Side‐Chain Modification and “Grafting Onto” via Olefin Cross‐Metathesis

Olefin cross‐metathesis is introduced as a versatile polymer side‐chain modification technique. The reaction of a poly(2‐oxazoline) featuring terminal double bonds in the side chains with a variety of functional acrylates has been successfully performed in the presence of Hoveyda–Grubbs second‐generation catalyst. Self‐metathesis, which would lead to polymer–polymer coupling, can be avoided by using an excess of the cross‐metathesis partner and a catalyst loading of 5 mol%. The results suggest that bulky acrylates reduce chain–chain coupling due to self‐metathesis. Moreover, different functional groups such as alkyl chains, hydroxyl, and allyl acetate groups, as well as an oligomeric poly(ethylene glycol) and a perfluorinated alkyl chain have been grafted with quantitative conversions.     

“Supramolecular” Amphiphiles Created by Wrapping Poly(styrene) with the Helix‐Forming β‐1,3‐Glucan Polysaccharide

We have demonstrated that giant polymer micelles with a uniform diameter (ca. 200 nm) can be fabricated by “supramolecular wrapping” of poly(styrene) (PS) with the β‐1,3‐glucan polysaccharide, with the β‐1,3‐glucan fastening the PS chains together in a noncovalent fashion to facilitate the formation of a supramolecular polymer network on the O/W emulsion surface. Various spectroscopic and microscopic investigations have revealed that the inner cores of the micelles are comprised of a hydrophobic PS network, whereas the surfaces consist of a hydrophilic β‐1,3‐glucan layer. Accordingly, functional guest molecules can easily be encapsulated inside the cavity through hydrophobic interactions. The encapsulated molecules can simply be released from the micelle cores by peeling off the β‐1,3‐glucan shell in a supramolecular manner. As functional groups can be introduced into the glucose side‐chain unit in a straightforward manner by chemical modification, the micellar surface can acquire further functions useful for molecular recognition. These results show that the micelles obtained could have applications as novel soft nanoparticles, which would be indispensable not only for nanotechnologies, but also for biotechnologies aimed at gene or drug delivery systems.     

Photoluminescent behavior of poly(3‐hexylthiophene) derivatives with a high azobenzene content in the side chains

A series of polythiophene derivatives with substantially higher azobenzene contents in the side chains were prepared via copolymerization of 3‐hexylthiophene with four different types of 4‐((4‐(phenyl)azo)phenoxy)alkyl‐3‐thienylacetate. The alkyl spacers with different lengths, i.e. butyl, hexyl, octyl and undecyl groups were used between the azobenzene group and the thiophene ring. The compositions, structures and thermal properties of these polythiophene derivatives were characterized. The structural dependence of photoluminescent emission, photochromic behavior of these copolymers were systematically studied and compared with poly(3‐hexylthiophene). The results show that the azobenzene substitution renders the polythiophene some interesting optical properties that can be modulated by UV light irradiation. In the azobenzene modified polythiophene, the intensity of photoluminescent emission associated with the conjugated polythiophene main chain was found to decrease significantly upon UV irradiation. The finding suggests that the photo‐induced trans‐cis isomerization of the azobenzene pendant groups has a significant effect on photoluminescent emission, particularly when short spacers are used between azobenzene groups and the main chain. However, the effect becomes less prominent when longer spacers are used between the azobenzene group and the main chain. Furthermore, UV irradiation of the copolymers also resulted in an increase in intensity and broadening of bandwidth for the absorption peak associated with the polythiophene backbones. Again the magnitude of intensity changes upon UV irradiation were found to be dependent on the spacer length between the azobenzene group and polythiophene main chain. Copyright © 2005 John Wiley & Sons, Ltd.     

Molecular dynamics simulations of end‐grafted polymers with charged side chains

We report molecular dynamics simulations on bottle‐brush polyelectrolytes end‐grafted to a planar surface. For each bottle‐brush polyelectrolyte, flexible charged side chains are anchored to one neutral main chain. The effects of the counterion valence and the grafting density on the density profiles and the structural characteristics of the brush were studied in this work. It is found that the electrostatic repulsion between charged monomers in the side chains leads an extended conformation of the brush in a solution containing monovalent counterions, while strong electrostatic binding of multivalent counterions to the side chains has a significant contribution to the collapse of the brush. For the trivalent case, the distribution of end monomers in the main chains becomes broader upon decreasing the grafting density, as compared with the monovalent case. However, the position of the distribution for the monovalent case is relatively insensitive to the change of the grafting density. Additionally, with increased counterion valence, enhanced electrostatic correlation between counterions and charged side chains also weakens the diffusive ability of counterions. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Macromolecular brushes synthesized by “grafting from” approach based on “click chemistry” and RAFT polymerization

Well‐defined macromolecular brushes with poly(N‐isopropyl acrylamide) (PNIPAM) side chains on random copolymer backbones were synthesized by “grafting from” approach based on click chemistry and reversible addition‐fragmentation chain transfer (RAFT) polymerization. To prepare macromolecular brushes, two linear random copolymers of 2‐(trimethylsilyloxy)ethyl methacrylate (HEMA‐TMS) and methyl methacrylate (MMA) (poly(MMA‐co‐HEMA‐TMS)) were synthesized by atom transfer radical polymerization and were subsequently derivated to azide‐containing polymers. Novel alkyne‐terminated RAFT chain transfer agent (CTA) was grafted to polymer backbones by copper‐catalyzed 1,3‐dipolar cycloaddition (azide‐alkyne click chemistry), and macro‐RAFT CTAs were obtained. PNIPAM side chains were prepared by RAFT polymerization. The macromolecular brushes have well‐defined structures, controlled molecular weights, and molecular weight distributions (M_w/M_n ≦ 1.23). The RAFT polymerization of NIPAM exhibited pseudo‐first‐order kinetics and a linear molecular weight dependence on monomer conversion, and no detectable termination was observed in the polymerization. The macromolecular brushes can self‐assemble into micelles in aqueous solution. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 443–453, 2010     

Controlled Stability of the Triple‐Stranded Helical Structure of a β‐1,3‐Glucan with a Chromophoric Aromatic Moiety at a Peripheral Position

We synthesized a semiartificial β‐1,3‐glucan, curdlan with dialkylaniline groups (CUR‐DA), that bears chromophoric aromatic groups at its peripheral positions. Spectroscopic studies as well as microscopic observations indicate that CUR‐DA adopts a triple‐stranded helical structure in water‐ or methanol‐rich solutions of dimethyl sulfoxide (DMSO). This triple‐stranded helical structure exhibits high thermal stability and resistance to base, attributes that are similar to those of the triple‐stranded helical structure of native β‐1,3‐glucans such as schizophyllan. Moreover, we found that the stability of the triple‐stranded helical structure can be easily modulated by solvent composition and metal‐ion (Zn²⁺) binding. As β‐1,3‐glucan polysaccharides are known to serve as “polymeric” hosts, including certain DNA molecules, carbon nanotubes, and conjugated polymers, and complexation occurs only with the single‐stranded structure, this information is very useful for the creation of these attractive polymeric composites, the controlled release of DNA, and so on.     

Thermoresponsive brush copolymers with poly(propylene oxide‐ran‐ethylene oxide) side chains via metal‐free anionic polymerization “grafting from” technique

Thermoresponsive brush copolymers with poly(propylene oxide‐ran‐ethylene oxide) side chains were synthesized via a “grafting from” technique. Poly(p‐hydroxystyrene) was used as the backbone, and the brush copolymers were prepared by random copolymerization of mixtures of oxyalkylene monomers, using metal‐free anionic ring‐opening polymerization, with the phosphazene base (t‐BuP₄) being the polymerization promoter. By controlling the monomer feed ratios in the graft copolymerization, two samples with the same side‐chain length and different compositions were prepared, both of which possessed high molecular weights and low molecular weight distributions. The results from light scattering and fluorescence spectroscopy indicated that the brush copolymers in their dilute aqueous solutions were near completely solvated at low temperature and underwent slight intramolecular chain contraction/association and much more profound intermolecular aggregation at different stages of the step‐by‐step heating process. Above 50 °C, very turbid solutions, followed by macrophase separation, were observed for both of the samples, which implied that it was difficult for the brush copolymers to form stable nanoscopic aggregates at high temperature. All these observations were attributed, at least partly, to the distribution of the oxyalkylene monomers along the side chains and the overall brush‐like molecular architecture. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2320–2328, 2010     

Synthesis of Self‐Threading Bithiophenes and their Structure–Property Relationships Regarding Cyclic Side‐Chains with Atomic Precision

We have recently reported a self‐threading polythiophene as a new family of insulated molecular wires. Herein, we focused on the structure–property relationships of the unique three‐dimensional architecture of the monomer. We have synthesized nine self‐threading bithiophene monomers that have cyclic side‐chains of different size and flexibility: i.e., 21‐, 22‐, 23‐, 24‐, 26‐, and 30‐membered rings composed of paraffinic, olefinic, or alkynic chains. To investigate their structure–property relationships, ¹H NMR spectroscopy, UV absorption, and fluorescence spectroscopy measurements were conducted. We found that cyclic side‐chains define the movable range of the dihedral angle of the bithiophene backbone, thereby affecting its photophysical properties. Therefore, the ability to design a structure with atomic precision as described herein would lead to the fine‐tuning of the electronic properties of insulated molecular wires.     

Conformational properties and dynamics of molecular bottle‐brushes: A cellular‐automaton‐based simulation

Extensive computer simulation was performed using the bond‐fluctuation model and cellular‐automaton (CA)‐based simulation technique to probe the equilibrium structure and dynamical behavior of comb‐branched polymers in which the flexible side chains of a given length are placed regularly along the backbone and the number of branches increases linearly with total molecular weight. By applying very efficient CA algorithm – the “lattice molecular dynamics” (LMD) method – we have been able to study the properties of sufficiently large structures (up to 5880 monomeric units). Depending on the length of main and side chains as well as on interbranch spacing, we have calculated mean chain dimensions, local fractal dimensionalities, particle scattering functions, time autocorrelation functions, etc. The following main conclusions may be drawn from the results presented in our study: (i) The critical exponent, governing the mean size of the main chain, remains unchanged from its value known for a 3d self‐avoiding walk (SAW). On the other hand, two‐dimensional branched macromolecules with one‐sided branches are effectively in a collapsed state even under conditions of a good solvent, forming specific helical superstructures. (ii) Comparison of the simulated data with the predictions of the scaling model indicates that the latter is valid in describing the mean dimensions of the backbone as a function of side‐chain length and interbranch spacing. (iii) The excluded volume interaction between side chains dramatically slows down the relaxation of the backbone chain.     

Degradable cellulose‐based polymer brushes with controlled grafting densities

Cellulose‐based polymer brushes with variable grafting densities and low dispersity were synthesized by grafting poly(n‐butyl acrylate) (PBA) side chains from cellulose‐derived backbones via ATRP. Esterification of commercially available cellulose acetate with 2‐bromoisobutyryl bromide (2‐BiBB) in NMP provided cellulose‐based macroinitiators averaging one initiation site per double glucose unit. ATRP macroinitiators averaging up to 6 initiation sites per repeating double glucose unit were prepared by acylation of microcrystalline cellulose (MCC) in LiCl/DMAc solvent system with 2‐BiBB. A series of linear macroinitiators with narrow MWD were obtained by fractional precipitation process. The content of initiating sites was determined by elemental analysis. (Meth)acrylate side chains were then grafted from the cellulose‐based macroinitiators. The prepared cellulose‐based polymer brushes showed tunable degradation rates dependent on grafting density of the brush, following two different degradation pathways, either cleavage of the main chain or detachment of the side chains. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 2426–2435     

Synthesis and properties of polythiophenes with conjugated side‐chains containing carbon–carbon double and triple bonds

Two soluble side‐chain conjugated polythiophenes, poly{3‐[2‐(4‐octyloxy‐phenyl)‐vinyl]‐thiophene} (P3OPVT) and poly{3‐(4‐octyloxy‐phenylethynyl)‐thiophene} (P3OPET) have been synthesized successfully. In P3OPVT and P3OPET, substituted benzene rings are connected with the polythiophene backbone through trans carbon–carbon double bond and carbon–carbon triple bond, respectively. Absorption spectra of the P3OPVT and P3OPET both show two absorption peaks located in UV and visible region, respectively. The results of optical and electrochemical measurements indicate that the conjugated side‐chains can reduce the bandgap effectively. This type of side‐chain conjugated polythiophenes may be promising for the applications in polymer photovoltaic cells and field effect transistors. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2206–2214, 2006     

Synthesis of a “molecular rope curtain”: Preparation and characterization of a sliding graft copolymer with grafted poly(ethylene glycol) side chains by the “grafting onto” strategy

A sliding graft copolymer (SGC) with poly(ethylene glycol) (PEG) side chains was prepared by ester formation between terminal carboxyl groups of oxidized PEG methyl ether with molecular weight of 2000 (mPEG2000‐COOH) and hydroxyl groups of a polyrotaxane consisting of PEG and cyclodextrins (CDs). Formation of the SGC structure was confirmed by ¹H NMR, attenuated total reflectance Fourier‐transformed infrared, and gel permeation chromatography. The SGC was soluble in good solvents of PEG and insoluble in poor solvents of PEG. Estimation of the number of grafted mPEG chains suggested a “rope‐curtain” like structure, in which an mPEG chain is connected to each CD ring. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Synthesis and characterization of asymmetric centipede‐like copolymers with two side chains at each repeating unit via ATRP and ring‐opening polymerization

A series of well‐defined centipede‐like copolymers with poly(glycidyl methacrylate) (PGMA) as main chain and poly(L ‐lactide) (PLLA) and polystyrene (PSt) as side chains have been synthesized successfully by combination of ring‐opening polymerization and atom transfer radical polymerization (ATRP). The synthetic process includes three steps. (1) Synthesis of PGMA via ATRP; (2) preparation of macroinitiator with one bromine group and a hydroxyl group at every GMA unit of PGMA; (3) ring‐opening polymerization of LLA and ATRP of St to obtain the asymmetric centipede‐like copolymers. The number–average degrees of polymerization of PGMA backbone, PLLA and PSt side chains were determined by ¹H‐NMR spectra, and the molecular weights of the resultant intermediates and centipede‐like copolymers were measured by gel permeation chromatography. The molecular weight distributions were narrow and the molecular weights of both the backbone and the side chains were controllable. The thermal behavior of the centipede‐like copolymers was investigated by differential scanning calorimeter. With the increase of PSt side chain length, the glass transition temperature of PLLA side chains shifted to high temperature, and crystallization ability of PLLA side chains became poor. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5580–5591, 2008     

Packing manner of graft copolymers with rigid‐rod main chains and amorphous‐crystalline diblock copolymers as side chains

We have investigated the morphology and packing manner of graft copolymers consisting of rigid‐rod‐like poly(γ‐benzyl L ‐glutamate) (PBLG) main chains and grafted diblock copolymers of amorphous poly(propylene glycol) (PPG) and crystalline poly(ethylene glycol) (PEG). The results of differential scanning calorimetry and wide‐angle X‐ray scattering measurements for graft copolymers with higher side‐chain volume fractions suggest that the rodlike main chains and crystallized PEG chains exist in segregated domains. Small‐angle X‐ray scattering profiles for these samples show diffraction intensity maxima accompanied by higher order peaks, the positions of which suggest the formation of an ordered layered structure. From these observations, the graft copolymers are estimated to form repeated layered structure consisting of segregated PBLG, PPG, and PEG layers. A proposed model for molecular packing of the graft copolymers is consistent with the experimental observation that the repeating distance for the layered structure decreases with an increase in the volume fraction of side chains. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1904–1912, 2002     

Benzodithiophene‐thiophene‐based photovoltaic polymers with different side‐chains

A series of benzodithiophene‐thiophene‐based alternating copolymers were synthesized with different side‐chains, and their photovoltaic characteristics were examined. The choice of solubilizing side‐chains influences significantly the chain conformation, frontier orbital energy levels, intermolecular organization, and the resulting optical, morphological, and photovoltaic properties. The incorporation of an e‐withdrawing carbonyl group in the side‐chain decreased the highest occupied molecular orbital (HOMO, ca. ?5.4 eV) level and improved the chain planarity through intrachain hydrogen bonding. The shortest π–π stacking distance (3.72 Å) was also measured for the alkylcarbonyl‐substituted BDTCOT:PC₇₁BM blended film by two dimensional grazing incidence X‐ray scattering. With compared to other polymers, the BDTCOT:PC₇₁BM device showed a substantially improved open‐circuit voltage and short‐circuit current density, leading to a 4.66% power conversion efficiency. The side‐chains need to be designed to be multifunctional to induce a deep HOMO level and chain planarity (for interchain ordering) as well as good solution processability. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 854–862     

Automated Solid‐Phase Synthesis of a β‐(1,3)‐Glucan Dodecasaccharide

β‐Glucans are a group of structurally heterogeneous polysaccharides found in bacteria, fungi, algae and plants. β‐(1,3)‐D ‐Glucans have been studied in most detail due to their impact on the immune system of vertebrates. The studies into the immunomodulatory properties of these glucans are typically carried out with isolates that contain a heterogeneous mixture of polysaccharides of different chain lengths and varying degrees of branching. In order to determine the structure–activity relationship of β‐(1,3)‐glucans, access to homogeneous, structurally‐defined samples of these oligosaccharides that are only available through chemical synthesis is required. The syntheses of β‐glucans reported to date rely on the classical solution‐phase approach. We describe the first automated solid‐phase synthesis of a β‐glucan oligosaccharide that was made possible by innovating and optimizing the linker and glycosylating agent combination. A β‐(1,3)‐glucan dodecasaccharide was assembled in 56 h in a stereoselective fashion with an average yield of 88 % per step. This automated approach provides means for the fast and efficient assembly of linker‐functionalized mono‐ to dodecasaccharide β‐(1,3)‐glucans required for biological studies.     

Influence of Side‐Chain Composition on Polythiophene Properties and Supramolecular Assembly of Anionic Polythiophene Derivatives

A series of 10 polythiophene derivatives is reported, in which each polymer has a different percentage of carboxylic acid‐bearing repeat units. The properties of these polymers are explored under acidic conditions, where the carboxylic acid moieties remain neutral, and under basic conditions, where the carboxylic acid units become anionic carboxylates. The properties that are examined for both solutions and films include UV–vis absorption spectroscopy, photoluminescence spectroscopy, and red‐edge optical band gaps. All the properties studied are strongly dependent both on protonation state and percentage of carboxylic acid/carboxylate side chains along the polymer backbone. The anionic form of each polythiophene derivative was also used in layer‐by‐layer film deposition with a cationic phosphonium polyelectrolyte. The film growth process was studied by spectroscopic techniques to assess the influence of side‐chain composition on the film growth and optical properties. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019     

Circular DNA by “Bis‐Click” Ligation: Template‐Independent Intramolecular Circularization of Oligonucleotides with Terminal Alkynyl Groups Utilizing Bifunctional Azides

A highly effective and convenient “bis‐click” strategy was developed for the template‐independent circularization of single‐stranded oligonucleotides by employing copper(I)‐assisted azide–alkyne cycloaddition. Terminal triple bonds were incorporated at both ends of linear oligonucleotides. Alkynylated 7‐deaza‐2′‐deoxyadenosine and 2′‐deoxyuridine residues with different side chains were used in solid‐phase synthesis with phosphoramidite chemistry. The bis‐click ligation of linear 9‐ to 36‐mer oligonucleotides with 1,4‐bis(azidomethyl)benzene afforded circular DNA in a simple and selective way; azido modification of the oligonucleotide was not necessary. Short ethynyl side chains were compatible with the circularization of longer oligonucleotides, whereas octadiynyl residues were used for short 9‐mers. Compared with linear duplexes, circular bis‐click constructs exhibit a significantly increased duplex stability over their linear counterparts. The intramolecular bis‐click ligation protocol is not limited to DNA, but may also be suitable for the construction of other macrocycles, such as circular RNAs, peptides, or polysaccharides.     

Side‐chain crystallization behavior of graft copolymers consisting of amorphous main chain and crystalline side chains: Poly(methyl methacrylate)‐graft‐poly(ethylene glycol) and poly(methyl acrylate)‐graft‐poly(ethylene glycol)

Graft copolymers consisting of amorphous main chain, poly(methyl methacrylate) (PMMA), or poly(methyl acrylate) (PMAc), and crystalline side chains, poly(ethylene glycol) (PEG), have been prepared by copolymerization of PEG macromonomers with methyl methacrylate or methyl acrylate (MMAx or MACx, respectively). Because of the compatibility of PMMA/PEG and PMAc/PEG, from small‐angle X‐ray scattering results, the main and side chains in graft copolymers were suggested to be homogeneous in the molten state. Differential scanning calorimetry (DSC) cooling scans revealed that PEG side chains for graft copolymers with large PEG fractions were crystallized when the sample was cooled, with a cooling rate of 10 °C/min. The spherulite pattern observed by a polarized optical microscope suggested the growth of PEG crystalline lamellae. Crystallization of PEG in MMAx was more restrained than in MACx. From these results, we have concluded that the crystallization behavior of the grafted side chains is strongly influenced by the glass transition of a homogeneously molten sample as well as dilution of the crystallizable chains. Domain spacings for isothermally crystallized graft copolymers were described by interdigitating chain packing in crystalline–amorphous lamellar structure. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 79–86, 2005     

Design of Well‐Defined Monofunctionalized Poly(3‐hexylthiophene)s: Toward the Synthesis of Semiconducting Graft Copolymers

The post‐functionalization of poly(3‐hexylthiophene) (P3HT) via various synthetic routes is reported. Well‐defined and monofunctionalized ω‐thiol‐terminated P3HT, ω‐carboxylic acid‐terminated P3HT, ω‐acrylate‐terminated P3HT, and ω‐methacrylate‐terminated P3HT are obtained in high yields through a straightforward procedure. From those, different novel P3HT‐based graft copolymers are synthesized following two routes: “grafting onto” and “grafting through” (macromonomer polymerization) methods. The synthesis of three types of graft copolymers is described. Each one has “rod” P3HT‐grafted side chains on a “coil” main chain, which can be polyisoprene, poly(vinyl alcohol), or poly(butyl acrylate). Each copolymer is characterized by size‐exclusion chromatography and NMR.