Ring-chain equilibria in the metathesis reaction of cycloolefines

Polyoctenylenes and polydodecenylenes prepared by the methathesis reaction exhibit a bimodal molecular weight distribution. The low molecular weight fraction consists of cyclic oligomers, the high molecular weight fraction is assumed to contain linear polymers. The distribution of cyclic oligomers together with the polymer material indicates a ring-chain equilibrium. The slope of the plot log K_x (molar cyclization equilibrium constant) vs. log x (degree of polymerization) is close to ?2,5 as predicted by the Jacobson and Stockmayer theory for unstrained macrocycles.     

MgCl2-supported Ziegler-Natta catalysts: A structural investigation by X-ray diffraction and Fourier-transform IR spectroscopy on the chemical activation process through MgCl2-ethanol adducts

By X-ray diffraction on polycrystalline powders and Fourier-transform IR spectroscopy, some adducts MgCl₂(C₂H₅OH)_x were studied, having different composition in the range from x = 0 (α-MgCl₂) to x = 6. For 2 < x ≤ 6, the structure of such adducts shows prevalently ionic Mg? Cl bonds. For x ≤ 2, covalent bonds are observed among chlorine and magnesium atoms, which give rise to chain compounds through chlorine bridges among the Mg-atoms. The obtained structural data are also discussed in terms of the synthesis of some MgCl₂-based supports which are very active in the Ziegler-Natta process of olefin polymerization, after a treatment with titanium halides and a successive addition of a cocatalyst and of other suitable donors.     

Polymerization reactivity of styrene complexes composed of maleic anhydride,p-chloranil or 2,3-dichloro-5,6-dicyano-p-benzoquinone

Terpolymerizations of styrene (St)/p-chloranil (PCA)/maleic anhydride (MAnh), St/2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ)/MAnh, and St/DDQ/PCA systems were studied, in order to compare the reactivities as acceptor monomers among MAnh, PCA, and DDQ. These terpolymerizations were treated by the complex mechanism. The modified and real monomer reactivity ratios of the complexes in these systems were obtained as r_I(K_I/K_II) (PCA-St) = 1,60 and r_II(K_II/K_I) (MAnh-St) = 0,10, r_I(PCA-St) = 3,55 and r_II(MAnh-St) = 0,045, r_I(K_I/K_II) (DDQ-St) = 50 and r_II(K_II/K_I) (MAnh-St) = 0,01, r_I(DDQ-St) = 132 and r_II(MAnh-St) = 0,0038, and r_I(K_I/K_II) (DDQ-St) = 200 and r_II(K_II/K_I) (PCA-St) = 0,001, r_I(DDQ-St) = 238 and r_II(PCA-St) = 0,0008, respectively. The reactivity of these St complexes was concluded to be controlled primarily by the polarity of the acceptor monomers.     

Die Metrik des Uteruswachstums bei digenen Trematoden

Zusammenfassung 1. Die Uteruslänge von Dicrocoelium dendriticum, Pleurogenoides medians und Fasciola hepatica wird auf verschiedenen Entwicklungsstadien in Beziehung zu dem korrigierten Körperlängenmaß Länge×Breite gesetzt.2. Die logarithmische Darstellung der Meßwerte aller drei Arten zeigt einen S-förmigen Verlauf, wobei ein mittlerer Abschnitt sich gerade erstreckt.3. Die Funktion y=a×x+×(x _w–x)³+b (y=log Uteruslänge, x=log Körperlänge, x _w=log Körperlänge am Wendepunkt) entspricht für Dicrocoelium mit numerisch bestimmten Konstanten (a, , b) quantitative der Meßwertschar. Sie gilt auch für Pleurogenoides und Fasciola, jedoch mit anderen Werten für die Konstanten a, und b.4. Der Mittelteil der S-Kurven kann nach dem allometrischen Wachstumsgesetz interpretiert werden. In diesem Entwicklungsabschnitt ist der Uterus schlauchförmig.5. Bei den jüngsten Entwicklungsstadien wächst der Uterus als massiver Zellstrang relativ langsamer (Volumenwachstum) als später, nachdem sich allmählich ein zellfreies Lumen gebildet hat (Flächenwachstum).6. Bei den ältesten Stadien nimmt die Länge des Uterus ebenfalls relativ langsam zu, weil er durch inzwischen gebildete Eimassen gedehnt wird.7. Die abgeleitete Funktion kann im Hinblick auf (4), (5) und (6) als Modifikation des Allometriegesetzes Y=B×X ^a (log Y=y, log X=x) bzw. y=ax+b aufgefaßt werden.

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Metric growth of the uterus in digenetic trematodes

Summary 1. The length of the uterus of Dicrocoelium dendriticum, Pleurogenoides medians and Fasciola hepatica at different stages of development is correlated with the corrected size of the body ( ).2. The data plotted in a log-scale show for all three species a sigmoid curve with linear middle section.3. The function y=ax+(x _w–x)³+b(y=log length of the uterus, x=log length of the body, x _w=log length of the body at the turning point) fits very well to the data obtained from Dicrocoelium dendriticum. The constants a, and b were determined numerically. The function is valid also for Pleurogenoides medians and Fasciola hepatica but with other constants.4. The linear middle section of the sigmoid curve may be interpreted by allometric growth. During this phase of development the uterus is tubelike.5. During the earliest stages of development the uterus forms a solid cord of cells (three-dimensional growth) and grows slower than in a later stage after formation of a cell-free interior (two-dimensional growth).6. The latest phases of development are characterized by a small increase in length of the uterus due to the three-dimensional dilatation by the produced eggs.7. The computed function is considered as a modification of the law of allometric growth Y=B×X ^a, respectively y=ax+b (log Y=y, log X=x).

     

Termination processes in free radical polymerization, 5,,. The numerical solution of a kinetic scheme with chain length dependent termination

In order to assess the general consequences of an eventual chain length dependence of the bimolecular termination rate constant k_t in free radical polymerization, an iterative procedure for solving the kinetic scheme was developed for this case. The iteration process is based on the equation for the propagation probability of a radical chain of length x, α_x, which constitutes a cyclic definition of the α-values, with F(x,y) giving the functional dependence of k_t = k F(x, y) on the lengths x and y of the two radicals involved in the termination process. As, in addition, α_x links the stationary concentrations of radicals of adjacent length, [R_x?1] and [R_x], solving for the set of α_x allows to build up the complete chain length distribution of polymer radicals from which all relevant kinetic quantities may be calculated. The following types of functional dependence have been used in practice The case of F₂, which is the slightly less realistic one, allows to separate the variables x and y in the equation for α_x, thus leading to an immediate comfortable solution. Irrespective of the type of F(x, y) chosen (the differences being quantitative in nature rather than qualitative) it can be shown that nearly all the familiar relationships known from “classical” polymerization kinetics with chain length independent termination (b = 0) break down. The new relationships, however, can be cast into comparatively simple power laws. The general consequences of the concept of chain length dependent termination are discussed in some detail.     

Strong linkage disequilibrium of a HbE variant with the (AT)9(T)5 repeat in the BP1 binding site upstream of the β-globin gene in the Thai population

A binding site for the repressor protein BP1, which contains a tandem (AT)_x(T)_y repeat, is located approximately 530 bp 5 to the human -globin gene (HBB). There is accumulating evidence that BP1 binds to the (AT)₉(T)₅ allele more strongly than to other alleles, thereby reducing the expression of HBB. In this study, we investigated polymorphisms in the (AT)_x(T)_y repeat in 57 individuals living in Thailand, including three homozygotes for the hemoglobin E variant (HbE; 26Glu->Lys), 22 heterozygotes, and 32 normal homozygotes. We found that (AT)₉(T)₅ and (AT)₇(T)₇ alleles were predominant in the studied population and that the HbE variant is in strong linkage disequilibrium with the (AT)₉(T)₅ allele, which can explain why the ^E chain is inefficiently synthesized compared to the normal ^A chain. Moreover, the mildness of the HbE disease compared to other hemoglobinopathies in Thai may be due, in part, to the presence of the (AT)₉(T)₅ repeat on the HbE chromosome. In addition, a novel (AC)_n polymorphism adjacent to the (AT)_x(T)_y repeat (i.e., (AC)₃(AT)₇(T)₅) was found through the variation screening in this study.MIM and accession numbers and URLs for data presented herein are as follows: Online Mendelian Inheritance of Man (OMIM), (for HBB [MIM 141900]). GenBank, (accession number [NG_000007.2] reference sequence information).     

The stochastic kinetics of intramolecular reactions in linear polymers

This paper deals with the kinetics of reactions of linear flexible macromolecules in solution possessing initially several reactive sites per macromolecule. It is assumed that the reactive sites interact with each other encounter-controlled and that both intra- and intermolecular reactions are possible. On the basis of the theory of stochastic kinetics in microsystems, equations were derived, describing the kinetics of intramolecular reactions as a function of the molecular weight distribution. An important application of this theoretical treatment pertains to the distribution between inter- and intramolecular reactions. A demonstration for typical reaction conditions reveals the influence of rate parameters and of the average number of reactive sites per macromolecule r?(0) on the proportionate distribution expressed by f_A = f (r?(0)); (f_A = fraction of intramolecular reactions). This function is only weakly influenced by varying the width parameter β in the case of Schulz-Zimm molecular weight distributions from β = 1 to → ∞.     

Charge density of renal interstitium

The charge density of renal interstitium was analysed from the volume of distribution of negative native albumin as compared with neutralized albumin, labelled with ¹²⁵I and ¹³¹ I, respectively. The experiments were conducted by infusing the two probes intravenously at a rate which kept the plasma concentrations stable. The concentration in renal hilar lymph, C_lymph(t), will then obey the function C_lymph(t) = C_lymph(t_x) (1-exp—Kt), where C_lymph(t_x) is the steady state Concentration and K the time constant for passage of the tracer through the renal interstitium - the former is dependent on the permeability of the peritubular capillary membrane, whereas the time constant is inversely related to the interstitial distribution volume of the tracers. The lymph-to-plasma concentration ratio (L/P-ratio) of negative, native albumin was found to be lower than that of neutralized albumin, a finding suggesting that the peritubular capillary membrane is negatively charged. Regarding the interstitium, it was calculated from the respective time constants, K, that the interstitium/lymph concentration ratio of negative native albumin was 0.96 ± 0.06 of that of neutralized albumin. This suggests the presence of negative fixed charges repelling negative native albumin. However, since the calculated charge density of — 1.8 ± 1.2 mEq 1^--¹ was not significantly different from zero, it is concluded that the renal interstitium is uncharged. This does not, however, rule out the possibility that, for example, negative groups are fixed to the interstitial matrix, merely that the average fixed charge density of renal interstitial fluid is negligible.     

Computer simulation of the steady state in reversible polymer-analogous reactions, 3. Flows and statistics in detailed balance

The steady state of reversible polymer-analogous reactions was simulated by Monte Carlo calculations. The length of the simulated copolymer chains was chosen large enough to yield data with an acceptable degree of statistical fluctuation. Two types of sets of relative rate constants K were investigated, first K₁ = K₂ = K₃, and second K₁ ≠ K₂, K₁ ≠ K₃, K₂ ≠ K₃, with K₁ · K₃ = K²₂. The relative rate constants K are defined as K₁ = k(AAA)/k(ABA), K₂ = k(AAB⁺)/k(ABB⁺), and K₃ = k(BAB)/k(BBB), where k is the ordinary rate constant pertaining to the reaction of the central monomeric unit in the triad given in parentheses, with two different types of monomeric units A and B in the binary copolymer. For both types of sets of relative rate constants the simulation showed a detailed balance to prevail for the kinetics in the steady state. It was also shown that the same kinetics and statistics were obtained in the steady state when starting the simulation first from a homopolymer consisting of A-monomeric units, and then from a homopolymer of B-monomeric units. Based on the finding of a detailed balance and on the definition of K, it could be shown by analysis that the first type of the sets of K leads to a Bernoullian statistics of the copolymers in the steady state, while the second type leads to a first-order Markov statistics. Correspondingly, equations are given which allow to calculate the relative rate constants from statistics and vice versa. The equations have also been confirmed by Monte Carlo simulation. In addition, the equations for the interrelations between the standard Gibbs free energies of the individual reactions for the central units in the triads are given.     

Synthesis and ring-opening polymerization of 2-acetoxymethyl-2-alkyltrimethylene carbonates and of 2-methoxycarbonyl-2-methyltrimethylene carbonate; a comparison with the polymerization of 2,2-dimethyltrimethylene carbonate

The polymerization of 2-acetoxymethyl-2-alkyltrimethylene carbonates (alkyl = methyl: AMTC, alkyl = ethyl: AETC) and of 2-methoxycarbonyl-2-methyltrimethylene carbonate (MMTC) in toluene with sec-butyllithium as initiator results in the respective polymer with yields between 78% and 88%. The analysis of the polymer microstructure by means of NMR spectroscopy reveals linear chains without branching due to an attack of the active chain end at the ester moiety of the repeating units. Poly(MMTC) and poly(-AETC) afford crystalline materials upon precipitation from solution while poly-(AMTC) is amorphous; after quenching from the melt all materials are amorphous. Copolymerization of AMTC, AETC and MMTC with 2,2-dimethyltrimethylene carbonate (DTC) results in random copolymers. The kinetics of the polymerization of AETC and MMTC revealed that the ester side chain enhances the rate of propagation compared to the polymerization of DTC. The apparent rate constants of propagation in toluene with lithium alcoholate as active sites at 23°C were determined to be k_app^DTC = 4 ċ 10⁻³ s⁻¹, k_app^AETC = 4,28 ċ 10⁻² s⁻¹, and k_app^MMTC = 2,57 ċ 10⁻² s⁻¹. Studies of ring-chain equilibria in solution of tetrahydrofuran revealed that on the basis of the theory of Jacobson-Stockmayer the characteristic ratios of the polymers are C_∞^PDTC = 7,1, C_∞^PMMTC = 8,6, C_∞^PAETC = 9,9, and C_∞^PAMTC = 13,4.     

Chain‐Length‐Dependent Termination of Vinyl Acetate and Vinyl Pivalate Bulk Homopolymerizations Studied by SP‐PLP‐EPR

Bulk homopolymerizations of vinyl acetate and vinyl pivalate are studied by EPR experiments between ?65 °C and 60 °C with dicumyl peroxide acting as the photoinitiator. No mid‐chain radicals are seen, which demonstrates that backbiting plays no role. The chain‐length dependence of the termination rate coefficients measured up to 13% monomer conversion is adequately represented by the composite model. The power‐law exponents α_s and α_l for short‐chain and long‐chain radicals are: α_s(VAc) = 0.57 ± 0.05, α_s(VPi) = 0.67 ± 0.15, α_l(VAc) = 0.16 ± 0.07, and α_l(VPi) = 0.16 ± 0.07. The crossover chain lengths differ largely: i_c(VAc) = 20 ± 10 and i_c(VPi) = 110 ± 30. The rate coefficient for termination of two radicals of chain length unity, , which is the fourth composite‐model parameter, depends on temperature, as does the monomer fluidity.

     

The use of activated benzyl vinyl ethers to control molecular weight in free radical polymerizations

The vinyl ethers, α-benzyloxyacrylonitrile ( 4 ), methyl α-benzyloxyacrylate ( 5 ), and α-benzyloxy-acrylamide ( 6 ), act as sulfur-free chain transfer agents in the polymerization of methyl methacrylate (chain transfer constant C_x = 0,081–0.47), styrene (C_x = 0,036–0,14), methyl acrylate (C_x = 0,3–1,1), and vinyl acetate (C_x = 12–20) initiated by AIBN at 60°C. With methyl methacrylate and styrene, the conversions of the polymerizations are relatively unaffected by the vinyl ethers. With methyl acrylate and vinyl acetate, however, some retardation occurs. The activity of the compounds is compared with that of α-benzyloxystyrene ( 1 ), which undergoes chain transfer by a novel free radical addition-fragmentation pathway.     

Facile Synthesis of Cx(AB)yCx Triblock Silicone Copolymers Utilizing Moisture Mediated Living‐End Chain Extension

The physical and chemical properties of silicone block copolymers depend upon the block size and chemical characteristics. Silicone block copolymer synthetic procedures typically involve chain‐growth polymerization techniques that are difficult to control, require low temperatures, inert atmospheres, etc., and which are subject to polymer chain redistribution; the latter process broadens dispersity and leads to the formation of cyclic monomers that must be removed. Once formed, tuning the size of the individual blocks is challenging. The controlled one‐pot synthesis of organosilicone C_x (AB)_y C_x triblock copolymers with aryl rich cores is demonstrated by exploiting the Piers–Rubinsztajn reaction and a hydrolysis/condensation polymerization, both using B(C₆F₅)₃ catalysis. A fast Piers–Rubinsztajn reaction between HSi‐terminated silicones and aryl phenyl ethers leads preferentially to the formation of the aryl‐rich core. Subsequent addition of HSi‐terminated silicones permits controlled chain extension of the flanking homosilicone blocks via a slower hydrolysis/condensation facilitated by atmospheric moisture and leads in one pot, two steps to clean C_x (AB)_y C_x triblocks. The products are surprisingly resilient to hydrolysis.     

Abbau und vernetzung von polydimethylsiloxan in lösung unter dem einfluß von 60Co-γ-strahlen

The radiation chemical yields for main chain scission and for crosslinking have been determined for pure poly (dimethyl siloxane) as well as for poly (dimethyl siloxane) irradiated in toluene solutions at various concentrations. In the case of pure poly (dimethyl siloxane), the ratio of the specific densities of degradation and crosslinking amounts to P₀/q₀ = 0.015. In the case of the toluene solution, P₀/q₀ steadily increases with decreasing concentration and approaches infinity at a base mole fraction of the polymer of x_P = 0.03. At the critical concentration (x_Perit = 0.15) P₀/q₀ is equal to 2. The 100 eV-yield of crosslinking decreases with decreasing polymer concentration and becomes zero in very dilute solutions. On the contrary, the 100 eV-yield for main chain scission slightly increases with decreasing polymer concentration over a broad range of x_P; a more significant increase occurs at low concentrations. The results agree with the previously developed theory of simultaneous degradation and crosslinking of polymers in solution under the influence of ionizing radiation.     

Structural chemistry of polymerizable monomers, 4. Crystal and molecular structure of N-methyldimethacrylamide and its effects on the initial stage of solid-state cyclopolymerization

The structure of N-methyldimethacrylamide was solved by direct methods and refined to a discrepancy index (reliability factor) R = 0,055 and weighted discrepancy index R_w = 0,068. C₉H₁₃NO₂, relative molecular mass M_r = 167,21, monoclinic, C 2/c, a = 11,882 (1), b = 8,130 (1), c = 10,079 (1) Å, β = 94,21 (1)°, V = 971,0 (1) ų, D_cal (Z = 4) = 1,143, D _obs = 1,16 Mg · m^?3, μ(MOK_a) = 0,087 mm^?1, mp 364 K, colourless needle crystal by sublimation. The molecule has a crystallographic C₂ symmetry. The shortest spacing between vinyl carbon atoms in the molecule is 2,908 (2) Å (C(3)…C(3′)). Planes of two intramolecular vinyl groups assume a dihedral angle of 40,6 (4)°. It seems that the location of the vinyl carbon atoms allows an intramolecular closure in the initiation process, that is, tail-to-tail cyclization as observed. The subsequent solid-state reaction probably proceeds through dimerization predominantly along a plane near the (1 01) plane in the lattice.     

Methoden zur Bestimmung des Lichtweges bei der Photometrie trüber Lösungen oder Gewebe mit durchfallendem oder reflektiertem Licht

Summary Reflexion spectra of living organs are not linearly transformed as compared to multiplicatively mixed spectra of transparent solutions having both uniform concentration and composition of the components. From this non-linear transformation inhomogeneous light paths can be assumed. The present paper described two methods which permit to calculate the probability function for the path covered in the probe.The first method uses an approximation of the light path distribution by discontinuous functions, whereas with the second method the light path distribution is developed in a sequence with normalized polynomials. The addition of a known dye to the tissue permits to calculate the dependence of the light path distribution on the wavelength.As an example, the probability function is calculated for the way covered in the guinea-pig-brain.

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Erklärung der benutzten Symbole A () additiv gemischtes Spektrum in Abhängigkeit von der Wellenlänge - B (N+1)-zeilige und einspaltige Matrix aus 10^{–H(i h)} - B ^T transformierte (gespiegelte) Matrix vonB - c _i Konzentrationswerte - h Parameter - H (M) Transformation zwischen multiplikativer und additiver Farbstoffmischung - H (M), H (M) erste bzw. zweite Ableitung vonH (M) - h ₀ Cuvettenhöhe - i Parameter aus natürlichen Zahlen oder Index - K Zahl der Komponenten eines Mehrkomponentenspektrums - L () Intensität des durch eine Cuvette der Dickez ₀ gefallenen Lichtes - L ₀ Intensität des einfallenden Lichtes - l (M) reflektierte Lichtintensität in Abhängigkeit vonM - l ₀ an einem nicht absorbierenden Vergleichsobjekt reflektierte Lichtintensität - lgx=log₁₀ x Zehnerlogarithmus - M () multiplikativ gemischtes Spektrum, bezogen auf einen Lichtwegz ₀ - M ₂,M ₁ größter bzw. kleinster Wert, für denH (M) bekannt ist - N nachN+1 Gliedern wird die Reihenentwicklung von (x) abgebrochen - n Parameter - P _v (x) Legendre Polynome - q _v Koeffizienten der Reihenentwicklung von (x) nach den PolynomenU _v (x) - q (N+1)-zeilige und einspaltige Matrix aus den Wertenq _v - q ^T transformierte (gespiegelte) Matrix vonq - r _v durch definiert - U _v (x) im Intervall (0,1) orthonormierte Polynome - U (N+1)-zeilige und einspaltige Matrix aus den PolynomenU _v (x) - V (N+1)×(N+1) Matrix, definiert durchU=V x - V ^T transformierte (gespiegelte) Matrix vonV - W (z) Wahrscheinlichkeit, daß in der Probe ein Lichtweg <z zurückgelegt wurde - (x) Verteilungsfunktion in Abhängigkeit vonx - x alsx=10^– hz/z _o definiert - x (N+1)-zeilige und einspaltige Matrix aus den Wertenx ⁱ - z Lichtweg - mittlerer Lichtweg - z Abweichungen vom mittleren Lichtweg - mittlere quadratische Abweichung - z (W) Lichtweg in Abhängigkeit von der WahrscheinlichkeitW, Umkehrfunktion vonW (z) - z _v s. Formel (15) - z ₀ Bezugswert für den Lichtweg - Anteil des bei absorptionsfreiem Vergleichsobjekt in den Empfänger gelangtes Licht - (y) Heavisidesche Sprungfunktion - _i () molare Extinktionskoeffizienten in Abhängigkeit von der Wellenlänge - Wellenlänge - , Index aus natürlichen Zahlen - (z) Wahrscheinlichkeitsdichtefunktion für den zurückgelegten Lichtwegz     

Isoconversional Analysis of Combined Melt and Glass Crystallization Data

Summary: As proposed earlier (Macromol. Rapid Commun., 2004 , 25, 733) the K_g and U* parameters of the Hoffman‐Lauritzen theory can be derived from isoconversional analysis of DSC data on melt crystallization. In this study, the analysis is extended to include both melt and glass crystallization data. The analysis of DSC data on non‐isothermal crystallization of poly(ethylene terephthalate) (PET) and poly(ethylene 2,6‐naphthalate) (PEN) demonstrates that for each of these polymers the combined melt and glass crystallization data can be described by single set of the K_g and U* values. The respective values for PET are 3.6 × 10⁵ K² and 7.5 kJ · mol^?1 and for PEN 2.2 × 10⁵ K² and 6.6 kJ · mol^?1. The use of combined data sets helps to improve the precision of computations.

Experimental E on T data for melt (circles) and glass (squares) crystallization of PET.     

Die Wärmekapazität von vollständig kristallinen Copolymeren des Polyoxymethylens

The molar heat capacity of fully crystalline copolymers of trioxane and 1,3-dioxolane can be discribed by the equation C_p(x₂, T) = 36,75 + 0,1425 (t₂ ? 25) + 80 x₂ in the temperature range 25°C < t < 140°C and at mole fraction of oxyethylene units x₂ < 0,16. For x₂ < 0,1 the molar heat capacity depends linearly on the reduced temperature T/T_m(x₂), but is independent from counit content.     

Solvent effects on free-radical polymerization, 5. Analysis of homopolymerization rate on the basis of the reactant-solvent complex model

The rate of polymerization was described as a function of the monomer concentration of 36 monomer/solvent systems in terms of the reactant-solvent complex (RSC) model. One-, two-and three-parameter optimization procedures were used to fit the rate function to experimental data. Fitting increments were the complex equilibrium constant K and the relative monomer reactivity parameter r₁₁. Irrespective of the optimization procedure, K and r₁₁, were found to correlate with the monomer exponent n and to be K = 0 and r₁₁, = 1 at n = 1. In these cases the RSC model reduces to the classical rate model. Despite the weak character of the monomer (radical)/solvent complexation, this interaction should not be neglected in rate modelling. The RSC model is a convenient tool to describe the chain propagation separately.     

Polymer retention mechanism in GPC on active gels, 1. Polystyrene in pure and mixed eluents

The elution behaviour of polystyrene (PS) on spherosil X0A 200 gel in twenty one eluents and its dependence on solvent goodness, as defined by the α exponent of the Mark-Houwink equation, and solvent strength, ?⁰, have been studied. The interstitial volume, V_o, seems to depend solely on solvent goodness, being minimum for good solvents and maximum for solvent mixtures at Θ composition or close to it. By application of a network-limited partition and adsorption mechanism, average pore radii of the gel, r , have been obtained. For ?⁰ > 0,45 a linear dependence with negative slope between r and ?⁰ exists, whereas for ?⁰ < 0,45 r values seem to display a joint dependence on ?⁰ and α·?⁰ α = 0,17 is the limit for obtaining experimental PS elution curves, below that value the polymer being not recovered. A coefficient f, which allows the evaluation of relative distribution coefficients (K_p) is introduced and its numerical values determined for all the eluent systems. The f dependence on ?⁰ and αis similar to that followed by r . At high ?⁰ values partion effects will be mainly responsible for K_p values. However, at low ?⁰ values, besides partition, adsorption effects contribute to the increase of K_p values.