Effect of Monomer Structure on the CuAAC Polymerization To Produce Hyperbranched Polymers

Injectable Hydrogels Based on Hyperbranched Polymers for Biomedical Applications

Injectable hydrogels (IHs) have garnered significant attention in biomedical applications due to their minimally invasive nature, adaptability, and high degree of customization. However, traditional design methods of IHs have limitations in addressing complex clinical needs, such as precise regulation of the gelation time and mechanical strength within a wide window. Hyperbranched polymers (HBPs), due to their unique highly branched structures and abundant functional sites, can be easily prepared and functionalized to enable decoupled modulation of mechanical properties of IHs and address the clinical challenges of IHs. Our research group developed a library of HBPs via a dynamically controllable polymerization method and built a series of adjustable, controllable, and responsive IHs based on the resulting HBPs. The prepared IHs fed by HBPs demonstrate an adjustable gelation process, a wide-range tuning of mechanical properties, and responsiveness on demand, which show the capabilities in the various biomedical applications. In this review, we summarize the role of HBPs in the gelation process, mechanical properties, self-healing ability, and responsiveness of IHs. However, achieving IHs through HBPs and extending them to a broad range of biomedical applications are still in its infancy. This review provides an overview of IHs fabricated by a variety of multifunctional HBPs, and their biomedical applications in diverse fields are also presented. Meanwhile, we point out the future development of IHs based on HBPs and their potential challenges.

Kumada-Tamao Catalyst-Transfer Condensation Polymerization of AB2 Monomer: Synthesis of Well-Defined Hyperbranched Poly(thienylene-phenylene)

The synthesis of well-defined hyperbranched aromatic polymer by Kumada-Tamao catalyst-transfer condensation polymerization of AB2 monomer is investigated. Grignard monomer 2 is generated by treatment of 2-(3,5-dibromophenyl)-3-hexyl-5-iodothiophene (1) with 1.0 equivalent of isopropylmagnesium chloride in THF at 0 °C for 1 h and subsequently polymerized with Ni(dppe)Cl2 at room temperature for 1 h. The molecular weight of the obtained polymer increases linearly up to ≈30 000 in proportion to the ratio of [consumed 2] /[Ni(dppe)Cl2]0 and in proportion to the conversion of 2, while a narrow molecular weight distribution is maintained (Mw/Mn ≤ 1.12). The matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrum shows almost a single series of peaks due to polymer with hydrogen at one end and bromine at the other, as in the case of Kumada-Tamao catalyst transfer condensation polymerization of AB monomers. The degree of branching (DB) of the obtained polymers is 0.70-0.75, irrespective of the degree of polymerization. These results indicate that the polymerization of 2 proceeds in a chain-growth polymerization manner through the intramolecular catalyst transfer mechanism, affording hyperbranched polymer with higher DB than the theoretical DB value of 0.5 in conventional polycondensation of AB2 monomers.

Polylactic-Containing Hyperbranched Polymers through the CuAAC Polymerization of Aromatic AB2 Monomers

We report on the synthesis and characterization of a novel class of hyperbranched polymers, in which a copper(I)-catalyzed alkyne azide cycloaddition (CuAAC) reaction (the prototypical "click" reaction) is used as the polymerization step. The AB₂ monomers bear two azide functionalities and one alkyne functionality, which have been installed onto a 1,3,5 trisubstituted benzene aromatic skeleton. This synthesis has been optimized in terms of its purification strategies, with an eye on its scalability for the potential industrial applications of hyperbranched polymers as viscosity modifiers. By taking advantage of the modularity of the synthesis, we have been able to install short polylactic acid fragments as the spacing units between the complementary reactive azide and alkyne functionalities, aiming to introduce elements of biodegradability into the final products. The hyperbranched polymers have been obtained with good molecular weights and degrees of polymerization and branching, testifying to the effectiveness of the synthetic design. Simple experiments on glass surfaces have highlighted the possibility of conducting the polymerizations and the formation of the hyperbranched polymers directly in thin films at room temperature.

Chain-growth click copolymerization for the synthesis of branched copolymers with tunable branching densities

A facile synthesis of branched polymers with regulated molecular weights, low dispersity and freely tuned branching densities is reported.

Highly Efficient Modular Construction of Functional Drug Delivery Platform Based on Amphiphilic Biodegradable Polymers via Click Chemistry

Amphiphilic copolymers with pendant functional groups in polyester segments are widely used in nanomedicine. These enriched functionalities are designed to form covalent conjugates with payloads or provide additional stabilization effects for encapsulated drugs. A general method is successfully developed for the efficient preparation of functional biodegradable PEG-polyester copolymers via click chemistry. Firstly, in the presence of mPEG as initiator, Sn(Oct)2-catalyzed ring-opening polymerization of the α-alkynyl functionalized lactone with D,L-lactide or ε-caprolactone afforded linear mPEG-polyesters bearing multiple pendant alkynyl groups. Kinetic studies indicated the formation of random copolymers. Through copper-catalyzed azide-alkyne cycloaddition reaction, various small azido molecules with different functionalities to polyester segments are efficiently grafted. The molecular weights, polydispersities and grafting efficiencies of azido molecules of these copolymers were investigated by NMR and GPC. Secondly, it is demonstrated that the resulting amphiphilic functional copolymers with low CMC values could self-assemble to form nanoparticles in aqueous media. In addition, the in vitro degradation study and cytotoxicity assays indicated the excellent biodegradability and low cytotoxicity of these copolymers. This work provides a general approach toward the preparation of functional PEG-polyester copolymers in a quite efficient way, which may further facilitate the application of functional PEG-polyesters as drug delivery materials.

High toughness and excellent electrical performance bismaleimide resin modified by hyperbranched unsaturated polyester of flexible aliphatic side chains

Because of its flawed toughness, the scope of application of bismaleimide resin (BMI) in the field of aeronautics, industry and electricity is greatly restricted. Herein, a novel Hyperbranched unsaturated polymers (HBP) with the flexible chains grafted was incorporated to reinforce BMI resin’s toughness, where the new interpenetrating networks (IPN) structure was formed. The rich unsaturated double bond of HBP polyester could react with the BMI resin for chemical crosslinking and be used for the toughening agent. 13C-NMR and FT-IR were used to characterize HBP’s hyperbranched structure. Consequently, after adding 20 wt.% HBP, the impact strength of BMI modified by HBP increased by 211.1%. In comparison with neat BMI, the flexural strength of BMI modified by HBP was enhanced by 209.3%. It is worth noticing that HBP available in BMI resin significantly improved its electrical performance.

Imidazole-based Cu(i)-catalyzed click polymerization of diazides and diynes under mild conditions

An efficient imidazole-based Cu(i)-catalyzed azide–alkyne click polymerization under mild reaction conditions was developed.

Tandem diaza-Cope rearrangement polymerization: turning intramolecular reaction into powerful polymerization to give enantiopure materials for Zn2+ sensors

[3,3]-Sigmatropic rearrangement is a powerful reaction to form C–C bonds stereospecifically; however, owing to intrinsic simultaneous bond formation and breakage, this versatile method has not been utilized in polymerization. Herein, we report a new tandem diaza-Cope rearrangement polymerization (DCRP) that can synthesize polymers with defect-free C–C bond formation from easy and efficient imine formation. A mechanistic investigation by in situ¹H NMR experiments suggests that this polymerization proceeds by a rapid DCR process, forming an enantiospecific C–C bond that occurs almost simultaneously with imine formation. This polymerization produces not only highly stable polymers against hydrolysis due to resonance-assisted hydrogen bonds (RAHBs) but also chiral polymers containing enantiopure salen moieties, which lead to high-performance Zn²⁺-selective turn-on chemosensors with up to 73-fold amplification. We also found that their optical activities and sensing performances are heavily dependent on the reaction temperature, which significantly affects the stereoselectivity of DCR.

Herein, we report a new tandem diaza-Cope rearrangement polymerization synthesizing enantiopure polymers with defect-free C–C bond formation. Furthermore, these polymers can be applied as high-performance turn-on Zn²⁺ sensors.

One-component rapid Norrish Type II photoinitiation of bulk photo-CuAAC polymer networks

A one-component photoinitiation scheme was devised utilizing amine-centered trialkyne monomers for the formation of bulk photo-CuAAC polymer networks. The novel monomers maintain rapid polymerization kinetics and allow for tuning of the T_g.

Synthesis and direct assembly of linear–dendritic copolymers via CuAAC click polymerization-induced self-assembly (CPISA)

A one-pot method was developed for in situ preparation of linear–dendritic copolymer assemblies via click polymerization-induced self-assembly (CPISA).

Assessment of TEMPO as a Thermally Activatable Base Generator and Its Use in Initiation of Thermally-Triggered Thiol-Michael Addition Polymerizations

We present a thermally initiated thiol-Michael reaction based on initiation via the temperature-dependent thiol-TEMPO oxidation-reduction reaction. In the presence of a thiol, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO, pK _a = 5.5) is reduced to produce a much stronger base, i.e., tetramethylpiperidine (TMP, pK _a = 11.4) in a temperature dependent process. This oxidation-reduction process is dramatically accelerated at elevated temperature, which allows for thermally controlled initiation of the base-catalyzed thiol-Michael addition reaction and potentially other base-catalyzed reaction systems. Several critical factors that affect base generation from TEMPO reduction were investigated via systematic variation of reaction conditions including the solvent, temperature, and the thiol type and concentration. The highly temperature-dependent attributes of this redox reaction were demonstrated in various thiol-TEMPO based systems and were further utilized to thermally control thiol-Michael polymerizations under different heating conditions. The strong amine species, TMP, formed at elevated temperatures from the TEMPO-thiol interaction combined with high temperature, enables rapid formation of thiol-Michael-based polymer networks and large scale material preparation without any detrimental effects often associated with highly exothermic polymerizations. This novel approach to develop thermally-initiated thiol-Michael polymer networks is unique, versatile and robust, resulting in wide utility in applications such as facile handling of highly reactive resins, bulk material preparation, pH sensitive materials construction, and composite/macro-particle synthesis.

Progress on Catalytic Systems Used in Click Polymerization

Click polymerization, a powerful synthetic technique to construct polymers with unique structures and advanced functions, is of crucial importance in the areas of polymer and material sciences. A variety of click polymerizations such as azide-alkyne, thiol-yne, amino-yne, and hydroxyl-yne reactions have been established, wherein the catalytic systems play an indispensable role in realizing these highly practical reactions based on triple-bond building blocks, as they directly influence the efficiencies of the click polymerizations and the performances of the resultant polymers. The vital employment of catalysts is reviewed and their developments from innovative discoveries to the eminent position are outlined. Moreover, the challenges and perspectives in this area are also briefly discussed.

Recent advances in alkyne-based click polymerizations

The recent progress in alkyne-based click polymerizations and their application in the preparation of new functional polymers are summarized. The challenges and opportunities in this area are also briefly discussed.