Surface-initiated Ring-opening Polymerization Research Articles

Enhancing the colloidal stability and surface functionality of molybdenum disulfide (MoS2) nanosheets with hyperbranched polyglycerol for photothermal therapy

Molybdenum disulfide (MoS2) nanosheets are gaining increasing attention due to their attractive properties and myriads of potential applications. However, challenges in the enhancement of their colloidal stability and surface functionality still remain and significantly restrict their practical applications. Herein, we present a viable approach to functionalize MoS2 nanosheets with multihydroxy hyperbranched polyglycerol (HPG) shell by surface-initiated ring-opening polymerization technique. The grafting of HPG from the surface of MoS2 nanosheet yielded MoS2-g-HPG nanohybrid with excellent water dispersibility, good biocompatibility, and greatly enhanced colloidal stability against pH change, ionic strength variation and long-term storage. The MoS2-g-HPG also exhibited excellent light-to-heat conversion capability for in vitro photothermal therapy application. Meanwhile, the MoS2-g-HPG showed favorable surface functionality owing to its numerous surface hydroxyl groups, as demonstrated by the conjugation of functional molecules such as fluorescent dye rhodamine B. As such, this paper opens up new opportunities to empower MoS2 nanosheets and other two-dimensional inorganic nanosheets with desired properties for various applications.

Amphiphilic hyperbranched polyglycerol-block-polycaprolactone copolymer-grafted nanoparticles with improved encapsulation properties

Amphiphilic block copolymer was grafted onto a silica nanoparticle core to template their micelle-like conformation when suspended in water. Such amphiphilic “nanoparticle micelles” will not disaggregate upon dilution, unlike the traditional self-assembled micelles. These nanodispersants were prepared by grafting successive blocks of linear poly(ε-caprolactone) (PCL) and hyperbranched poly(glycerol) (HPG), from functionalized silica nanoparticles (SiO2NPs) using surface-initiated ring-opening polymerization. The swellable hydrophobic PCL inner block enabled the encapsulation of a hydrocarbon payload while the hydrophilic HPG outer block afforded stable dispersions in aqueous media. These nanoparticle-based micelles are proposed as improved dispersants that exhibit stability to high dilution unlike traditional small molecule surfactants. Their capacity for hydrocarbon encapsulation was confirmed by their sequestration of hydrophobic ultraviolet active dyes when dispersed in water. Additional characterization using light scattering-based methodologies combined with ultraviolet spectroscopy, Fourier transform infrared spectroscopy, transmission electron microscopy, and thermogravimetric analysis experiments confirmed the nanodispersant structure and their “unimolecular micelle-like” behavior.

Directed Growth of Polymer Nanorods Using Surface-Initiated Ring-Opening Polymerization of N-Allyl N-Carboxyanhydride.

A stepwise chemistry route was used to prepare arrays of polymer nanostructures of poly(N-allyl glycine) on Si(111) using particle lithography. The nanostructures were used for studying surface reactions with advanced measurements of atomic force microscopy (AFM). In the first step to fabricate the surface platform, isolated nanopores were prepared within a thin film of octadecyltrichlorosilane (OTS). The OTS served as a surface resist, and the areas of nanopores provided multiple, regularly shaped sites for further reaction. An initiator, (3-aminopropyl)triethoxysilane (APTES), was grown selectively inside the nanopores to define sites for polymerization. The initiator attached selectively to the sites of nanopores indicating OTS prevented nonspecific adsorption. Surface-initiated ring-opening polymerization of N-allyl N-carboxyanhydride with APTES produced polymer nanorods on the nanodots of APTES presenting amine functional groups. The surface changes for each step were monitored using high resolution atomic force microscopy (AFM). Slight variations in the height of the poly(N-allyl glycine) nanorods were observed which scale correspondingly to the initial dimensions of nanopores. The distance between adjacent polymer nanorods was controlled by the size of mesoparticle masks used in the experiment. This surface platform has potential application in biotechnology for smart coatings or biosensors.

Characterization of bi-layered magnetic nanoparticles synthesized via two-step surface-initiated ring-opening polymerization

Abstract A versatile strategy to integrate multiple functions in a polymer based material is the formation of polymer networks with defined nanostructures. Here, we present synthesis and comprehensive characterization of covalently surface functionalized magnetic nanoparticles (MNPs) comprising a bi-layer oligomeric shell, using Sn(Oct)2 as catalyst for a two-step functionalization. These hydroxy-terminated precursors for degradable magneto- and thermo-sensitive polymer networks were prepared via two subsequent surface-initiated ring-opening polymerizations (ROPs) with ω-pentadecalactone and ε-caprolactone. A two-step mass loss obtained in thermogravimetric analysis and two distinct melting transitions around 50 and 85°C observed in differential scanning calorimetry experiments, which are attributed to the melting of OPDL and OCL crystallites, confirmed a successful preparation of the modified MNPs. The oligomeric coating of the nanoparticles could be visualized by transmission electron microscopy. The investigation of degrafted oligomeric coatings by gel permeation chromatography and 1H-NMR spectroscopy showed an increase in number average molecular weight as well as the presence of signals related to both of oligo(ω-pentadecalactone) (OPDL) and oligo(ε-caprolactone) (OCL) after the second ROP. A more detailed analysis of the NMR results revealed that only a few ω-pentadecalactone repeating units are present in the degrafted oligomeric bi-layers, whereby a considerable degree of transesterification could be observed when OPDL was polymerized in the 2nd ROP step. These findings are supported by a low degree of crystallinity for OPDL in the degrafted oligomeric bi-layers obtained in wide angle X-ray scattering experiments. Based on these findings it can be concluded that Sn(Oct)2 was suitable as catalyst for the preparation of nanosized bi-layered coated MNP precursors by a two-step ROP.

Degradable Polycaprolactone and Polylactide Homopolymer and Block Copolymer Brushes Prepared by Surface-Initiated Polymerization with Triazabicyclodecene and Zirconium Catalysts.

Surface-initiated ring-opening polymerization (SI-ROP) of polycaprolactone (PCL) and polylactide (PLA) polymer brushes with controlled degradation rates were prepared on oxide substrates. PCL brushes were polymerized from hydroxyl-terminated monolayers utilizing triazabicyclodecene (TBD) as the polymerization catalyst. A consistent brush thickness of 40 nm could be achieved with a reproducible unique crystalline morphology. The organocatalyzed PCL brushes were chain extended using lactide in the presence of zirconium n-butoxide to successfully grow PCL/PLA block copolymer (PCL-b-PLA) brushes with a final thickness of 55 nm. The degradation properties of "grafted from" PCL brush and the PCL-b-PLA brush were compared to "grafted to" PCL brushes, and we observed that the brush density plays a major role in degradation kinetics. Solutions of methanol/water at pH 14 were used to better solvate the brushes and increase the kinetics of degradation. This framework enables a control of degradation that allows for the precise removal of these coatings.

Polypeptide brushes grown via surface-initiated ring-opening polymerization of α-amino acid N-carboxyanhydrides

Polypeptide brushes are attractive platforms to generate functional and responsive interfaces that are of potential interest due to their possible biodegradability, biocompatibility and tunable secondary structures. Surface-initiated ringopening polymerization (SI-ROP) of α-amino acid N-carboxyanhydrides represents a powerful and versatile strategy to generate polypeptide brushes. This review is an attempt to capture the state-of-the-art in this field and highlights the latest developments in several selected areas. In addition to presenting an overview of the synthetic methods that have been used to generate polypeptide brushes via SI-ROP, this article will discuss the preparation of patterned polypeptide brushes, the conformational properties of surface-tethered polypeptides, ways to control chain orientation at surfaces as well as properties and applications of these thin polymer films.

Shape-Persistent, Thermoresponsive Polypeptide Brushes Prepared by Vapor Deposition Surface-Initiated Ring-Opening Polymerization of α-Amino Acid N-Carboxyanhydrides

Surface-grafting thermoresponsive polymers allows the preparation of thin polymer brush coatings with surface properties that can be manipulated by variation of temperature. In most instances, thermoresponsive polymer brushes are produced using polymers that dehydrate and collapse above a certain temperature. This report presents the preparation and properties of polymer brushes that show thermoresponsive surface properties, yet are shape-persistent in that they do not undergo main chain collapse. The polymer brushes presented here are obtained via vapor deposition surface-initiated ring-opening polymerization (SI-ROP) of γ-di- or tri(ethylene glycol)-modified glutamic acid N-carboxyanhydrides. Vapor deposition SI-ROP of γ-di- or tri(ethylene glycol)-modified l- or d-glutamic acid N-carboxyanhydrides affords helical surface-tethered polymer chains that do not show any changes in secondary structure between 10 and 70 °C. QCM-D experiments, however, revealed significant dehydration of poly(γ-(2-(2-methoxyethoxy)ethyl)-l-glutamate) (poly(L-EG2-Glu)) brushes upon heating from 10 to 40 °C. At the same time, AFM and ellipsometry studies did not reveal significant variations in film thickness over this temperature range, which is consistent with the shape-persistent nature of these polypeptide brushes and indicates that the thermoresponsiveness of the films is primarily due to hydration and dehydration of the oligo(ethylene glycol) side chains. The results presented here illustrate the potential of surface-initiated NCA ring-opening polymerization to generate densely grafted assemblies of polymer chains that possess well-defined secondary structures and tunable surface properties. These polypeptide brushes complement their conformationally unordered counterparts that can be generated via surface-initiated polymerization of vinyl-type monomers and represent another step forward to biomimetic surfaces and interfaces.

One-Step Immobilization of Initiators for Surface-Initiated Ring Opening Polymerization and Atom Transfer Radical Polymerization by Poly(norepinephrine) Coating.

We report a facile method for surface-initiated ring opening polymerization (ROP) and atom transfer radical polymerization (ATRP) via a poly(norepinephrine) coating. Solid substrates were modified by poly(norepinephrine) under alkaline conditions, with concurrent co-adsorption of an ATRP initiator. The poly(norepinephrine) layer acted as a ROP initiator due to the presence of hydroxyl groups in its side chain, resulting in a surface that was able to initiate ATRP and ROP simultaneously. ε-Caprolactone (ε-CL) and 2-(dimethylamino)ethyl methacrylate (DMAEMA) were grafted onto the surface via ROP and ATRP, respectively, and the polymers subsequently grown from the surfaces were characterized in detail using Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), contact angle goniometry, and atomic force microscopy (AFM).

Surface characteristics of cellulose nanoparticles grafted by surface-initiated ring-opening polymerization of ε-caprolactone

In this study, surface-initiated ring-opening polymerization has been employed for the grafting of e-caprolactone from cellulose nanoparticles, made by partial hydrolysis of cellulose cotton linters. A sacrificial initiator was employed during the grafting reactions, to form free polymer in parallel to the grafting reaction. The degree of polymerization of the polymer grafts, and of the free polymer, was varied by varying the reaction time. The aim of this study was to estimate the cellulose nanoparticle degree of surface substitution at different reaction times. This was accomplished by combining measurement results from spectroscopy and chromatography. The prepared cellulose nanoparticles were shown to have 3.1 (±0.3) % of the total anhydroglucose unit content present at the cellulose nanoparticle surfaces. This effectively limits the amount of cellulose that can be targeted by the SI-ROP reactions. For a certain SI-ROP reaction time, it was assumed that the resulting degree of polymerization (DP) of the grafts and the DP of the free polymer were equal. Based on this assumption it was shown that the cellulose nanoparticle surface degree of substitution remained approximately constant (3–7 %) and seemingly independent of SI-ROP reaction time. We believe this work to be an important step towards a deeper understanding of the processes and properties controlling SI-ROP reactions occurring at cellulose surfaces.

Poly(ε-caprolactone)-Grafted Fe3O4 Nanoparticles: Preparation and Superparamagnetic Nanocomposites with Epoxy Thermosets

Poly(ε-caprolactone)-grafted magnetic iron oxide nanoparticles (Fe3O4 NPs) were prepared via a surface-initiated ring opening polymerization approach. First, the surface of the unmodified Fe3O4 NPs were treated with 3-aminopropyltriethoxysilane to afford the 3-aminopropyl-functionalzed Fe3O4 NPs. Second, the surface-initiated ring-opening polymerization of ε-caprolactone was carried out with the surface-functionalized Fe3O4 NPs as the initiator to afford the poly(ε-caprolactone)(PCL)-grafted Fe3O4 NPs. The PCL-grafted Fe3O4 NPs have been characterized by means of Fourier transform infrared spectroscopy, X-ray diffraction, thermogravimetric analysis, and transmission electron microscopy. It was found that the PCL-grafted Fe3O4 NPs can be well dispersed into epoxy thermosets. The nanocomposites were successfully obtained with the content of the PCL-grafted Fe3O4 NPs up to 40 wt %. The fine dispersion of Fe3O4 nanoparticles in epoxy was demonstrated by transmission electron microscopy and dynamic mechanical thermal analysis. The results of magnetic analysis with vibrating sample measuring technology indicated that the nanocomposites involving epoxy resins and Fe3O4 nanoparticles possessed superparamagnetic properties.

Surface-initiated anionic polymerization of [1]silaferrocenophanes for the preparation of colloidal preceramic materials.

A novel strategy for the preparation of poly(ferrocenylsilane) (PFS) immobilized on the surface of cross-linked polystyrene (PS) nanoparticles is reported. The ferrocene-containing core/shell architectures are shown to be excellent candidates as preceramic polymers yielding spherical ceramic materials consisting of iron silicide (Fe3 Si) and metallic iron after thermal treatment. For this purpose, dimethyl- and hydromethyl[1]silaferrocenophane monomers are polymerized by surface-initiated ring-opening polymerization upon taking advantage of residual vinylic moieties at the PS particle surface. A strategy for selective chain growth from the particle surface is developed without the formation of free PFS homopolymer in solution. The grafted particles are characterized using transmission electron microscopy (TEM) and dynamic light scattering (DLS). These particles are excellent precursors for ceramics as studied by thermogravimetric analysis (TGA). The composition of the ceramics is studied using X-ray diffraction (XRD) measurements, while the morphology is probed by scanning electron microscopy (SEM) revealing the original spherical shape of the precursor particles. Obtained ceramic materials- predominantly based on iron silicides-show ferromagnetic behavior as investigated by superconducting quantum interference device (SQUID) magnetization measurements at different temperatures.

Synthesis of biopolymer-grafted nanodiamond by ring-opening polymerization

We have developed an efficient and biological method for the covalent functionalization of nanodiamond (ND) biopolymers. ND-grafted-poly(l-lactic acid) (ND-g-PLLA) and ND-grafted-poly(ε-caprolactone) (ND-g-PCL) were prepared by surface-initiated ring-opening polymerization. After homogenization of the surface by mixed acids and thionyl chlorides, the surface of ND was modified with 1,6-hexanediol to possess hydroxyl groups, which could be used as coinitiators to polymerize l-lactide or ε-caprolactone to yield ND grafted with bio-polyesters. The FT-IR, Raman and NMR spectra revealed that the polyester chains were covalently attached to the ND and the weight gains as a result of the functionalized ND particles were determined by TGA analysis. The ND-g-PLLA and ND-g-PCL were both well dispersed in the organic solvents and the average diameters were measured. The TEM images demonstrate that the dispersion of pristine ND has been significantly improved after modification of ND with PLLA or PCL.

Surface-Initiated Ring-Opening Polymerization of Poly(2-methyl-2-oxazoline) from Poly(bromoethyl methacrylate/methyl methacrylate) Microspheres and Modification into PEI: Immobilization of α-Amylase by Adsorption and Cross-Linking

Bromide-functionalized microspheres were prepared by the suspension polymerization of bromoethyl methacrylate (BEMA), methyl methacrylate (MMA), and ethylene glycol dimethacrylate (EGDMA). The microspheres were employed as a solid macroinitiator for the ring-opening polymerization of 2-methyl 2-oxazoline. The surface brushes were then converted into poly(ethylenimine) (PEI) by simple hydrolysis with HCl and then NaOH as inferred from chemical analyses and FTIR spectra. The microspheres were used for the immobilization of α-amylase through adsorption and covalent cross-linking. The optimum pH for free enzyme was 6.0 and shifted by about 1.0 unit to the acidic pH range upon immobilization. Compared to free α-amylase, the immobilized enzyme preparations were found to exhibit better tolerance to variations in pH and temperature, as well as improved storage stabilities. The activity retentions of the adsorbed and cross-linked enzyme preparations were found to be 69% and 53%, respectively. Furthermore, the cros...

Biodegradable polymer brush as nanocoupled interface for improving the durability of polymer coating on metal surface

Metal-based drug-eluting stents (DESs) have severe drawbacks such as peeling-off and cracking of the coated polymer. To prevent the fracture of polymer-coated layer and improve the durability of DES, poly(l-lactide) (PLLA) brushes were synthesized onto cobalt-chromium (Co-Cr or CC) surface through atom transfer radical polymerization (ATRP) of 2-hydroxyethylmethacrylate (HEMA) followed by surface-initiated ring opening polymerization (SI-ROP) of l-lactide. The polymer brushes were then characterized by attenuated total reflection-Fourier transform infrared (ATR-FTIR), water contact angle, ellipsometry, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and scanning electron microscopy (SEM). All of the unmodified and modified Co-Cr surfaces were coated with a matrix of poly(d,l-lactide) (PDLLA) and sirolimus (SRL). The in vitro drug release profile was measured for 70 days. The PLLA-modified Co-Cr showed a biphasic release pattern in the initial burst followed by a slow release. On the other hand, the unmodified Co-Cr showed fast drug release and detachment of the coated polymer layer due to the instability of the polymer layer on Co-Cr surface. In comparison, the PLLA-modified Co-Cr preserved a uniform coating without detachment even after 6 weeks of degradation test. The platelet morphology and low density of platelet adhered on the modified layer and the SRL-in-PDLLA coated Co-Cr surfaces demonstrated that these samples would be blood compatible. Therefore, the introduction of PLLA brush onto Co-Cr surface is proved to dramatically improve the durability of the coating layer, and it is a promising strategy to prevent the coating defects found in DESs.

Crack prevention of biodegradable polymer coating on metal facilitated by a nano-coupled interlayer

Crack prevention of biodegradable polymer coatings on drug-eluting stents was investigated by introducing a nano-coupled layer at the interface between the metal surface and the polymer coating layer using surface-initiated ring-opening polymerization of ε-caprolactone. Poly(d,l-lactide-co-glycolide) coating on cobalt-chromium control and ricinoleic acid-poly(caprolactone)–grafted cobalt-chromium was carried out using electrospraying. The cracking of the biodegradable polymer coating on drug-eluting stents during ballooning was addressed by introducing a nano-coupled interlayer on the cobalt-chromium surface. The ricinoleic acid-poly(caprolactone) nano-coupled interlayer and poly(d,l-lactide-co-glycolide)-coated top layer were characterized using attenuated total reflection Fourier transform infrared, contact angle, ellipsometry, X-ray photoelectron spectroscopy, and atomic force microscopy. Based on scratch tests, the nano-coupled samples had stronger interfacial adhesion compared to the control sample without the nano-coupled layer. Scanning electron microscope images indicated that the cracking on the poly(d,l-lactide-co-glycolide) coating was addressed. Introducing a nano-coupling interlayer may be an important strategy to preventing polymer coating cracking on drug-eluting stents.

Controlled synthesis, optical properties and cytotoxicity studies of CdSe-poly(lactic acid) multifunctional nanocomposites by ring-opening polymerization.

A facile synthetic route has been developed for the covalent grafting of biocompatible poly(lactic acid) (PLA) onto CdSe Quantum Dots (QDs) using surface initiated ring opening polymerization (ROP) to afford CdSe-g-PLA nanocomposites. At first, 2-mercaptoethanol (ME) capped CdSe QDs were synthesized through a wet chemical process. The surface initiated ROP of lactide was accomplished with Sn(Oct)2 to give CdSe-g-PLA nanocomposites having surface hydroxyl functionality. FT-IR data suggested that a robust covalent bond was formed between ME capped CdSe QDs and polymer moieties. The grafting density of PLA on CdSe QDs was found to be moderate as measured by TGA analysis. The CdSe QDs were well dispersed in CdSe-g-PLA nanocomposites matrices as captured by TEM. The cubic phase crystal structure of CdSe QDs in the nanocomposites was determined by XRD. The optical properties of the CdSe-g-PLA nanocomposites were investigated by UV-vis and photoluminescence spectroscopy which suggested their potentialities as optical materials in biomedical application. Cell viability studies revealed that the biocompatibility of CdSe QDs was improved upon PLA immobilization.

Reinforcement of interfacial adhesion of a coated polymer layer on a cobalt-chromium surface for drug-eluting stents.

During the balloon expansion of several commercially available drug-eluting stents, various types of defects in the polymer layer have been observed. The aim of this study is to prevent these defects by increasing the interfacial adhesion between the metal substrate and the drug-in-polymer matrix using poly(caprolactone) (PCL) brushes onto a cobalt-chromium (Co-Cr or CC) alloy surface. The chemical modification of the Co-Cr surface was accomplished by grafting ricinoleic acid (RA) onto the metal substrate followed by surface-initiated ring opening polymerization of ε-caprolactone. The unmodified, RA-grafted (CC-RA), and PCL-grafted Co-Cr substrates (CC-RA-PCL3D and CC-RA-PCL6D) were characterized by various surface analyses. Poly(d,l-lactide) containing sirolimus was spray coated onto the unmodified and modified substrates. The adhesion property of the polymer coating on the PCL-grafted surfaces was improved compared to those of other samples. Among all of the drug-in-polymer coated samples, both CC-RA-PCL3D and CC-RA-PCL6D exhibited a stabilized drug release profile over 49 days. It was also revealed that CC-RA-PCL6D showed the slowest drug release of all the samples. On the basis of these results, the proposed nanocoupling method has shown not only improved adhesion of the drug-in-polymer matrix to the Co-Cr substrate but also controlled drug release.

Near-infrared light-responsive shape-memory poly(ɛ-caprolactone) films that actuate in physiological temperature range

Near-infrared (NIR) light-responsive shape-memory films were prepared through the photo-initiated polymerization of poly(ɛ-caprolactone) (PCL) macromonomers with acryloyl terminal groups in the presence of gold nanorods (AuNRs). To incorporate the AuNRs homogenously within the films, the surfaces of AuNRs were also modified with PCL via surface-initiated ring-opening polymerization. The shape-switching temperature of PCL was adjusted in the physiological temperature range by controlling the melting temperature of PCL. Exposure to NIR light successfully induced the photothermal heating of embedded AuNRs and, consequently, the shape-switching transition. After NIR irradiation at higher-power densities, the film completely recovered its original shape. When exposed to lower-power densities, a local temperature increase was observed in the area where the beam hit. Therefore, local shape memory transformations were obtained. These results show the potential of gold nanorod-embedded PCL films as spatially controllable, shape-memory materials that actuate in physiologically relevant temperature ranges and from remote stimuli.

Surface-grafted semiconductor layer prepared by surface initiated ring-opening polymerization of L-lysine NCA derivatives.

Asymmetric and symmetric L-lysine N-carboxylic anhydrides based on perylene bisimide derivatives were successfully synthesized and grafted onto silicon wafers via surface-initiated ring-opening polymerization. The grafted n-type semiconductor film was flat and compact, and its thickness could be tuned by variation of the polymerization conditions.

Polymer-Coated Hydroxyapatite Nanoparticles for the Delivery of Statins

Statin drugs have been previously reported to increase the activity of osteoblasts, and are being investigated to treat low bone density pathologies. A controlled release nanoparticle drug delivery system may be used to prolong the exposure of osteoblasts to statin. Furthermore, nano-hydroxyapatite (nHA) has known osteoconductive and osteoinductive properties. The success of nanoparticle drug delivery systems is associated with safety, stability, biodegradability, and pharmacological parameters. nHA have been used as gene and drug delivery vehicles, however in vivo studies have shown that nHA aggregate, accumulate in the lungs, and impede the function of pulmonary surfactant. To address some of these challenges, we present a nHA coated with the biodegradable co-polymer poly(glycolide)- poly(ethylene glycol) (PGA-PEG). PGA coating thickness could be precisely tuned by surface initiated ring opening polymerization to control the release of lovastatin. PGA-PEG coated nHA were shown to be internalized in human osteoblasts, and were also shown to maintain 70% cell viability in human umbilical vein endothelial cells at doses up to 500 μg mL-1 in vitro. Preliminary in vivo maximum tolerated dose studies showed that PGA-PEG coated nHA were less toxic compared to uncoated nHA. This hybrid polymer-HA nanoparticle drug delivery system has potential as a controlled release system for the delivery of statins to treat low bone density pathologies.