Unimolecular Combination of an Atom Transfer Radical Polymerization Initiator and a Lactone Monomer as a Route to New Graft Copolymers

Tailoring the Architecture of Molecular Bottlebrushes via Click Grafting-Onto Strategy

Molecular bottlebrush (MBB) refer to a synthetic macromolecule, in which a mass of polymeric side chains (SCs) are covalently connected to a macromolecular backbone densely, representing an important type of unimolecular nanomaterial. The chemical composition, size, shape, and surface property of MBB can be precisely tailored by varying the backbones and SCs as well as the grafting density (Gdst ). Meanwhile, the topological structure of backbones and SCs can also significantly affect the chemical and physical properties of MBBs. For the past few years, by combining the structure features of MBB, the polymers with diverse architectures using MBB as building block are synthesized, including linear, branched, and cyclic MBB etc. These promising architectural features will bring MBBs with diverse architectures and lots of applications in advanced materials. For this reason, this work is interested in giving a briefly summary of the recent progress on tailor of well-defined MBBs with diverse architectures using grafting-onto strategy combined with controlled polymerization technique.

Poly(ε-caprolactone)-Based Graft Copolymers: Synthesis Methods and Applications in the Biomedical Field: A Review

Synthetic biopolymers are attractive alternatives to biobased polymers, especially because they rarely induce an immune response in a living organism. Poly ε-caprolactone (PCL) is a well-known synthetic aliphatic polyester universally used for many applications, including biomedical and environmental ones. Unlike poly lactic acid (PLA), PCL has no chiral atoms, and it is impossible to play with the stereochemistry to modify its properties. To expand the range of applications for PCL, researchers have investigated the possibility of grafting polymer chains onto the PCL backbone. As the PCL backbone is not functionalized, it must be first functionalized in order to be able to graft reactive groups onto the PCL chain. These reactive groups will then allow the grafting of new reagents and especially new polymer chains. Grafting of polymer chains is mainly carried out by "grafting from" or "grafting onto" methods. In this review we describe the main structures of the graft copolymers produced, their different synthesis methods, and their main characteristics and applications, mainly in the biomedical field.

Tandem polymerization consisting of cyclotrimerization and the Tishchenko reaction: synthesis of acid- and alkali-degradable polymers with cyclic acetal and ester structures in the main chain

Tandem polymerization consisting of cyclotrimerization and the Tishchenko reaction was developed, which yielded acid- and alkali-degradable polymers.

Baeyer-Villiger-Including Domino Two-Step Oxidations of β-O-Substituted Primary Alcohols: Reflection of the Migratory Aptitudes of O-Substituted Alkyl Group in the Outcome of the Reaction

Following the recent development of the one-pot two-step oxidation protocol for primary β-alkoxy alcohols, additional primary β-oxy alcohols were examined under similar conditions. The outcome of these reactions closely follows the migratory aptitudes of the related α-oxy-alkyls in Baeyer-Villiger oxidation vs. that of hydrogen. Thus, these experiments helped to establish the correct order of these aptitudes for α-oxy-alkyls. Furthermore, in the case of primary β,β-dialkoxyalcohols, the formation of dialkoxymethyl formates by the domino oxidation reaction was followed by secondary reactions, forming a number of interesting products.

Organocatalyzed ring-opening copolymerization of α-bromo-γ-butyrolactone with ε-caprolactone for the synthesis of functional aliphatic polyesters - pre-polymers for graft copolymerization

Diphenyl phosphate (DPP) was exploited as an organocatalyst to synthesize copolymers by ring-opening polymerization with α-bromo-γ-butyrolactone (αBrγBL) and ε-caprolactone (εCL) as monomers and polyethylene glycol (PEG) as initiator. The conversion rates of monomers and molecular weights of copolymers synthesized under different conditions were determined by ¹H-NMR. The ¹H-NMR results showed that the copolymers of αBrγBL and εCL initiated by PEG (PEGCB) were successfully synthesized and the conversions of εCL were relatively high (＞70%), while the conversions of αBrγBL were relatively low (＜26%). The highest molar ratio of αBrγBL to εCL units in these copolymers is 0.17, when the copolymerization was carried out at 100℃ for 17h.

The bromine atoms hanged on the chain of the copolymers PEGCB provide a good opportunity to construct graft copolymers via atom transfer radical polymerization (ATRP). The subsequent grafting of 2-(dimethylamino)ethyl methacrylate (DMAEMA) was conducted by using PEGCB3 as macroinitiator, CuBr/N,N’,N’,N”,N”- pentamethyldiethylenetriamine (PMDETA) as catalysts and toluene/anisole as solvents via ATRP. According to the analysis of ¹H-NMR, the grafting efficiency, grafting ratio and grafting frequency were 22.4%, 160.7% and 1133.8, respectively.

Functional biodegradable polymers via ring-opening polymerization of monomers without protective groups

Biodegradable polymers are of current interest and chemical functionality in such materials is often demanded in advanced biomedical applications. Functional groups often are not tolerated in the polymerization process of ring-opening polymerization (ROP) and therefore protective groups need to be applied. Advantageously, several orthogonally reactive functions are available, which do not demand protection during ROP. We give an insight into available, orthogonally reactive cyclic monomers and the corresponding functional synthetic and biodegradable polymers, obtained from ROP. Functionalities in the monomer are reviewed, which are tolerated by ROP without further protection and allow further post-modification of the corresponding chemically functional polymers after polymerization. Synthetic concepts to these monomers are summarized in detail, preferably using precursor molecules. Post-modification strategies for the reported functionalities are presented and selected applications highlighted.

Branched polyesters: Preparative strategies and applications

In the last 20years, the availability of precision chemical tools (e.g. controlled/living polymerizations, 'click' reactions) has determined a step change in the complexity of both the macromolecular architecture and the chemical functionality of biodegradable polyesters. A major part in this evolution has been played by the possibilities that controlled macromolecular branching offers in terms of tailored physical/biological performance. This review paper aims to provide an updated overview of preparative techniques that derive hyperbranched, dendritic, comb, grafted polyesters through polycondensation or ring-opening polymerization mechanisms.

Application of Hyperbranched Block Copolymers as Templates for the Generation of Nanoporous Organosilicates

A general route to organic–inorganic hybrids with nanophase morphologies has been elaborated with the objective of ultimately generating nanoporosity in organosilicates. Hyperbranched block copolymers prepared by either the sequential or concurrent polymerization of an ABC monomer ( γ-( ∊-caprolactone) 2-bromo-2-dimethylpropionate) with a BCD monomer (2-hydroxyethyl methacrylate) were used as the macromolecular templates. The two monomers, each polymerizing by different chemistries, for example ring-opening polymerization and atom transfer radical polymerization, bear initiating centres that are targeted for the functionality located on the accompanying monomer. Consequentially, a branched polymer is obtained which avoids the traditional multistep procedures. The branching density was altered simply by the addition of the appropriate AB ( ∊-caprolactone) and/or CD (methyl methacrylate) comonomers. These polymers were readily soluble initially in the organosilicate prepolymer (methyl silsesquioxane), however, upon the onset of crosslinking, both the solubility parameters and molecular weight of the organosilicate (polymethylsilsesquioxane) change, causing the hyperbranched polymer to phase separate by a nucleation and growth process. The organic polymer was selectively removed by thermolysis, producing a nanoporous inorganic structure. The size and shape of the pores are identical to those of the initial hybrid morphology. A significant reduction in the dielectric constant was achieved by simply replacing a portion of the glass matrix with air, which has a dielectric constant of 1.0.

The Promising Nanocarrier for Doxorubicin and siRNA Co-delivery by PDMAEMA-based Amphiphilic Nanomicelles

Synergistic effects of anticancer drug and siRNA have displayed superior advantages for cancer therapy. Herein, we deeply analyzed the feasibility that whether doxorubicin (DOX) and siRNA could be co-delivered by mPEG-PCL-graft-PDMAEMA (PECD) micelles, which mediated excellent DNA/siRNA delivery in vitro and in vivo reported in our previous work. DOX-loaded NPs (PECD-D) were developed by nanoprecipitation technology and exhibited high drug loading content (DLC, 9.5%). In vitro cytotoxicity study in MDA-MB-231 cells, PECD-D treated groups had lower IC50 compared to free DOX groups (F-DOX) at different transfection time (24, 48, and 72h), which maybe attribute to its high cellular uptake and endosomal escape properties. The speculation was confirmed with the results of drug release profile in acidic media, flow cytometry analysis and confocal images. Futhermore, Cy5 labeled siRNA was introduced in PECD-D micelles (PECD-D/siRNA) to track the behavior of dual-loaded nanodrug in vitro and in vivo. Flow cytometry analysis presented that DOX and siRNA were successfully co-delivered into cells, the positive cells ratio were 94.6 and 99.5%, respectively. Confocal images showed that not only DOX and siRNA existed in cytoplasm, but DOX traversed endosome/lysosome and entered into cell nucleus. For in vivo tumor-targeting evaluation in BALB/c nude mice, both DOX and Cy5-siRNA could be detected in tumor sites after intravenous injection with PECD-D/siRNA formulation. Therefore, we believed that PECD micelles have a potential ability as DOX and siRNA co-delivery carrier for cancer therapy.

Synthesis of degradable poly(ε-caprolactone)-based graft copolymers via a “grafting-from” approach

The controlled ring-opening polymerisation (ROP) of α-bromo-ε-caprolactone (αBrCL), a derivative of ε-caprolactone (εCL), and its copolymerisation with εCL is reported.

Dual responsive polymeric nanoparticles prepared by direct functionalization of polylactic acid-based polymers via graft-from ring opening metathesis polymerization

Polylactic acid (PLA) has found widespread use in plastics and in biomedical applications due to its biodegradability into natural benign products. However, PLA-based materials remain limited in usefulness due to difficulty of incorporating functional groups into the polymer backbone. In this paper, we report a strategy for PLA functionalization that establishes the preparation of highly derivatized materials in which ring opening metathesis polymerization (ROMP) is employed as a graft-from polymerization technique utilizing a norbornene-modified handle incorporated into the PLA backbone. As a demonstration of this new synthetic methodology, a PLA-derived nanoparticle bearing imidazole units protected with a photolabile group was prepared. The morphology of this material could be controllably altered in response to exposure of UV light or acidic pH as a stimulus. We anticipate that this graft-from approach to derivatization of PLA could find broad use in the development of modified, biodegradable PLA-based materials.

Structural mediation on polycation nanoparticles by sulfadiazine to enhance DNA transfection efficiency and reduce toxicity

Reducing the toxicity while maintaining high transfection efficiency is an important issue for cationic polymers as gene carriers in clinical application. In this paper, a new zwitterionic copolymer, polycaprolactone-g-poly(dimethylaminoethyl methyacrylate-co-sulfadiazine methacrylate) (PC-SDZ) with unique pH-sensitivity, was designed and prepared. The incorporation of sulfadiazine into poly(dimethylaminoethyl methacrylate) (PDMAEMA) chains successfully mediates the surface properties including compacter shell structure, lower density of positive charges, stronger proton buffer capability, and enhanced hydrophobicity, which lead to reduction in toxicity and enhancements in stability, cellular uptake, endosome escape, and transfection efficiency for the PC-SDZ2 nanoparticles (NPs)/DNA complexes. Excellent transfection efficiency at the optimal N/P ratio of 10 was observed for PC-SDZ2 NPs/DNA complexes, which was higher than that of the commercial reagent-branched polyethylenimine (PEI). The cytotoxicity was evaluated by CCK8 measurement, and the results showed significant reduction in cytotoxicity even at high concentration of complexes after sulfadiazine modification. Therefore, this work may demonstrate a new way of structural mediation of cationic polymer carriers for gene delivery with high efficiency and low toxicity.

Visible-Light Initiated Free-Radical/Cationic Ring-Opening Hybrid Photopolymerization of Methacrylate/Epoxy: Polymerization Kinetics, Crosslinking Structure, and Dynamic Mechanical Properties

The effects of polymerization kinetics and chemical miscibility on the crosslinking structure and mechanical properties of polymers cured by visible-light initiated free-radical/cationic ring-opening hybrid photopolymerization are determined. A three-component initiator system is used and the monomer system contains methacrylates and epoxides. The photopolymerization kinetics is monitored in situ by Fourier transform infrared-attenuated total reflectance. The crosslinking structure is studied by modulated differential scanning calorimetry and dynamic mechanical analysis. X-ray microcomputed tomography is used to evaluate microphase separation. The mechanical properties of polymers formed by hybrid formed by free-radical polymerization. These investigations mark the first time that the benefits of the chain transfer reaction between epoxy and hydroxyl groups of methacrylate, on the crosslinking network and microphase separation during hybrid visible-light initiated photopolymerization, have been determined.

Effects of hydrophobic core components in amphiphilic PDMAEMA nanoparticles on siRNA delivery

Due to their biodegradable character, polyesters such as polycaprolactone (PCL), poly(D,L-lactide) (PDLLA), and polylactic-co-glycolic acid (PLGA) were widely used as the hydrophobic cores of amphiphilic cationic nanoparticles (NPs) for siRNA delivery. However, fewer researches focused on facilitating siRNA delivery by adjusting the polyester composition of these nanoparticles. Herein, we investigated the contribution of polyester segments in siRNA delivery in vitro by introducing different ratio of DLLA moieties in PCL segments of mPEG-block-PCL-graft-poly(dimethylamino ethyl methacrylate)(PEG-b-PCL-g-PDMAEMA). It was noticed that compared with the other ratios of DLLA moieties, a certain molar ratio (about 70%) of the NPs, named mPEG45-P(CL21-co-DLLA48)-g-(PDMAEMA29)2 (PECLD-70), showed the highest gene knockdown efficiency but poorest cellular uptake ability in vitro. Further research revealed that NPs with various compositions of the polyester cores showed different physicochemical properties including particle size, zeta potential and stiffness, leading to different endocytosis mechanisms thus influencing the cellular uptake efficiency. Subsequently, we observed that the cells treated by PECLD-70 NPs/Cy5 siRNA complexes exhibited more diffuse Cy5 signal distribution than other NPs by confocal laser scanning microscope, which suggested that siRNA delivered by PECLD-70 NPs/Cy5 siRNA complexes possessed of stronger capabilities in escaping from endosome/lysosome, entering the RNA-induced silencing complex (RISC) and cutting the target mRNA efficiently. The different siRNA release profile was dominated by the degradation rate of polyester segments. Therefore, it could be concluded that the adjustment of hydrophobic core of cationic nanoparticles could significantly affect their transfection behavior and appropriate polyester composition should be concerned in designing of analogous siRNA vectors.

Contribution of hydrophobic/hydrophilic modification on cationic chains of poly(ε-caprolactone)-graft-poly(dimethylamino ethylmethacrylate) amphiphilic co-polymer in gene delivery

Nanoparticles (NPs) assembled from amphiphilic polycations have been certified as potential carriers for gene delivery. Structural modification of polycation moieties may be an efficient route to further enhance gene delivery efficiency. In this study two electroneutral monomers with different hydrophobicities, 2-hydroxyethyl methacrylate (HEMA) and 2-hydroxyethyl acrylate (HEA), were incorporated into the cationic poly(dimethylamino ethyl methacrylate) (PDMAEMA) side-chains of amphiphilic poly(ε-caprolactone)-graft-poly(dimethylamino ethylmethacrylate) (PCD) by random co-polymerization, to obtain poly(ε-caprolactone)-graft-poly(dimethylamino ethyl methacrylate-co-2-hydroxyethyl methacrylate) (PCD-HEMA) and poly(ε-caprolactone)-graft-poly(dimethylamino ethyl methacrylate-co-2-hydroxyethyl acrylate) (PCD-HEA). Minimal HEA or HEMA moieties in PDMAEMA do not lead to statistically significant changes in particle size, zeta potential, DNA condensation properties and buffering capacity of the naked NPs. However, the incorporation of HEMA and HEA lead to reductions and increases, respectively, in the surface hydrophilicity of the naked NPs and NPs/DNA complexes, which was confirmed by water contact angle assay. These simple modifications of PDMAEMA with HEA and HEMA moieties significantly affect the gene transfection efficiency on HeLa cells in vitro: PCD-HEMA NP/DNA complexes show a much higher transfection efficiency than PCD NPs/DNA complexes, while PCD-HEA NPs/DNA complexes show a lower transfection efficiency than PCD NP/DNA complexes. Fluorescence activated cell sorter and confocal laser scanning microscope results indicate that the incorporation of hydrophobic HEMA moieties facilitates an enhancement in both cellular uptake and endosomal/lysosomal escape, leading to a higher transfection efficiency. Moreover, the process of endosomal/lysosomal escape confirmed in our research that PCD and its derivatives do not just rely on the proton sponge mechanism, but also on membrane damage due to the polycation chains, especially hydrophobic modified ones. Hence, it is proved that hydrophobic modification of cationic side-chains is a crucial route to improve gene transfection mediated by polycation NPs.

Surface grafted poly(ε-caprolactone) prepared using organocatalysed ring-opening polymerisation followed by SI-ATRP

The controlled ring-opening polymerisation (ROP) of an ATRP initiator-containing lactone, γ-BMPCL, and its copolymerisation with ε-caprolactone is reported. One resulting copolymer was successfully used as a substrate for surface initiated ATRP to produce surface-grafted poly(oligo(ethylene glycol) methacrylate) brushes.

Sequence-regulated copolymers via tandem catalysis of living radical polymerization and in situ transesterification

Sequence regulation of monomers is undoubtedly a challenging issue as an ultimate goal in polymer science. To efficiently produce sequence-controlled copolymers, we herein developed the versatile tandem catalysis, which concurrently and/or sequentially involved ruthenium-catalyzed living radical polymerization and in situ transesterification of methacrylates (monomers: RMA) with metal alkoxides (catalysts) and alcohols (ROH). Typically, gradient copolymers were directly obtained from the synchronization of the two reactions: the instantaneous monomer composition in feed gradually changed via the transesterification of R(1)MA into R(2)MA in the presence of R(2)OH during living polymerization to give R(1)MA/R(2)MA gradient copolymers. The gradient sequence of monomers along a chain was catalytically controlled by the reaction conditions such as temperature, concentration and/or species of catalysts, alcohols, and monomers. The sequence regulation of multimonomer units was also successfully achieved in one-pot by monomer-selective transesterification in concurrent tandem catalysis and iterative tandem catalysis, providing random-gradient copolymers and gradient-block counterparts, respectively. In contrast, sequential tandem catalysis via the variable initiation of either polymerization or in situ transesterification led to random or block copolymers. Due to the versatile adaptability of common and commercially available reagents (monomers, alcohols, catalysts), this tandem catalysis is one of the most efficient, convenient, and powerful tools to design tailor-made sequence-regulated copolymers.

Structural contributions of blocked or grafted poly(2-dimethylaminoethyl methacrylate) on PEGylated polycaprolactone nanoparticles in siRNA delivery

The multiformity in polymer structure and conformation design provides a great potential in improving the gene silencing efficiency of siRNA by polymer vectors. In order to provide information on the polymer design for siRNA delivery, the structural contributions of blocked or grafted poly(2-dimethylaminoethyl methacrylate) on PEGylated polycaprolactone nanoparticles (NPs) in siRNA delivery were studied. Herein, two kinds of self-assembly nanoparticles (NPs) formed by amphiphilic cationic polymers, methoxy poly(ethylene glycol)-block-polycaprolactone-block-poly(2-dimethylaminoethyl methacrylate) (mPEG-PCL-b-PDMAEMA, PECbD) and methoxy poly(ethylene glycol)-block-(polycaprolactone-graft-poly(2-dimethylaminoethyl methacrylate)) (mPEG-PCL-g-PDMAEMA, PECgD), were used to deliver siRNA for in vitro and in vivo studies. The physiochemical properties including size and zeta potential of PECbD NPs/siRNA and PECgD NPs/siRNA complexes were characterized. In vitro cytotoxicity, cellular uptake and siRNA knockdown efficiency were evaluated in HeLa-Luc cells. The endosome escape and intracellular distribution of PECbD NPs/siRNA and PECgD NPs/siRNA in HeLa-Luc cells were also observed. In vivo polymer mediated siRNA delivery and the complexes distribution in isolated organs were studied using mice and tumor-bearing mice. At the same total degree of polymerization (DP) of DMAEMA, PECgD NPs/siRNA complexes possessed higher zeta potentials than PECbD NPs/siRNA complexes (at the same N/P ratio), which may be the reason that PECgD NPs/siRNA complexes can deliver more siRNA into the cytoplasm and lead to higher in vitro luciferase and lamin A/C silencing efficiency than PECbD NPs/siRNA complexes. The in vivo imaging measurement and histochemical analysis also confirmed that siRNA could be delivered to lungs, livers, pancreas and HeLa-Luc tumors more efficiently by PECgD NPs than PECbD NPs. Meanwhile, the PDMAEMA chains of PECgD could be shortened which provides benefits for clearing. Therefore, PECgD NPs have great potential to be used as efficient non-viral carriers for in vivo siRNA delivery.

A Facile Strategy for Preparation of α-Heterobifunctional Polystyrenes with Well-Defined Molecular Weight

A facile strategy for synthesis of α-heterobifunctional polystyrenes is reported. The novel functional polystyrenes have been successfully synthesized via a combination of atom transfer radical polymerization (ATRP) and chemical modification of end-functional groups. First, ε-caprolactone end-capped polystyrenes with controlled molecular weight and low polydispersity were prepared by ATRP of styrene using α-bromo-ε-caprolactone (αBrCL) as an initiator. Then, removal of the terminal bromine atom was performed with iso-propylbenzene in the presence of CuBr/PMDETA. Finally, ring-opening modifications of the caprolactone group were carried out with amines, n-butanol and H(2) O to produce novel polystyrenes containing two different functional groups at one end.