Polymer Branching Research Articles

Structural Engineering of Star Block Biodegradable Polymer Unimolecular Micelles for Drug Delivery in Cancer Cells.

The present investigation reports the structural engineering of biodegradable star block polycaprolactone (PCL) to tailor-make aggregated micelles and unimolecular micelles to study their effect on drug delivery aspects in cancer cell lines. Fully PCL-based star block copolymers were designed by varying the arm numbers from two to eight while keeping the arm length constant throughout. Multifunctional initiators were exploited for stepwise solvent-free melt ring-opening polymerization of ε-caprolactone and γ-substituted caprolactone to construct star block copolymers having a PCL hydrophobic core and a carboxylic PCL hydrophilic shell, respectively. A higher arm number and a higher degree of branching in star polymers facilitated the formation of unimolecular micelles as opposed to the formation of conventional multimicellar aggregates in lower arm analogues. The dense core of the unimolecular micelles enabled them to load high amounts of the anticancer drug doxorubicin (DOX, ∼12-15%) compared to the aggregated micelles (∼3-4%). The star unimolecular micelle completely degraded leading to 90% release of the loaded drug upon treatment with the lysosomal esterase enzyme in vitro. The anticancer efficacies of these DOX-loaded unimolecular micelles were tested in a breast cancer cell line (MCF-7), and their IC50 values were found to be much lower compared to those of aggregated micelles. Time-dependent cellular uptake studies by confocal microscopy revealed that unimolecular micelles were readily taken up by the cells, and enhancement of the drug concentration was observed at the intracellular level up to 36 h. The present work opens new synthetic strategies for building a next-generation biodegradable unimolecular micellar nanoplatform for drug delivery in cancer research.

Entanglement Length Scale Separates Threading from Branching of Unknotted and Non-concatenated Ring Polymers in Melts.

Current theories on the conformation and dynamics of unknotted and non-concatenated ring polymers in melt conditions describe each ring as a tree-like double-folded object. While evidence from simulations supports this picture on a single ring level, other works show pairs of rings also thread each other, a feature overlooked in the tree theories. Here we reconcile this dichotomy using Monte Carlo simulations of the ring melts with different bending rigidities. We find that rings are double-folded (more strongly for stiffer rings) on and above the entanglement length scale, while the threadings are localized on smaller scales. The different theories disagree on the details of the tree structure, i.e., the fractal dimension of the backbone of the tree. In the stiffer melts we find an indication of a self-avoiding scaling of the backbone, while more flexible chains do not exhibit such a regime. Moreover, the theories commonly neglect threadings and assign different importance to the impact of the progressive constraint release (tube dilation) on single ring relaxation due to the motion of other rings. Despite that each threading creates only a small opening in the double-folded structure, the threading loops can be numerous and their length can exceed substantially the entanglement scale. We link the threading constraints to the divergence of the relaxation time of a ring, if the tube dilation is hindered by pinning a fraction of other rings in space. Current theories do not predict such divergence and predict faster than measured diffusion of rings, pointing at the relevance of the threading constraints in unpinned systems as well. Revision of the theories with explicit threading constraints might elucidate the validity of the conjectured existence of topological glass.

One-Pot Synthesis of Hyperbranched Polyurethane-Triazoles with Controlled Structural, Molecular Weight and Hydrodynamic Characteristics.

We report a simple and convenient approach to the one-pot synthesis of hyperbranched polyurethane-triazoles with desirable properties. This method is based on in situ generation of an AB2 + A2 + B4 azide-acetylene monomer mixture of known composition, due to quantitative reactions of urethane formation between isophorone diisocyanate (IPDI), 1,3-diazidopropanol-2 (DAPOL) (in the first stage) and propargyl alcohol (in the second stage). The obtained monomer mixture can be involved in step-growth polymerization by azide-alkyne cycloaddition without additional purification (in the third stage). The properties of the resulting polymers should depend on the composition of the monomer mixture. Therefore, first the model revealing the correlation between the monomer composition and the ratio and reactivity of the IPDI and DAPOL active groups is developed and proven. In addition, the newly developed structural kinetic model considering the substitution effect at polyaddition of the complex mixture of monomers allows the prediction of the degree of branching of the target polymer. Based on our calculations, the hyperbranched polyurethane-triazoles were synthesized under found conditions. All products were characterized by 1H NMR, FTIR, SEC, DLS, DSC, TGA and viscometry methods. It was shown that the degree of branching, molecular weight, intrinsic viscosity, and hydrodynamic radius of the final hyperbranched polymers can be specified at the first stage of one-pot synthesis. The obtained hyperbranched polyurethane-triazoles showed a degree of branching from 0.21 to 0.44 (calculated DB-0.25 and 0.45, respectively).

Editorial: theme issue on complex rheology in biological systems

Editorial: theme issue on complex rheology in biological systems

Self-Restructuring of Polyhydromethylsiloxanes by the Hydride Transfer Process: A New Approach to the Cross-Linking of Polysiloxanes and to the Fabrication of Thin Polysiloxane Coatings.

The branching and cross-linking of siloxane polymers are important processes in silicone technology. A new type of such a process has been developed, which is a self-restructuring of linear polyhydromethylsiloxane (PHMS). This process involves the reorganization of the PHMS to form a highly branched siloxane polymer or finally a cross-linked siloxane network. It occurs through the transfer of a hydride ion between silicon atoms catalyzed by tris(pentafluoromethyl)borane. Its advantage over existing branching and cross-linking reactions is that it runs at room temperature without a low-molecular-weight cross-linker in the absence of water, silanol groups, or other protic compounds and it does not use metal catalysts. The study of this process was carried out in toluene solution. Its course was followed by 1H NMR, 29Si NMR and FTIR, SEC, and gas chromatography. A general mechanism of this new self-restructuring process supported by quantum calculations is proposed. It has been shown that a linear PHMS self-restructured to a highly branched polymer can serve as a pure methylsiloxane film precursor.

Impact of Star Polyacid Branching on Polymer Diffusion within Multilayer Films

A series of linear and star poly(acrylic acids) (LPAA and star PAAs) were synthesized to explore the effect of molecular architecture on the stratification and polymer dynamics of electrostatic layer-by-layer (LbL) films. Studies of LbL deposition of LPAA and star PAAs with poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) at acidic pH showed an ∼30% increase in film dry thickness with increased polymer branching. Consistent with a greater mass of star polymer deposited within the films, in situ ellipsometric measurements of PAA uptake from solution revealed ∼3.5-fold greater diffusion coefficients for 8-arm PAA in comparison to linear PAA. For comparison, the dynamics of the linear PDMAEMA partner was explored via neutron reflectometry (NR) studies of stacked multilayers containing hydrogenated and deuterated polycations, hPDMAEMA and dPDMAEMA. The stacked multilayers deposited from low-ionic-strength solutions were stratified, exhibiting interfacial widths between hydrogenated and deuterated stacks of ∼15 and 10 nm for films constructed with star and linear PAAs, respectively, suggesting relatively low mobility of the polycation in both assemblies. Further exposure of these films to 0.5 M sodium chloride solutions enhanced the mobility of PDMAEMA, revealing an order magnitude faster diffusion of PDMAEMA in films of 8-arm PAA relative to linear PAA. The faster diffusion of polymers within films of star polyacids was correlated not only with the compactness of star polymers but also with an ∼2-fold lower ionization of assembled 8-arm PAA as determined by Fourier transform infrared spectroscopy and thus a lower number of polycation–polyacid ionic contacts in the case of star polymers as compared to their linear counterparts. The significant influence of molecular architecture on the number of polymer–polymer contacts was further confirmed by isothermal titration calorimetry studies of polyelectrolyte complexes in solution.

Investigation of Curing Mechanism and Mechanical Properties of Polypropylene/Aliphatic Epoxy Composites

Temperature-sensitive cure kinetics based on bisphenol A oligomeric diglycidyl ether and branching diphenylmethane di-isocyanate, polymer aliphatic or lower molecular weight aromatic amines, and polypropylene and epoxy composites were examined. Polypropylene networks are formed initially, followed by amine hardeners interfering with epoxy oxirane rings to frame linear oligomers. Finally, the system is formed by a reaction between amines obtained in the second phase and epoxy oxirane rings during the curing step. This three-stage treatment was illustrated. The activation energy was calculated using the Flynn-Wall-Ozawa and Kissinger-Akahira-Sunose isoconversational approaches to cure degree. The Ea-to-αcorrelation was determined using these approaches, revealing that the research curing systems exhibited autocatalytic effects. Among the materials studied, compounds with low molecular weight aromatic amines had the strongest and longest-lasting tensile characteristics. There are numerous advantages to slow curing and discrete stages of composite creation, including better mechanical properties.

Branched PCL-Based Thermogelling Copolymers: Controlling Polymer Architecture to Tune Drug Release Profiles

Temperature-responsive hydrogels, or thermogels, are a unique class of biomaterials that show facile and spontaneous transition from solution to gel when warmed. Their high biocompatibility, and ease of formulation with both small molecule drugs and biologics have made these materials prime candidates as injectable gel depots for sustained local drug delivery. At present, controlling the kinetics and profile of drug release from thermogels is achieved mainly by varying the ratio of hydrophobic: hydrophilic composition and the polymer molecular weight. Herein, we introduce polymer branching as a hitherto-overlooked polymer design parameter that exhibits profound influences on the rate and profile of drug release. Through a family of amphiphilic thermogelling polymers with systematic variations in degree of branching, we demonstrate that more highly-branched polymers are able to pack less efficiently with each other during thermogel formation, with implications on their physical properties and stability towards gel erosion. This in turn resulted in faster rates of release for both encapsulated small molecule hydrophobic drug and protein. Our results demonstrate the possibility of exploiting polymer branching as a hitherto-overlooked design parameter for tailoring the kinetics and profile of drug release in injectable thermogel depots.

Effects of Polymer Branching Structure on the Hydrodynamic Adsorbed Layer Thickness Formed on Colloidal Particles

Effects of Polymer Branching Structure on the Hydrodynamic Adsorbed Layer Thickness Formed on Colloidal Particles

Calculation of the main characteristics of polymer branching taking into account the exchange of activity between polymerization centres

The calculation of the main characteristics of the branching of the polymer is presented in the article using the example of polymerization of butadiene in the presence of an organolithium catalytic system modified with polyfunctional sodium and magnesium alcoholates. The addition of a modifier to the catalytic system leads to the participation in the polymerization of at least two types of active centers, which is the reason for some branching of macromolecules. The complemented mathematical model of the periodic process, taking into account the exchange of activity between the two centers, allows to calculate the main characteristics of branching.

Effect of the branching morphology of a cationic polymer flocculant synthesized by controlled reversible‐deactivation radical polymerization on the flocculation and dewatering of dilute mature fine tailings

AbstractOne of the methods used to treat oil sands tailings is to add water‐soluble polymer flocculants to create large flocs that settle to the bottom of the ponds, allowing the supernatant water to be recovered and reused in the oil extraction process. However, a systematic study of how polymer branching affects flocculation has not yet been done to date. Poly((vinylbenzyl)trimethylammonium chloride) (PVB), a cationic and partially hydrophobic polymer, is the focus of this paper. Its positive charges make it adsorb strongly on the negatively charged clay particles, and its partial hydrophobicity produces flocs that retain less water. We synthesized PVB in linear, 3‐arm star, and 4‐arm star configurations using activators regenerated by electron transfer atom transfer radical polymerization. The 3‐arm star polymer outperformed its linear counterpart at every dosage in the initial settling rate tests, while the 4‐arm star polymer performed well only at high dosages. All polymers had similar dewatering performances.

Interaction of polyamidoamine dendrimers and amphiphylic dendrons with lipid membranes

Polyamidoamine (PAMAM) dendrimers and amphiphilic dendrons are one of the types of nanomaterials characterized by a hyperbranched structure of polymer branches. In the case of dendrimers, the dendrons are covalently linked at the central focal point. In the case of amphiphilic dendrons, dendrons are non-covalently linked by hydrophobic interactions, forming micellar structures. These nanoparticles are widely used in biology and medicine as contrast agents, carriers of drugs and genetic material. Their use in scientific practice requires an understanding of the basic mechanisms of their interaction with membranes – the main obstacle to the entry of dendrimers into the cell. This review discusses the regularities of the interaction of dendrimers and amphiphilic dendrons with lipid membranes. Various models of dendrimer-membrane interactions are described as the basis for the penetration of dendrimers and amphiphilic nanoparticles into cells. Keywords: polyamidoamine dendrimers, amphiphilic dendrons, lipid membranes, cells, antitumor therapeutics, antibacterial agents, diagnostics, genetic therapy.

Direct visualization of bottlebrush polymer conformations in the solid state

Although the behavior of single chains is integral to the foundation of polymer science, a clear and convincing image of single chains in the solid state has still not been captured. For bottlebrush polymers, understanding their conformation in bulk materials is especially important because their extended backbones may explain their self-assembly and mechanical properties that have been attractive for many applications. Here, single-bottlebrush chains are visualized using single-molecule localization microscopy to study their conformations in a polymer melt composed of linear polymers. By observing bottlebrush polymers with different side chain lengths and grafting densities, we observe the relationship between molecular architecture and conformation. We show that bottlebrushes are significantly more rigid in the solid state than previously measured in solution, and the scaling relationships between persistence length and side chain length deviate from those predicted by theory and simulation. We discuss these discrepancies using mechanisms inspired by polymer-grafted nanoparticles, a conceptually similar system. Our work provides a platform for visualizing single-polymer chains in an environment made up entirely of other polymers, which could answer a number of open questions in polymer science.

Lithium-Conducting Branched Polymers: New Paradigm of Solid-State Electrolytes for Batteries.

The past decades have witnessed rapid development of lithium-based batteries. Significant research efforts have been progressively diverted from electrodes to electrolytes, particularly polymer electrolytes (PEs), to tackle the safety concern and promote the energy storage capability of batteries. To further increase the ionic conductivity of PEs, various branched polymers (BPs) have been rationally designed and synthesized. Compared with linear polymers, branched architectures effectively increase polymer segmental mobility, restrain crystallization, and reduce chain entanglement, thereby rendering BPs with greatly enhanced lithium transport. In this Mini Review, a diversity of BPs for PEs is summarized by scrutinizing their unique topologies and properties. Subsequently, the design principles for enhancing the physical properties, mechanical properties, and electrochemical performance of BP-based PEs (BP-PEs) are provided in which the ionic conduction is particularly examined in light of the Li+ transport mechanism. Finally, the challenges and future prospects of BP-PEs in this rapidly evolving field are outlined.

Role of organometallic complexes in olefin polymerization: a review report

• A detailed discussion on various organometallic complexes used for olefin polymerization, has been done. • Regulation on composition and microstructure of the polymer products can be done by altering the metal center and the auxiliary ligand of an organometallic complex. • The organometallic catalysts of group 3 metals for the polymerization of olefins are still less discovered and very uncommon and group 4 metal-based catalysts holding diverse auxiliary ligands have been widely researched and investigated. • Increase in performance of homogeneous catalysts by introducing them on metalasiloxanes. The chemistry of catalytic polymerization of ethylene and propylene employing titanium complexes and aluminum-alkyls as co-catalysts is well known since last 50 years. Thorough studies show that invention of homogeneous organometallic complexes as catalysts such as metallocenes and other transition metal complexes with methylaluminoxane (MAO) as co-catalyst for the polymerization of olefins has achieved a lot of interest in this area as compared to the well-known and conventional heterogeneous catalysts i.e. Ziegler–Natta and Phillips catalyst. The possibilities of controlling the polymer composition, microstructure, tacticity, stereoregularity and special properties with high precision are found to be high in homogeneous catalysts. Availability of single active accessible site on metallocene catalysts in contrast to their classical equivalents is responsible for improved catalytic polymerization. Furthermore, these single-site catalysts, such as half titanocenes/MAO, zirconocenes, and other single-site catalysts, can control important parameters like co-monomer distribution, molecular weight, molecular weight distribution, molecular architecture, stereo-specificity, degree of linearity, polymer branching etc . The purity in products found less when Ziegler–Natta catalysts were used. This review summarises and compares the recent advances in the polymerization of various olefins (especially ethylene or α-olefins) using various organometallic catalysts consist of rare-earth elements and lanthanides, group 4 metals, and other transition metals (V,Cr and Ni). At the end the review discusses modulation of catalytic activity to control polymerization reactions and concludes the study with future challenges in the area.

Investigations of ligand backbone effects on bulky diarylmethyl-based nickel(II) and palladium(II) catalyzed ethylene polymerization and copolymerization

Recently, the sterically hindered ortho-dibenzhydryl or diarylmethyl-derived (CHPh2 or CHAr2 on the N-aryl ring) α-diimine nickel(II) and palladium(II) catalysts have received much attention due to their unique catalytic properties for olefin (co)polymerization. In this work, the synthesis and characterization of bulky diarylmethyl-derived Ni(II) and Pd(II) ethylene (co)polymerization catalysts containing a series of backbone substituents (H, CH3 and acenaphthene) were described. Substituent effect of the backbone on catalyst activity and stability, polymer molecular weight, polymer branching density and polar monomer incorporation were investigated in detail. Overall, acenaphthene-based Ni3 showed the highest catalytic activity and produced the polymer with the highest molecular weight in Ni(II) system, while CH3-based Pd2 showed the best catalytic property in Pd(II) system. The H-substituted Ni(II) and Pd(II) catalyst was notable in its low activity or no activity compared to the other substituted Ni(II) and Pd(II) catalyst. Most interestingly, Pd2 demonstrated highest polymerization activities under gas-phase conditions through the self-supported catalytic mechanism.

Horizontally and Vertically Concerted Steric Strategy in α‐Diimine Nickel Promoted Ethylene (Co)Polymerization†

Main observation and conclusionSteric bulk plays a significantly paramount role in late transition metal promoted olefin (co)polymerization in terms of polymer molecular weight and catalyst thermal stability. Numerous sterically encumbered nickel and palladium catalysts have been developed, but they are usually uni‐directional. In this contribution, from a bi‐directional side the distinctive horizontally and vertically concerted steric strategy was well‐developed and applied to α‐diimine nickel catalysts. In ethylene polymerization, the increase of vertically steric bulk (H, Ph, Biph) led to an enhanced polymer molecular weight, a slightly ascended branching density; likewise increasing horizontally steric bulk (Ph, Nap, Ant) further resulted in the increase of polymer molecular weight and branching density. As a result, in sharp contrast to Ni1(H) without steric bulk, Ni5(Ant) with horizontally and vertically concerted steric bulk was thermally stable, showed very high activities at a level of 107 g·mol–1·h–1, and produced polyethylenes with the highest molecular weight of 518.8 × 104 g·mol–1 and the broader range of branching density from 2.4/1000C to 27.8/1000C. In the copolymerization of ethylene with methyl 10‐undecenoate, Ni5(Ant) also provided the highest copolymer molecular weight but gave the lowest incorporation of co‐monomer. This work sheds light on the steric effect on olefin (co)polymerization.

Fluorinated α-Diimine Nickel Mediated Ethylene (Co)Polymerization.

Fluorine substituents in transition metal catalysts are of great importance in olefin polymerization catalysis; however, the comprehensive effect of fluorine substituents is elusive in seminal late transition metal α-diimine catalytic system. In this contribution, fluorine substituents at various positions (ortho-, meta-, and para-F) and with different numbers (Fn ; n=0, 1, 2, 3, 5) were installed into the well-defined N-terphenyl amine and thus were studied for the first time in the nickel α-diimine promoted ethylene polymerization and copolymerization with polar monomers. The position of the fluorine substituent was particularly crucial in these polymerization reactions in terms of catalytic activity, polymer molecular weight, branching density, and incorporation of polar monomer, and thus a picture on the fluorine effect was given. As a notable result, the ortho-F substituted α-diimine nickel catalyst produced highly linear polyethylenes with an extremely high molecular weight (Mw =8703 kDa) and a significantly low degree of branching of 1.4/1000 C; however, the meta-F and/or para-F substituted α-diimine nickel catalysts generated highly branched (up to 80.2/1000 C) polyethylenes with significantly low molecular weights (Mw =20-50 kDa).

Uniformly aligned liquid crystal molecules on reformed poly(ethylene-co-vinyl acetate) layers driven by ion beam exposure

ABSTRACT The characteristics of a poly(ethylene-co-vinyl acetate) (PEVA) film formed by spin coating process and ion beam (IB) irradiation was investigated with irradiation times. A physicochemical analysis was used to analyse the suitability of the PEVA film as a uniform liquid crystal (LC) alignment layer. Atomic force microscopy (AFM) revealed an increased surface roughness after IB irradiation. X-ray photoelectron spectroscopy (XPS) analysis showed IB irradiation broke the polymer branches and increased the oxygen concentration and bonding on the surface. These derived the strong Van der Waals forces and dipole-dipole interactions between the PEVA surface and LCs. The IB irradiation generated surface anisotropy on the PEVA film and unidirectional LC alignment was achieved. Polarised optical microscopy (POM) and pre-tilt angle measurements were carried out to show the uniform LC alignment. The transmittance curve exhibited a high optical performance. These results show the promising capabilities of IB irradiated PEVA film as an alignment layer in LC displays.

Tailoring a new hyperbranched PEGylated dendrimer nano-polymer as a super-adsorbent for magnetic solid-phase extraction and determination of letrozole in biological and pharmaceutical samples

A novel hyperbranched magnetic dendrimer was successfully fabricated and utilized as an efficient adsorbent for solid-phase extraction of letrozole from different media (plasma, urine, and tablet matrices) in this study. Iron oxide nanoparticles as magnetic cores were modified with PEGylated block-dendrimer polymer branches. The Fe3O4@SiO2/DAG/MMA/ED/Gly-PEGylated was characterized by a Vibrating sample magnetometer (VSM), Thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FTIR), and Scanning electron microscope (SEM) analyses. The average particle size was obtained as 47 ± 4.3 nm for the nano-adsorbent. The experimental parameters influencing the extraction efficiency of the letrozole, such as pH of the sample solution, and contact time were carefully examined and optimized. Moreover, the isotherm adsorption and kinetic studies demonstrated that the adsorption process was well fitted to Langmuir isotherm and Pseudo-first-order kinetic models, respectively. A maximum adsorption capacity of 20 mg g−1 was acquired based on the Langmuir model. Under optimal conditions, the suggested approach provided favorable linearity in the range of 0.1–15 μg L−1, low limits of detection (1.36 μg L−1), and quantification (4.54 μg L−1). The recoveries for spiking 1 μg L−1 of letrozole in plasma and urine samples and letrozole tablet were respectively, 75.6%, 98.4%, and 93.7%. Relative standard deviation values for plasma, urine, and tablet were respectively, obtained to be 1.1, 05, and 0.4. These results showed that the suggested technique was a potent analytical tool and a helpful alternative for common analysis of such medicines.