1992 рік

Bioerodible matrices are particularly suited for the systemic administration of drugs over extended time periods. Type I-A bioerodible matrices reported in the past degrade either too slowly or too rapidly. The rate of erosion, therefore, has no influence on the kinetics of release. This paper reports kinetics of release of theophyUine from a series of crosslinked glassy/swollen hydrogel matrices based on a novel crosslinking monomer, viz. 2-hydroxyethyl glycolate dimethacrylate. The release of theophylline in an alkaline medium is accompanied by the erosion of the crosslinks. Under appropriate conditions, theophylline is released at a constant rate by the erosion of the crosslinks in the bulk of the swollen polymer as well as by the surface erosion of the glassy polymer. The release behaviour has been explained on the basis of the concept of time-dependent diffusivity of the active ingredient and swelling-controlled zero-order release, respectively

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In a kinetically-controlled step-growth polymerization, the molecular weight distribution (MWD) of the polymers produced can be controlled via its reaction path. In this paper, the effect of intermediate monomer feed on the MWD in a batch reactor or a continuous plug flow reactor (PFR) is investigated, and a kinetic model is proposed for MWD development. The present model considers the interchange reactions during polymerization. The variance of the MWD can be increased by the intermediate monomer feed method while the interchange reactions counteract this effect. The present model has given satisfactory predictions for the polycondensation of adipic acid and 1,6-hexanediol.

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The curing of epoxy oligomers by isocyanates, as a function of structure and ratio of components, as well as of synthesizing conditions, has been studied. It was found that differences exist in reactions occurring during epoxy-isocyanate formation in bulk and in thin layer on substrate. The aspects studied were the effect of the structure of epoxy oligomers and isocyanates on relative reactivity of oligomers and on polymer formation and their structural changes depending on synthesizing temperature. Epoxy oligomers and isocyanates of various chemical structures were investigated by i.r. spectrography, differential thermal analysis, chemical analysis, electrical relaxation and electron microscopy. It is shown that the interaction between epoxy oligomers and isocyanates comprises two consecutive stages : formation of urethanes and 2-oxazolidones. The presence of tertiary nitrogen atoms in the structure of epoxy oligomer or isocyanate causes additional trimerization of isocyanate and formation of isocyanurate cycles. The structure of epoxy oligomer and isocyanate essentially influences their relative reactivity in the formation of oxazolidones and determines the temperature ranges in which these reactions take place. It is also shown that the conditions of formation and the ratio of components have a considerable effect on the chemistry of interaction in the epoxy-isocyanate oligomeric system. The interaction temperature in the epoxy-isocyanate system is the cause of varied degree of cross-linking, topological and supermolecular structure of polymers

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The kinetics of photopolymerization of dimethacrylates with the camphorquinone (CQ)/amine pair has been studied by isothermal d.s.c, and u.v.-vis, spectroscopy. In accordance with the proposed initiation mechanism of electron and proton transfer from the amine to the triplet CQ ketone, only amines with an abstractable proton on the s-carbon undergo significant rates of polymerization. However, in contrast to other studies of exciplex formation from triplet state ketones and quenchers, only a poor correlation was observed between the rate of radical formation (measured in terms of the maximum polymerization rate) and ionization potential. In general, the efficiency of amines in enhancing initiation was: tertiary > secondary > primary amine. A kinetic scheme for CQ consumption and radical formation was in satisfactory agreement with the experimentally observed rate dependence on the radiation intensity and on the CQ and amine concentrations. For low amine concentrations, the polymerization rate had a half order dependence on concentration, but at intermediate levels, the rate was independent of amine concentration. The polymerization was slightly retarded at high amine concentrations. The influence of CQ concentration and radiation intensity on the polymerization rate generally fitted the theoretical half power relationship, however evidence for primary radical termination and pseudo first-order termination was found at high and low initiation rates, respectively

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ct-(2-(4-Pyridyl)ethyl)-polydimethylsiloxane (Py-PDMS) was prepared according to the anionic ring opening polymerization of hexamethylcyclotrisiloxane (D3) initiated with sodium 2-(4-pyridyl)ethyldimethylsilanolate, which were derived from the reaction of 2-(4-pyridyl)ethyl-dimethylsilanol with sodium hydride. The average degree of polymerization of Py-PDMS was controlled in the range between 3 and 34 by changing the ratio of D 3 and the initiator. Furthermore, the preparation of c(-(2-(1-methyl-4- pyridinio)ethyl)-polydimethylsiloxane (MePy-PDMS) was carried out by the reaction of Py-PDMS with methyl iodide. In order to evaluate the activity of MePy-PDMS as a transdermal penetration enhancer, the permeability of indomethacin through rabbit abdominal skin with and without MePy-PDMS was measured by in vitro experiment. MePy-PDMS showed effective enhancing activity for drug penetration through the skin. It was also revealed that the enhancing activity was due to an increase of the partition coefficient of the drug into the stratum corneum, from the determination of kinetic parameters in the drug permeation

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Differential scanning calorimetry has been used to investigate the thermal behaviour of a mesogenic polyester built up of a flexible spacer of eight methylene units and a rigid aromatic ester triad. Non-isothermal crystallizations at different cooling rates and isothermal crystallizations at various temperatures have been carried out; the variations of the melting temperatures and enthalpies as a function of the crystallization parameters have been investigated. The melting profiles of the treated samples reveal two endotherms at T~ and Td,which cannot be interpreted as usually reported for conventional polymers. A new model of crystallization has been proposed, taking into account that a certain registry of neighbouring chains persists in the nematic state above Tin, and becomes poorer and poorer on increasing the temperature and time; this persistent registry can be regarded as potential nuclei of crystallization, responsible for the high-temperature endotherm. Therefore, the presence of multiple endotherms in the melting profile of thermotropic polymers crystallized from the liquid-crystalline state appears to be a consequence of the annealing conditions in the nematic phase.

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The interconnection between the kinetics of semi-interpenetrating network (semi-IPN) formation, phase separation during the process and viscoelastic properties has been investigated. The semi-IPN based on crosslinked polyurethane (PU) and poly(butyl methacrylate) (PBMA) was synthesized in two ways: (1) by introducing PBMA into the reaction mixture for PU formation and (2) by simultaneously carrying out both the polyaddition reaction of PU formation and the polymerization of butyl methacrylate, the latter being produced using initiators and ultra-violet light. The kinetic constants, degrees of conversion at the onset of phase separation and viscoelastic properties have been estimated. It was shown that viscoelastic properties are changed essentially depending on the semi-IPN formation conditions, the degree of segregation (degree of phase sepration) being dependent on the kinetic conditions. The dependence of viscoelasticity on the kinetics is explained by various degrees of segregation fixed by crosslinking and by the formation of two phases of different composition.

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Star-shaped copolymers having three different branches, poly(styrene), poly(dimethylsiloxane), and poly(tert-butyl methacrylate), were prepared by coupling of poly(styryl) anion with end-reactive poly(dimethylsiloxane), followed by anionic propagation of tert-butyl methacrylate. End-reactive poly(dimethylsiloxane) was prepared by anionic polymerization of hexamethylcyclotrisiloxane initiated with the lithium salt of p-(dimethylhydroxy)silyl-~-phenylstyrene. The resultant polymers were characterized by osmometry, g.p.c., 1H-n.m.r., and found to be pure and to have narrow molecular weight distributions

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The transparent solution blends of aramids and poly(ether sulphone)s in N,N-dimethylacetamide or N-methyl-2-pyrrolidone were obtained from three kinds of commercially available poly(ether sulphone)s and two kinds of aramids derived from isophthaloyl chloride and both 4,4'- and 3,4'-diaminodiphenyl ether. The as-prepared polymer blend films were transparent or somewhat translucent. Some of the blend films showed 'blend induced crystallization' behaviour, giving high temperature resistant films after annealing at 300°C. The tensile properties of the as-prepared and annealed blend films were investigated

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The filler effects on the compatibility and phase separation kinetics of poly(vinyl acetate) (PVA)-poly(methyl methacrylate) (PMMA) mixtures were studied. Cloud-point curves for filled and unfilled PVA-PMMA mixtures were plotted. It was shown that this system exhibited lower critical solution temperature behaviour. The phase separation of filled mixtures occurs at lower temperatures than that of unfilled mixtures. The temperatures of phase separation of mixtures with predominant PVA or PMMA contents exhibit an especially sharp decline. The results of the studies on the kinetics of phase separation show that the phase separation rate of filled mixtures is much lower than that of unfilled mixtures at temperatures equidistant from the corresponding cloud-point curves. It is established that the phase separation of mixtures occurs in two stages. This can be associated with the structure rearrangement of the phases formed during phase separation. The differences in phase separation of filled and unfilled mixtures can be explained by the specific interactions between a filler and one of the components of a binary polymer mixture

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