Switchable Polymerization Organocatalysis: From Monomer Mixtures to Block Copolymers.

One-pot production of sequence-controlled block copolymer from mixed monomers is a crucial but rarely reached goal. Using a switchable Lewis-pair organocatalyst, we have accomplished sequence-selective polymerization from a mixture of O-carboxyanhydride (OCA) and epoxide. Polymerization of the OCA monomer occurs first and exclusively because of its exceedingly high polymerizability. When OCA is fully consumed, alternating copolymerization of epoxide and CO2 liberated in OCA polymerization is triggered from the termini of the first block. The two polymerizations thus occur in tandem, both in chemoselective fashion, so that a sequence-controlled block polymer with up to 99 % CO2 conversion is furnished in this one-pot protocol. Calculations and experimental results demonstrate a chemoselective and cooperative mechanism, where the high polymerizability of the OCA monomers guarantees exquisite sequence selectivity and the cooperative decarboxylation partly arose from the stabilization effect by triethylborane, which facilitates the smooth transformation of the chain end from carbonate to alkoxide.

Synthesis of Y-Shaped OEGylated Poly(amino acid)s: The Impact of OEG Architecture.

OEGylation is an attractive approach to modifying poly(amino acid)s. OEG conjugation improves water-solubility of poly(amino acid)s, and confers possible thermal-responsive functionality for the conjugated poly(amino acid)s. Nevertheless, the impact of OEG architecture and the manner in which the OEG moiety interferes with the performances of poly(amino acid)s remain a work in progress. In this study, a series of new linear and Y-shaped OEG-substituted poly(glutamic acid)s were designed and synthesized. It is found that the thermoresponsive behavior of OEGylated poly(glutamic acid)s experiences steric repulsion effect, the strengths of which are architecture and length-dependent, and grows pronounced only when the number of the OEG units is ≥6. Notably, the Y-shaped architecture is able to stabilize the helicity of poly(glutamic acid) backbones, while maintaining higher α-helical conformation than its linear counterparts. In sum, our result indicate that Y-shaped architecture is more appropriate toward OEGylating poly(amino acid)s for biomedical applications.

A Multifunctional Polypeptide via Ugi Reaction for Compact and Biocompatible Quantum Dots with Efficient Bioconjugation.

The growing application of quantum dots (QDs) in biomedical research necessitates, in turn, continuous development of surface functionalizing ligands to optimize their performance for ever more challenging and diverse biological applications. Here, we demonstrate the novel multifunctional polypeptide ligands for compact and biocompatible QDs. The target ligand preparation exploits the efficient, activating agent-free Ugi reaction of four functional components to incorporate lipoic acid, pyridine, zwitterion motifs, and reactive functionalities in a one-pot procedure under mild conditions. Cap exchange with these multifunctional polypeptide ligands generates hydrophilic QD dispersions, which are colloidally stable for prolonged periods of time. The zwitterionic ligation delivers compact and small QDs, and the existence of reactive functionalities enables coupling of the QDs to biologics through bio-orthogonal coupling chemistry, such as ligation of azide-modified QDs to DNA. Therefore, this QD functionalization strategy via Ugi reaction is believed to be a viable approach for compact and biocompatible QDs with efficient bioconjugation.

Multidentate Comb-Shaped Polypeptides Bearing Trithiocarbonate Functionality: Synthesis and Application for Water-Soluble Quantum Dots.

We describe here the synthesis of multidentate comb-shaped polypeptides bearing trithiocarbonate functionality and their application in the preparation of water-soluble quantum dots (QDs). A new l-lysine-based N-carboxyanhydride monomer containing trithiocarbonate functionality was designed and synthesized. Ring-opening polymerization of the resulting monomer initiated by hexamethyldisilazane affords polypeptides bearing pendent trithiocarbonate groups (P(TTCLys)) with controllable molecular weights. P(TTCLys) was then applied to mediate the reversible addition-fragmentation chain-transfer polymerization of oligo(ethylene glycol)acrylate for the metal-free preparation of hydrophilic comb-shaped polypeptides. Simple reduction of trithiocarbonate functionality enabled the introduction of multiple thiol anchoring groups to the above-mentioned comb-shaped polypeptides. Finally, the obtained multidentate polypeptide-based ligands were successfully applied in the ligand exchange procedures to generate water-soluble QDs. The fluorescent microscopic images suggested that the resultant water-soluble QDs could be effectively internalized into HeLa cells to realize bright cellular imaging. Therefore, our work can result in a new kind of valuable polypeptide-based QDs with hydrophilic character and biocompatibility for cellular imaging.

Construction of Well-Defined Redox-Responsive CO2 -Based Polycarbonates: Combination of Immortal Copolymerization and Prereaction Approach.

Due to the axial group initiation in traditional (salen)CoX/quaternary ammonium catalyst system, it is difficult to construct single active center propagating polycarbonates for copolymerization of CO2 /epoxides. Here a redox-responsive poly(vinyl cyclohexene carbonate) (PVCHC) with detachable disulfide-bond backbone is synthesized in a controllable manner using (salen)CoTFA/[bis(triphenylphosphine)iminium, [PPN]TFA binary catalyst, where the axial group initiation is depressed by judiciously choosing 3,3'-dithiodipropionic acid as starter. While for those comonomers failing to obtain polycarbonate with unimodal gel permeation chromatography (GPC) curve, a versatile method is developed by combination of immortal copolymerization and prereaction approach, and functional aliphatic polycarbonates having well-defined architecture and narrow polydispersity can be prepared. The resulting PVCHC can be further functionalized with alkenes by versatile cross-metathesis reaction to tune the physicochemical properties. The combination of immortal polymerization and prereaction approach creates a powerful platform for controllable synthesis of functional CO2 -based polycarbonates.

In vitro studies on regulation of osteogenic activities by electrical stimulus on biodegradable electroactive polyelectrolyte multilayers.

In this study, a novel electroactive tetreaniline-containing degradable polyelectrolyte multilayer film (PEM) coating [(poly(l-glutamic acid)-graft-tetreaniline/poly(l-lysine)-graft-tetreaniline)n, (PGA-g-TA/PLL-g-TA)n] was designed and fabricated by layer-by-layer (LbL) assembly method. Compared with the nongrafted PEMs, the tetreaniline-grafted PEMs showed higher roughness and stiffness in micro/nanoscale structures. The special surface characteristics and the typical electroconductive properties were more beneficial for adhesion, proliferation, and differentiation of preosteoblast MC3T3-E1 cells. Moreover, the enhanced effects were observed on the modulation of MC3T3-E1 cells that differentiated into maturing osteoblasts, when the electroactive PEMs were coupled with electrical stimulus (ES), especially in the early phase of the osteoblast differentiation. The alkaline phosphatase (ALP) activity, calcium deposition, immunofluorescence staining, and RT-qPCR were evaluated on the differentiation of preosteoblast. These data indicate that the comprehensive effects through coupling electroactive scaffolds with electrical stimulus are better to develop bioelectric strategies to control cell functions for bone regeneration.

Synthesis of biodegradable and electroactive tetraaniline grafted poly(ester amide) copolymers for bone tissue engineering.

Biodegradable poly(ester amide)s have recently been used as biomaterials due to their desirable chemical and biological characteristics as well as their mechanical properties, which are amendable for material processing. In this study, electroactive tetraaniline (TA) grafted poly(ester amide)s were successfully synthesized and characterized. The poly(ester amide)s-graft-tetraaniline copolymers (PEA-g-TA) exhibited good electroactivity, mechanical properties, and biodegradability. The biocompatibility of the PEA-g-TA copolymers in vitro was systematically studied, which demonstrated that they were nontoxic and led to favorable adhesion and proliferation of mouse preosteoblastic MC3T3-E1 cells. Moreover, the PEA-g-TA copolymers stimulated by pulsed electrical signal could serve to promote the differentiation of MC3T3-E1 cells compared with TCPs. Hence, the biodegradable and electroactive PEA-g-TA copolymers possessed the properties in favor of the long-time potential application in vivo (electrical stimulation directly to the desired area) as bone repair scaffold materials in tissue engineering.

Rheological behaviors and miscibility of mixture solution of polyaniline and cellulose dissolved in an aqueous system.

In our previous work, supramolecular films composed of hydrophilic cellulose and hydrophobic polyaniline (PANI) dissolved in NaOH/urea aqueous solution at low temperature through rearrangement of hydrogen bonds have been constructed. To further understand the miscibility and processability of the complex solution, the dynamic rheological behaviors of the PANI/cellulose complex solution were investigated, for the first time, in the present work. Transmission electron microscope (TEM) results demonstrated that the inclusion complexes consisted of PANI and cellulose, existed in the aqueous solution, showing a good miscibility. Time-temperatures superposition (tTs) results indicated that the PANI/cellulose solution exhibited a homogeneous system, and the complex solution was more stable than the cellulose solution in the temperature range from 5 to 25 °C. Winter-Chambon theory was proved to be capable of describing the gelation behavior of the PANI/cellulose complex solution. The relaxation exponent at the gel point was calculated to be 0.74, lower than the cellulose solution, indicating strong interactions between PANI and cellulose chains. Relatively larger flow activation energy of the PANI/cellulose solution suggested the formation and rupture of linkages in "junction zones" during the gelation processes. Furthermore, PANI/cellulose gels could form at elevated temperature as a result of the physical cross-linking and chain entanglement, and it was a thermoirreversible process. Moreover, the PANI/cellulose solution remained a liquid state for a long time at the temperature range from 0 to 8 °C, which is important for the industry process.

Organocatalytic Copolymerization of Cyclic Lysine Derivative and ε-Caprolactam toward Antibacterial Nylon-6 Polymers.

Functional polymers of nylon-6, particularly those with sustained antibacterial functions, have many practical applications. However, the development of functional ε-caprolactam monomers for the subsequent ring-opening copolymerization (ROCOP) formation of these materials remains a challenge. Here we report a t-BuP4-mediated ROCOP of dimethyl-protected cyclic lysine with ε-caprolactam, followed by quaternization, affording antibacterial nylon-6 polymers bearing quaternary ammonium functionality with high molecular weight (up to 77.4 kDa). The antibacterial nylon-6 polymers exhibited good physical and mechanical properties and strong antimicrobial activities. At 25 mol % quaternary ammonium group incorporation, the nylon-6 polymer demonstrated complete killing of Staphylococcus aureus (Gram-positive) and Escherichia coli (Gram-negative). The results from this study may provide a strategy for the facile preparation of antibacterial nylon-6 polymers to addressing the public health and safety challenges.

Synthesis and characterization of electroactive and biodegradable ABA block copolymer of polylactide and aniline pentamer.

A triblock copolymer PLA-b-AP-b-PLA (PAP) of polylactide (PLA) and aniline pentamer (AP) with the unique properties of being both electroactive and biodegradable is synthesized by coupling an electroactive carboxyl-capped AP with two biodegradable bi-hydroxyl-capped PLAs via a condensation reaction. Three different molecule weight PAP copolymers are prepared. The PAP copolymers exhibit excellent electroactivity similar to the AP and polyaniline, which may stimulate cell proliferation and differentiation. The electrical conductivity of the PAP2 copolymer film ( approximately 5x10(-6)S/cm) is in the semiconducting region. Transmission electron microscopic results suggest that there is microphase separation of the two block segments in the copolymer, which might contribute to the observed conductivity. The biodegradation and biocompatibility experiments in vitro prove the copolymer is biodegradable and biocompatible. Moreover, these new block copolymer shows good solubility in common organic solvents, leading to the system with excellent processibility. These biodegradable PAP copolymers with electroactive function thus possess the properties that would be potentially used as scaffold materials for neuronal or cardiovascular tissue engineering.

Facile transformation of hydrophilic cellulose into superhydrophobic cellulose.

Superhydrophobic cellulose-based materials coupled with transparent, stable and nanoscale polymethylsiloxane coating have been successfully achieved by a simple process via chemical vapor deposition, followed by hydrolyzation and polymerization.

CO2-based amphiphilic polycarbonate micelles enable a reliable and efficient platform for tumor imaging.

Biodegradable polymeric nanomaterials can be directly broken down by intracellular processes, offering a desirable way to solve toxicity issues for cancer diagnosis and treatment. Among them, aliphatic polycarbonates are approved for application in biological fields by the United States Food and Drug Administration (FDA), however, high hydrophobicity, deficient functionality and improper degradation offer significant room for improvement in these materials. METHODS: To achieve progress in this direction, herein, we demonstrate that CO2-based amphiphilic polycarbonates (APC) with improved hydrophilicity and processability can be used as a reliable and efficient platform for tumor imaging. To better investigate their potential, we devised a convenient strategy through conjugation of APC with gadolinium (Gd). RESULTS: The resulting polymeric micelles (APC-DTPA/Gd) exhibit excellent magnetic resonance imaging performance, simultaneously enabling real-time visualization of bioaccumulation and decomposition of polymeric micelles in vivo. Importantly, these micelles can be degraded to renally cleared products within a reasonable timescale without evidence of toxicity. CONCLUSION: Our findings may help the development of CO2-based amphiphilic polycarbonate for cancer diagnosis and treatment, accompanied by their low-toxicity degradation pathway.

One-pot atom-efficient synthesis of bio-renewable polyesters and cyclic carbonates through tandem catalysis.

One-pot synthesis of well-defined bio-renewable polyesters and cyclic carbonates in high yields was successfully realized for the first time by way of a tandem reaction using metal salen complexes as catalysts. This tandem process offered unprecedented opportunities for the atom-efficient production of two relevant compounds.

Nano-hydroxyapatite surfaces grafted with electroactive aniline tetramers for bone-tissue engineering.

An electroactive amino/carboxyl-capped aniline tetramer (AT) is covalently grafted to the surface of hydroxyapatite (HA) nanoparticles to generate novel electroactive HA-AT nanoparticles. The amount of AT ranges from 16.5 to 34.0 wt% and is characterized by thermogravimetric analysis (TGA). The HA-AT nanoparticles are characterized by Fourier transform IR (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction, and scanning electron microscopy (SEM). For the excellent electroactivity of HA-AT, the mixture of HA-AT and PLA shows much better adhesion ability and proliferation ability than that of HA and a PLA matrix. At a 15 wt% AT grafting amount, the matrix shows the best biocompatibility.

Zinc complexes containing asymmetrical N,N,O-tridentate ligands and their application in lactide polymerization.

A series of zinc complexes based on asymmetrical N,N,O-tridentate ligands were prepared via binaphthyl diamine derivatives. These complexes were characterized and employed as catalysts in lactide polymerization. The X-ray diffraction analyses revealed that molecular structures of 1b and 2b were mononuclear complexes with zinc atoms in distorted octahedral geometries. Upon co-catalysis with isopropanol, complex 2a showed the highest activity among these zinc complexes for the ring-opening polymerization of L-lactide, and complex 3a exhibited the highest stereoselectivity for the ring-opening polymerization of rac-lactide affording substantially isotactic polylactide (PLA) with a P(m) of 0.62. The polymerization kinetics using 2a as a catalyst was studied in detail. The kinetics of the polymerization results revealed that the rates of polymerization were first-order both in the monomer and the catalyst, and there was a linear relationship between the L-LA conversion and the number-average molecular weight of PLA with a narrow molecular distribution (1.07-1.17). The activation energy (31.49 kJ mol(-1)) was deduced according to the Arrhenius equation.

Synthesis and characterization of novel biodegradable and electroactive hydrogel based on aniline oligomer and gelatin.

A biodegradable electroactive hydrogel (AP-g-GA), aniline pentamer (AP) grafting gelatin (GA), is synthesized by a coupling reaction between the carboxyl group of AP and the amino side group of GA in aqueous solution. The electroactivity of the physical hydrogel is confirmed by UV-vis and CV. The hydrophobic AP changes the hydrogel's porous structure of the natural GA and the gel-time, which is confirmed by the rheological behavior of the AP-g-GA and GA. With an increase in the content of AP, the hydrogel gradually forms a porous structure, from "honeycomb" to "bamboo raft". The porous scaffolds can be crosslinked with 3.5% EDC in 90% ethanol. MTT assays show that the AP-g-GA exhibits reduced cytotoxicity compared to EM AP due to the introduction of the biocompatible GA moiety. The in vitro cell cultures suggest that the AP-g-GA#1 (with 1.9% AP) shows the best biocompatibility and cell adhesion ability.

Preparation and characterization of biodegradable and electroactive polymer blend materials based on mPEG/tetraaniline and PLLA.

A novel PGTA diblock copolymer containing electroactive TEA and mPEG is described. Biodegradable PLLA/PGTA polymer blends are shown to be electroactive. Nanopores of different sizes can be formed on the surface of the polymer blends. An MTT assay shows that the PLLA/PGTA blends exhibited reduced cytotoxicity as compared to TEA due to the introduction of the biocompatible PLLA moiety. Cell culture results show that the cells grow better when they were adhered to the surface of the PLLA/PGTA blends. The 80/20 wt.-% PLLA/PGTA blend shows the best biocompatibility and an electroactivity that accelerates the differentiation of rat C6 glioma cells.

Carbon dioxide-based copolymers: environmental benefits of PPC, an industrially viable catalyst.

Carbon dioxide-based copolymers utilize the green house gas CO(2) and can be applied in research and industry. Here we focus on industrially viable CO(2)-based catalysts in China and beyond. Poly(propylene carbonate) (PPC), an alternating copolymer of CO(2) and propylene oxide, is one of the emerging low-cost biodegradable plastics. We describe the thermal and mechanical performances of as-polymerized PPC, where amorphous state, low glass transition temperature, and biodegradability are the three main properties. We also describe modification of the PPC, the so-called toughening and strengthening at high temperature, and plasticizing at low temperature, including incorporation of a third monomer unit by chemical terpolymerization, and introduction of special intermolecular interactions or crystallizable components by physical blending. The fast development in catalyst design and performance improvement for PPC has created new chances for industry. In particular, high molecular weight PPC from rare earth ternary catalyst is becoming an economically viable biodegradable plastic with tens of thousands of tons produced per year, providing a new solution to overcome the problem of high cost in biodegradable plastics.

From tunneling to hopping: a comprehensive investigation of charge transport mechanism in molecular junctions based on oligo(p-phenylene ethynylene)s.

The charge transport mechanism of oligo(p-phenylene ethynylene)s with lengths ranging from 0.98 to 5.11 nm was investigated using modified scanning tunneling microscopy break junction and conducting probe atomic force microscopy methods. The methods were based on observing the length dependence of molecular resistance at single molecule level and the current-voltage characteristics in a wide length distribution. An intrinsic transition from tunneling to hopping charge transport mechanism was observed near 2.75 nm. A new transitional zone was observed in the long length molecular wires compared to short ones. This was not a simple transition between direct tunneling and field emission, which may provide new insights into transport mechanism investigations. Theoretical calculations provided an essential explanation for these phenomena in terms of molecular electronic structures.

Surface modification of hydroxyapatite nanoparticles with thermal-responsive PNIPAM by ATRP.

Hydroxyapatite (HA) nanoparticles grafted by poly(N-isopropylacrylamide) (PNIPAM) brushes (PNIPAM-g-HA) have been synthesized by the surface-initiated atom transfer radical polymerization (ATRP) of N-isopropylacrylamide (NIPAM). The surface grafting amount of PNIPAM ranges from 15.5% to 46.4%. PNIPAM-g-HA has been characterized by FT-IR spectroscopy, thermal gravimetric analysis, X-ray diffraction, and scanning electron microscopy (SEM). The UV transmittance spectra and the particle size analysis of PNIPAM-g-HA in aqueous solution demonstrates that the PNIPAM-g-HA possess reversible thermal stimuli responsive properties. An in vitro bioactivity assessment indicates that PNIPAM-g-HA can induce the mineralization of Ca(2+) and HPO(4) (2-) and possesses an excellent bioactivity. The cell culture results show that the cells adhered to the surface of PNIPAM-g-HA grow better than on HA, and the area of the cells on the surface of PNIPAM-g-HA is much greater than for HA, which proves that the PNIPAM-g-HA has a better biocompatibility than HA.