Modeling of termination in intermediate and high conversion free radical polymerizations

A simple means of modeling termination in free radical polymerizations of a large number of monomers over a wide variety of intermediate and high conversion conditions is presented. It is shown that monomers can be divided into two classes according to the qualitative variation of their termination rate coefficient (k_t) with conversion (U). It is then suggested that it is profitable to represent k_t data as k_t/[k_p·(1 ? U)]. Doing this reveals that over a wide range of temperatures and of intermediate and high conversions, seemingly complex variations of k_t with U can be described by a single parameter (C_RD), the value of which varies from monomer to monomer. Proceeding from the idea that the process of reaction-diffusion is the rate determining mechanism of termination, two easily implementable models for a priori prediction of C_RD are developed in parallel from already existing ideas. One of these models is shown to predict with remarkable accuracy a large variety of experimental C_RD data. Slight modification of these predictions makes this model even more accurate, and therefore gives it great potential for modeling purposes.     

Carbon-13 NMR spectra and reactivities of vinyl compounds in ionic and radical polymerizations

The ¹³C-chemical shift of β-carbon in the vinyl group of monomers was correlated with the relative reactivity of the monomer in the cationic copolymerizations of substituted styrenes, benzyl vinyl ethers, and phenyl vinyl ethers and also in the copolymerization of substituted styrenes by Ziegler catalysts. It was found that the β-carbon resonates at the higher field, the higher the reactivity of the monomer is. The reverse was the case in the anionic addition reaction of substituted styrene to living polystyrene. The relations were also studied between the ¹³C-chemical shift and the e-value of the monomer and some direct evidence was obtained for the validity of the Q–e scheme. From these results it was revealed that the ¹³C-chemical shift of vinyl compounds is an important tool for the examination of the mechanism of polymerization reaction.     

Low molecular weight hyaluronic acid prevents oxygen free radical damage to granulation tissue during wound healing

Hyaluronic acid protects granulation tissue from oxygen free radical damage and stimulates wound healing, but its molecular weight prevents it from permeating the epidermal barrier A low molecular weight hyaluronic acid preparation is able to permeate the skin, but it is unknown whether or not it retains the scavenging effects of oxygen free radicals in granulation tissue. Our experiments were conducted in rats with excisional or incisional wounds. Wound contraction over 11 days and breaking strength on the fifth day were measured. Oxygen free radical production was induced by intraperitoneal administration of two different xenobiotics: phenazine methosulfate and zymosan. The wounds were treated topically with low molecular weight hyaluronic acid (0.2%) cream or placebo. In the incisional wound group, the effects of superoxide dismutase were also determined. Absolute controls received wounds and placebo but no xenobiotics. Wound healing was significantly slower in the xenobiotic group than in the control groups. These effects were strongly reduced by topical administration of low molecular weight hyaluronic acid (0.2%) cream and in incisional wounds by topically injected superoxide dismutase. Low molecular weight hyaluronic acid is effective as the native compound against oxygen free radicals. Its pharmacological effects through transdermal administration should be tested in appropriate models.     

Rare earth metal initiated polymerizations of polar and nonpolar monomers to give high molecular weight polymers with extremely narrow molecular weight distribution

Organolanthanide (III) initiated polymerizations of alkyl methacrylates and alkyl acrylates proceed in a living fashion and yield high molecular weight polymers with extremely narrow molecular weight distribution at high conversion. Random and block copolymerizations of alkyl acrylates are carried out successfully to give almost “monodisperse” polymers. Rare earth metal initiated polymerizations of lactones such as ε-caprolactone and δ-valerolactone produce again high molecular weight polymers with narrow molecular weight distribution. Bulky divalent and trivalent rare earth metal complexes catalyze the living polymerization of ethylene and α-olefins and the Sm (OR)₃/AIR₃? H₂O initiator system catalyzes the polymerization of propylene oxide to give polymers with narrow molecular weight distribution.     

Rheological study of the kinetics of photoinitiated free radical polymerizations with the quartz microbalance

Using a quartz microbalance operating in solutions, a very sensitive determination of the increasing viscosity during the very early stages of polymerization reactions is possible in a continuous fashion. Kinetic data have been obtained for methyl methacrylate as the monomer and benzoin, its methyl ether, and 2,2-dimethoxy-2-phenylacetophenone as photoinitiators using an excimer laser operating at 351 nm. This in situ method represents a very sensitive, convenient, and inexpensive technique for obtaining kinetic data under a variety of boundary conditions including photopolymerization, the example outlined here.     

Molecular weight distribution studies, 4 Kinetics of the gamma-ray induced free radical polymerization of vinyl acetate at low temperatures

The kinetics of the gamma-radiation initiated free radical polymerization of vinyl acetate were studied in bulk over a temperature range of ?60°C to 0°C. Four radiation dose rates of 0,83, 0,33, 0,26, and 0,079 Mrad were used. The overall rate of polymerization was found to vary with the 0,46 to 0,53 power of the radiation intensity, indicating a free-radical polymerization mechanism. The overall activation energy was found to tie between 16 and 18 kj/mol. The values of k_trm/k_p and (k_tc + k_td)k_p² were evaluated from the overall polymerization rates and the number-average molecular weights. The values of the radiation chemical yield G_VAC were found to lie between 2,4 and 4,1.     

Branching in free radical polymerization due to chain transfer. Application to poly(vinyl acetate)

A general statistical theory in direct relation to the kinetics of polymerization is developed for free radical polymerization with chain transfer. In the present paper this relationship is displayed in detail; analytical expressions are derived for the probabilities of reaction as functions of the reaction rates and the monomer conversion. The statistical treatment (cascade theory) of the polyreaction allows the calculation of the weight average degree of polymerization P_w, the z-average mean square radius of gyration 〈S²〉_z, and the particle scattering factor P_z(θ). While for linear chains the mentioned averages are determined by a single link probability, in the branched system these averages are characterized by a stochastic matrix whose elements are the various link probabilities. The theory is applied to poly(vinyl acetate). A good agreement is obtained with the experimental results of D. J. Stein for this polymer.     

Crystallinity of free radical polymerized poly(vinyl p-trimethyl silyl benzoate)

Concentrations of the initial components and of p-nitrobenzoic acid and N-methyl-p-toluidine (or N-methylmesidine) were followed on the reaction between p.p′-dinitrobenzoyl-peroxide and N.N-dimethyl-p-toluidine (or N.N-dimethylmesidine). In the products of the reaction between benzoylperoxide and N.N-dimethylaniline, N.N-dimethyl-p-toluidine or N.N-dimethylmesidine the contents of the initial amine, benzoic acid, CH₂O and secondary amine were determined. The data found support the suggestion, that the primary products of the reactions between benzoylperoxide and tertiary amine (C₆H₅N(R′)CH(R)OCOC₆H₅) can condensate either with the initial amine or with each other to produce benzoic acid and dehydrogenated oligomers of the initial amine. The GPC analysis data of the products of the reaction between benzoylperoxide and N.N-dimethylaniline corroborate the presence of the initial amine oligomers in the reaction mixture.     

On the activity of monomers and polymers on the basis of vinyl ethers of glycols in radical reactions

The absolute values of reaction constants of various radicals with vinyl ethers of glycols and aminoalcohols and with N-vinyl-2-pyrrolidone have been determined by the spin-trapping method. It was shown that the radicals are associated with the β-carbon atoms of the vinyl groups. The kinetics of the process is considerably determined by the polarity of the monomers. It was established that the rate of elimination of H atoms from methylene groups in the side groups is considerably higher than that from methylene groups in the main chain of the macromolecules. The over-all activity of poly(ethylene glycol monovinyl ether) is about 3 times higher than that of poly(diethylene glycol^a monovinyl ether).     

Study of the distributions of chemical composition and molecular weight in relation to the preparation method of vinyl alcohol/vinyl acetate copolymers

The chemical heterogeneity of two vinyl alcohol/vinyl acetate (VAL-VAC) copolymers prepared by partial acetylation of poly(vinyl alcohol) and by partial alkaline hydrolysis of poly(vinyl acetate), respectively, was studied by cross-fractionation. The used solvent/non-solvent systems were selected on the basis of cloud-point determinations. The chemical composition distribution (CCD) and the molecular weight distribution (MWD) curves were constructed from fractionation data. The observed chemical heterogeneity was correlated with the kinetic parameters of the two different methods used for the preparation of the VAL-VAC copolymers. Thus, squared standard deviation values = 2,90·10^?4 and = 7,42·10^?4 were found for the two VAL-VAC copolymers prepared by acetylation and hydrolysis, respectively. These results are in agreement with theoretical predictions. The cumulative data were fitted by the two-parameters MWD functions of Tung and of Wesslau.     

Kinetics and mechanism of inhibition of an antioxidant type inhibitor in free radical vinyl copolymerizations

Inhibition mechanism and kinetics for an antioxidant type inhibitor, which can function only in the presence of oxygen, in the copolymerization of styrene and methyl methacrylate (MMA) are proposed. Experimental studies of the homopolymerizations with hydroquinone (HQ) and p-tert-butylpyrocatechol (TBC) as inhibitor at 50–70°C and of the copolymerization with HQ as inhibitor at 60°C show a good fit of the equations proposed and are self-consistent, except for the homopolymerization system MMA/TBC, in which TBC can function also as an inhibitor even after the depletion of oxygen. The copolymerization system styrene (1)/MMA (2)/HQ shows a synergistic effect, positive for the rate of oxygen consumption and negative for the inhibition time. This can be attributed to the favored cross-propagation of the peroxyl radicals, R₁ O and R₂ O, with the comonomers as indicated by the found reactivity ratios (both < 1; r₁ = 0,275, r₂ = 0,314). Detailed calculations using the proposed kinetic equations indicate that, if the cross-propagation is not favored, the inhibition time will show a positive synergistic effect.     

Analysis of the molecular weight distribution in free radical polymerization: Modelling of the MWD from the analysis of experimental GPC curves

In this paper different techniques for the deconvolution of experimental gel permeation chromatography (GPC) curves of polymers obtained from batch reactors are examined with the aim of improving the modelling of the polymerization process. The deconvoution of the GPC curves gives insight into the effects of diffusive phenomena on the values of the average molecular weights (M_n and M_w) and the molecular weight distribution (MWD) of the polymer. Two methods, one based on a two-peak deconvolution and the other one based on a multi-peak deconvolution, are compared. The different characteristics and relative merits of the two methods are illustrated using several sets of experimental data. In conclusion, the first method is less accurate but allows a separation of the two contributions given by the chemical and diffusive control of the MWD; the second method has a sound theoretical background (confirms the prediction of the free volume theory) and is potentially much superior but the results prove to be more sensitive to the quality of the experimental part.     

The free radical theory of aging

Aging is the accumulation of changes that increase the risk of death. Aging changes can be attributed to development, genetic defects, the environment, disease, and an inborn process: the aging process. The latter is the major risk factor for disease and death after age 28 in the developed countries. In these countries, average life expectancies at birth (ALE-B) now range from 76 to 79 years, 6-9 years less than the limit of approximately 85 years imposed by aging. Aging changes may be caused by free radical reactions. The extensive studies based on this possibility hold promise that the ALE-B can be extended to >85 years and the maximum life span increased.     

Equation-of-state parameters of poly(vinyl acetate) as function of molecular weight

Specific volume, thermal expansion coefficient and thermal pressure coefficient are measured for poly(vinyl acetate) of different molecular weights. From these data the Flory equation-of-state characteristic parameters v^*, T^* and p^* are determined. Comments on theoretical expectations and experimental behaviour are made.     

The free radical signal of pigment gallstones

Summary Bile stones have been studied by Electron Spin Resonance (ESR) spectroscopy. Pigment containing stones show a 8.76±0.02 gauss wide free radical singlet atg 2.003 region of the spectrum. Amplitude of this free radical signal linearly correlates with pigment content of the stones, which can be applied in the estimation of pigment content. The possible role of free radicals is suggested in the formation of pigment bile stones.     

Role of cocatalysts in polymerizations initiated with benzyl derivatives of transition metals, 2. Polymerization of styrene

Polymerization of styrene in toluene solution at 50°C was found not to be initiated with sufficiently pure tetrabenzylzirconium or -titanium. However, a cocatalytic effect of water was observed with a maximum rate for a mole ratio H₂O/Mt (Mt = Zr or Ti) close to 0,1. No activities were observed for the hypothetical compounds (C₆H₅CH₂)₃Mt—O—Mt(CH₂C₆H₅)₃. This cocatalytic effect was also observed with tribenzylchlorozirconium ( 1 ), tribenzylzirconiooxytriphenylsilane ( 2 ), or 6,6-dibenzyl-2,2,4,4-tetramethyl-1,3,5-trioxa-2,4-disila-6-zirconacyclohexane ( 3 ). Styrene polymerizations were also initiated by supported catalysts obtained by reaction of Zr(CH₂C₆H₅)₄ with aluminium oxide or silicium oxide pre-dried at various temperatures. The activities of such systems were equal or slightly lower than those obtained with the system Zr(CH₂C₆H₅)₄/H₂O (mole ratio 10 : 1). The resulting polystyrenes have very high molecular weights with very broad distribution. Copolymerization experiments with methyl methacrylate gave as main product a copolymer containing only 4% of monomeric units of styrene. A discussion of all results led to the conclusion that the main polymerization involves an anionic-type propagation.     

Role of cocatalysts in polymerizations initiated with benzyl derivatives of transition metals, 1. Polymerization of ethylene

Polymerization of ethylene initiated in naphta solution by Zr(CH₂C₆H₅)₄ at 200°C under high purity conditions showed a cocatalytic effect of water with a maximum rate for a mole ratio H₂O/Zr close to 0,1. No activity was observed for the hypothetical compound (C₆H₅CH₂)₃ZrOZr(CH₂C₆H₅)₃. Ethylene polymerization was also initiated with supported catalysts obtained by reaction of Zr(CH₂C₆H₅)₄ with silicium oxide or pre-dried aluminium oxide with an increase of activity (by a factor of 5). The resulting polyethylenes were found to be virtually linear with 1,5 CH₃ groups for 1000 C atoms and with melting points of 134 – 136,5°C. They have high molecular weights with very broad distribution. These results were interpreted by a mechanism involving Ziegler-like active centers.     

The molecular weight and the amino acid and carbohydrate compositions of free secretory component (SC)

Free secretory component was isolated from human milk whey using gel filtration and immunosorbent techniques. The purified protein was shown to be homogeneous in the ultracentrifuge and on cellulose acetate, polyacrylamide gel and immunoelectrophoresis.

From three different preparations the molecular weight was determined using Archibald's meniscus method. A mean value of 66,000±4500 was found. The S⁰_20,w value was calculated to be 4.3 S±0.17 and the D⁰₂₀ value was 5.66±0.08. From these data the molecular weight was calculated to be 67,000±3000.

The amino acid composition revealed the absence of methionine from the molecule.

The carbohydrate analysis showed the absence of N-acetylgalactosamine and a rather high fucose content. These results are discussed in relation to the carbohydrate composition of secretory IgA and serum IgA.