The effect of commonly used induced condensing agents (ICAs) on the rate of polymerization of ethylene on a supported Ziegler–Natta catalyst in a gas‐phase process is investigated. It is observed that the instantaneous rate of ethylene polymerization is promoted in presence of all ICAs studied. This is attributed to the effect of the heavy ICAs in enhancing the local concentration of ethylene at the active polymerization sites that are dispersed within the growing polymer particles. It is found that the higher the solubility of ICA in the polymer, the greater the observed effect. It is also observed that the enhancement due to the presence of an ICA is greater during the initial phase of the polymerization.
Inert condensing agents (ICAs) are used principally to cool down gas‐phase fluidized bed reactors used to polymerize ethylene over supported catalysts. While these ICAs are chemically inert, it appears that they might nevertheless increase the polymerization rate. n‐Hexane is used as an ICA, and an enhancement in the instantaneous rate of ethylene polymerization in the gas phase is observed. This is attributed to a rise in the local ethylene concentration in the amorphous polymer phase surrounding the active sites, due to increases in both the solubility and diffusivity of ethylene in the amorphous polymer. In addition, the polymer particles have a smoother surface with less formation of fiber‐like substructures as the n‐hexane concentration increases.
The crystallization of high‐density polyethylene (HDPE) alone, and in the presence of n‐hexane (a common induced condensing agent [ICA]) is studied using differential scanning calorimetry. The presence of a noncrystallizable ICA which is partially soluble in the amorphous phase of HDPE reduces the rate of crystallization. This is reflected by a shifting of the crystallization and melting peaks of HDPE to lower temperatures when the ICA concentration in the medium increases. It is also observed that the rate of crystallization of HDPE can be very slow when the ratio of ICA to HDPE increases from zero. This behavior is useful to better understand the physical effects that can potentially occur at the beginning of the gas phase polymerization of ethylene, and suggests that the crystallization of the nascent polymer, and thus the properties of the polymer in the reactor will be quite different from those of the powder at later stages of the reaction when running in condensed mode. 相似文献
An experimental investigation of the impact of changes in temperature on the observed rate of polymerization of ethylene in the gas phase using a commercial Ziegler–Natta catalyst in the presence of induced condensing agents (ICAs) reveals some unexpected behavior. In the absence of ICA, the effect of temperature is as expected: raising the temperature of the gas phase from 70 to 90 °C causes the observed rate of polymerization to increase monotonically. It is demonstrated in the past that ICA can increase rate of polymerization of the ethylene in the gas phase due to a cosolubility effect. However, in the current study, it is shown that when ICA is present in the reactor, the same increase in temperature can actually lead to an observable decrease in the reaction rate under certain conditions of temperature and pressure. This is attributed to a lower impact of the ICA on the solubility of ethylene in the amorphous phase of the high‐density polyethylene in the reactor at higher temperatures. An order of magnitude analysis also reveals that the presence of ICA can have an impact on the particle temperature as well. 相似文献
A systematic analysis methodology is proposed in order to be able to evaluate the role and the significance of the impact of the physical and chemical phenomena during the gas phase ethylene copolymerization in presence of 1‐pentene and 1‐hexene comonomers in the gas phase composition. In addition, the effect of inert n‐hexane on the instantaneous rate of gas phase ethylene polymerization in the absence and presence of 1‐hexene comonomer and hydrogen is investigated. This, in turn, provides a novel insight on the counter‐effects of the physical and chemical phenomena taking place simultaneously during the gas phase processes for ethylene polymerization in the industrially relevant conditions.
Graft‐type anion‐conducting polymer electrolyte membranes (AEMs) are prepared by the radiation‐induced graft polymerization of chloromethylstyrene into poly(ethylene‐co‐tetrafluoroethylene) (ETFE) films and subsequent quaternization with trimethylamine. AEMs in the hydroxide form (AEM‐OH) are prepared by immersing the chloride form (AEM‐Cl) in 1 M potassium hydroxide (KOH) solution, followed by KOH and washing with nitrogen‐saturated water to prevent bicarbonate formation (AEM‐HCO3). The AEM‐OH shows conductivity and water uptake four and two times higher than AEM‐Cl and ‐HCO3 and is thermally and chemically less stable, resulting in the tendency to absorb water and to convert to the bicarbonate form.
Ethylene was polymerized using both homogeneous and modified methylaluminoxane (MMAO)‐treated silica supported nickel‐diimine catalysts (1,4‐bis(2,6‐diisopropylphenyl) acenaphthene diimine nickel(II ) dibromide) in a slurry semibatch reactor. The effects of catalyst support and polymerization conditions (ethylene pressure and reaction temperature) on catalyst activity and polymer properties were systematically investigated. The supported catalyst gave far lower activity than the homogeneous catalyst. The activities of both catalyst systems increased with polymerization temperature with a maximum at 40 °C. Compared with the homogeneous catalyst, the supported catalyst system produced polyethylene with a different microstructure. Due to steric effects, the supported catalyst system exhibited lower chain walking rates than the homogeneous catalyst, producing polymers with less branching content and, thus higher melting points. Depending on polymerization conditions, two active site populations were observed during polymerization using supported catalyst; one population remained fixed on the surface of the support, and the other was extracted from the support, exhibiting the same polymerization behavior as the homogeneous catalyst.
DSC thermograms for polyethylene produced with homogeneous and supported catalysts at an ethylene pressure of 50 psig (3.45 · 105 Pa) and reaction temperature 40 °C. 相似文献
Thin hydrogel films of the thermoresponsive polymer poly(N‐isopropylacrylamide) (pNIPAm) were prepared by electrochemically triggered reversible addition‐fragmentation chain transfer (RAFT) polymerization. Two different RAFT agents were employed, which work in either acidic or basic solution. In both cases, addition of RAFT agents had an influence on the thickness and the surface morphology of the films. At low concentration, the polymerization efficiency increased. At high concentration, the efficiency decreased at acidic pH, while it remained constant under basic conditions. Neither RAFT agent displayed electrochemical activity on its own, but they did modify the electrochemical behavior of the initiator. The addition of RAFT agent strongly enhances the homogeneity of the hydrogel surfaces, which presumably is due to a reduced amount of microgel formation.
The RAFT polymerization of styrene in a solvent consisting of a water/alcohol mixture with different water contents is performed and the solvent effects on polymerization kinetics, polymer chain propagation, and polymer particle growth are evaluated. It is found that the solvent affects the RAFT polymerization kinetics greatly, and the apparent polymerization rate constant (Kpapp) increases with an increase in the water content of the water/alcohol mixture. In addition, RAFT polymerization in a water/alcohol mixture with a higher water content affords better control of the polydispersity index (PDI) of the synthesized polymers. Furthermore, the solvent also exerts a great influence on the growth of the polymer particles. Hollow particles are formed either at the initial polymerization with low monomer conversion or in the solvent with a low water content, whereas solid polymer particles are produced either at high monomer conversion or in the solvent with a high water content.
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