Thiol–Ene Click Chemistry: Computational and Kinetic Analysis of the Influence of Alkene Functionality

Kinetic Model under Light-Limited Condition for Photoinitiated Thiol-Ene Coupling Reactions

Thiol-ene click chemistry has become a powerful paradigm in synthesis, materials science, and surface modification in the past decade. In the photoinitiated thiol-ene reaction, an induction period is often observed before the major change in its kinetic curve, for which a possible mechanism is proposed in this report. Briefly, light soaking generates radicals following the zeroth-order reaction kinetics. The radical is the reactant that initializes the chain reaction of thiol-ene coupling, which is a first-order reaction. Combining both and under the light-limited conditions, a surprising kinetics represented by a Gaussian-like model evolves that is different from the exponential model used to describe the first-order reaction of the final product. The experimental data are fitted well with the new model, and the reaction kinetic constants can be pulled out from the fitting.

Covalent inhibitors of EGFR family protein kinases induce degradation of human Tribbles 2 (TRIB2) pseudokinase in cancer cells

A major challenge associated with biochemical and cellular analysis of pseudokinases is a lack of target-validated small-molecule compounds with which to probe function. Tribbles 2 (TRIB2) is a cancer-associated pseudokinase with a diverse interactome, including the canonical AKT signaling module. There is substantial evidence that human TRIB2 promotes survival and drug resistance in solid tumors and blood cancers and therefore is of interest as a therapeutic target. The unusual TRIB2 pseudokinase domain contains a unique cysteine-rich C-helix and interacts with a conserved peptide motif in its own carboxyl-terminal tail, which also supports its interaction with E3 ubiquitin ligases. We found that TRIB2 is a target of previously described small-molecule protein kinase inhibitors, which were originally designed to inhibit the canonical kinase domains of epidermal growth factor receptor tyrosine kinase family members. Using a thermal shift assay, we discovered TRIB2-binding compounds within the Published Kinase Inhibitor Set (PKIS) and used a drug repurposing approach to classify compounds that either stabilized or destabilized TRIB2 in vitro. TRIB2 destabilizing agents, including the covalent drug afatinib, led to rapid TRIB2 degradation in human AML cancer cells, eliciting tractable effects on signaling and survival. Our data reveal new drug leads for the development of TRIB2-degrading compounds, which will also be invaluable for unraveling the cellular mechanisms of TRIB2-based signaling. Our study highlights that small molecule-induced protein down-regulation through drug "off-targets" might be relevant for other inhibitors that serendipitously target pseudokinases.

In situ-forming hyaluronic acid hydrogel through visible light-induced thiol-ene reaction

Here we present hyaluronic acid (HA) hydrogels crosslinked via thiol-ene reaction initiated by visible blue light exposure in the presence of riboflavin phosphate (RFP). The gelation procedure is rapid and proceeds as effectively with exposure to blue light as it does with UV light. We successfully initiated the thiol-ene reaction by RFP with blue light, which triggered gelation that proceeds over about 5 min at 36 °C after an initial small change in modulus upon light exposure. Gel transparency was also evaluated, and the HA gel exhibited over 80% transmittance in the visible spectrum. The degradation and protein release kinetics of the photo-crosslinked HA hydrogel are also presented. The capacity of blue light to initiate thiol-ene reaction was equal to or more effective than UV light of the same energy. The cytocompatibility of hydrogels was evaluated using corneal fibroblasts, and the light-induced fabrication procedure and resultant gel materials did not affect cell viability. The results indicate that an RFP-based, BL-initiated photo-reaction to gelate HA may be an effective and promising modality for applications where in situ gelation is desired.

Why Not Trans? Inhibited Radical Isomerization Cycles and Coupling Chains of Lipids and Alkenes with Alkane -thiols

Reversible addition of thiyl radicals to cis fatty acids converts them into trans fatty acids, L _Z + S^• ⇄ SL^• ⇄ L _E + S^•, in a cycle that, uninterrupted, would rapidly isomerize lipids exposed to radicals and thiols. One reason this does not happen in foods and organisms is because the cycle is interrupted, by exothermic allylic abstraction, L + S^• → L^• + SH. Autoinhibition limits the cis-trans cycle length to around 400-500 (L _E per S^•) in a MUFA model (methyl oleate) and just ∼13-15 in a PUFA lipid model (methyl linoleate). The weak C-H bonds in bisallylic groups in PUFAs thereby act as the first line of defense against thiyl cis-trans cycles in biolipid solutions (±O₂). With the intriguing exception of vitamin E in MUFA, thiyl-active antioxidants inhibit isomerization in much the same way as they protect against peroxidation. Applied to thiol-ene coupling (TEC), the allylic abstraction, degraded-chain paradigm resolved a raft of hitherto contradictory trends and findings in "click" TEC polymerization and organic synthesis methods.

Effect of Selected Thiols on Cross-Linking of Acrylated Epoxidized Soybean Oil and Properties of Resulting Polymers

The effect of the chemical structure and functionality of three structurally different thiols on the cross-linking of acrylated epoxidized soybean oil and on the properties of the resulting polymers was investigated in this study. 1,3-Benzenedithiol, pentaerythritol tetra(3-mercaptopropionate), and an hexathiol synthesized from squalene were used in the cross-linking of acrylated epoxidized soybean oil by thiol⁻Michael addition reaction. The reactivity of thiols determined from calorimetric curves followed the order: 1,3-benzenedithiol > pentaerythritol tetra(3-mercaptopropionate) > hexathiolated squalene. Thermal and mechanical properties and the swelling in different solvents of the cross-linked polymers were studied. The cross-linked polymer obtained from 1,3-benzenedithiol showed the highest swelling values in chloroform and toluene. The cross-linked polymer with pentaerythritol tetra(3-mercaptopropionate) fragments showed the best mechanical performance (highest mechanical strength and Young's modulus) and thermal stability. The cross-linked polymers from hexathiolated squalene showed the highest glass transition temperature.

Control of thiol-maleimide reaction kinetics in PEG hydrogel networks

Michael-type addition reactions are widely used to polymerize biocompatible hydrogels. The thiol-maleimide modality achieves the highest macromer coupling efficiency of the reported Michael-type pairs, but the resulting hydrogel networks are heterogeneous because polymerization is faster than the individual components can be manually mixed. The reactivity of the thiol dictates the overall reaction speed, which can be slowed in organic solvents and acidic buffers. Since these modifications also reduce the biocompatibility of resulting hydrogels, we investigated a series of biocompatible buffers and crosslinkers to decelerate gelation while maintaining high cell viability. We found that lowering the polymer weight percentage (wt%), buffer concentration, and pH slowed gelation kinetics, but crosslinking with an electronegative peptide was optimal for both kinetics and cell viability. Including a high glucose medium supplement in the polymer solvent buffer improved the viability of the cells being encapsulated without impacting gelation time. Slowing the speed of polymerization resulted in more uniform hydrogels, both in terms of visual inspection and the diffusion of small molecules through the network. However, reactions that were too slow resulted in non-uniform particle dispersion due to settling, thus there is a trade-off in hydrogel network uniformity versus cell distribution in the hydrogels when using these networks in cell applications.

STATEMENT OF SIGNIFICANCE

The polymer network of thiol-maleimide hydrogels assembles faster than individual components can be uniformly mixed due to their fast gelation kinetics. The lack of homogeneity can result in variable cell-based assay results, resulting in batch-to-batch variability and limiting their use in predictive screening assays. Although these hydrogels are incredibly useful in tissue engineering, this network heterogeneity is a known problem in the field. We screened a variety of possible techniques to slow down the reaction speed and improve the homogeneity of these hydrogels, without sacrificing the viability and distribution of encapsulated cells. As others have reported, an electronegative crosslinker was the most effective technique to slow the reaction, but the chemical modification required is technically challenging. Of interest to a broad community, we screened buffer type, strength, and crosslinker electronegativity to find an optimal reaction speed that allows for high cell viability and small molecule diffusion, without allowing cells to settle during gelation, allowing application of these materials to the drug screening industry and tissue engineering community.

Cell-instructive pectin hydrogels crosslinked via thiol-norbornene photo-click chemistry for skin tissue engineering

Cell-instructive hydrogels are attractive for skin repair and regeneration, serving as interactive matrices to promote cell adhesion, cell-driven remodeling and de novo deposition of extracellular matrix components. This paper describes the synthesis and photocrosslinking of cell-instructive pectin hydrogels using cell-degradable peptide crosslinkers and integrin-specific adhesive ligands. Protease-degradable hydrogels obtained by photoinitiated thiol-norbornene click chemistry are rapidly formed in the presence of dermal fibroblasts, exhibit tunable properties and are capable of modulating the behavior of embedded cells, including the cell spreading, hydrogel contraction and secretion of matrix metalloproteases. Keratinocytes seeded on top of fibroblast-loaded hydrogels are able to adhere and form a compact and dense layer of epidermis, mimicking the architecture of the native skin. Thiol-ene photocrosslinkable pectin hydrogels support the in vitro formation of full-thickness skin and are thus a highly promising platform for skin tissue engineering applications, including wound healing and in vitro testing models.

STATEMENT OF SIGNIFICANCE

Photopolymerizable hydrogels are attractive for skin applications due to their unique spatiotemporal control over the hydrogel formation. This study reports the design of a promising photo-clickable pectin hydrogel which biophysical and biochemical properties can be independently tailored to control cell behavior. A fast method for the norbornene-functionalization of pectin was developed and hydrogels fabricated through UV photoinitiated thiol-norbornene chemistry. This one-pot click reaction was performed in the presence of cells using cell-adhesive and matrix metalloproteinase-sensitive peptides, yielding hydrogels that support extensive cell spreading. Keratinocytes seeded on top of the fibroblast-loaded hydrogel formed a compact epidermis with morphological resemblance to human skin. This work presents a new protease-degradable hydrogel that supports in vitro skin formation with potential for skin tissue engineering.

Ionic liquid syntheses via click chemistry: expeditious routes toward versatile functional materials

The application of click reaction (e.g. CuAAC, thiol–X, oxime formation and nucleophilic ring opening) has recently begun to draw attention for efficient and robust synthesis of new functional ionic liquids, requiring minimal purification.

Synthesis of Elastomeric Liquid Crystalline Polymer Networks via Chain Transfer

Materials capable of complex shape changes have broad reaching applications spanning biomimetic devices, componentless actuators, artificial muscles, and haptic displays. Liquid crystal elastomers (LCE) are a class of shape programmable materials which display anisotropic mechanical deformations in response external stimuli. This work details a synthetic strategy to quickly and efficiently prepare LCEs through the usage of chain transfer agents (CTA). The polyacrylate materials described herein exhibit large, reversible shape changes with strains greater 475%, rivalling properties observed in polysiloxane-based networks. The approach reported here is distinguished in that the materials chemistry is readily amenable to surface alignment techniques. The facile nature of the materials chemistry and the compatibility of these materials with directed self-assembly methods could further enable paradigm shifting end uses as designer substrates for flexible electronics or as actuating surfaces.

Thiol-Ene Clickable Gelatin: A Platform Bioink for Multiple 3D Biofabrication Technologies

Bioprinting can be defined as the art of combining materials and cells to fabricate designed, hierarchical 3D hybrid constructs. Suitable materials, so called bioinks, have to comply with challenging rheological processing demands and rapidly form a stable hydrogel postprinting in a cytocompatible manner. Gelatin is often adopted for this purpose, usually modified with (meth-)acryloyl functionalities for postfabrication curing by free radical photopolymerization, resulting in a hydrogel that is cross-linked via nondegradable polymer chains of uncontrolled length. The application of allylated gelatin (GelAGE) as a thiol-ene clickable bioink for distinct biofabrication applications is reported. Curing of this system occurs via dimerization and yields a network with flexible properties that offer a wider biofabrication window than (meth-)acryloyl chemistry, and without additional nondegradable components. An in-depth analysis of GelAGE synthesis is conducted, and standard UV-initiation is further compared with a recently described visible-light-initiator system for GelAGE hydrogel formation. It is demonstrated that GelAGE may serve as a platform bioink for several biofabrication technologies by fabricating constructs with high shape fidelity via lithography-based (digital light processing) 3D printing and extrusion-based 3D bioprinting, the latter supporting long-term viability postprinting of encapsulated chondrocytes.

Evaluation of Photochemistry Reaction Kinetics to Pattern Bioactive Proteins on Hydrogels for Biological Applications

Bioactive signals play many important roles on cell function and behavior. In most biological studies, soluble biochemical cues such as growth factors or cytokines are added directly into the media to maintain and/or manipulate cell activities in vitro. However, these methods cannot accurately mimic certain in vivo biological signaling motifs, which are often immobilized to extracellular matrix and also display spatial gradients that are critical for tissue morphology. Besides biochemical cues, biophysical properties such as substrate stiffness can influence cell behavior but is not easy to manipulate under conventional cell culturing practices. Recent development in photocrosslinkable hydrogels provides new tools that allow precise control of spatial biochemical and biophysical cues for biological applications, but doing so requires a comprehensive study on various hydrogel photochemistry kinetics to allow thorough photocrosslink reaction while maintain protein bioactivities at the same time. In this paper, we studied several photochemistry reactions and evaluate key photochemical parameters, such as photoinitiators and ultra-violet (UV) exposure times, to understand their unique contributions to undesired protein damage and cell death. Our data illustrates the retention of protein function and minimize of cell health during photoreactions requires careful selection of photoinitiator type and concentration, and UV exposure times. We also developed a robust method based on thiol-norbornene chemistry for independent control of hydrogel stiffness and spatial bioactive patterns. Overall, we highlight a class of bioactive hydrogels to stiffness control and site specific immobilized bioactive proteins/peptides for the study of cellular behavior such as cellular attraction, repulsion and stem cell fate.

Gas-Phase Photochemical Overall H2 S Splitting by UV Light Irradiation

Splitting of hydrogen sulfide is achieved to produce value-added chemicals. Upon irradiation at 254 nm in the gas phase and in the absence of catalysts or photocatalysts at near room temperature, H₂ S splits into stoichiometric amounts of H₂ and S with a quantum efficiency close to 50 %. No influence of the presence of CH₄ and CO₂ (typical components in natural gas and biogas in which H₂ S is an unwanted component) on the efficiency of overall H₂ S splitting was observed. A mechanism for the H₂ and S formation is proposed.

"Click" analytics for "click" chemistry - A simple method for calibration-free evaluation of online NMR spectra

Driven mostly by the search for chemical syntheses under biocompatible conditions, so called "click" chemistry rapidly became a growing field of research. The resulting simple one-pot reactions are so far only scarcely accompanied by an adequate optimization via comparably straightforward and robust analysis techniques possessing short set-up times. Here, we report on a fast and reliable calibration-free online NMR monitoring approach for technical mixtures. It combines a versatile fluidic system, continuous-flow measurement of ¹H spectra with a time interval of 20s per spectrum, and a robust, fully automated algorithm to interpret the obtained data. As a proof-of-concept, the thiol-ene coupling between N-boc cysteine methyl ester and allyl alcohol was conducted in a variety of non-deuterated solvents while its time-resolved behaviour was characterized with step tracer experiments. Overlapping signals in online spectra during thiol-ene coupling could be deconvoluted with a spectral model using indirect hard modeling and were subsequently converted to either molar ratios (using a calibration-free approach) or absolute concentrations (using 1-point calibration). For various solvents the kinetic constant k for pseudo-first order reaction was estimated to be 3.9h^-1 at 25°C. The obtained results were compared with direct integration of non-overlapping signals and showed good agreement with the implemented mass balance.

Expedient synthesis of the heneicosasaccharyl mannose capped arabinomannan of the Mycobacterium tuberculosis cellular envelope by glycosyl carbonate donors

The global incidence of tuberculosis is increasing at an alarming rate, and Mycobacterium tuberculosis (Mtb) is the causative agent for tuberculosis, a disease with high mortality. Lipoarabinomannan (LAM) is one of the major components of the Mtb cellular envelope and is an attractive scaffold for developing anti-tubercular drugs, vaccines and diagnostics. Herein, a highly convergent strategy is developed to synthesize heneicosasaccharyl arabinomannan for the first time. The arabinomannan synthesized in this endeavour has several 1,2-trans or α-Araf linkages and three 1,2-cis or β-Araf linkages end capped with 1,2-trans or α-Manp linkages. All the key glycosidations were performed with alkynyl carbonate glycosyl donors under [Au]/[Ag] catalysis conditions, which gave excellent yields and stereoselectivity even for the reactions between complex and branched oligosaccharides. The resultant allyl oligosaccharide was globally deprotected to obtain the heneicosasaccharyl arabinomannan as a propyl glycoside. In summary, heneicosasaccharyl mannose capped arabinomannan synthesis was achieved in 56 steps with 0.016% overall yield.

A Study on the Electro-Optical Properties of Thiol-Ene Polymer Dispersed Cholesteric Liquid Crystal (PDChLC) Films

In this study, a polymer dispersed cholesteric liquid crystal (PDChLC) film obtained via a one-step fabrication technique based on photopolymerization of a thiol-acrylate reaction system was prepared and characterized for the first time. The effects of the chiral dopant, the influence of thiol monomer functionality and content on the morphology and subsequent performance of the PDChLC films were systematically investigated. It was demonstrated that the addition of a small amount of chiral dopant slightly increased the driving voltage, but decreased the off-state transmittance significantly. Furthermore, scanning electron micrographs (SEM) shown that the liquid crystal (LC) droplet size decreased at first and then increased with the increasing amount of thiol monomer functionality, while increasing the thiol content increased the LC droplet size. Correspondingly, the electro-optical switching behavior was directly dependent on LC droplet size. By tuning the raw material composition, PDChLC film with optimized electro-optical performance was prepared.

Simultaneous Cyclization and Derivatization of Peptides Using Cyclopentenediones

Unprotected linear peptides containing N-terminal cysteines and another cysteine residue can be simultaneously cyclized and derivatized using 2,2-disubstituted cyclopentenediones. High yields of cyclic peptide conjugates may be obtained in short reaction times using only a slight excess of the cyclopentenedione moiety under TEMPO catalysis and in the presence of LiCl.

A low-temperature, photoinduced thiol–ene click reaction: a mild and efficient method for the synthesis of sugar-modified nucleosides

While studying the radical mediated hydrothiolation of nucleoside enofuranosides, an unusual temperature effect was observed by the exploitation of which various thio-substituted nucleoside analogues were produced.

Circularly polarized light triggered enantioselective thiol–ene polymerization reaction

Herein, circularly polarized light is utilized to trigger an enantioselective polymerization reaction, resulting in the synthesis of an optically active polymer from racemic monomers in the absence of any chiral dopant or catalyst.

Effect of a Polymercaptan Material on the Electro-Optical Properties of Polymer-Dispersed Liquid Crystal Films

Polymer-dispersed liquid crystal (PDLC) films were prepared by the ultraviolet-light-induced polymerization of photopolymerizable monomers in nematic liquid crystal/chiral dopant/thiol-acrylate reaction monomer composites. The effects of the chiral dopant and crosslinking agents on the electro-optical properties of the PDLC films were systematically investigate. While added the chiral dopant S811 into the PDLC films, the initial off-state transmittance of the films was decreased. It was found that the weight ratio among acrylate monomers, thiol monomer PETMP and the polymercaptan Capcure 3-800 showed great influence on the properties of the fabricated PDLC films because of the existence of competition between thiol-acrylate reaction and acrylate monomer polymerization reaction. While adding polymercaptans curing agent Capcure 3-800 with appropriate concentration into the PDLC system, lower driven voltage and higher contrast ratio were achieved. This made the polymer network and electro-optical properties of the PDLC films easily tunable by the introduction of the thiol monomers.

ChemEngine: harvesting 3D chemical structures of supplementary data from PDF files

Digital access to chemical journals resulted in a vast array of molecular information that is now available in the supplementary material files in PDF format. However, extracting this molecular information, generally from a PDF document format is a daunting task. Here we present an approach to harvest 3D molecular data from the supporting information of scientific research articles that are normally available from publisher's resources. In order to demonstrate the feasibility of extracting truly computable molecules from PDF file formats in a fast and efficient manner, we have developed a Java based application, namely ChemEngine. This program recognizes textual patterns from the supplementary data and generates standard molecular structure data (bond matrix, atomic coordinates) that can be subjected to a multitude of computational processes automatically. The methodology has been demonstrated via several case studies on different formats of coordinates data stored in supplementary information files, wherein ChemEngine selectively harvested the atomic coordinates and interpreted them as molecules with high accuracy. The reusability of extracted molecular coordinate data was demonstrated by computing Single Point Energies that were in close agreement with the original computed data provided with the articles. It is envisaged that the methodology will enable large scale conversion of molecular information from supplementary files available in the PDF format into a collection of ready- to- compute molecular data to create an automated workflow for advanced computational processes. Software along with source codes and instructions available at https://sourceforge.net/projects/chemengine/files/?source=navbar.Graphical abstract.

Nanoplasmonics tuned "click chemistry"

Nanoplasmonics is a growing field of optical condensed matter science dedicated to optical phenomena at the nanoscale level in metal systems. Extensive research on noble metallic nanoparticles (NPs) has emerged within the last two decades due to their ability to keep the optical energy concentrated in the vicinity of NPs, in particular, the ability to create optical near-field enhancement followed by heat generation. We have exploited these properties in order to induce a localised "click" reaction in the vicinity of gold nanostructures under unfavourable experimental conditions. We demonstrate that this reaction can be controlled by the plasmonic properties of the nanostructures and we propose two physical mechanisms to interpret the observed plasmonic tuning of the "click" chemistry.

A New Methodology for Assessing Macromolecular Click Reactions and Its Application to Amine--Tertiary Isocyanate Coupling for Polymer Ligation

Click reactions have provided access to an array of remarkably complex polymer architectures. However, the term "click" is often applied inaccurately to polymer ligation reactions that fail to respect the criteria that typify a true "click" reaction. With the purpose of providing a universal way to benchmark polymer-polymer coupling efficiency at equimolarity and thus evaluate the fulfilment of click criteria, we report a simple one-pot methodology involving the homodicoupling of α-end-functionalized polymers using a small-molecule bifunctional linker. A combination of SEC analysis and chromatogram deconvolution enables straightforward quantification of the coupling efficiency. We subsequently employ this methodology to evaluate an overlooked candidate for the click reaction family: the addition of primary amines to α-tertiary isocyanates (α-(t)NCO). Using our bifunctional linker coupling strategy, we show that the amine-(t)NCO reaction fulfills the criteria for a polymer-polymer click reaction, achieving rapid, chemoselective, and quantitative coupling at room temperature without generating any byproducts. We demonstrate that amine-(t)NCO coupling is faster and more efficient than the more common amine-tertiary active ester coupling under equivalent conditions. Additionally, we show that the α-(t)NCO end group is unprecedentedly stable in aqueous media. Thus, we propose that the amine-(t)NCO ligation is a powerful new click reaction for efficient macromolecular coupling.

Comparison of Thiyl, Alkoxyl, and Alkyl Radical Addition to Double Bonds: The Unusual Contrasting Behavior of Sulfur and Oxygen Radical Chemistry

High-level ab initio calculations have been used to compare prototypical thiyl, alkoxyl, and alkyl radical addition reactions. Thiyl radical addition to the sulfur center of thioketones is exothermic and rapid, occurring with negative enthalpic barriers and only weakly positive Gibbs free energy barriers. In stark contrast, alkoxyl radical addition to the oxygen center of ketones is highly endothermic and occurs with very high reaction barriers, though these are also suppressed. On the basis of analysis of the corresponding alkyl radical additions to these substrates and the corresponding reactions of these heteroatom radicals with alkenes, it suggested that addition reactions involving thiyl radicals have low intrinsic barriers because their unpaired electrons are better able to undergo stabilizing resonance interactions with the π* orbitals of the substrate in the transition state.

Photochemical radical thiol–ene click-based methodologies for silica and transition metal oxides materials chemical modification: a mini-review

The photochemical radical thiol–ene addition reaction (PRTEA) is a highly powerful synthetic technique for surface modification.

Multiblock thermoplastic elastomers via one-pot thiol–ene reaction

We report a facile approach to designing multiblock thermoplastic elastomers using a one-pot sequential thiol–ene reaction.

Modular and orthogonal synthesis of hybrid polymers and networks

Biomaterials scientists strive to develop polymeric materials with distinct chemical make-up, complex molecular architectures, robust mechanical properties and defined biological functions by drawing inspirations from biological systems. Salient features of biological designs include (1) repetitive presentation of basic motifs; and (2) efficient integration of diverse building blocks. Thus, an appealing approach to biomaterials synthesis is to combine synthetic and natural building blocks in a modular fashion employing novel chemical methods. Over the past decade, orthogonal chemistries have become powerful enabling tools for the modular synthesis of advanced biomaterials. These reactions require building blocks with complementary functionalities, occur under mild conditions in the presence of biological molecules and living cells and proceed with high yield and exceptional selectivity. These chemistries have facilitated the construction of complex polymers and networks in a step-growth fashion, allowing facile modulation of materials properties by simple variations of the building blocks. In this review, we first summarize features of several types of orthogonal chemistries. We then discuss recent progress in the synthesis of step growth linear polymers, dendrimers and networks that find application in drug delivery, 3D cell culture and tissue engineering. Overall, orthogonal reactions and modulular synthesis have not only minimized the steps needed for the desired chemical transformations but also maximized the diversity and functionality of the final products. The modular nature of the design, combined with the potential synergistic effect of the hybrid system, will likely result in novel hydrogel matrices with robust structures and defined functions.