Liming Jiang

Synthesis and characterization of temperature responsive graft copolymers of dextran with poly(N-isopropylacrylamide)

Abstract Poly (N-isopropylacrylamide) (PNIPAAm) was grafted to dextran using ceric ion as redox initiator. The graft copolymers formed temperature responsive materials and can be used to construct polymeric micelles as drug carriers for colon-site specifically delivery. The chemical structure of the graft copolymers was characterized by FTIR, 1H- and 13C-NMR spectroscopy. The influence of reaction conditions on the grafting parameters was investigated. It was found that the percentage of homopolymer formation (H%), the grafting efficiency (GE%) and the grafting (G%) of the copolymers increased with increasing the amount of the ceric catalyst used. Extension of the duration of graft reaction increased GE% and G% of the copolymers, suggesting that G% of the copolymers could be readily manipulated by changing copolymerization duration. Higher grafting temperature was in favor of increasing GE% and G%, however, when the reaction temperature was above the LCST of the copolymers, GE% and G% decreased. The optical transmittance of the copolymers in the aqueous solution was examined by UV–Vis instrument. The result showed that the phase transition of the graft copolymer in aqueous solution moved slightly to higher temperature when G% of the graft copolymers decreased. The results of atomic force microscopy and dynamic light scattering measurement indicated that the graft copolymers form micelles in a spherical morphology, and for the copolymer with the G% of 33.8% formed micelles in the mean diameter of less than 30 nm in aqueous solution.

A polymeric film probe with a turn-on fluorescence response to hydrogen sulfate ions in aqueous media

A new kind of hydrophilic copolymer poly(HEMA-co-VNP) was designed and synthesized by the radical copolymerization of 2-hydroxyethylmethacrylate (HEMA) and 2-{[2-(4-vinylbenzyloxy)naphthalen-1-yl]methyleneamino}-(S)-2-phenylethanol (VNP). The desired copolymer showed a highly selective red-shifted emission and a unique chiroptical response upon HSO4 - binding in organic solution. UV-vis and 1H NMR spectroscopic studies revealed that the hydrogen bonding between the imine moiety in the VNP repeating units and HSO4 - is crucial for the high selectivity of the receptor to this anion. As expected, the incorporation of HEMA into the polymer matrix endowed the copolymer excellent hydrophilicity, flexibility and good film-forming properties. Thus, high-quality film sensors could be easily fabricated on quartz plates through spin-casting techniques. The resultant polymeric films can recognize HSO4 - ions among a series of common anions in aqueous solution with high selectivity and sensitivity. The promising new film probe for HSO4 - has distinct characteristics such as a rapid response, enough stability in an aqueous media and practicality.

A novel bithiazole-tetrathiapentalene polymer and its metal complexes

Abstract This paper describes the synthesis of a novel heterocyclic polymer incorporating both bithiazole and tetrathiapentalene moieties in the main chain. The complexion of the polymer with FeSO 4 or NdCl 3 in dimethyl sulfoxide produced corresponding metal complexes. Based on IR spectra and X-ray photoelectron spectroscopy (XPS) data the structure of these metal complexes is discussed briefly. The magnetic properties of the metallopolymers were measured as a function of magnetic field strength (0–60 kOe) at 5 K and as a function of temperature (5–300 K) under an applied magnetic field of 30 kOe, respectively. The neodymium complex exhibits a hysteresis cycle at 5 K, the observed coercivity H c and the remnant magnetization M r are 15 Oe and 0.00035 emu/g, respectively.

Synthesis of ultra-high molecular weight polystyrene with rare earth–magnesium alkyl catalyst system: general features of bulk polymerization

Bulk polymerization of styrene (St) with an in-situ-activated Ziegler-catalyst containing neodymium 2-ethylhexyl phosphonate [Nd(P204)3], magnesium–aluminum alkyls and hexamethyl phosphoramide (HMPA) was studied. The new rare-earth catalyst exhibited high activity for polymerization of styrene, and its catalytic efficiency reached 14 730 g PSt/g Nd. The influence of reaction parameters, such as Mg/Nd, Mg/Al, St/Nd molar ratios, temperature, etc, on the catalyst performance was examined in detail. The molecular weight of the resulting polystyrene is ultra-high (MW = 40 × 104 ∼ 120 × 104 g mol−1) and the distribution of molecular weight is broad (MW/Mn = 2.1 ∼ 2.8). The microstructure of the polystyrene was characterized by IR and 13C NMR spectroscopies and found to be atactic. © 2001 Society of Chemical Industry

Polymerization of methyl methacrylate with a new rare-earth coordination catalyst

Abstract Methyl methacrylate was polymerized in toluene with a new rare-earth catalyst composed of rare-earth 2-ethylhexyl phosphonate, di- n -butylmagnesium, and N , N , N ′ , N ′ -tetramethylethylene diamine. The influence of the catalyst components and reaction conditions on polymerization has been examined in detail. The results showed that the rare-earth catalyst has a high activity and can prepare poly(methyl methacrylate) with a high molecular weight at a low temperature (−18°C).

Chiroptical Inversion Induced by Rotation of a Carbon–Carbon Single Bond: An Experimental and Theoretical Study

We propose a new strategy to construct chiral molecular switches with highly reversible and sensitive chiroptical responses to variations in the external environment. Its fundamental concept involves a stimuli-triggered exchange of two conformations presenting significantly different chiroptical properties through the rotation of a carbon-carbon single bond, as demonstrated by chiral Schiff bases s-1, s-2, and a salicylamide analogue s-3. Upon addition of base in solution, the circular dichroism (CD) spectra of these molecular switches displayed unique changes featuring an inversion of the Cotton effects signs, and the original CD profiles can be recoverd by acidification. Various spectroscopic studies as well as the conformational analysis combining with TDDFT computations allowed clear elucidation of the chiroptical inversion mechanism. It is expected that this kind of chiroptical switches is of great interest for molecular recognition, chemosensing, and the construction of molecular-scale devices. Furthermore, the present study indicates that the use of the conformational transition about a single bond may serve as the basis for designing chiroptical inversion systems.

Living cationic ring-opening polymerization of 2-oxazolines initiated by rare-earth metal triflates

The cationic ring-opening polymerization (CROP) of substituted 2-oxazolines using rare-earth metal triflates (RE(OTf)3) as initiator was investigated for the first time. In this work, we examined the polymerization characteristics of 2-ethyl-2-oxazoline (EtOx) initiated by Sc(OTf)3 under conventional thermal heating and microwave irradiation, and compared the respective outcomes with those obtained with the most frequently used initiator methyl tosylate (MeOTs). The results indicated that Sc(OTf)3 exhibits a higher catalytic efficiency to the EtOx polymerization than MeOTs under identical conditions. The controlled/living nature of the Sc(OTf)3-catalyzed CROP was confirmed by its linear first-order kinetics and the narrow molecular weight distribution of the resultant polymers as well as the block copolymerization of EtOx and 2-phenyl-2-oxazoline (PhOx). Based on in situ NMR spectroscopic studies and SEC analysis of PEtOx samples obtained from the control termination experiments, a possible initiating/propagating mechanism has been proposed for the living cationic ring-opening polymerization. Morever, this rare-earth catalytic system can also be applied to the ring-opening polymerization of some sterically hindered or aryl-substituted 2-oxazolines.

Study on syntheses and magnetic properties of novel bithiazole-containing polymeric complexes

A series of novel bithiazole-containing polymers was synthesized their complexes were prepared through the polymers coordination with transition metal ions or rare-earth metal ions. The structures of the polymers and their complexes were determined by IR, NMR, ESR, XPS and elemental analysis The magnetic behavior of these complexes was measured as a function of the magnetic field strengh (0-60kOe0 at 5K and a function of temperature (5-300K). They exhibit a hysteresis cycle at 5K with coercivity (He) from 20 to 500Oe. the remanence magnetization (Mr) from 0.003 to 0.15 emu/g, and the saturation magnetization (Ms) from 0.18 to 8 emu/g, respectivley These data indicate that these complexes are soft ferromagnetic materials.

A new rare earth coordination catalyst for polymerization of acrylonitrile

Abstract Polymerization of acrylonitrile was carried out for the first time using the rare-earth coordination catalysts composed of rare-earth 2-ethyl hexyl phosphonate [Ln(P 204 ) 3 ], dibutylmagnesium [Mg(Bu − n) 2 ], and N,N,N′,N′-tetramethyl ethylene diamine (TMEDA, 1:6:6 molar ratio). The new catalyst gives high molecular weight polyacrylonitrile in fairly good yield. The triad tacticities of the polymers obtained increase with a decrease of polymerization temperature and mm content attains 0.52 at − 78 °C.

Novel poly(2-oxazoline)s with pendant L-prolinamide moieties as efficient organocatalysts for direct asymmetric aldol reaction

Poly(2-oxazoline)-supported bifunctional organocatalysts have been prepared through a bottom-up protocol, which involves synthesis of well-defined poly(2-oxazoline) precursors bearing amino groups in the side-chains followed by amide coupling with N-Boc-L-proline then deprotection. The resultant L-prolinamido-functionalized polymers have proven to be significantly more active than their monomeric counterparts for the aldolisation of cyclic ketones with several substituted benzaldehydes under neat conditions. By using 15 mol% of the polymer as a catalyst, the direct aldol reaction products were isolated in high yields and with good diastereo- and enantioselectivity. Based on circular dichroism spectrum analysis, the enhancement in catalytic activity is probably related to the conformational changes of the pseudo-peptide scaffold of poly(2-oxazoline)s. In addition, these soluble polymeric catalysts can be recovered and reused by precipitation in ether for five catalytic cycles without significantly diminishing their efficiency.