Polymerization of vinyl acetate in microemulsions stabilized with dodecyltrimethylammonium bromide and cetyltrimethylammonium bromide

The polymerization of vinyl acetate in microemulsions stabilized with dodecyltrimethylammonium bromide (DTAB), cetyltrimethylammonium bromide (CTAB) or mixture of these surfactants is examined. The polymerization rate diminishes as the surfactant mixture becomes richer in DTAB. Also, particles grow with conversion and become increasingly larger as the DTAB content in the mixture increases. Our results indicate that chain transfer reactions to monomer are more important than chain transfer reactions to polymer even at high conversions with CTAB. However, as the content of DTAB increases, bimolecular termination induced by coagulation, terminal double bond polymerization and chain transfer reactions to polymer become increasingly more important.     

Polymerization of vinyl acetate in ternary microemulsions stabilized with hexadecyltrimethylammonium bromide

The polymerization of vinyl acetate (VA) in three component o/w microemulsions stabilized with the cationic surfactant, CTAB, is presented. Initiation is achieved thermally with a water soluble initiator (V-50). Stable latex containing small particles (ca. 35 nm) with molecular weights (M_w) of around 4 × 10⁵ are obtained. Analysis of the molecular weight distribution suggests that chain transfer to monomer (and not to polymer, which is the typical termination mechanism in emulsion polymerization, specially at high conversions) is the dominant mechanism of termination. Received: 11. October 1996/Revised: 16 January 1997/Accepted: 21 January 1997     

乙酸乙烯酯的微波加热聚合

对乙酸乙烯酯甲醇溶液在微波加热条件下的聚合反应进行了研究,讨论了微波聚合条件、引发剂浓度、溶剂浓度和种类对聚合产物的转化率和分子量的影响。聚合物可以在较短时间内获得。且在同样的实验条件下,用不同的溶剂获得的聚合物的分子量不同。     

Dispersion polymerization of vinyl acetate in a mixture of ethanol and water

Dispersion polymerization of vinyl acetate (VAc) was studied in a mixed solvent of ethanol and water using poly(vinyl alcohol) (PVA) with a low degree of hydrolysis as a steric stabilizer. Six compositions were chosen from a homogeneous phase before the polymerization. In a composition with the highest solubility parameter, 450 nm particles with a size distribution close to monodispersity were obtained. With decreasing solubility parameters of the compositions, the particles became polydisperse. In the compositions with solubility parameter lower than 17.1, particles were not formed even at 100 % conversion. Degrees of polymerization obtained in this homogeneous phase were less than 100 and close to calculated values. With increasing solubility parameters of the system, degrees of polymerization increased but were still less than 200. Grafting of VAc onto PVA and grafted PVA were very small, indicating the presence of a competing reaction to the grafting. Copyright © 2004 Society of Chemical Industry     

Microemulsion copolymerization of vinyl acetate and butyl acrylate using a mixture of anionic and non-ionic surfactants

Monomer mixtures of vinyl acetate (VAc)/butyl acrylate (BuA) were polymerized in batch reactions at 60 °C with potassium persulfate as the initiator in microemulsions consisting of VAc:BuA (85:15 wt/wt)/water/sodium dodecyl sulfate (SDS)/polyoxyethylene (23) dodecyl ether (3:1 wt/wt). The effect of the concentration of the monomer mixture on the kinetics was studied. It was found that, as the total monomers concentration ([M]₀) increases, the polymerization rate increases also, and that the maximum polymerization rate is proportional to [M]₀^1.26. Particle size increases with total monomers concentration. In all cases, final average particle diameter was less than 50 nm. Particle number density is independent of total monomers concentration. A mathematical model that takes into account the partition of monomers between the different phases during polymerization using a minimum of adjustable parameters was applied to simulate the experimental data. A correlation for the radical desorption coefficient, which is a function of the rate of monomer chain transfer and of the probability of desorption, was used in the model. Radical capture by micelles and particles was assumed to occur by diffusion. The model takes into account both micellar and homogeneous nucleation. Good agreement between the model and the experimental results was observed.     

Diffusion of a carbonate mixture in a poly(vinyl alcohol-co-vinyl acetate) in the solid state

The diffusion study of a carbonate mixture composed of ethylene and propylene carbonates in poly(vinyl alcohol-co-vinyl acetate) (PVA–Ac) was studied. The diffusion occurred in the solid state and the effect of the carbonate mixture on the PVA–Ac properties are presented as well as the homogeneity of the diffusion. Due to the complexity of the diffusion kinetics with temperature, a model was made using the free volume theory at 75 °C. This temperature was chosen for modelling because the physical properties of the polymer are relatively stable at this point. Swelling heterogeneity, due to the particle size distribution of the powder, were estimated. Copyright © 2004 Society of Chemical Industry     

低聚合度、低醇解度聚乙烯醇的合成与性能研究——I.低聚合度聚醋酸乙烯酯的合成

以甲醇为溶剂,偶氮二异丁腈(AIBN)为引发剂,采用自由基聚合方法合成了低聚合度聚醋酸乙烯酯,并对其结构进行了表征。讨论了聚合时间、引发剂用量、物料配比、聚合温度等因素对聚合率的影响,获得了相对较佳的工艺参数。     

Comparison of tensile stress relaxation behavior of graft copolymer of poly(vinyl acetate) with styrene and mechanical mixture of poly(vinyl acetate) with polystyrene

As one in a series of studies relating the rheological behavior of mechanical mixtures of two polymer components to the degree of mixing, the tensile stress relaxation behavior of a graft copolymer of poly(vinyl acetate) with styrene, obtained by ⁶⁰Co γ-irradiation of poly(vinyl acetate) in styrene solution and extraction of homopolymers, was investigated as a function of the fraction of styrene component and compared with that of a mechanical mixture of poly(vinyl acetate) with polystyrene, one of the typical combinations of incompatible components. The results obtained may classify the graft copolymer as a sort of mechanical mixture giving stable mixing even for incompatible components.     

Polymerization conversion and structure of magnesium methacrylate in ethylene–vinyl acetate rubber vulcanizates

Ethylene–vinyl acetate rubber (EVM) filled with magnesium methacrylate (MDMA) was cured with dicumyl peroxide (DCP). The experimental results showed that the mechanical properties of MDMA/EVM vulcanizates were superior to those of high‐abrasion furnace carbon black/EVM vulcanizates. The tensile strength of the MDMA/EVM vulcanizate could reach 22.5 MPa and the tear strength was 83.5 kN/m, whereas its elongation at break remained over 300%, even when the MDMA content was 50 phr. FTIR analysis confirmed that polymerization of MDMA occurred under the initiation of DCP, and the polymerization conversion of MDMA decreased with the increase of MDMA content. When the MDMA content increased from 10 to 50 phr, the conversion of MDMA in EVM decreased from 68 to 20%. SEM and TEM observations indicated that 20 μm particles of MDMA powder changed into about 20 nm particles during the curing process, and the finer particles dispersed evenly in the EVM vulcanizates. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2379–2384, 2004     

Miniemulsion and macroemulsion copolymerization of vinyl acetate with vinyl versatate

The miniemulsion and macroemulsion polymerization of vinyl acetate with vinyl versatate in batch and semibatch systems was investigated. Vinyl versatate was added either as an emulsion with the vinyl acetate, or as a neat liquid stream. In the batch runs, there is a poor dispersion of vinyl versatate during the nucleation period for the runs in which the vinyl versatate was added neat at the beginning of the polymerization. This led to smaller particles, lower polymerization rate, and different polymer composition evolution when compared with runs in which the vinyl versatate was emulsified with the vinyl acetate. In seeded semibatch runs, residual surfactant in the seed latex, along with the propensity for homogeneous nucleation in vinyl acetate emulsions, resulted in continuing nucleation during the entire semibatch interval. The polymerization rate was primarily affected by monomer feed rate rather than the feeding mode. The effect of monomer feeding mode on copolymer composition was weak when the semibatch feed rate was low, indicating some level of vinyl versatate mass transfer resistance. In all runs, only one glass transition temperature was observed, indicating effective copolymerization. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 2219–2229, 2002     

The kinetics of solution copolymerization of vinyl chloroacetate with vinyl acetate in a CSTR

The kinetics of isothermal, solution copolymerization of vinyl chloroacetate (VCLAC) with vinyl acetate (VAC) in methyl isobutyl ketone (MIBK) has been investigated using a continuous flow stirred-tank reactor (CSTR). The initiator used was 2,2′-azobis 2,4-dimethyl valeronitrile (ADVN). Polymerizations were carried out to moderately high conversions, necessitating the use of a modified parameter estimation method to determine the reactivity ratios. The copolymer composition conforms to terminal model kinetics. The values obtained for r₁ and r₂ are 1.18 and 0.80, respectively, with VCLAC as M₁ and VAC as M₂. Both values are close to unity indicating random copolymerization. The cross termination factor ? strongly depends on the composition of the comonomer feed ratio, implying that the chain-termination step is possibly diffusion controlled. The variation of ? could also be caused by chain transfer to the solvent retarding the reaction rate. Such a possibility is consistent with the observed reduction in the molecular weight of the copolymer in the presence of the solvent. The approach to steady-state in the reactor was characterized. It was not possible to obtain good agreement between experiment and simulation with a single value of the ? factor.     

Photodecomposition of dodecyltrimethylammonium bromide in aqueous solutions in presence of hydrogen peroxide

It is established that the cationic surfactant (dodecyltrimethylammonium bromide, DTAB) efficiently decomposes in an aqueous solution in the presence of H₂O₂ upon exposure to UV radiation. The presence of bicarbonate ions decreases the rate of photodecomposition, which is due to the acceptance of OH radicals generated in the photochemical reaction. The rate constant of OH radicals and DTAB reaction equals ～10⁹ L mol^?1 s^?1. The kinetics of DTAB decomposition are described by the first-order reaction equation, and the effective rate constant is proportional to the H₂O₂ content in the solution.     

Deactivation of a vinyl acetate catalyst

Kinetic studies as well as morphological and physico-chemical surface studies have been carried out on the deactivation of a palladium carrier catalyst for vinyl acetate synthesis in the ethylene gas phase process. The experiments cover concentration and temperature ranges of technical interest at a total pressure of 900 kPa. The deactivation rate depends on the temperature and the oxygen and acetic acid concentrations. The decline in activity is best described by a rate law of second order relative to the activity number. This behaviour is attributed to the observed aggregation of finely dispersed palladium. It can be concluded that palladium(II) acetate acts as a transport species in a chemically assisted sintering mechanism. At temperatures T ≤ 438 K the more advantageous final texture of the catalyst will only be formed on reaction.     

Zirconia stabilized with a mixture of the rare earth oxides

A hydrothermal technique was used to prepare micropowders of ZrO₂ solid solutions stabilized by the mixture of rare earth oxides and yttria (Ln₂O₃) recovered from phosphogypsum. Phase compositions and particle sizes of the powders were measured. Zirconia solid solutions of cubic and tetragonal symmetry are the major phases. A larger Ln₂O₃ concentration promotes a greater cubic phase content. Powders having low Ln₂O₃ concentrations have small amounts of monoclinic phase present.

The phase composition, fracture toughness (K_Ic) and Vickers hardness of samples sintered at temperatures ranging from 1250 to 1350°C were measured. Tetragonal and, in some cases, monoclinic zirconia solid solutions are found in the microstructure of the sintered bodies. Two ordered phases with the formulae Ln₂Zr₂O₇ and Ce₂Zr₃O₁₀ were also detected in the system. Tetragonal zirconia polycrystals (TZP) with high (10 MPaM^0·5) fracture toughness were found.     

氯乙烯-醋酸乙烯酯共聚糊树脂中醋酸乙烯酯含量的测定

采用红外光谱法测定了氯乙烯-醋酸乙烯酯共聚糊树脂中醋酸乙烯酯的含量,并与热重法、化学分析法等分析方法进行了比较。指出氯乙烯-醋酸乙烯酯共聚糊树脂中醋酸乙烯酯含量的多种测定方法各有其特点,需要根据实际情况选用;红外光谱法快速、准确,可满足企业科研和生产所需。     

Polymerization and copolymerization of vinyl carbamates and vinyl carbonates

Poly(vinyl carbamates) and poly(vinyl carbonates) of high molecular weight have been obtained by polymerization of the corresponding monomers prepared from vinyl chloroformate. Poly(vinyl carbonates) can also be prepared by chemical modification of poly(vinyl chloroformate) with alcohols and phenols. Copolymerization parameters of phenyl vinyl carbonate with vinyl acetate have been determined in methylene chloride, at 35°C, with dicyclohexyl peroxydicarbonate as initiator.     

Thermal decomposition of a vinyl chloride/vinyl acetate copolymer

The thermal decomposition of an 85% vinyl $\text{chloride}\text{15\%}$ vinyl acetate copolymer and its different fractions obtained by precipitation was studied by the thermogravimetric scanning (t.g.s.) technique. Analyses of the t.g.s. thermograms revealed the existence of two major steps in the decomposition reaction. The first occurs between 180 and 380°C during which hydrogen chloride and hydrogen acetate are given off. In the second, which occurs between 420 and 480°C, degradation of the carbon chain takes place. Both the order of reaction and the activation energy were found to be dependent on the molecular weight of the copolymer and on the heating rate at which the experiments were carried out. Analyses of the samples carried out by gel permeation chromatography (g.p.c.) after each decomposition revealed that, besides the two steps mentioned above, yet another step, viz. crosslinking, may be taking place at 180°C or below where the weight loss of copolymer is too small to be detected by t.g.s. The process of crosslinking, however, may take place at other temperatures as well. A higher degree of crosslinking has been associated with a higher acetate content in the copolymer. A comparison of g.p.c. results with those obtained by nuclear magnetic resonance spectra showed that, out of the three steric configurations of the copolymer, syndiotactic sequences are least resistant to thermal treatment followed in order by heterotactic and isotactic sequences.     

Synthesis of vinyl phenyl acetate and an evaluation of vinyl chloride/vinyl phenyl acetate copolymers

The synthesis of vinyl phenyl acetate, by an ester interchange reaction between phenyl acetic acid and vinyl acetate and utilizing a catalyst, is described. Copolymerization with vinyl chloride, in a suspension system and using a peroxide catalyst, is described on a laboratory and pilot plant scale. Monomer/copolymer compositions, for an initial charge consisting of vinyl chloride/vinyl phenyl acetate (80/20 by weight) are presented over a range of conversions, as an indication of reactivity ratios. Discs, molded from unstabilized copolymers, show very good clarity and color stability, which improve with increased comonomer loading. Some retention of unpolymerized vinyl phenyl acetate monomer occurred, and some increase in softening points resulted following two reprecipitations from acetone into excess methanol. Compound from a 96/4 vinyl chloride/vinyl phenyl acetate copolymer has better color stability than does an equivalent vinyl chloride/vinylidene chloride copolymer compound. The enhanced color and heat stability of the copolymers is attributed to the aromatic character of the comonomer vinyl phenyl acetate.