27 research outputs found
All-Conjugated, All-Crystalline Donor–Acceptor Block Copolymers P3HT‑<i>b</i>‑PNDIT2 via Direct Arylation Polycondensation
The synthesis and
characterization of all-conjugated, all-crystalline
donor–acceptor block copolymers (BCPs) containing polyÂ(3-hexylÂthiophene)
(P3HT) and polyÂ{[<i>N</i>,<i>N</i>′-bisÂ(2-octylÂdodecyl)Ânaphthalene-1,4,5,8-bisÂ(dicarboximide)-2,6-diyl]-<i>alt</i>-5,5′-(2,2′-bithiophene)} (PNDIT2) is presented.
Direct arylation polycondensation (DAP) of dibromoÂnaphthaleneÂdiimide
and bithiophene is carried out in the presence of P3HT end-cappers
to allow the in situ formation of BCPs P3HT-<i>b</i>-PNDIT2.
As-prepared, well-defined H-P3HT-Br with hydrogen and bromine chain
termini shows nonoptimal reactivity under the DAP conditions used.
Therefore, H-P3HT-Br is converted into either H-P3HT-Th (thiophene)
or H-P3HT-Mes (mesitylene), giving α,ω-hetero-C–H
functionalized P3HT with modulated C–H reactivity. The influence
of the different C–H chain termini of P3HT on the ability to
act as end-capper and the resulting block structures is investigated
in detail using wavelength-dependent size exclusion chromatography
(SEC) and NMR spectroscopy. Different C–H reactivities of α,ω-hetero-C–H
functionalized P3HT cause different contents of multiblocks, which
in turn lead to varied degrees of crystallinity. These results show
that careful tuning of C–H reactivity is a promising way to
obtain well-defined, all-conjugated block copolymers via DAP
Rational Use of Aromatic Solvents for Direct Arylation Polycondensation: C–H Reactivity versus Solvent Quality
The solvent for direct arylation
polycondensation (DAP) is of crucial
importance. For conjugated polymers exhibiting reduced solubility,
the choice of solvent decides on the maximum molecular weight that
can be achieved, hence, good aromatic solvents are generally desirable.
However, unintentional activation of C–H bonds present in aromatic
solvents under DAP conditions leads to in situ solvent termination
which competes with step growth. Here we evaluate relative C–H
reactivity and solvent quality of seven aromatic solvents for the
DAP of defect-free naphthalene diimide (NDI)-based copolymers of different
solubility. C–H reactivity is strongly reduced with increasing
degree of substitution for both chlorine and methyl substituents.
Mesitylene is largely C–H unreactive and, thus, albeit being
a moderate solvent, enables very high molecular weights at elevated
temperature for NDI copolymers with limited solubility
Alkoxide-Initiated Regioselective Coupling of Carbon Disulfide and Terminal Epoxides for the Synthesis of Strongly Alternating Copolymers
The synthesis of highly regioregular
and alternating polythiocarbonates
from carbon disulfide and terminal epoxides has been achieved. The
copolymerizations were performed under ambient and solvent-free conditions
in the presence of LiO<sup><i>t</i></sup>Bu (0.125–0.5
mol %) as initiator. At higher loadings the reaction pathway switched
in favor to the catalytic formation of cyclic dithiocarbonates. Under
optimized reaction conditions polymers with molecular weights up to
109 kg mol<sup>–1</sup> were isolated. The NMR spectroscopic
analysis of the polythiocarbonates revealed that 94% of backbone structure
is formed by strongly alternating head-to-head arrangement of epoxypropane
and 1,2-epoxybutane monomers, respectively, at a thiocarbonate group
−CHR–OCÂ(S)ÂO–CHR– and tail-to-tail arrangement
at a trithiocarbonate group −CH<sub>2</sub>–SCÂ(S)ÂS–CH<sub>2</sub>–. Atactic polymers were obtained using racemic mixtures
of the epoxides, but an isotactic polymer was obtained when chiral
(<i>R</i>)-epoxyÂpropane was converted. A mechanism
is proposed which rationalizes the regio- and stereochemistry observed
for the alkoxide-initiated copolymerization of CS<sub>2</sub> and
terminal epoxides
Defect-free Naphthalene Diimide Bithiophene Copolymers with Controlled Molar Mass and High Performance via Direct Arylation Polycondensation
A highly efficient, simple, and environmentally
friendly protocol
for the synthesis of an alternating naphthalene diimide bithiophene
copolymer (PNDIT2) via direct arylation polycondensation (DAP) is
presented. High molecular weight (MW) PNDIT2 can be obtained in quantitative
yield using aromatic solvents. Most critical is the suppression of
two major termination reactions of NDIBr end groups: nucleophilic
substitution and solvent end-capping by aromatic solvents via C–H
activation. In situ solvent end-capping can be used to control MW
by varying monomer concentration, whereby end-capping is efficient
and MW is low for low concentration and vice versa. Reducing C–H
reactivity of the solvent at optimized conditions further increases
MW. Chain perfection of PNDIT2 is demonstrated in detail by NMR spectroscopy,
which reveals PNDIT2 chains to be fully linear and alternating. This
is further confirmed by investigating the optical and thermal properties
as a function of MW, which saturate at <i>M</i><sub>n</sub> ≈ 20 kDa, in agreement with controls made by Stille coupling.
Field-effect transistor (FET) electron mobilities μ<sub>sat</sub> up to 3 cm<sup>2</sup>/(V·s) are measured using off-center
spin-coating, with FET devices made from DAP PNDIT2 exhibiting better
reproducibility compared to Stille controls
Enhancing Phase Separation and Photovoltaic Performance of All-Conjugated Donor–Acceptor Block Copolymers with Semifluorinated Alkyl Side Chains
Phase separation of all-conjugated
donor–acceptor block
copolymers is more difficult to achieve compared to classical coil–coil
systems owing the intrinsic similarity of the two blocks having both
rigid conjugated backbones and alkyl side chains and their generally
low degrees of polymerization. Here we demonstrate that side chain
fluorination of a polyÂ(carbazole-<i>alt</i>-dithienylbenzothiadiazole)
segment (SF-PCDTBT), to be used as electron acceptor block in combination
with polyÂ(3-hexylthiophene) P3HT as donor block in all-conjugated
donor–acceptor block copolymers of type SF-PCDTBT-<i>b</i>-P3HT, strongly increases dissimilarity between P3HT and SF-PCDTBT
leading to phase separation for already moderate molar masses. Key
to the successful synthesis of a new TBT-monomer with semifluorinated
side chains is a direct arylation step that elegantly bypasses classical
cross-coupling reactions in which the semifluorinated side chain causes
low yields. Suzuki polycondensation of the semifluorinated TBT monomer
with a suitable carbazole comonomer and in situ termination by P3HT-Br
is optimized extensively with respect to the yield of the end-capping
efficiency and molar mass control of the PCDTBT segment. When the
fluorinated side chains are replaced by hydrogen (H-PCDTBT) or by <i>n</i>-hexyl chains (hex-PCDTBT), the tendency for phase separation
with covalently connected P3HT is much reduced as shown by differential
scanning calorimetry and grazing incidence small-angle scattering
measurements on thin films. Favorably, of all the block copolymers
made only SF-PCDTBT-<i>b</i>-P3HT is microphase separated,
exhibits face-on orientation of P3HT domains, and additionally displays
surface segregation of the SF-PCDTBT segment at the polymer/air interface.
All of these properties are beneficial for single layer single component
solar cells. SF-PCDTBT-<i>b</i>-P3HT exhibits the best solar
cells performance with a high open-circuit voltage of 1.1 V and a
power conversion efficiency of ∼1% which largely outperforms
devices based on the analogous H-PCDTBT-<i>b</i>-P3HT and
hex-PCDTBT-<i>b</i>-P3HT
Synthesis of Allyl-Terminated Polar Macromonomers by Metallocene-Catalyzed Polymerizations of 10-Undecene-1-ol
PolyÂ(10-undecene-1-ol) macromonomers were synthesized
by metallocene-catalyzed
polymerization using vinyl chloride as a chain transfer agent. Using
this technique, predominantly allyl terminated polymers could be obtained,
which was verified by NMR spectroscopy and matrix-assisted laser desorption
ionization time-of-flight mass spectrometry (MALDI-TOF-MS). By means
of the mass spectra, a detailed interpretation of the nature of the
head and end groups of these polymers was possible
Synthesis of Magnetic Polystyrene Nanoparticles Using Amphiphilic Ionic Liquid Stabilized RAFT Mediated Miniemulsion Polymerization
Imidazole
based amphiphilic ionic liquids (ILs) were used as surfactants
in miniemulsion polymerization (MEP) of styrene using a free radical
process as well as reversible addition–fragmentation chain
transfer (RAFT). Monodisperse polystyrene (PS) nanoparticles were
obtained, demonstrating the efficiency of the amphiphilic IL as surfactant
in MEP. IL stabilized miniemulsion was furthermore used to prepare
polystyrene based magnetic nanoparticles (MNP). A large increase of
the possible MNP content associated with very good colloidal stability
was achieved using IL stabilized RAFT mediated MEP where a carboxyl
functionalized chain transfer agent (CTA) was applied, allowing interaction
with the MNP surface. The molecular weight and dispersity index of
polystyrene, the content of MNP, and the morphologies of the hybrid
nanoparticles were controlled by proper optimization of the concentration
of initiator and CTA. The materials have been analyzed by NMR, GPC,
DLS, SEM, TEM, and TGA. Finally, the magnetic properties of the materials
were determined by vibrating sample magnetometer (VSM) analysis
Formation of Oligomeric and Macrocyclic Ureas Based on 2,6-Diaminopyridine
The conversion of 1,3-bis-(6-amino-pyridin-2-yl)-urea
(<b>1</b>) with <i>N</i>,<i>N</i>′-carbonyldiimidazole
at high temperatures in DMSO yielded a mixture of defined cyclic trimers
and tetramers. On the basis of model reactions, exchange reactions
were evidenced, which convert the cyclic tetramer into a stable cyclic
trimer. Linear even numbered oligomers were obtained in acetone under
reflux where side reactions were suppressed. The pronounced tendency
of cyclization is attributed to a preferred folded conformation of
the urea bond between two pyridyl units
Network Formation, Properties, and Actuation Performance of Functionalized Liquid Isoprene Rubber
Due to some useful
mechanical, dynamic, and dielectric properties
along with the ease of processing and forming, liquid rubbers are
ideal materials for fabricating dielectric elastomer actuators in
various configurations and for many potential applications ranging
from automation to automobile and medical industry. In this study,
we present a cross-linkable liquid rubber composition where amine-catalyzed
esterification reactions lead to the formation of a network structure
based on anhydride functional isoprene rubber, carboxyl-terminated
nitrile-butadiene rubber, and epoxy end-capped prepolymers. The success
of this intricate network formation procedure was verified by HR-MAS
NMR spectroscopy. The new isoprene-based elastomeric material exhibits
actuation-relevant attributes including a low elastic modulus of 0.45
MPa, soft response to an applied load up to a large deformation of
300%, and a dielectric constant value (2.6) higher than the conventional
Elastosil silicone (2.2). A dot actuator comprising of an isoprene
dielectric elastomer film in unstretched state and carbon paste electrodes
was fabricated that demonstrated an electrode deformation of 0.63%,
which is nearly twice as high as for the commercial Elastosil 2030
film (∼0.30%) at 5 kV. Compared to the Elastosil silicone film,
the enhanced performance is attributed to the low modulus and high
dielectric constant value of the new isoprene elastomer
Fullerene-Functionalized Donor–Acceptor Block Copolymers through Etherification as Stabilizers for Bulk Heterojunction Solar Cells
A new synthetic method for the covalent linking of fullerenes
to
polymers is introduced. The Bingel-reaction was used to prepare bromine-functionalized
fullerene building blocks that could be covalently linked to hydroxyl
groups of model copolymers by the cesium carbonate promoted Williamson
ether synthesis. Subsequently, block copolymers with a second block
based on styrene and hydroxystyrene or hydroxyethyl methacrylate could
be synthesized with a polyÂ(3-hexylthiophene)–TEMPO macroinitiator
through NMRP. Fullerene derivatives were linked to these polymers
in a controlled manner and donor–acceptor block copolymers
with high fullerene contents of near 50 wt % were achieved