uptake levels.
This novel block copolymer is completely biocompatible with normal and
cancer cells, with the cytotoxicity depending on the local pH levels. A
comparison study was performed between pristine DOX solution as the
control and TPE-PEGA-Hyd-DOX solutions at the same concentration, where
drug release reached only 10% after 96 hours under normal cell
conditions compared with 40% after 24 hours under cancer cell
conditions helps confirmed that conjugation of DOX to the drug carrier
controls the release of DOX and protects normal cells against DOX. Many
other studies also employed the Cu(0)-RDRP methods to synthesize AIE
polymers such as Cu(0)-catalyzed SET-LRP reported by Wang, Yang and
co-workers for the study of multi-arm star polymers with
TPE-functionalized core,[183] and the study of
through-space charge-transfer thermally activated delayed fluorescence
(TSCT-TADF) phenomenon using AIE-functionalized
monomers.[184-186]
NMP can also be used for the synthesis of AIE polymers with a unique
morphology as demonstrated by Nicolas and co-workers in 2017, whom
employed carbodiimide chemistry to link
4-(N -methylpiperazine)-1,8-naphthalimide-based AIE
dye,[187] with AMA-SG1 alkoxyamine, yielding
Napht-AMA-SG1 in 82% yield via the grafting from or
‘drug-initiated’ method. Isoprene monomers were then added to produce
the AIE-active polymer
4-(N -methylpiperazine)-1,8-naphthalimide-polyisoprene (Napht-PI)
and subsequently, co-nanoprecipitated with
cladribine-diglycolate-polyisoprene (CdA-digly-PI) to form Napht-PI
CdA-digly-PI prodrug nanoparticles (Figure
9B ).[188] Cytotoxicity of the polymers
synthesized were also determined by incubating with murine leukemia
(L12210) cells, with cell viability reaching approximately 100%, up to
a concentration of 250 µg mL-1 after 72 incubation
time. Confocal laser scanning spectroscopy (CLSM) on Napht-PI
CdA-digly-PI prodrug nanoparticles were carried out though incubation
with A549 human lung carcinoma cells for intracellular imaging. The low
cytotoxicity of these nanoparticles combined with the sharp fluorescence
signal from the AIE-active part of the prodrug, provides excellent
imaging and tracking abilities in living cells. It is worth noting that
the 1,8-naphthalimide-based fluorescent dyes studied in Nicolas’
work,[188] were used previously to conjugate with
different chemical species due to their versatile chemical
structures.[189] These dyes exhibit AIE properties
due to a twisted intramolecular charge transfer (TICT) process
originating from RIM.
The use NMP in AIE polymer synthesis was also demonstrated by Qiao, Pang
and co-workers in 2021 where TPE-functionalized
4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl (NH2-TEMPO)
and
3-(((2-cyanopropan-2-yl)oxy)(cyclohexyl)amino)-2,2-dimethyl-3-phenylpropanenitrile
(Dispolreg 007) were used to study reaction kinetics in homogenous and
heterogenous polymerization systems
respectively.[37]