Alkyne End Research Articles

A novel PEG-based hydrogel possessing disulfide bonds: synthesis via click chemistry and study on swelling behavior

ABSTRACT In this research, polyoxyethylene with alkyne end groups and two disulfide bonds in the polymer chain was synthesized through two consecutive esterification reactions from polyethylene glycol (PEG). A new PEG-based hydrogel with exceptional structural features was prepared from the polycycloaddition reaction of this macro-dialkyne with a triazidohydrin obtained from trimethylolpropane triglycidyl ether. The network structure of the hydrogel was investigated by FT-IR spectroscopy, elemental analysis, field emission-scanning electron microscopy (FE-SEM) and energy dispersive X-ray (EDX) spectroscopy. The swelling behavior of the prepared hydrogel in water was studied by drawing the graph of water uptake percentage versus time. It was found that the hydrogel has the ability to absorb water more than five times its dry weight within 12 h. The water uptake rate of the prepared hydrogel was compared with several other hydrogels with similar structure. In addition to water, the swelling behavior was also checked in a number of organic solvents with different dipole moments. The prepared hydrogel showed significant solvent uptake (slightly more than twice the dry weight) in methanol and N,N-dimethylformamide solvents.

Soft Hydroxyapatite Composites Based on Triazine-Trione Systems as Potential Biomedical Engineering Frameworks.

Composites of triazine-trione (TATO) thiol-ene networks and hydroxyapatite (HA) have shown great potential as topological fixation materials for complex bone fractures due to their high flexural modulus, biocompatibility, and insusceptibility to forming soft-tissue adhesions. However, the rigid mechanical properties of these composites make them unsuitable for applications requiring softness. The scope of these materials could therefore be widened by the design of new TATO monomers that would lead to composites with a range of mechanical properties. In this work, four novel TATO-based monomers, decorated with either ester or amide linkages as well as alkene or alkyne end groups, have been proposed and synthesized via fluoride-promoted esterification (FPE) chemistry. The ester-modified monomers were then successfully formulated along with the thiol TATO monomer tris [2-(3-mercaptopropionyloxy)ethyl] isocyanurate (TEMPIC) and HA to give soft composites, following the established photo-initiated thiol-ene coupling (TEC) or thiol-yne coupling (TYC) chemistry methodologies. The most promising composite shows excellent softness, with a flexural modulus of 57 (2) MPa and εf at maximum σf of 11.8 (0.3)%, which are 117 and 10 times softer than the previously developed system containing the commercially available tri-allyl TATO monomer (TATATO). Meanwhile, the surgically convenient viscosity of the composite resins and their excellent cytotoxicity profile allow them to be used in the construction of soft objects in a variety of shapes through drop-casting suitable for biomedical applications.

PEGylated reduced graphene oxide as nanoplatform for targeted gene and drug delivery

Combination therapies involving anticancer drugs and gene therapy agents have become a potential treatment for different cancer types reducing the side effects of anticancer drugs while enhancing their effectiveness. Here, we improved our previously published strategy that uses multiple pyrene groups of poly(ethylene glycol) (PEG) brushed graft copolymer to enhance the biological use of reduced graphene oxide (rGO) and introduced an azide containing monomer in the copolymer for further modifications to carry hydrophobic drugs like doxorubicin (Dox) along with oligonucleotides such as small interfering RNAs (siRNA). Azide functionality on the side chain of coated copolymer enables not only the conjugation of alkyne end group functional cationic copolymer but also allows the addition of targeting moieties like EPPT1 peptide that selectively binds MUC1, an early hallmark of tumorigenesis. To achieve the outlined carrier, first, a well-defined PEG, pyrene, azide, and methyl methacrylate containing poly[(poly(ethylene glycol) methyl ether methacrylate)-co-(3-azidopropyl methacrylate)-co-(methyl methacrylate)-co-(1-pyrenemethyl methacrylate)] (P(PEGMA-co-AzPMA-co-MMA-co-PMA), PAMP) copolymer was synthesized via ATRP and through its azide group another well-defined, cationic copolymer composed of hexyl methacrylate, 2-dimethyl amino ethyl methacrylate, and N,N,N-trimethylaminoethyl methacrylate iodide (P(HMA-co-DMAEMA-co-TMAEMA) was coupled via copper-catalyzed azide-alkyne cycloaddition (CuAAC). This cationic copolymer was synthesized using a unique ATRP initiator to allow click reaction as described as well as pH-dependent degradation through its hydrazone linkage. The obtained graft copolymer (PAMP-CP) was then coated on a graphene oxide (GO) surface and GO was in situ reduced to rGO (PAMP-CP-rGO) to have an rGO-based PEGylated nanoplatform that can be highly loaded with doxorubicin and successfully condensed with anti-GAPDH siRNA. Consequently, this dual drug delivery nanoplatform showed a controlled drug release that switches between release kinetics by the change of pH and high transfection efficiency regarding targeted GAPDH protein levels. Moreover, in vitro cytotoxicity tests showed that EPPT1-targeted Dox-loaded PAMP-CP-rGO shows concentration-dependent and selective cellular toxicity against cancer cells compared to its non-targeted versions. These results demonstrated the potential of a highly adaptable, multifunctional nanoplatform for safely and selectively transporting different cargoes to the tumor site.

Glass Nanopore Detection of Copper Ions in Single Cells Based on Click Chemistry.

As a common redox metal ion pair in cells, copper ions (Cu2+/Cu+) often transform between oxidation (Cu2+) and reduction (Cu+) states. They play important roles in the redox process, so monitoring the change of intracellular copper ions helps understand the redox balance and events in cells. In this study, by self-assembling a thiolated ssDNA (with an alkyne end group) onto a gold-coated glass nanopore (G-nanopore) via the Au-S bond, an alkyne-end single-stranded DNA (ssDNA)-functionalized G-nanopore sensing platform (AG-nanopore) was developed to detect copper ions in cells. In the presence of Cu2+ or Cu+, the introduction of another ssDNA with an azide group will be ligated with an alkyne group on the functionalized nanopore via a copper-catalyzed azide-alkyne 1,3-cycloaddition (CuAAC) click reaction and hence cause the change of the rectification behavior of the AG-nanopore. The rectification ratio variation of the AG-nanopore had a good response to the intracellular copper ion concentration, and the sensing platform was further applied to the study of the relationship between intracellular oxidative stress and the value of Cu2+/Cu+.

Synthesis of a new triple-responsive biocompatible block copolymer: Self-assembled nanoparticles as potent anticancer drug delivery vehicle

There has been a continuous effort towards a synthesis of new stimuli-responsive polymer nanoparticle systems for improved cancer chemotherapy over the last decade. In this context, we have presently developed a temperature, pH, and redox-responsive amphiphilic block-copolymer capable of forming nanoparticles in the aqueous medium, targeted towards drug delivery applications. The copper-catalyzed azide-alkyne cycloaddition reaction was utilized to tie the ends of two copolymers - a thermo-responsive poly(N-isopropylacrylamide) based copolymer with an azide end group and a pH-responsive hydrophobic polymer with redox responsive disulfide bond and an alkyne end group, producing a new triple responsive amphiphilic block copolymer (PHNP) that self-assemble in water to produce nanoparticles. Upon heating above the cloud point of poly(N-isopropylacrylamide), these nanoparticles experienced further aggregation to produce larger sized particles as measured by dynamic light scattering, UV–visible spectroscopy, and scanning electron microscopy techniques. PHNP was found to be capable of encapsulating drugs like doxorubicin (DOX) and also fluorescent probes alike Nile Red. The drug release kinetics divulged that in a period of 24 h more than 90% of the encapsulated DOX was released in pH 5.4 buffer having 10 mM glutathione (GSH) at 40 °C, an environment prevailing in cancer tissues. In vitro studies including live-dead assay and rhodamine-DAPI staining showed that PHNP was non-cytotoxic. DOX-loaded PHNP was observed to be more effective in prohibiting bone cancer cell (MG63) line in comparison to free DOX, demonstrated by the significant reduction of IC50 values. The uptake studies showed that DOX-encapsulated PHNP was more effective for morphometric distortion of MG63 cells in comparison to bare DOX. Therefore, the present research on the development of a biocompatible thermal, pH, and redox-responsive polymer opens up new opportunities in the area of polymeric carrier systems for drug delivery to cancer cells.

Linear Amphiphilic Polyglycidol/Poly(ε-caprolactone) Block Copolymers Prepared via “Click” Chemistry-Based Concept

We introduce a “click” chemistry-based concept for the preparation of amphiphilic block copolymers comprising linear polyglycidol (PG). Herein, a new class of linear polyglycidol/poly(e-caprolactone) (PG/PCL) block copolymers of precisely designed macromolecular characteristics and composition was synthesized to demonstrate the convenience of this strategy. Monohydroxyl poly(ethoxyethylglycidyl ether) (PEEGE) precursors were synthesized by ring-opening anionic polymerization of ethoxyethylglycidyl ether (EEGE) and subsequently functionalized with a “clickable” alkyne end group. In parallel, a bifunctional PCL diol was modified to an azide-terminated macroreagent. PG/PCL block copolymers were obtained by “click” coupling reaction of the alkyn- and azide-functional macroreagents in the presence of a CuBr/PMDETA catalytic complex and subsequent cleavage of the protective ethoxyethyl groups of PEEGE. The amphiphilic block copolymers were not directly soluble in water, and defined nano-sized micelles were obta...

P‐TSA‐Based DESs as “Active Green Solvents” for Microwave Enhanced Cyclization of 2‐Alkynyl‐(hetero)‐arylcarboxylates: an Alternative Access to 6‐Substituted 3,4‐Fused 2‐Pyranones

In this paper, we describe the use of p‐TSA based Deep Eutectic Solvents (DESs) as alternative environmental‐friendly “active” solvents for the microwave‐mediated synthesis of 6‐substituted 3,4‐fused 2‐pyranones, and in particular isocoumarins, starting from 2‐alkynyl‐(hetero)arylcarboxylates. When the alkyne terminus bears a neutral or an electron‐donating group (EDG), the reactions are fast, clean and highly regioselective, to give the 6‐endo‐dig cyclization products in good to excellent yields. For substrates bearing an electron‐withdrawing group (EWG) on the alkyne end, the regioselectivity can be tuned by adding a small amount of silver(I) triflate as co‐catalyst. DES was demonstrated to be reusable without loss of efficiency in terms of reaction yields. Based on experimental evidence and previous findings, two competitive mechanisms working simultaneously are proposed to explain the outcomes and the regioselectivity issues.

Crystallization in PEG networks: The importance of network topology and chain tilt in crystals

We synthesized poly(ethylene glycol) (PEG) networks by Cu(I) catalyzed azide-alkyne cycloaddition (CuAAC) using three-arm star-PEG with azide end groups and linear PEG oligomers of different molar masses with alkyne end groups. After extensive characterization of the networks by 13C MAS and 1H multiple-quantum (MQ) NMR spectroscopy, their crystallization behavior is studied by differential scanning calorimetry (DSC), temperature dependent small and wide angle X-ray scattering (SAXS, WAXS) as well as 1H FID NMR. Since CuAAC introduces 1,4-disubstituted 1,2,3-triazole (TR) rings as chain defects into the network, we compare our results with those obtained from linear PEG which contained a purposely introduced TR ring in the middle of the polymer chain. PEG network chains are able to form the characteristic 72 helices upon cooling to −20 °C and crystallize into the monoclinic unit cell in complete analogy to the homopolymer. The crystallinity of the networks, though it is lower as compared to the linear samples, reached values of about 30–50%. Given the fact that the network chains are highly constrained in their configurational freedom, this is very surprising. However, cross-links and the TR defects have only a minor effect on the molecular dynamics of the PEG chains in the crystals since the characteristic αc-relaxation process (helix jumps) is also detected in the PEG networks by analysis of the 1H FIDs. The SAXS data reveal that randomly arranged stacks of lamellae are formed upon crystallization and the chains are tilted in the crystals at large angles of about 40°–55°. Supported by all experimental data, we propose a model to explain the formation of polymer crystals in the networks and concluded that primary and secondary loops which extent the network chains beyond the length of the linear precursors play an essential role for the crystallization.

A multimeric MRI contrast agent based on a closo-borane scaffold bearing modified AAZTA chelates on the periphery.

A multimeric MRI contrast agent based on the closo-borane motif is reported. Twelve copies of a modified AAZTA chelate with an alkyne end group are appended on an azide-functionalized closo-borane motif using Cu(i) catalyzed click chemistry. The presence of two water molecules on the Gd-bound AAZTA chelate results in high relaxivity for the closomer in vitro/in vivo.

Copper (50 MeV) and Oxygen (84 MeV) Ion Irradiation Probed Thermal, Structural and Optical Behavior of Polyethylene Terephthalate

Polyethylene terephthalate (PET) films of 50 μm thickness were exposed to swift-heavy 50 MeV copper and 84 MeV oxygen ions, with fluence varying from 1 × 1011 to 1 × 1013 ions cm−2. Differential scanning calorimetry (DSC), X-ray diffraction (XRD) and Fourier-transform infrared (FTIR) and UV-visible spectroscopic techniques were used to characterize the irradiated samples. The XRD and DSC analysis indicated the loss of crystallinity during/after the irradiation. The FTIR spectra revealed the formation of alkyne end groups, CC groups and structural deformation with increase of ion fluence. Considerable reduction in the value of the optical bandgap was inferred from the study of the UV-visible absorption curves.

Synthesis, Surface Grafting, and Fabrication of Ultrathin Polymeric SAMFETs with High Field-Effect Mobility.

Densely surface-grafted monolayer (3-4 nm) poly(3-hexylthiophene) (P3HT) brushes are prepared by click chemistry. For this, P3HT chains with alkyne end groups were synthesized and chemically coupled to a surface-immobilized self-assembled monolayer (SAM) having azide functionality in an organic field-effect transistor channel. The grafted P3HT-alkyne with a molecular weight of Mn,MALDI = 11 400 g mol-1 ( Mn,SEC = 17 400 g mol-1) and a narrow distribution of Đ = 1.15, has the highest reported molecular weight for surface-immobilized P3HT brushes. We show the successful grafting of P3HT on the substrate surface with atomic force microscopy, contact angle, and absorption studies. From the film thickness, we can calculate the reduced tethered densities of ∑ = 10.3-12.1, which is indicative of the monolayers being in the true brush regime with high grafting density that is enough to form a compact self-assembled monolayer. The aggregation behavior of the films is characterized by UV-vis spectroscopy and compared to linear P3HT and a bottlebrush copolymer polystyrene- g-P3HT (PS- g-P3HT) with similar P3HT lengths. For such an SAM-based organic field-effect transistor (SAMFET) nanodevice with an ultrathin P3HT layer of 3-4 nm, a very high field-effect mobility of up to 1.8 × 10-3 cm2 V-1 s-1 is achieved in channel lengths of 5-20 μm, which is nearly 2 orders of magnitude higher than reported values for polymer-based SAMFETs.

Cyclic Multiblock Copolymers via Combination of Iterative Cu(0)-Mediated Radical Polymerization and Cu(I)-Catalyzed Azide-Alkyne Cycloaddition Reaction.

Cyclic multiblock polymers with high-order blocks are synthesized via the combination of single-electron transfer living radical polymerization (SET-LRP) and copper-catalyzed azide-alkyne cycloaddition (CuAAC). The linear α,ω-telechelic multiblock copolymer is prepared via SET-LRP by sequential addition of different monomers. The SET-LRP approach allows well control of the block length and sequence as A-B-C-D-E, etc. The CuAAC is then performed to intramolecularly couple the azide and alkyne end groups of the linear copolymer and produce the corresponding cyclic copolymer. The block sequence and the cyclic topology of the resultant cyclic copolymer are confirmed by the characterization of 1 H nuclear magnetic resonance spectroscopy, gel permeation chromatography, Fourier transform infrared spectroscopy, and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry.

Silicon–SAM–AuNP electrodes: Electrochemical “switching” and stability

The immobilization of gold nanoparticles (AuNPs) on functionalized electrodes has been shown to be an effective way for switching on the electrochemistry at an otherwise passivated surface. Herein, the attachment of AuNPs onto the industrially available p+- Si(100) electrodes was studied by means of cyclic voltammetry and X-ray photoelectron spectroscopy. The hydrosilylation of p+-Si(100) with 1,8-nonadiyne followed by the “click” of azido propylamine onto the alkyne end formed a self-assembled monolayer (SAM), which protected the p+-Si(100) against oxidation and prevented the electron transfer from dissolved Ru(NH3)63+. Upon the attachment of AuNPs, however, electron transfer was initially restored. The performance of the p+–Si–SAM–AuNP electrode was found to decay with repeated voltammetric cycling due to the formation of SiOx underneath the SAM. These results suggest that p+–Si–SAM–AuNP electrodes can be used in electrochemical devices for single use applications but are not suitable for long-term use.

Induced Electronic and Thermal Effects of 86 MeV Nickel Ions in Bisphenol-A Polycarbonate

ABSTRACTBisphenol-A polycarbonate films were irradiated with 86 MeV swift heavy nickel ions at varying fluences, ranging from 1 × 1011 to 1 × 1013 ions cm−2, under vacuum at room temperature, to analyze the induced electrical and thermal modifications. AC conductivity measurements and UV-visible spectroscopy, Fourier transform infra-red (FTIR) spectroscopy, thermo-gravimetric analysis (TGA), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) techniques were applied to analyze the changes. A significant, exponential increase in conductivity at higher frequency was observed with the increase of nickel ion fluence. UV-visible analysis corroborated the results of the AC conductivity measurement, revealing the increase in size of the carbon clusters embedded in the polymer network, with the increase of heavy ion fluence. FTIR analysis revealed the formation of alkene and alkyne end groups at higher doses, which further supported the suggestion that the variation in electrical properties induced by the ion irradiation of the polymer was due to development of a carbonaceous phase inside the polymer due to the irradiation. Thermal analysis, i.e., TGA and DSC patterns, showed that chain-scission was the leading phenomena in the heavy ion-irradiated polycarbonate samples, resulting in degradation of their thermal stability.

NMR and mass spectral analysis of step-growth polymers from azide alkyne cycloaddition and regioselectivity afforded by copper(I) and ruthenium(II) catalysts

ABSTRACTAzide alkyne cycloaddition was applied to step growth polymerization of the diazido monomer, di(3-azido-2-hydroxypropyl) ether of bisphenol-A (DAHP-BPA) with either tetraethyleneglycol dipropargyl ether (TEGDPE) or tetraethyleneglycol dipropiolate (TEGDP). Polymerizations were conducted without catalyst and in the presence of Cu(I) or Ru(II) complex. The resulting oligomers and polymers were characterized using 1H- and 13C-NMR spectroscopy and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS), with an emphasis on the relative abundance of 1,4- vs. 1,5-disubstituted regioisomeric 1,2,3-triazoles. Uncatalyzed reaction of DAHP-BPA/TEGDPE at 70°C yielded a 55/45 mixture of 1,4/1,5-disubstituted triazoles; reaction was slow and residual alkyne end groups were observed, suggesting attritional loss of azide. Catalyzed with Cu(PPh3)3Br, the same system yielded 93/7, 1,4/1,5-disubstituted triazoles, and few residual end groups were detected, consistent with higher molecular weight and controlled 1:1 depletion of azide and alkyne. Cp*RuCl(COD) catalyst was not soluble in the bulk system, necessitating solution polymerization in THF. Ru(II) yielded 6/94, 1,4/1,5-disubstituted triazoles, and MALDI-TOF-MS showed an end group composition similar to that observed with Cu(I). Uncatalyzed reaction of the DAHP-BPA/TEGDP system, involving the more reactive propiolate, yielded a high proportion (85%) of 1,4-disubstituted triazole linkages, and MALDI-TOF-MS revealed a controlled 1:1 depletion of azide and alkyne groups.

Poly(3-hydroxyalkanoate) sulfonate: From nanoparticles toward water soluble polyesters

A series of amphiphilic biodegradable polyesters based on short chain lengths, scl-PHAs, or unsaturated medium chain lengths poly(3-hydroxyalkanoate)s, mcl-PHA, bearing sulfonate groups (SO3−) have been synthesized using click chemistry. First, well-defined PHAs oligomers containing alkene or alkyne end groups were prepared by transesterification from natural polyesters using allyl alcohol or propargyl alcohol and by acid-catalyzed methanolysis in the case of unsaturated mcl-PHAs containing vinyl groups on the side chains. Radical mediated thiol–ene or thiol–yne reactions were then used to functionalize all PHAs oligomers using 3-mercapto-1-ethanesulfonate under photochemical initiation. This straightforward and highly efficient modification was confirmed by 1H NMR spectroscopy. The influence of the nature of PHAs and the molar composition of the ionic oligoesters on the self-assembled behavior in aqueous media has been studied. When the molar ratio of sulfonate groups is about 27% the ionic polyester is totally soluble in water for molar masses ranging from 5000 to 223,000gmol−1. Under 27% of sulfonate groups content, particles were obtained with different diameters (118–1746nm), depending on the nature of polyester, and short or medium chain lengths PHAs.

Facile Synthesis of Rod–Coil Block Copolymers with Chiral, Helical Polycarbodiimide Segments via Postpolymerization CuAAC “Click” Coupling of Functional End Groups

Using the living nickel(II)-mediated polymerization of carbodiimides, the chiral (R)- or (S)-N-1-phenethyl-N′-methylcarbodiimide (PMC) monomers were polymerized with a new TIPS protected alkyne functional nickel initiator forming PPMC with an excess single-handed screw sense and the alkyne moiety covalently attached to the terminus of the polymer, as confirmed by 1H NMR and MALDI-TOF MS. After deprotection, the alkyne end groups of rigid-rod PPMC-2 were coupled with azide-terminated, random-coil PS and PEG homopolymers forming a novel class of helical-b-coil block copolymers. In the thin-film, all synthesized diblock copolymers formed interesting nanofibular morphologies when subject to specific conditions. The triblock RCP-4, however, adopted unique macroporous morphology as identified by AFM and SEM with average pore diameters of ca. 832 ± 194 nm. The origin of this was found to be associated with the ordering of large, hollow vesicle aggregates upon solvent evaporation followed by the melting of these aggregates filling in the hollow interior forming the submicron pores observed. Furthermore, the size of these aggregates can be easily modulated in a linear fashion from 272 to 1648 nm simply by increasing the concentration of RCP-4 in THF. Finally, the three PPMC–PEG copolymers synthesized were found to adopt lyotropic cholesteric mesophases in concentrated toluene solutions (ca. 30 wt %).

Nitric oxide-releasing polymeric furoxan conjugates

Polymeric nitric oxide (NO) donors were prepared by the conjugation of the azide-containing furoxans and poly(ethylene glycol) having an alkyne end groupviathe copper-catalyzed Huisgen cycloaddition.

Azide and Alkyne Terminated Polybutadiene Binders: Synthesis, Cross-linking, and Propellant Studies

In composite solid propellants, the fuel and oxidizer are held together by a polymer binder. Among the different types of polymeric binders used in solid propellants, hydroxyl terminated polybutadiene (HTPB) is considered as the most versatile. HTPB is conventionally cured using isocyanates to form polyurethanes. However, the incompatibility of isocyanates with energetic oxidizers such as ammonium dinitramide and hydrazinium nitroformate, the short “pot life” of the propellant slurry, and undesirable side reactions with moisture are limiting factors which adversely affect the mechanical properties of HTPB based propellant. With an aim of resolving these problems, HTPB was chemically transformed to azidoethoxy carbonyl amine terminated polybutadiene and propargyloxy carbonyl amine terminated polybutadiene by adopting appropriate synthesis strategies and characterizing them by spectroscopic and chromatographic techniques. This is the first report on 1,3-dipolar addition reaction involving azide and alkyne end groups for cross-linking HTPB. The blend of these two polymers underwent curing under mild temperature (60 °C) conditions through 1,3-dipolar cycloaddition reaction resulting in triazole–triazoline networks. The curing parameters were studied using differential scanning calorimetry. The kinetic parameter, viz., activation energy, was computed to be 107.6 kJ/mol, the preexponential factor was 2.79 × 1012 s–1, and the rate constant at 60 °C was computed to be 3.64 × 10–5 s–1.The cure profile at a given temperature was predicted using the kinetic parameters. Rheological studies revealed that the gel time for curing through the 1,3-dipolar addition is 280 min compared to 120 min for curing through the urethane route. The mechanical properties of the resultant cured polybutadiene network were superior to those of polyurethanes. The cured triazoline–triazole polymer network exhibited biphasic morphology with two glass transitions (Tg) at −56 and 42 °C in contrast to the polyurethane which exhibited a single transition at −60 °C. This was corroborated by associated morphological changes observed by scanning probe microscopy. The propellant processed using this binder has the advantages of improved pot life as indicated by the end of the mix viscosity which is 165 Pa·s as compared with 352 Pa·s for the polyurethane system along with a slow build-up rate. The mechanical properties of the propellant are superior to polyurethane with an improvement of 14% in tensile strength, 22% enhancement in elongation at break, and 12% in modulus.

A Multisegmented Polystyrene with pH-Cleavable Linkages

A multisegmented polystyrene (PS) with pH-cleavable ester and carbamate linkages was successfully synthesized by a combination of atom transfer radical polymerization (ATRP) and Cu(I)-catalyzed 1,3-dipolar cycloaddition of azide and alkynes (click chemistry). ATRP was employed to synthesize polystyrene from hydroxyl-terminated initiator using CuBr/N,N,N',N'',N''-pentamethyldiethylenetriamine (PMDETA) as the catalyst. The reaction of the resulting PS with sodium azide yielded the azido-terminated polymer. The hydroxyl group in the other end of the polymer was reacted with 4-nitrophenyl chloroformate (NPC), followed by reaction with propargylamine to produce an alkyne end group with a carbamate linkage. The PS with an alkyne group in one end and an azide group in the other end was then self-coupled in the presence of CuBr/2,2'bipyridyl (bpy) in DMF to yield a desired multisegmented PS. Molecular weight and molecular weight distribution of the self-coupled polymer increased with time, as in the typical step-growth-type polymerization processes. Finally, we demonstrated that the ester and carbamate linkages of the multisegmented PS were hydrolyzed in the presence of HCl to yield individual PS chains.