Degradable Thermoresponsive Polyesters by Atom Transfer Radical Polyaddition and Click Chemistry

Recent Advances in the Application of ATRP in the Synthesis of Drug Delivery Systems

Advances in atom transfer radical polymerization (ATRP) have enabled the precise design and preparation of nanostructured polymeric materials for a variety of biomedical applications. This paper briefly summarizes recent developments in the synthesis of bio-therapeutics for drug delivery based on linear and branched block copolymers and bioconjugates using ATRP, which have been tested in drug delivery systems (DDSs) over the past decade. An important trend is the rapid development of a number of smart DDSs that can release bioactive materials in response to certain external stimuli, either physical (e.g., light, ultrasound, or temperature) or chemical factors (e.g., changes in pH values and/or environmental redox potential). The use of ATRPs in the synthesis of polymeric bioconjugates containing drugs, proteins, and nucleic acids, as well as systems applied in combination therapies, has also received considerable attention.

Hierarchical Self-Assembly of Triphilic Main-Chain-Type Semifluorinated Alternating Graft Copolymers in Aqueous Solution

Fluorinated copolymers can self-assemble in solution and form micelles with rare properties due to the peculiar behavior of fluorinated groups. However, the process description of the self-assembly is still largely phenomenological and difficult to explain due to the tendency of the fluorinated segments to segregate from both the hydrophilic and lipophilic segments, which can result in various morphologies. Herein, the controlled formation of ellipsoidal micelles, disklike micelles, and sheets by hierarchical self-assembly of triphilic main-chain-type semifluorinated alternating graft copolymers (AB)_n A-g-mOEG is presented (where A represents unit of α,ω-diiodoperfluoroalkane, B represents the unit of α,ω-unconjugated diene, and mOEG represents methoxy oligo(ethylene glycol)), which are synthesized by step transfer-addition and radical-termination (START) polymerization and azide-alkyne click chemistry. Furthermore, the possible self-assembly mechanism of these micron-level aggregates is proposed, which is ascribed to the hierarchical self-assembly, crowding effect of hydrophilic chains and the interfacial tension between the fluoroalkane and alkane segments. This study can provide a facile and highly efficient approach to the synthesis of main-chain-type fluorinated graft copolymers and expand the research field for the solution self-assembly of fluorinated copolymers.

A Degradable Difunctional Initiator for ATRP That Responds to Hydrogen Peroxide

Mid-chain degradable polymers can be prepared by atom transfer radical polymerization from difunctional initiators that include triggers for the desired stimuli. While many difunctional initiators can respond to reducing conditions, procedures to prepare difunctional initiators that respond to oxidizing conditions are significantly less available in the literature. Here, a difunctional initiator incorporating an oxidizable boronic ester trigger was synthesized over four steps using simple and scalable procedures. Methyl methacrylate was polymerized by atom transfer radical polymerization using this initiator, and the polymerization kinetics were consistent with a controlled polymerization. The polymer synthesized using the difunctional initiator was found to decrease in molecular weight by 58% in the presence of hydrogen peroxide, while a control experiment using poly(methyl methacrylate) without a degradable linkage showed a much smaller decrease in molecular weight of only 9%. These observed molecular weight decreases were consistent with cleavage of the difunctional initiator via a quinone methide shift and hydrolysis of the methyl ester pendent groups in both polymers, and both polymers increased in polydispersity after oxidative degradation.

Stimuli-responsive polypeptides for controlled drug delivery

Controlled drug delivery systems, which could release loaded therapeutics upon physicochemical changes imposed by physiological triggers in the desired zone and during the required period of time, offer numerous advantages over traditional drug carriers including enhanced therapeutic effects and reduced toxicity. A polypeptide is a biocompatible and biodegradable polymer, which can be conveniently endowed with stimuli-responsiveness by introducing natural amino acid residues with innate stimuli-responsive characteristics or introducing responsive moieties to its side chains using simple conjugating methods, rendering it an ideal biomedical material for controlled drug delivery. This feature article summarizes our recent work and other relevant studies on the development of polypeptide-based drug delivery systems that respond to single or multiple physiological stimuli (e.g., pH, redox potential, glucose, and hypoxia) for controlled drug delivery applications. The material designs, synthetic strategies, loading and controlled-release mechanisms of drugs, and biomedical applications of these stimuli-responsive polypeptides-based drug delivery systems are elaborated. Finally, the challenges and opportunities in this field are briefly discussed.

Progress in the Preparation of Functional and (Bio)Degradable Polymers via Living Polymerizations

This review presents the latest developments in (bio)degradable approaches and functional aliphatic polyesters and polycarbonates prepared by typical ring-opening polymerization (ROP) of lactones and trimethylene carbonates. It also considers several recent innovative synthetic methods including radical ring-opening polymerization (RROP), atom transfer radical polyaddition (ATRPA), and simultaneous chain- and step-growth radical polymerization (SCSRP) that produce aliphatic polyesters. With regard to (bio)degradable approaches, we have summarized several representative cleavable linkages that make it possible to obtain cleavable polymers. In the section on functional aliphatic polyesters, we explore the syntheses of specific functional lactones, which can be performed by ring-opening copolymerization of typical lactone/lactide monomers. Last but not the least, in the recent innovative methods section, three interesting synthetic methodologies, RROP, ATRPA, and SCSRP are discussed in detail with regard to their reaction mechanisms and polymer functionalities.

Dual Reduction/Acid-Responsive Disassembly and Thermoresponsive Tunability of Degradable Double Hydrophilic Block Copolymer

We report a thermoresponsive double hydrophilic block copolymer degradable in response to dual reduction and acidic pH at dual locations. The copolymer consists of a poly(ethylene oxide) block covalently connected through an acid-labile acetal linkage with a thermoresponsive polymethacrylate block containing pendant oligo(ethylene oxide) and disulfide groups. The copolymer undergoes temperature-driven self-assembly in water to form nanoassemblies with acetal linkages at the core/corona interface and disulfide pendants in the core, exhibiting dual reduction/acid responses at dual locations. The physically assembled nanoaggregates are converted to disulfide-core-crosslinked nanogels through disulfide-thiol exchange reaction, retaining enhanced colloidal stability, yet degraded to water-soluble unimers upon reduction/acid-responsive degradation. Further, the copolymer exhibits improved tunability of thermoresponsive property upon the cleavage of junction acetal and pendant disulfide linkages individually and in combined manner. This work suggests that dual location dual reduction/acid-responsive degradation is a versatile strategy toward effective drug delivery exhibiting disulfide-core-crosslinking capability and disassembly as well as improved thermoresponsive tunability.

Development and disassembly of single and multiple acid-cleavable block copolymer nanoassemblies for drug delivery

Acid-degradable block copolymer-based nanoassemblies are promising intracellular candidates for tumor-targeting drug delivery as they exhibit the enhanced release of encapsulated drugs through their dissociation.

Iron-catalysed atom transfer radical polyaddition for the synthesis and modification of novel aliphatic polyesters displaying lower critical solution temperature and pH-dependent release behaviors

Novel aliphatic polyesters were synthesized and quantitatively modified by click reactions to obtain amphiphilic polymer brushes for nano-carrier applications.

Dual stimuli-responsive copolymers with precisely arranged degradable units: synthesis by controlled alternating copolymerization of oxyethylene-containing vinyl ethers and conjugated aldehydes

Thermoresponsive polymers with precisely arranged degradable units were synthesized via controlled cationic copolymerization of oxyethylene-containing vinyl ethers and conjugated aldehydes.

Transiently thermoresponsive polymers and their applications in biomedicine

The focus of this review is on the class of transiently thermoresponsive polymers. These polymers are thermoresponsive, but gradually lose this property upon chemical transformation - often a hydrolysis reaction - in the polymer side chain or backbone. An overview of the different approaches used for the design of these polymers along with their physicochemical properties is given. Their amphiphilic properties and degradability into fully soluble compounds make this class of responsive polymers attractive for drug delivery and tissue engineering applications. Examples of these are also provided in this review.

One-Enzyme Triple Catalysis: Employing the Promiscuity of Horseradish Peroxidase for Synthesis and Functionalization of Well-Defined Polymers

We demonstrate a new concept in polymer chemistry that the promiscuity of enzymes, as represented by horseradish peroxidase, can be employed for RAFT polymerization and thiol-ene and Diels-Alder reactions to synthesize well-defined functional polymers, via three different catalytic reactions mediated by one single enzyme.

The effect of pendant group structure on the thermoresponsive properties of N-substituted polyesters

Structure–property correlation studies of a diverse set of biodegradable thermoresponsive polyesters provides a rationale for the design of thermoresponsive polyesters with desired cloud points.

Oxidation and temperature dual responsive polymers based on phenylboronic acid and N-isopropylacrylamide motifs

Oxidation and temperature dual responsive copolymers using ROS as a target for drug delivery have been demonstrated.

Synthesis and pH-responsive self-assembly behavior of a fluorescent amphiphilic triblock copolymer mPEG-b-PCL-b-PDMAEMA-g-PC for the controlled intracellular delivery of doxorubicin

Schematic illustration of the pH-responsive self-assembly of a mPEG-b-PCL-b-PDMAEMA-g-PC copolymer with fluorescent coumarin units for controlling DOX release.

Degradable vinyl polymers for biomedical applications

Vinyl polymers have been the focus of intensive research over the past few decades and are attractive materials owing to their ease of synthesis and their broad diversity of architectures, compositions and functionalities. Their carbon-carbon backbones are extremely resistant to degradation, however, and this property limits their uses. Degradable polymers are an important field of research in polymer science and have been used in a wide range of applications spanning from (nano)medicine to microelectronics and environmental protection. The development of synthetic strategies to enable complete or partial degradation of vinyl polymers is, therefore, of great importance because it will offer new opportunities for the application of these materials. This Review captures the most recent and promising approaches to the design of degradable vinyl polymers and discusses the potential of these materials for biomedical applications.

Synthesis of thermal and oxidation dual responsive polymers for reactive oxygen species (ROS)-triggered drug release

A PEG-based main chain thermo-responsive polymer with oxidation-triggerable thioether groups: synthesis, characterization and its use for ROS-responsive drug release.

Hyperbranched poly(triazole) with thermal and metal ion dual stimuli-responsiveness

Hyperbranched poly(triazole) bearing oligo(ethylene glycol) terminal groups is dual stimuli-responsive to thermal conditions and metal ions and is capable for the selective absorption of Ag⁺ ion on tuning temperature.

Polymerization and degradation of aliphatic polyesters synthesized by atom transfer radical polyaddition

By tuning leaving group affinity, the aliphatic polyesters synthesized by ATRPA can avoid lactonization and obtain high molecular weights.

Novel pH-tunable thermoresponsive polymers displaying lower and upper critical solution temperatures

Novel pH-tunable thermoresponsive 3-azido-2-hydroxypropyl methacrylate-based polymers displaying LCST and UCST in phosphate buffer solutions were successfully synthesized by ATRP.

Efficient Design for Stimuli-Responsive Polymers with Quantitative Acid-Degradability: Specifically Designed Alternating Controlled Cationic Copolymerization and Facile Complete Degradation

Specifically designed alternating cationic copolymerization produced well-defined thermo- or pH-responsive polymers with complete acid-degradability. For example, a thermosensitive alternating copolymer with acid-labile acetal linkages in the main chain was obtained from the controlled cationic copolymerization of p-methoxybenzaldehyde (pMeOBzA) and a vinyl ether (VE) with an oxyethylenic side chain. The resulting copolymer exhibited a sharp thermosensitive phase transition in water. The same strategy but using different VEs with esters and benzaldehyde (BzA) yielded pH-responsive copolymers with nearly alternating sequences and narrow molecular weight distributions (MWDs). The alternately arranged acetal bonds in the copolymers allowed complete facile and rapid degradation under acidic conditions, which selectively produced low-molecular-weight compounds (MW ∼ 1-2 × 10²).

Combination of ATRA and ATRC for the synthesis of periodic vinyl copolymers

A new approach to periodic vinyl copolymers via combination of atom transfer radical addition (ATRA) and atom transfer radical coupling (ATRC) is reported. The two examples are methyl methacrylate (MMA) and styrene (St) periodic copolymer (P(SMMS)) and acrylonitrile (AN) and St periodic copolymer (P(SAAS)). First, two monomer sequence units (MSU) with built-in sequence, SMMS and SAAS, are synthesized through the controlled ATRA of two ATRP initiators with St. Then, the ATRC of SMMS and SAAS are conducted at high radical conditions to generate two types of high-molecular-weight copolymers, P(SMMS) and P(SAAS). Though side reactions can not be totally avoided, characterizations of the polymer structure with a variety of means confirm that the main chain structures of P(SMMS) and (PSAAS) are predominantly with the periodic sequences from the MSUs. Attempts to suppress the side reactions are successful via the MNP-mediated ATRC of SMMS and SAAS.