Yougen Chen

Synthesis, thermomorphic characteristics, and fluorescent properties of poly[2,7-(9,9-dihexylfluorene)]-block-poly(N-isopropylacrylamide)-block-poly(N-hydroxyethylacrylamide) rod-coil-coil triblock copolymers

New thermoresponsive conjugated rod-coil-coil triblock copolymers were successfully synthesized from terminal azido functionalized poly(N-isopropylacrylamide)-b-poly(N-hydroxyethylacrylamide) (PNIPAAm-b-PHEAA) and alkynyl functionalized polyfluorene (PF) via a click reaction. The azido functionalized PNIPAAm-b-PHEAA copolymers with different block ratio was prepared by atom transfer radical polymerization from an initiator bearing the azide group, whereas alkynyl functionalized PF was synthesized by Suzuki coupling reaction. The lower critical solution temperature (LCST) of the block copolymers increased with an enhanced hydrophilic PHEAA block ratio, since the longer PHEAA segment facilitated the copolymer chains to stretch at an elevated temperature. The micelles of PNIPAAm-b-PHEAA with different block ratio changed into spheres, aggregate spheres, vesicles, and wormlike micelles as the temperature was increased, due to the variation on the hydrophilic/hydrophobic characteristic of PNIPAAm. However, the micellar morphologies became worms, bundles of wormlike micelles, and hollow tubes in the triblock PF-b-PNIPAAm-b-PHEAA, which were probably induced by the π–π interaction among the fluorene segments. The variation of the micelle morphology with temperature was consistent from the results of transmission electron microscopy, atomic force microscopy, and dynamic light scattering. Also, the micelle morphologies of PF-b-PNIPAAm-b-PHEAA showed a thermoreversible property based on its LCST. The photoluminescence characteristics behaved as an on/off fluorescence indicator of temperature, showing an “on-off-on” profile at an elevated temperature in water at a higher block ratio of PNIPAAm and switching to “on-off” as the block ratio of PNIPAAm decreased. The present study suggests that the PF-b-PNIPAAm-b-PHEAA copolymers have tunable morphologies and could be potentially used as thermoresponsive sensory materials.

Synthesis, Morphology, and Sensory Applications of Multifunctional Rod−Coil−Coil Triblock Copolymers and Their Electrospun Nanofibers

We report the synthesis, morphology, and applications of conjugated rod-coil-coil triblock copolymers, polyfluorene-block-poly(N-isopropylacrylamide)-block-poly(N-methylolacrylamide) (PF-b-PNIPAAm-b-PNMA), prepared by atom transfer radical polymerization first and followed by click coupling reaction. The blocks of PF, PNIPAAm, and PNMA were designed for fluorescent probing, hydrophilic thermo-responsive and chemically cross-linking, respectively. In the following, the electrospun (ES) nanofibers of PF-b-PNIPAAm-b-PNMA were prepared in pure water using a single-capillary spinneret. The SAXS and TEM results suggested the lamellar structure of the PF-b-PNIPAAm-b-PNMA along the fiber axis. These obtained nanofibers showed outstanding wettability and dimension stability in the aqueous solution, and resulted in a reversible on/off transition on photoluminescence as the temperatures varied. Furthermore, the high surface/volume ratio of the ES nanofibers efficiently enhanced the temperature-sensitivity and responsive speed compared to those of the drop-cast film. The results indicated that the ES nanofibers of the conjugated rod-coil block copolymers would have potential applications for multifunctional sensory devices.

Recent progress in organocatalytic group transfer polymerization

Group transfer polymerization (GTP) has been a minor research topic since the emergence of controlled/living radical polymerization methods in the mid-1990s, even though it is counted as one of the living polymerization methods. However, the performance of GTP has much improved in many aspects, such as catalytic activity, polymerizable monomers, molecular weight control, and polymer structures that can be synthesized, through the introduction of newly developed Lewis base/acid organocatalysts and novel initiators since 2007–2008. In this review article, recent progress in GTP brought about by organocatalysts is described.

Thermoresponsive properties of 3-, 4-, 6-, and 12-armed star-shaped poly[2-(dimethylamino)ethyl methacrylate]s prepared by core-first group transfer polymerization

The t-Bu-P4-catalyzed group transfer polymerization (GTP) of 2-(dimethylamino)ethyl methacrylate (DMAEMA) using multi-functional GTP initiators that bear multiple silyl ketene acetal moieties (MTS3, MTS4, MTS6, and MTS12) homogeneously proceeded and rapidly completed to afford well-defined star-shaped poly[2-(dimethylamino)ethyl methacrylate]s (s-PDMAEMAs) with relatively narrow polydispersities. The molecular weights (MWs) of s-PDMAEMAs were well-controlled by optimizing the molar ratios of the monomer to initiators. For the structural analyses, the arm number and length uniformity of each s-PDMAEMA were then investigated by arm cleavage experiments using a transesterification method. The thermoresponsive properties in an aqueous solution of the resulting s-PDMAEMAs together with their analogous linear PDMAEMAs (l-PDMAEMAs), in terms of polymer concentration, molecular weight, and arm number, were eventually elucidated based on turbidimetry curves.

Synthesis of end-functionalized poly(methyl methacrylate) by organocatalyzed group transfer polymerization using functional silyl ketene acetals and α-phenylacrylates

The present study describes the α- and ω-end-functionalization of poly(methyl methacrylate)s (PMMAs) by organocatalyzed group transfer polymerization (GTP) using both functional silyl ketene acetal (SKA) initiators and α-phenylacrylate terminators. The syntheses of structurally defect-free α-end-functionalized PMMAs with hydroxyl, ethynyl, vinyl, and norbornenyl groups (HO-PMMA, HCC-PMMA, H2CCH-PMMA, and NB-PMMA, respectively) were achieved by either the N-(trimethylsilyl)bis(trifluoromethanesulfonyl)imide-(Me3SiNTf2-) or t-Bu-P4-catalyzed GTP of MMA using functional trimethyl SKAs (1a–1d). On the other hand, the ω-end-functionalized PMMAs with ethynyl, hydroxyl, vinyl, and bromo groups (PMMA-CCH, PMMA-OH, PMMA-CHCH2, and PMMA-Br, respectively) were for the first time obtained by the Me3SiNTf2-catalyzed GTP of MMA followed by a termination reaction using functional α-phenylacrylates (2a–2d). All the polymerizations produced end-functionalized PMMAs with controlled molar masses, narrow dispersities, and defect-free polymer structures as designed. The quantitative incorporation of functionalities into the α- or ω-end of the PMMAs was confirmed by the 1H NMR and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) measurements.

Synthesis and thermoresponsive properties of four-arm star-shaped poly(N-isopropylacrylamide)s bearing covalent and non-covalent cores

The present study describes the precise synthesis and thermoresponsive properties of two types of four-arm star-shaped poly(N-isopropylacrylamide)s (PNIPAM), i.e., the covalently linked one (3) and the non-covalently Ru(II)-chelated one (5). The atom transfer radical polymerization (ATRP) method was used to prepare the azido-terminated PNIPAM (1) arm using (2-azidoethyl)-2-chloropropionamide (AECP) as the initiator. 3 was subsequently prepared based on the click reaction of 1 with a multifunctional linker of tetra[(5-hexynyloyloxy)methyl]methane. For comparison, its linear counterpart 2 was also synthesized as a reference polymer by the same method using ethyl 5-hexynyloate. The four-arm star-shaped PNIPAM Ru complex 5, on the other hand, was prepared by a click-to-chelate approach, which involves the click reaction of 1 with 2,6-diethynylpyridine to produce the macroligand of 2,6-bis(1-PNIPAM-1,2,3-triazol-4-yl)pyridine (4) and the chelating reaction of RuCl3 with 4 to afford 5. The thermoresponsive properties of the resulting polymers were investigated using a UV-vis spectrophotometer by measuring the optical transmittance of the polymer solution with varying solution temperature and the cloud point (Tc) at 50% transmittance intensity in order to assess their thermoresponsive properties. The detailed thermoresponsive properties of these polymers, including the effects of the polymer terminal and core linkage and constituents of the four-arm star-shaped PNIPAMs on the Tc, are significantly described in the later part of this study.

Synthesis of 3-, 4-, 5-, 6-, 7-, 8-, 9-, 10-, 11-, and 12-armed star-shaped poly(styrene oxide) Ru(II) complexes by a click-to-chelate approach

This study describes the first convenient preparation of 3-, 4-, 5-, 6-, 7-, 8-, 9-, 10-, 11-, and 12-armed star-shaped poly(styrene oxide) (PSO) Ru(II) complexes using a click-to-chelate approach. This approach involves the combination of the click reaction and stepwise chelating reactions of Ru(II)(DMSO)4Cl2 with macroligands, 2-(1-PSOn-1,2,3-triazol-4-yl)pyridine (PSOn-tapy) or 2-(1-PSOm-1,2,3-triazol-4-yl)-6-(1-PSOn-1,2,3-triazol-4-yl)pyridine (PSOm-bitapy-PSOn) (m, n = 1, 2, or 3). Click chemistry was used to prepare the PSOn-tapy and PSOm-bitapy-PSOn macroligands. More specifically, PSOn-tapy was prepared by reacting the azido-functionalized PSOn (PSOn-N3) with excess 2-ethynylpridine. On the other hand, PSOm-bitapy-PSOn was obtained by the click reaction of excess PSOn-N3 with 2,6-diethynylpridine to afford (PSOn)2-bitapy when m equals n, and by the stepwise click reactions of PSOm-N3 and PSOn-N3 with 2,6-diethynylpridine to produce PSOm-bitapy-PSOn when m is not equal to n. In order to obtain these polymer-substituted macroligands, PSOn-N3 was initially synthesized by the living ring-opening polymerization (ROP) of styrene oxide (SO) using t-Bu-P4 as a catalyst and the azido-functionalized mono- or multi-hydroxyl compounds, e.g., 6-azido-1-hexanol, 2-((6-azidohexyloxy)methyl)-2-methylpropane-1,3-diol (1) and 2-((6-azidohexyloxy)methyl)-2-(hydroxymethyl)propane-1,3-diol (2) as initiators.

Synthesis of ABB′ and ABC star copolymers via a combination of NMRP and ROP reactions

We report here a facile method to prepare ABB′ and ABC type miktoarm star copolymers by a combination of nitroxide-mediated radical polymerization (NMRP) and ring opening polymerization (ROP). The first two arms were prepared by sequential NMRP of the same or different monomers, e.g. styrene and tert-butyl methacrylate (tBMA). The hydroxyl group was introduced between the two blocks by one-step radical addition to N-(2-hydroxyethyl)maleimide. The attached in-chain hydroxyl group was used to initiate the ring opening polymerization of L-lactide in the presence of DBU, forming the third block of poly(L-lactide). Thermal properties, microphase separation and solution self-assembly of the miktoarm star copolymers were investigated using TGA, DSC and TEM. It was found that the star copolymer underwent microphase separation to form two phases with a lamellar structure in the bulk state. In addition, hollow spheres were formed from ABC copolymer solution when a mixed solvent was used.

Synthesis, morphology, and electrical memory application of oligosaccharide-based block copolymers with π-conjugated pyrene moieties and their supramolecules

We report the synthesis, morphology, and field effect transistor memory application of maltoheptaose-based block copolymers, maltoheptaose-block-poly(1-pyrenylmethyl methacrylate) (MH-b-PPyMA), and their supramolecules with (4-pyridyl)-acceptor-(4-pyridyl), MH(4Py-Acceptor-4Py)x-b-PPyMA. MH-b-PPyMA was prepared by the combination of the t-Bu-P4-catalyzed group transfer polymerization and the Cu(I)-catalyzed azide–alkyne cycloaddition reaction. After the thermal annealing process, the MH-b-PPyMA bulk sample underwent microphase separation to form the sub-10 nm periodic self-assembled nanostructure. The self-assembled morphologies transform from the hexagonal cylinder packing to the body-centered cubic sphere arrangement and the disordered spherical nanodomain with the increase of the PPyMA segment length. On the contrary, only the spherical nanodomain was observed in the thermo-annealed thin film samples of both MH-b-PPyMA and MH(4Py-Acceptor-4Py)x-b-PPyMA. The electrical characteristics of the p-type pentacene-based OFET memory device were studied using the thermo-annealed polymer thin film as the electret layer. The MH(4Py-Acceptor-4Py)x-b-PPyMA-based organic field effect transistor (OFET) devices had the high hole mobility of 0.20–1.08 cm2 V−1 s−1 and the ON/OFF current (ION/IOFF) ratio of 107–108, in which the acceptor of the benzo[c][1,2,5]thiadiazole (BT) based device possessed the higher hole mobility than that of the isoindigo-based one due to the more ordered packing pentacene crystals. The memory window (ΔVTH) of the supramolecule-based device was increased with enhancing the 4Py-Acceptor-4Py blending composition, and that of the MH(4Py-BT-4Py)1.5-b-PPyMA10-based device had the largest ΔVTH of ca. 9 V, a long-term retention time greater than 104 s and the high ION/IOFF memory ratio of 106–107 (reading at Vg = 0 V) for more than 100 programming/erasing cycles. Our results demonstrate that the bio-related block copolymers and their supramolecular thin film could be used as electret layers for high-performance nonvolatile flash green memory devices.

B(C6F5)3-catalyzed group transfer polymerization of alkyl methacrylates with dimethylphenylsilane through in situ formation of silyl ketene acetal by B(C6F5)3-catalyzed 1,4-hydrosilylation of methacrylate monomer

The group transfer polymerization (GTP) of alkyl methacrylates has been studied using hydrosilane and tris(pentafluorophenyl)borane (B(C6F5)3) as the new initiation system. For the B(C6F5)3-catalyzed polymerization of methyl methacrylate (MMA) using triethylsilane, tri-n-butylsilane (nBu3SiH), dimethylphenylsilane (Me2PhSiH), triphenylsilane, and triisopropylsilane, nBu3SiH and Me2PhSiH were suitable for producing well-defined polymers with predicted molar masses and a low polydispersity. The livingness of the GTP of MMA using Me2PhSiH/B(C6F5)3 was verified by the matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) measurement of the resulting polymers, kinetic analyses, and chain extension experiments. The B(C6F5)3-catalyzed GTP using Me2PhSiH was also applicable for other alkyl methacrylates, such as the n-propyl, n-hexyl, n-decyl, 2-ethylhexyl, iso-butyl, and cyclohexyl methacrylates. The in situ formation of the silyl ketene acetal by the 1,4-hydrosilylation of MMA was proved by the MALDI-TOF MS and 2H NMR measurements of the polymers obtained from the B(C6F5)3-catalyzed GTPs of MMA with Me2PhSiH or Me2PhSiD, which was terminated using CH3OH or CD3OD.