Hydrogels based on crosslinked polymethacrylic acid were synthesized via free-radical polymerization in aqueous solution,
using N,N′-methylene bisacrylamide as a crosslinking agent and 2,2′-azobis-[2-(2-imidazolin-2-yl)propane] dihydrochloride
as an initiator. The influence of the reaction parameters (the neutralization degree of methacrylic acid and the initial monomer
concentration) on the equilibrium swelling degree, the swelling kinetic parameters and the basic structural properties of
xerogels was investigated. The change of synthesis parameters leads to the change of the basic structural parameters of xerogel,
as well as the equilibrium swelling degree and the initial swelling rate of the hydrogels. It is found that there are power
form relationships between the equilibrium swelling degree, the initial swelling rate and the structural xerogel’s properties
and the change of the neutralization degree of monomer, i.e. the monomer concentration. The examined correlations proved that
the crosslinking density is the crucial parameter which determines all the other investigated structural and swelling parameters.
The article is published in the original. 相似文献
A novel thiol-ene photopolymerization reaction involving copolymerization of tetrathiol monomer with vinyl silazane is experimentally characterized and is modeled successfully. The overall polymerization rate is found to be controlled by the ratio of the propagation to chain transfer kinetic parameters. The polymerization rate of this mixture, in the presence of added photoinitiator, is approximately first order in ene functionality and is independent of thiol functional group concentration. Initiation rates in this system, when cured utilizing a light centered around 365 nm light, and in the presence of no added photoinitiator, are shown to be proportional to the ene monomer concentration. When the mixture is polymerized utilizing light centered at 254 nm light, and without photoinitiator, the initiation rates are proportional to the thiol monomer concentrations. This novel reaction scheme is further utilized to form ultra rapidly polymerizable polymer derived ceramic structures with high aspect ratios. 相似文献
The colloidal chemical properties of triple block copolymers of polypropylene oxide and polyethylene oxide (pluronics of various structures) were studied in comparison. All of them are shown to be surfactants but differ in interfacial tension, surface activity, surface area occupied in the adsorption layer, and adsorption layer thickness. The kinetic regularities of polymerization of styrene and methyl methacrylate were studied. The particle diameters and their size distribution were determined. Distinctions in the kinetic regularities of polymerization are shown: the shape of the conversion—time curves (for the duration of the initial and stationary stages of polymerization) and the dependences of the diameter on the surfactant concentration and monomer to water volume ratio. In the presence of the water-insoluble pluronics, the mechanism of formation of polymer—monomer particles and interfacial layer on the surface differs from that when using water-soluble surfactants, which makes it possible to distinguish these processes into an independent type of heterophase polymerization.
Fully aromatic polyquinazolinediones (IV) of high molecular weight were obtained by thermal cyclodehydration of aromatic poly(uredio acids) (III) prepared by the polyaddition reaction of 4,4′-diaminobiphenyl-3,3′-dicarboxylic acid (I) with aromatic diisocyanates (II). From the kinetic study of reactions of model systems (anthranilic acid with phenyl isocyanate) in the presence of a variety of basic catalysts, it was established that tertiary amines had the highest catalytic activity for the formation of ureido linkage. The optimum polymerization conditions were determined by the study of reaction variables such as monomer concentration, polymerization temperature, monomer ratio, and catalyst concentration. The effect of polarity and purity of organic solvents and reactants was also studied. 相似文献
Radiation-induced terpolymerizations of methyl α,β,β-trifluoroacrylate (MTFA) with tetrafluoroethylene (TFE) and α-olefins, such as ethylene, propylene, and isobutylene, were carried out in bulk at 25°C for the purpose of controlling the content of ester group in the MTFA-α-olefin alternating copolymers. These monomers polymerized to form alternating terpolymers which contained 50 mole % α-olefin in a wide range of monomer composition. The content of MTFA, namely, the ester group in polymer, can be varied without destruction of the alternating structures between fluoroolefins (MTFA, TFE) and α-olefin by changing the MTFA/TFE ratio in the monomer mixture. The relative reactivities of MTFA and TFE in the terpolymerization were discussed according to kinetic treatments by free propagating and complex mechanisms. The relation between the MTFA/TFE ratio in the monomer mixture and that in terpolymer was explained favourbly by the complex mechanism. It was also concluded that the relative reactivity of MTFA is larger than that of TFE in the terpolymerizations. 相似文献
The A–A/B–B step‐growth copolymerization between a monomer immobilized in the crystalline state and a monomer mobile in the solution state is demonstrated. One of the two monomers was immobilized as organic ligands of the metal–organic framework (MOF) and polymerized with the mobile guest monomer, resulting in the formation of linear polymers. The polymerization behavior was completely different from that of the solution polymerizations. In particular, the degrees of polymerization (DP) converged to a specific value depending on the MOF structures. The inevitable termination is caused not by imperfectness of the polymerization reaction, but by the selection of the two polymerization partners among the several adjacent immobilized monomers. This is fully supported by the Monte Carlo simulation on the basis of the polymerization mechanism. Precise immobilization of monomers in the supramolecular assemblies is a promising way for the controlled A–A/B–B step‐growth polymerization. 相似文献
Background: Peptides derived from three of four putative α-helical regions of the prion protein (PrP) form amyloid in solution. These peptides serve as models for amyloidogenesis and for understanding the α helix → β strand conformational change that is responsible for the development of disease. Kinetic studies of amyloid formation usually rely on the detection of fibrils. No study has yet explored the rate of monomer peptide uptake or the presence of nonfibrillar intermediate species. We present a new electron spin resonance (ESR) method for probing the kinetics of amyloid formation. A spin label was covalently attached to a highly amyloidogenic peptide and kinetic trials were monitored by ESR.Results: Electron microscopy shows that the spin-labeled peptide forms amyloid, and ESR reveals the kinetic decay of free peptide monomer during amyloid formation. The combination of electron microscopy and ESR suggests that there are three kinetically relevant species: monomer peptide, amyloid and amorphous aggregate (peptide aggregates devoid of fibrils or other structures with long-range order). A rather surprising result is that amyloid formation requires the presence of this amorphous aggregate. This is particularly interesting because PrPSc the form of PrP associated with scrapie, is often found as an aggregate and amyloid formation is not a necessary component of prion replication or pathogenesis.Conclusions: Kinetic analysis of the time-dependent data suggests a model whereby the amorphous aggregate has a previously unsuspected dual role: it releases monomer into solution and also provides initiation sites for fibril growth. These findings suggest that the β-sheet-rich PrPSc may be stabilized by aggregation. 相似文献
This paper is a brief survey on the preparation of latexes bearing covalently bonded functional groups using various routes. Recent methods are presented using water soluble functional (or surface active) initiators, monomers or macromonomers. For tailor synthesis, the copolymerisation approach of a basic monomer with a functional monomer under emulsifier-free conditions is generally the more suitable and versatile one. More progress should be pursued in this field in order to design better adapted chemical structures which will improve their use in radical-initiated heterogeneous conditions and favour surface incorporation. 相似文献
In tetrahydrofuran, with Na+ as counter-ion, the anionic polymerization of acrolein involves numerous transfer reactions to monomer and to polymer; on the other hand, termination of growing chains does not occur. The use of initiators, like carbanions or oxanions, does not affect the polymerization rate. The kinetic order of the reaction is unity for monomer and unity for initiator; these results indicate that the living ends are not associated at the studied concentrations of initiator. Without stating precisely the mechanism of the transfer reactions, we have proposed a kinetic scheme.In tetrahydrofuran, with Na+ as counter-ion, the anionic polymerization of acrolein involves numerous transfer reactions to monomer and to polymer; on the other hand, termination of growing chains does not occur. The use of initiators, like carbanions or oxanions, does not affect the polymerization rate. The kinetic order of the reaction is unity for monomer and unity for initiator; these results indicate that the living ends are not associated at the studied concentrations of initiator. Without stating precisely the mechanism of the transfer reactions, we have proposed a kinetic scheme. 相似文献
Summary: Experimental data for the photopolymerization of furfuryl acrylate (FA) conformed satisfactorily to the kinetics model proposed for the photopolymerization of furfuryl methacrylate (FM). This model allowed the kinetic constants of the basic steps of the studied mechanism, namely propagation, degradative transfer, re‐initiation and cross‐termination, to be determined. The calculated values of these constants were in agreement with the chemical nature of FA. For each of these constants, the confidence intervals were determined, and the statistical dependence between some of them was analyzed using the ellipse error method. The equations of moments of the distribution of molecular sizes of the primary chains in the network with order greater than one were developed to describe different molecular averages, such as weight‐average chain length and size‐average heterogeneity of the primary chains. The results found for the monomer conversion, the cross‐link degree and number‐average length of the primary chains of the network for FA were compared with those obtained for FM, and it was shown that the process of polymerization of the former monomer was more retarded and produced gels with a greater degree of cross‐linking than the latter as expected.
Network structures with different heterogeneity in size of the primary chains. 相似文献
Three basic conditions for preparation of alternating copolymer with narrow molecular weight distribution were derived from the element kinetic equations of binary radical copolymerization. Using maleimide (MI) and atropie acid (ATA) as model monomer pairs and dioxane as the solvent the alternating copolymer with molecular weight distribution in the range of 1.09--1.20 was prepared successfully by charger transfer complex (CTC) mechanism in the presence of benzoyl peroxide at 85℃. The monomer reactivity ratioes r_1(MI)=0.05±0.01 and r_2(ATA)=0.03±0.02 were measured. The alternating eopolymerization was carried out through formation of a contact-type CTG and then alternating addition of MI and ATA monomers. The molecular weight of the copolymers is nearly independent of the feed ratio in a large range and the polymerization rate dropped with an increase in ATA in feed ratio. 相似文献