Side-chain Copolymers Research Articles

Synthesis, characterization and photovoltaic properties of poly(cyclopentadithiophene-alt-isoindigo)

Isoindigo based conducting polymers have attracted extensive interest for polymer solar cell application since isoindigo is a green material and renewable from plants. We have synthesized four soluble low band gap isoindigo based polymers (PCI) with cyclopentadithiophene (CPDT) as the donor unit and isoindigo (I) as the acceptor unit, decorated with two kinds of alkyl side chains, octyl (8) and 2-ethylhexyl (e), via the Stille cross-coupling reaction denoted as PC8I8, PC8Ie, PCeI8 and PCeIe. By changing the side chain of copolymers from linear (PC8I8) to branched (PCeIe), the λmax of absorption is blue shifted from 1.37 to 1.48 eV and the HOMO level is lowered from −5.24 to −5.45 eV. The changes are due to the twist coplanarity of the polymer backbone. The density functional theory calculation revealed that the dihedral angle of copolymers has been increased from 14° to 20°. The properties of PC8Ie and PCeI8 lie between those of PC8I8 and PCeIe. The type of the side chain plays a major role in determining the photovoltaic performance of copolymers. The branched side chain improves the solubility of the polymer and increases the effective phase separation between the copolymer and PCBM. This results in favorable nanomorphology of the active layer. Thus, PCeIe with branched side chains on both donor and acceptor units exhibits the best photovoltaic properties with a Voc of 0.80 eV, Jsc of 11.6 mA cm−2 and fill factor of 43.0% and power conversion efficiency of 4.0%. The power conversion efficiency of this type of polymer could be further improved by optimizing the fabrication conditions and interlayer modification. This study offers a useful guideline for the molecular design of high efficiency isoindigo-based polymer solar cells.

Synthesis and photovoltaic properties of a series of bulk heterojunction solar cells based on interchain-linked conjugated polymers

We have used Suzuki coupling to synthesize a series of maleimide–thiophene copolymers presenting pendent 2-hydroxyethyl and 6-hydroxyhexyl units. The maleimide–thiophene copolymers containing different contents of OH groups were then reacted with 3,3′-dimethoxy-4,4′-biphenylene diisocyanate (DMBPI) in solution to form the interchain-linked polymers. The interchain-linked polymers exhibited excellent solubility in organic solvents. The average molecular weight and thermal stability of the copolymers increased after interchain linking with DMBPI. The wavelengths of maximum absorption of the interchain-linked copolymers were red-shifted relative to those of the corresponding side-chain copolymers. The energy level of the lowest unoccupied molecular orbitals and highest occupied molecular orbitals decreased after the copolymers had undergone interchain linking with DMBPI. We fabricated polymer solar cells (PSCs) from blends of the interchain-linked copolymers and [6,6]-phenyl-C61-butyric acid methyl ester. The photovoltaic performances of the PSCs incorporating the interchain-linked copolymers were superior to those of the corresponding PSCs based on the OH-presenting copolymers.

Branched copolymers with biodegradable core and pendant oxirane groups

The reactive oxirane groups were incorporated into the macromolecule as substituents in the side chains of loosely-grafted copolymer or in the arms of star-shaped copolymer using glycidyl methacrylate (GMA) in the controlled atom transfer radical polymerization (ATRP). The branched GMA copolymers with various architectures were obtained by using hydrophobic copolymers containing six and seven units of caprolactone 2-(methacryloyloxy)ethyl ester (CLMA) functionalized with bromoester groups, and trifunctional poly(ε-caprolactone) (PCL), as well as hydrophilic tri-, and six-functional acetal derivatives of D-glucopyranosides as (macro)initiators with biodegradable and biocompatible properties. The well-defined copolymers with core-shell structures and polymerization degrees of GMA in the range of 20–100 per side chain/arm at 20–70% of monomer conversion within 1–6 h and narrow molecular weight distributions (Mw/Mn = 1.14–1.4) were obtained. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers

Synthesis of densely grafted copolymers with tert-butyl methacrylate/2-(dimethylamino ethyl) methacrylate side chains as precursors for brush polyelectrolytes and polyampholytes

Methacrylate-based densely grafted copolymers were synthesized by atom transfer radical polymerization (ATRP) and activators generated by electron transfer for atom transfer radical polymerization (AGET ATRP) techniques. The linear poly(2-(2-bromoisobutyryloxy)ethyl methacrylate) PBIEM prepared by ATRP served as a macroinitiator backbone. The “grafting from” strategy was used to initiate polymerization of tert-butyl methacrylate (tBuMA) from PBIEM under ATRP and/or AGET ATRP conditions yielding densely grafted copolymers PBIEM-graft-P(tBuMA). The low polydispersity indices (PDI) of the synthesized brushes evidenced by SEC analysis were consistent with a controlled/living radical polymerization (CLRP) mechanism. The chlorine-terminated PBIEM-graft-P(tBuMA)-Cl macroinitiators were subsequently employed for chain extension with 2-(dimethylamino ethyl) methacrylate (DMAEMA) yielding densely grafted copolymers with diblock copolymer side chains PBIEM-graft-P(tBuMA)-block-PDMAEMA. Further, PBIEM macroinitiator was used to initiate the copolymerization of a binary mixture of tBuMA and DMAEMA through both ATRP and AGET ATRP initiating systems, yielding densely grafted copolymers with statistical distribution of the side chains. The reactivity ratios for random graft copolymerization of tBuMA and DMAEMA from PBIEM backbone established by three different methods (Finemann-Ross, Kelen-Tüdös and Error-in-Variable) did not substantially differ from literature values for conventional free-radical copolymerization of the same monomers. Polyampholyte brushes with PMAA-stat-PDMAEMA side chains were eventually synthesized by hydrolysis of the shielding tert-butyl groups.

Aggregation Behavior of Amphiphilic p(HPMA)-co-p(LMA) Copolymers Studied by FCS and EPR Spectroscopy

A combined study of fluorescence correlation spectroscopy and electron paramagnetic resonance spectroscopy gave a unique picture of p(HPMA)-co-p(LMA) copolymers in aqueous solutions, ranging from the size of micelles and aggregates to the composition of the interior of these self-assembled systems. P(HPMA)-co-p(LMA) copolymers have shown high potential as brain drug delivery systems, and a detailed study of their physicochemical properties can help to elucidate their mechanism of action. Applying two complementary techniques, we found that the self-assembly behavior as well as the strength of hydrophobic attraction of the amphiphilic copolymers can be tuned by the hydrophobic LMA content or the presence of hydrophobic molecules or domains. Studies on the dependence of the hydrophobic lauryl side chain content on the aggregation behavior revealed that above 5 mol % laury side-chain copolymers self-assemble into intrachain micelles and larger aggregates. Above this critical alkyl chain content, p(HPMA)-co-p(LMA) copolymers can solubilize the model drug domperidone and exhibit the tendency to interact with model cell membranes.

Thermoresponsive Poly(2‐Oxazoline) Molecular Brushes by Living Ionic Polymerization: Modulation of the Cloud Point by Random and Block Copolymer Pendant Chains

AbstractMolecular brushes (MBs) of poly(2‐oxazoline)s were prepared by living anionic polymerization of 2‐isopropenyl‐2‐oxazoline to form the backbone and living cationic ring‐opening polymerization of 2‐n‐propyl‐2‐oxazoline and 2‐methyl‐2‐oxazoline to form random and block copolymers. Their aqueous solutions displayed a distinct thermoresponsive behavior as a function of the side‐chain composition and sequence. The cloud point (CP) of MBs with random copolymer side chains is a linear function of the hydrophilic monomer content and can be modulated in a wide range. For MBs with block copolymer side chains, it was found that the block sequence had a strong and surprising effect on the CP. While MBs with a distal hydrophobic block had a CP at 70 °C, MBs with hydrophilic outer blocks already precipitated at 32 °C.

Incoherent UV/VIS Lasers Assisted Surface Relief Grating Formation

Two efficient optical methods are demonstrated to improve the formation of surface relief grating (SRG). The SRG formation is realized by a two-beam interference technique (λ=532nm) in an azobenzene side-chain copolymer. An incoherent UV beam (λ=355nm) or an incoherent VIS beam (λ=532nm), called assisting beam, was used to accelerate the photoisomerization process of the cis-form↔trans-form cycle, resulting in an efficient formation of SRG. The dependence of the diffraction efficiency and the relief depth of the SRGs on the polarizations of both assisting beams were investigated showing two different assisting mechanisms.

Dipolar Chromophore Facilitated Huisgen Cross-Linking Reactions for Highly Efficient and Thermally Stable Electrooptic Polymers.

Efficient thermal cross-linking protocol for an azide-alkyne-based Huisgen 1,3-dipolar cycloaddition reaction has been developed for making highly efficient electrooptic (EO) polymers. The material system is based on an azide-containing side-chain copolymer and cross-linkable dendronized chromophores that have two pairs of dendritic bispropargyl ether moiety on the periphery. The dendritic chromophore possesses a tetraene conjugating bridge and strong dialkylaminophenyl donor and CF3-TCF acceptor. This material system not only provides adequate accessibility for propargyl ether to react azido-containing moieties for efficient cross-linking but also gives the rotational freedom needed for electric field poling. The site isolation offered by the bispropargyl ether dendron effectively suppresses the unwanted side reactions that are usually associated with the decompositions of azides and chromophores. Due to these concerted efforts, it allows the Huisgen 1,3-dipolar cycloaddition reactions to be carried out at moderate temperatures in polar media that have a high concentration of dipolar polyene chromophores. Through sequential electric field poling and in situ cross-linking, the poled films exhibit very large EO activity (up to 147 pm/V at 1.31 μm) with a long-term alignment stability at 85 °C.

High‐Heteronucleobase‐Content Polystyrene Copolymers Prepared Using Click Chemistry Form Supramolecular Structures With Melamine Through Complementary Multiple Hydrogen‐Bonding Interactions

AbstractFree‐radical copolymerization is used to synthesize poly[styrene‐co‐(4‐vinylbenzyl azide)] (PS‐co‐PVBN3), followed by click chemistry to react it with propargyl thymine (PT) to obtain a series of high‐performance polystyrene derivatives, poly[styrene‐co‐(4‐vinylbenzyltriazolylmethylmethylthymine)] (PS‐co‐PVBT). The presence of the thymine units on the side chains of the PS copolymers allows specific intermolecular association through self‐complementary T–T interactions. In addition, the thermal and dynamic rheological properties of PS‐co‐PVBT increase significantly when it is blended with melamine (M) because of the strong, complementary, multiple hydrogen‐bonding interactions that occur between the T units and this low‐molecular‐weight compound (M).

Convenient Approach to Polypeptide Copolymers Derived from Native Proteins

A convenient approach for the synthesis of narrowly dispersed polypeptide copolymers of defined compositions is presented. The controlled denaturation of the proteins serum albumin and lysozyme followed by an in situ stabilization with polyethylene(oxide) chains yields polypeptide side chain copolymers of precisely defined backbone lengths as well as the presence of secondary structure elements. Supramolecular architectures are formed in solution because of the presence of hydrophobic and hydrophilic amino acids along the polypeptide main chain. Polypeptide copolymers reported herein reveal excellent solubility and stability in aqueous media and no significant cytotoxicity at relevant concentrations, and they can be degraded via proteolysis, which is very attractive for biomedical applications. This "semi-synthetic chemistry" approach is based on a novel and convenient concept for producing synthetic polypeptides from native protein resources, which complements traditional polypeptide synthesis and expression approaches and offers great opportunities for the preparation of diverse polypeptides with unique architectures.

Synthesis and Aggregation Behavior of Copolymer of Acrylamide with Pseudorotaxane Monomer by Threading Cucurbit[6]uril onto N′-(4-vinylbenzyl)-1,4-diaminobutane Dihydrochloride

The novel side-chain copolymers of acrylamide (AM) and complex pseudorotaxane monomer N′-(4-vinylbenzyl)-1,4-diaminobutane dihydrochloride with cucurbit[6]uril (CB[6]) threading (4VBCB) were prepared via free-radical polymerization in aqueous solution. The copolymers with CB[6] (PAM4VBCB) were characterized by 1H NMR, FTIR, elemental analysis, and static light scattering. The thermodynamic properties of polypseudorotaxanes were studied by thermogravimetric analysis (TGA) and dynamic light scattering (DSC). The average hydrodynamic radius (Rh) of the polypseudorotaxanes was studied by DLS. At the same time, pH and conductivity of the polypseudorotaxanes was studied. The experiment data show that CB[6] beads are localized on 1,4-diaminobutane units in side chains of copolymer. TGA data show that thermal stability of the polypseudorotaxanes increases with the adding of the threading CB[6] because of the enhanced rigidity and the bulky steric hindrance of 4VBCB in side chains of PAM4VBCB. DLS data show that the average hydrodynamic radius of polypseudorotaxanes increases with the increase of the copolymer concentration and has a jump with increasing with increasing pH due to the existing of the small size CB[6] dethreading from the polypseudorotaxanes with increasing pH. Both pH and electrical conductivity curves of the solution of PAM4VBCB-4 have a jump because that CB[6] could dethread from the polypseudorotaxanes with the addition of NaOH.

UV beam-assisted efficient formation of surface relief grating on azobenzene polymers

An efficient method is demonstrated to improve the formation of a surface relief grating (SRG) with a high amplitude. The SRG is realized by a two-beam interference technique (λ=532 nm) in an azobenzene side-chain copolymer. An UV laser beam (λ=355 nm), called assisting beam, was used to accelerate the photoisomerization process from cis-form to trans-form, resulting in a rapid cis-form ⇌ trans-form cycle. This UV beam-assisted method allowed to increase the diffraction efficiency of the formed SRG as well as its depth. Stable gratings with modulation depth as high as 410 nm were obtained at room temperature with moderate laser power.

One-pot synthesis of comb-like copolymer bearing side chains composed of branched and linear polylactides

Ethyl cellulose (EC), lactide (LA) and branching comonomer 2,2-bis(hydroxymethyl) butyric acid (BHB) were copolymerized in xylene using Sn(Oct)2 as catalyst. Catalyst amount and polymerization temperature were optimized for the effective introduction of certain amount of branched polylactide (PLA) into the side chains of comb-like copolymers having EC backbone. The characterization of the obtained copolymers by GPC and 1H NMR demonstrated that the influence of polymerization temperature in the range of 110–150 °C on the efficient incorporation of branching units was not pronounced, whereas high amount of catalyst was the key point. The content of branching units in the copolymers could be enhanced by increasing the feed amount of BHB monomer. A plausible mechanism for the polymerization was proposed according to the model experiments. Compared with the comb-like copolymers with merely linear PLA side chains, the ones bearing both branched and linear PLA side chains had the following characters: (1) better solubility in the polar solvent of ethanol; (2) much faster hydrolysis degradation. These characters became more pronounced when the content of branching units in the side chains increased.

Synthesis and multi-stimuli-responsive behavior of copolymer of N,N′-dimethylacrylamide and complex pseudorotaxane

Novel water-soluble side-chain copolymer with the pseudorotaxane structure threaded by cucurbit[6]uril (CB[6]) was synthesized by N′-(4-vinylbenzyl)-1,4-diaminobutane dihydrochloride with CB[6] threaded (4VBCB) and N,N′-dimethylacrylamide (DMAA) as monomers. SEM results show that the diameter of the spherical particles increases and the crosslinking in spherical particles occurred with the decrease of the molar ratio of DMAA/4VBCB. Interestingly, 1H NMR spectra show that the threading and dethreading of CB[6] on the side chains could be controlled by the addition of CaCl2 or BaCl2, but CB[6] cannot drop from the side chains by the addition of NaCl. DLS data indicate that the copolymers have thermal sensitivity and their jump temperature could be controlled by adjusting the molar ratio of DMAA/4VBCB, and RLS data show that the size of aggregation increases with increasing the concentration of NaCl. At the same time, the copolymers have pH-responsive behavior. Novel water-soluble side-chain copolymer with the pseudorotaxane structure threaded by cucurbit[6]uril (CB[6]) was synthesized. Interestingly, the threading and dethreading of CB[6] on the side chains could be controlled by the addition of CaCl2 or BaCl2, but CB[6] cannot drop from the side chains by the addition of NaCl. And Na2CO3 or Na2SO4 was added into the CaCl2 or BaCl2 solution of copolymer for the removal of Ca2+ or Ba2+. As expected, CB[6] could thread on the side chains again as shown in Fig. 6a.

Alkoxy Side Chains in Low Band-Gap Co-Polymers: Impact on Conjugation and Frontier Energy Levels

The molecular design of electron donor conjugated polymers with accurate optoelectronic properties for efficient bulk heterojunction photovoltaic devices has attracted a lot of attention worldwide. Ideally, the polymer should have i) appropriate photon harvesting properties, ii) frontier orbitals that allow both, efficient exciton dissociation and a high open circuit voltage, iii) high charge carrier mobilities, iv) high solubility and v) high molecular weights. Charge transfer D/A copolymers are promising materials that have allowed significant progress in this field. However, although a lot of attention has been given to the design of their molecular conjugated backbone, only few investigations have considered the influence of their solubilizing side chains. The aim of the present work is to show how the nature and the position of side chains may influence above mentioned polymer properties. For that purpose, our team has designed and characterized a series of donor-acceptor alternating conjugated polymers using 2,1,3- benzothiadiazole as electron-deficient unit, and thieno[3,2-b]thiophene as well as thiophenes moieties as electron-rich units. Various side-chains were introduced onto different positions along the same conjugated backbone and the molecular structure/polymer properties relationships have been investigated systematically.

Fluorescent nanoparticles from self-assembly of β-cyclodextrin-functionalized fluorene copolymers for organic molecule sensing and cell labeling

Bromide-bearing conjugated fluorene copolymers were prepared by Suzuki coupling polymerization of 2,7-dibromo-9,9-bis(6′-bromohexyl)fluorene and 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dihexylfluorene. Thiol-functionalized β-cyclodextrin was subsequently grafted to the bromoalkyl side chains of the fluorene copolymers via the thio-bromo ‘click’ reaction. The resulting fluorene copolymers can self-assemble into fluorescent nanoparticles surrounded by a β-cyclodextrin outer layer in an aqueous medium. Host–guest interactions of the β-cyclodextrin moieties of nanoparticles with the organic guest molecules in the aqueous medium gave rise to variations in the emission intensity of the nanoparticles. The low cytotoxicity of the fluorescent nanoparticles was revealed by incubation with KB cells, a cell line derived from carcinoma of the nasopharynx, in 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assays in vitro. The biocompatible and fluorescent nanoparticles were further used as effective fluorescence labels for KB cells.

Carbazole/iridium dendrimer side-chain phosphorescent copolymers for efficient light emitting devices

Dendrimers have proved to be successful materials for solution-processable phosphorescent organic light emitting diodes (OLEDs). Recently we have been investigating poly(dendrimer)s with improved viscosity for ink-jet printing applications. In this work we present three phosphorescent side-chain copolymers comprised of a poly(styrene) backbone, carbazole charge transporting units, and iridium(III) complexes containing no dendrons (simple complex), a single dendron attached to two of the ligands, or doubly dendronised complexes with two dendrons added to the ligands. In addition to their charge transporting capability the copolymer structure incorporating carbazole units can also prevent aggregation of the chromophores, as evidenced by there being only one emissive environment in time-resolved luminescence. We report high external quantum efficiencies of 11.0% (37.3 cd A−1) at 100 cd m−2 and 8.3 V for devices made with the neat copolymers. The best performance is achieved by blending the doubly dendronised copolymer with 50 wt% of 4,4′-bis(N-dicarbazolyl)biphenyl (CBP) which results in 14.7% EQE (48.3 cd A−1) at 100 cd m−2 and 9.3V.

Recoverable fluorescence chemosensors for Ni2+ ions based on hydrogen-bonded side-chain copolymers presenting pendent benzoic acid and pyridyl receptor units

In this study we synthesized the novel fluorescent conjugated homopolymers P1 and P4 and the copolymers P2 and P3 containing proton-acceptor (pyridyl receptor) M1 monomer units and proton-donor (benzoic acid) M2 monomer units and investigated them as recoverable chemosensors for Ni2+ ions (based on adjusting the fluorescence energy transfer between the M1 and M2 moieties through varying the degree of hydrogen bonding). The levels of available (i.e., non-hydrogen-bonded) pyridyl receptors in the copolymers P2 and P3 were modified using different molar ratios of the pyridyl monomer M1 and the benzoic acid monomer M2. Significant chelation of the Ni2+ ions by the fluorescent homopolymers P1 and P4 and the copolymers P2 and P3 was evident by their distinct fluorescence quenching behaviors; the specific quenching constants (KSV) decreased in the order P1 > P2 > P3 > P4, following the trend of the molar ratios of pyridyl receptors. When we added a third component, cyclodextrin (CD), to cap the benzoic acid moieties and, thereby, disrupt the hydrogen bonding between the M1 and M2 moieties in the copolymers P2 and P3, the levels of available pyridyl receptors in the recoverable chemosensor copolymers increased, resulting in relatively higher quenching constants for the P2 + CD, P3 + CD and blend (P1/P4) + CD systems. Fluorescence decay experiments revealed that the addition of CD also increased the fluorescence quenching efficiencies of the copolymers P2 and P3 towards Ni2+ ions. Such supramolecular (hydrogen-bonded) side-chain fluorescent copolymers might have practical applicability as recoverable and/or adjustable chemosensors.

Synthesis and applications of 3,6-carbazole-based conjugated side-chain copolymers containing complexes of 1,10-phenanthroline with Zn(II), Cd(II) and Ni(II) for dye-sensitized solar cells

Three novel donor-acceptor polymeric metal complexes based on polycarbazole containing complexes of 1,10-phenanthroline with Zn(II), Cd(II) and Ni(II) have been synthesized by the Ullmann reaction and characterized by gel permeation chromatography, FT-IR, UV–visible absorption and photoluminescence spectroscopy, cyclic voltammetry and elemental analysis. The application of these organometallic polymers in fabricated dye-sensitized solar cells has been studied. The solar cells exhibited good device performance with a power conversion efficiency of up to 0.44%, under simulated air mass 1.5 G solar irradiation.

Charge photogeneration and transport in side-chain carbazole polymers and co-polymers

The photoconductivity, hole mobility and charge photogeneration efficiency of a series of side-chain carbazole homopolymers and copolymers (with azo side-chains) have been investigated. Cyclic voltammetry measurement of frontier orbitals energies show that the HOMO energy is determined by the nature and the position of attachment of the linker between the main chain and the carbazole, the azo-moiety being not relevant in this respect. Hole mobility is not influenced by the HOMO energy but seems to depend on the degree of conformational mobility of the side-chains, reaching values of the order of 10 −3 cm 2 V −1 s −1 in the best cases. The HOMO energy is instead extremely important when considering photogeneration efficiency, that can change by 10 orders of magnitude depending on the density of the carbazole side-chains in co-polymers and on the linker nature and attachment position.