We are investigating an unusual reaction that occurs when methyl methacrylate (MMA) is kept in contact with concentrated nitric acid1 (65% HNO3, sp. gr. 1.41). Polymer of high molecular weight is formed, showing about one unit of methacrylic acid (MAA) per unit of MMA, when equilibrium is reached. The reaction depends on the temperature, the molar ratio MMA:HNO3, and the reaction time. Although we also found polymer at temperatures in the range 50–70°C,2 in this paper we only report the results when the temperature was kept between 25 and 40°C. Methacrylic acid (MAA) was found to homopolymerize under those mild conditions; its behavior was investigated. Although we also observed that polymer is formed with sulfuric acid (96%) and that acrylic acid polymerizes with both nitric and sulfuric acid at 20–30°C, we are limiting this article to the observed polymerizing action of nitric acid on methyl methacrylate and on methacrylic acid. Work proceeds on this matter in this laboratory. 相似文献
Copolymers of methacrylic acid (MAA) and a nonionic hydrophilic monomer N-vinylpyrrolidone (NVP) were synthesized by polymerization in aqueous solution in the absence of metal ions. The NVP content of the copolymers ranged from 2 to 36 mole % with sequences of MAA interrupted at random by a single unit of NVP at all compositions. The pH-induced conformational transition of these copolymers was followed by potentiometric titration and viscosity studies and the results were compared with those of pure poly(methacrylic acid) (PMAA). The negative free energy of transition from the un-ionized compact from to expanded structure showed a gradual decrease with increasing NVP content, and the collapsed conformation observable for PMAA at low degrees of ionization (0 < α < 0.3) disappeared at NVP contents greater than 15 mole%. These findings are supported by viscosity data. The results suggest that long-range methyl–methyl hydrophobic contacts still possible in higher NVP content copolymers are not sufficient to bring about the collapse of the molecule and a minimum average sequence length of about 20 MAA units is required to compact the molecule. Hydrophilic “shielding” of MAA chains by NVP segments could also partly destabilize the collapsed structure. 相似文献
A polychromatic action spectrum for the induction of an ultraviolet-absorbing/screening mycosporine-like amino acid (MAA) has been determined in a filamentous and heterocystous nitrogen-fixing rice-field cyanobacterium, Anabaena sp. High-performance liquid chromatographic (HPLC) studies revealed the presence of only one type of MAA, which was identified as shinorine, a bisubstituted MAA containing both glycine and serine groups having a retention time at 2.8 min and an absorption maximum at 334 nm. Exposure of cultures to simulated solar radiation in combination with various cut-off filters (WG 280, 295, 305, 320, 335, 345, GG 400, 420, 455, 475, OG 515, 530, 570, RG 645, 665 and a broad-band filter, UG 11) clearly revealed that the induction of the MAA takes place only in the UV range. Photosynthetic active radiation (PAR) had no significant impact on MAA induction. The ratio of the absorption at 334 nm (shinorine) to 665 nm (chlorophyll a) and the action spectrum also showed the induction of MAA to be UV dependent peaking in the UV-B range at around 290 nm. The results indicate that the studied cyanobacterium, Anabaena sp. may protect itself from deleterious short wavelength solar radiation by its ability to synthesize a mycosporine-like amino acid in response to UV-B radiation and thereby screen the negative effects of UV-B. 相似文献
Propagation rate coefficients, kp, of free-radical methacrylic acid (MAA) polymerization in aqueous solution are presented and discussed. The data has been obtained via the pulsed laser polymerization – size-exclusion chromatography (PLP-SEC) technique within extended ranges of both monomer concentration, from dilute solution up to bulk MAA polymerization, and of degree of ionic dissociation, from non-ionized to fully ionized MAA. A significant decrease of kp, by about one order of magnitude, has been observed upon increasing monomer concentration in the polymerization of non-ionized MAA. Approximately the same decrease of kp occurs upon varying the degree of MAA ionization, α, at low MAA concentration from α = 0 to α = 1. With partially ionized MAA, the decrease of kp upon increasing MAA concentration is distinctly weaker. For fully ionized MAA, the propagation rate coefficient even increases toward higher MAA concentration. The changes of kp measured as a function of monomer concentration and degree of ionization may be consistently interpreted via transition state theory. The effects on kp are essentially changes of the Arrhenius pre-exponential factor, which reflects internal rotational mobility of the transition state (TS) structure for propagation. Friction of internal rotation of the TS structure is induced by ionic and/or hydrogen-bonded intermolecular interaction of the activated state with the molecular environment. 相似文献
In this paper, we describe a new procedure to phase transfer large gold nanoparticles (diameters > 45 nm) from aqueous solution to organic solvents. This is accomplished using a covalent amide coupling reaction that incorporates dicyclohexylamine (DCHA) headgroups on the surface of mercaptoacetic acid (MAA) functionalized gold nanoparticles. Gold nanoparticles are first synthesized in aqueous solution by the citrate-reduction method, and nanoparticle size is controlled by the molar ratio of the reducing agent (sodium citrate) and the gold precursor (KAuCl4). MAA is then adsorbed to the surface of the gold nanoparticles followed by an amide-coupling reaction to covalently attach DCHA to the surface-immobilized MAA. The bulky dicyclohexyl groups entropically stabilize gold nanoparticles in organic solvents. This procedure was used to reliably transfer gold nanoparticles with diameters between 45 and 100 nm from aqueous solution to organic solvents such as dimethyl sulfoxide and chloroform. 相似文献
<正>In this study,P(St-MAA) seed latex particles were first prepared via soap-free emulsion polymerization of styrene(St) and methacrylic acid(MAA),then the seed particles were allowed to swell with St at room temperature,and the P(St-MAA)/P(StNaSS) core/shell latex particles were then synthesized via seeded emulsion copolymerization of St and sodium styrene sulphonate (NaSS) using AIBN as initiator in the presence of N,N'-methylenebisacrylamide(BAA,water-soluble crosslinker).Results showed that the polymerization could be carried out smoothly when the ratio of BAA to total monomers was less than 3 mol%,the narrow dispersed P(St-MAA) seed particles with the diameter of 150 nm and the P(St-MAA)/P(St-NaSS) core/shell latexes with the particle size of about 200 nm were synthesized.When the 25/75 mole ratio of NaSS/(St + MAA) and 2 mol%of BAA were used in the seeded emulsion polymerization,the resulted P(St-MAA)/P(St-NaSS) latex product showed a low weight loss after water extraction,and the NaSS unit content in the whole particle and in the shell reached 11.7 mol%and 34.6 mol%,respectively. 相似文献
We demonstrate a strategy for using an optical stimulus to trigger the dissociation of block copolymer (BCP) vesicles in aqueous solution. The BCP, comprising hydrophilic poly(ethylene oxide) (PEO) and a block of poly(methacrylic acid) bearing a number of spiropyran methacrylate comonomer units (P(MAA‐co‐SPMA)), was allowed to firstly self‐assemble into large vesicles in aqueous solution at pH = 3 with protonated carboxylic acid groups, and then become kinetically stable at pH = 8 due to the glassy vesicle membrane of P(MAA‐co‐SPMA). Fast dissociation of the vesicles was achieved through a cascade of events triggered by UV‐induced isomerization from neutral spiropyran to charged merocyanine in the membrane.
The aim of this study was to analyse the influence of the template/functional monomer proportion on the achievement of molecularly imprinted hydrogels with cavities with a high enough affinity for the drug to sustain drug release. Imprinted hydrogels were prepared from N,N-dimethylacrylamide and tris(trimethylsiloxy)sililpropyl methacrylate (DMAA and TRIS; main components), methacrylic acid (MAA; functional monomer), ethylene glycol dimethacrylate (EGDMA; cross-linker), and timolol (template drug). Photo-polymerization of the monomer solutions was carried out in poly(propylene) molds (0.3 mm thickness) to obtain contact lens-like devices. Non-imprinted control hydrogels were also prepared in the same way but without the addition of timolol. The imprinted hydrogels showed a higher affinity for timolol and a slower release rate than the non-imprinted hydrogels. The release rate decreased by increasing the MAA/timolol ratio in the gel recipe. Hydrogels prepared with 400 x 10(-3) M MAA, 600 x 10(-3) M EGDMA, and a timolol/MAA mole ratio of 1:16-1:32 had drug diffusion coefficients two orders of magnitude below those of non-imprinted hydrogels. The results obtained clearly indicate that the timolol release rate is critically affected by the conditions under which the hydrogels were synthesized. These effects are discussed on the basis of the influence of drug proportion on the conformation of the imprinted cavities. 相似文献
A thorough study has been made of the ephedrine-methacrylic acid (MAA) system for molecular imprinting, involving NMR studies of the pre-polymerisation equilibria, modelling and HPLC enantioseparations with different mobile phases. When dimerisation of MAA is accounted for, NMR titrations demonstrate there is a very strong ('stoichiometric', K approximately 10000 M(-1)) interaction between ephedrine and a single MAA monomer. Polymers prepared with a 1:1 monomer:template ratio are capable of enantioseparation, indicating, in combination with the NMR results, that the 1:1 interaction probably involves the carboxylic acid acting as a chelating monomer, forming hydrogen bonds to both the template amine and hydroxyl moieties. Higher monomer:template ratios cause further changes in the NMR signals, suggesting at least one further MAA can interact with the amine group, with a weaker association constant (K approximately 80 M(-1)). Polymers prepared with a 4:1 monomer:template ratio are thus proposed to contain a mixture of 1-monomer binding sites and 2-monomer binding sites, the latter being of enhanced acidity. In HPLC, better results are obtained with the 4:1 polymer using acetic acid as a modifier, while better results are obtained for the 1:1 polymer using butylamine as a modifier. We propose a model whereby acetic acid exerts its effect by reducing binding to the 1-monomer sites, while butylamine works largely through blocking the most acidic, 2-monomer sites. For preparative chromatography we suggest the 1:1 polymer, with a larger population of weaker but more uniform binding sites, is most promising. 相似文献
Six molecularly imprinted polymers (MIPs) of erythromycin (ERY) were prepared by noncovalent bulk polymerization using methacrylic
acid (MAA) as the functional monomer. On the basis of binding analysis, the MIPs with 1:2 optimum ratio of template to MAA
were selected for subsequent scanning electron microscopy and Brunauer–Emmett–Teller analyses, which indicated that the MIPs
had more convergent porous structures than the nonimprinted polymers. The equilibrium binding experiments showed that the
binding sites of MIPs were heterogeneous, with two dissociation constants of 0.005 and 0.63 mg mL−1, respectively. Furthermore, the performance of the MIPs as solid-phase extraction (SPE) sorbents was evaluated, and the selectivity
analysis showed that the MIPs could recognize ERY with moderate cross-reactivity for other macrolides. The overall investigation
of molecularly imprinted SPE for cleanup and enrichment of the ERY in pig muscle and tap water confirmed the feasibility of
utilizing the MIPs obtained as specific SPE sorbents for ERY extraction in real samples.
Figure Schematic diagram of the preparation and application of the erythromycin imprinted molecularly imprinted polymers
Suquan Song and Aibo Wu contributed equally to this work. 相似文献