Reversible deactivation radical polymerization (RDRP) has revolutionized modern polymer chemistry over the past two decades, thus laying the groundwork for the synthesis of complex macromolecules and enabling the preparation of previously inaccessible materials. Reversible addition-fragmentation chain transfer (RAFT) polymerization has emerged as one of the most promising techniques because of its functional group tolerance, applicability to a wide range of vinyl monomers, and its nondemanding experimental conditions. However, despite the promise and clearly demonstrated utility of RAFT, limitations of the method sometimes still exist, including the occasional need for extended polymerization times, limited access to high molecular weight polymers, low "livingness" due to unavoidable radical termination events, etc. This Perspective focuses on recent advances that have been specifically designed to address many of these perceived limitations to reinforce the promise of RAFT for the synthesis of complex and well-defined polymers under facile conditions.
Hydrophobicity
inherently affects a solutes behavior in water,
yet how polymer chain hydrophobicity impacts aggregate morphology
during solution self-assembly and reorganization is largely overlooked.
As polymer and nanoparticle syntheses are easily achieved, the resultant
nanoparticle architectures are usually attributed to chain topology
and overall degree of polymerization, bypassing how the chains may
interact with water during/after self-assembly to elicit morphology
changes. Herein, we demonstrate how block copolymer hydrophobicity
allows control over aggregate morphology in water and leads to remarkable
control over the length of polymeric nanoparticle worms. Polymerization-induced
self-assembly facilitated nanoparticle synthesis through simultaneous
polymerization, self-assembly, and chain reorganization during a block
copolymer chain extension from a hydrophilic poly(
N
,
N
-dimethylacrylamide)
macro-chain-transfer agent with diacetone acrylamide and
N
,
N
-dimethylacrylamide.
Slight variations in the monomer feed ratio dictated the block copolymer
chain composition and were proposed to alter aggregate thermodynamics.
Micelles, worms, and vesicles were synthesized, and the highest level
of control over worm elongation attained during a polymerization is
reported, simply due to the polymer chain hydrophobicity.
We report mechanistic investigations into aqueous visible-light reversible addition−fragmentation chain transfer (RAFT) polymerizations of acrylamides using eosin Y as a photoinduced electron-transfer (PET) catalyst. The photoinduced polymerization was found to be dependent upon the irradiation wavelength and reagents, where either reduction or oxidation of the PET catalyst leads to inherently different initiation and reversible-termination steps. Using blue light, multiple mechanisms of initiation are observed, depending on the presence or absence of a sacrificial reducing agent. Using green light, both an oxidative and a reductive PET initiation mechanism can be pursued. Investigations into the role of PET catalyst, wavelength, and reducing agent demonstrated that precise polymers with predictable molecular weights are best realized under an oxidative PET-RAFT mechanism. Therefore, this study provides fundamental insight into visible-light RAFT photopolymerizations and the role of eosin Y as a photoredox catalyst.
Complex coacervates can form through
the electrostatic complexation
of oppositely charged polymers. The material properties of the resulting
coacervates can change based on the polymer chemistry and the complex
interplay between electrostatic interactions and water structure,
controlled by salt. We examined the effect of varying the polymer
backbone chemistry using methacryloyl- and acryloyl-based complex
coacervates over a range of polymer chain lengths and salt conditions.
We simultaneously quantified the coacervate phase behavior and the
linear viscoelasticity of the resulting coacervates to understand
the interplay between polymer chain length, backbone chemistry, polymer
concentration, and salt concentration. Time-salt superposition analysis
was used to facilitate a broader characterization and comparison of
the stress relaxation behavior between different coacervate samples.
Samples with mismatched polymer chain lengths highlighted the ways
in which the shortest polymer chain can dominate the resulting coacervate
properties. A comparison between coacervates formed from methacryloyl
vs acryloyl polymers demonstrated that the presence of a backbone
methyl group affects the phase behavior, and thus the rheology in
such a way that coacervates formed from methacryloyl polymers have
a similar phase behavior to those of acryloyl polymers with ∼10×
longer polymer chains.
As many physical properties of polymers
scale with molecular weight,
the ability to achieve polymers of nearly inaccessibly high molecular
weight provides an opportunity to probe the upper size limit of macromolecular
phenomena. Yet many of the most stimulating macromolecular designs
remain out of reach of current ultrahigh molecular weight (UHMW) polymer
synthetic approaches. Herein, we show that UHMW polymers of diverse
composition can be achieved by irradiation of thiocarbonylthio photoiniferters
with long-wave ultraviolet or visible light in concentrated organic
solution. This facile photopolymerization strategy is general to acrylic-,
acrylamido-, methacrylic-, and styrenic-based monomers, enabling the
synthesis of well-defined macromolecules with molecular weights in
excess of 106 g/mol. The high chain-end fidelity afforded
by photoiniferter polymerization conditions facilitated the design
of UHMW amphiphilic block copolymers, which were found to self-assemble
into micellar morphologies up to 200 nm in diameter.
An initiator-and catalyst-free method for polymer end-group modification has been designed. Under long-wave ultraviolet irradiation, polymers with thiocarbonylthio end groups undergo photolytic cleavage to reveal an active macroradical capable of irreversible termination with a suitable hydrogen source. This straightforward method was successfully demonstrated by the removal of a range of end groups that commonly result from reversible addition−fragmentation chain transfer or photoiniferter polymerizations, including trithiocarbonate, dithiobenzoate, xanthate, and dithiocarbamate mediating agents. This strategy proved efficient for polymers derived from acrylamido, acrylic, methacrylic, styrenic, and vinylpyrrolidone monomers.
Gold nanoparticles decorated with “polymeric thermometers,” consisting of a polymeric spacer, thermally-labile azo linker, and fluorescent tag, were used to quantify the extent of localized hyperthermia under microwave irradiation.
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