Polymerization of Ethylene and Propylene: Synthesis and Analysis from One Company
Introduction
Polymerization is a very important process in the chemical industry. The products formed exhibit desirable properties such as durability, inertness toward many chemical substances, elasticity, transparency, as well as electrical and thermal resistance.
Polymers are produced in many different forms, for example as fibers, films, pipes, coatings and injection molding components.
An important metal-catalyzed polymerization is initiated by Ziegler-Natta catalysts [1]. The Ziegler-Natta catalyst is synthesized by treating crystalline a-TiCl3 with [AlCl(C2H5)2]2. An alternative route to the catalyst entails the reaction of titanium or zirconium tetrachloride with a trialkylaluminum reagent such as triethylaluminum, AlR3 or Al(CH2CH3)3. This catalyst system polymerizes alkenes, particularly ethylene, at relatively low pressures with remarkable ease and efficiency.
The advantage of Ziegler-Natta polymerization is the regularity with which substituted alkane chains are formed from substituted alkenes such as propylene, and the high linearity of the chains. The resulting polymers are of higher density and much stronger compared with polymers obtained by radical polymerization [2].
This article describes how the polymerization of ethylene and propylene was investigated in the laboratory using a METTLER TOLEDO Automated Lab Reactor (ALR).
To investigate the kinetics of the polymerization of ethylene and the copolymerization of ethylene and propylene on a small scale, the ALR was equipped with 50-mL reactors and a gas supply system. The gas uptake was measured by monitoring the pressure drop in the gas reservoir [3].
The reaction was also studied by monitoring the difference between the temperature of the reactor contents, Tr , and the temperature of the reactor jacket, Tj , i.e. Tr – Tj . The difference is a measure of the heat flowing into or out of the reactor. The synthesized products were characterized by differential scanning calorimetry (DSC) because an ALR cannot be used for this purpose. This article shows how reaction calorimetry and thermal analysis ideally complement one another for the synthesis and analysis of polymers.
Experimental Details
The reactor was first filled with solvent and the appropriate amounts of catalyst and co-catalyst in solution were added.
The desired reaction temperature was then set. The substrates were supplied to the reactor at the required pressure as soon as the temperature to start the reaction was reached.
Table 1 summarizes the amounts of reactants and the experimental conditions for both reactions.
The mixture of ethylene and propylene was stored in the reservoir in predefined molar ratios depending on the type of polymerization planned (see Figure 2). The experiments were carried out in a METTLER TOLEDO MultiMax RB04-50 Reactor Box equipped with Hastelloy® reactors with a working volume of 25 to 50 mL.
MultiMax is an automated parallel reactor system designed for process screening and optimization. It features simultaneous temperature control of the reaction mixture and reactor jacket as well as multiple dosing, mechanical or magnetic stirring, and pH, volumetric and gravimetric dosing controls.
The products were analyzed using a METTLER TOLEDO DSC 1 calorimeter [4]. DSC is widely used for the thermal characterization of polymers [5].
As shown in Figure 2, the MultiMax system was equipped with the LMPress automatic pressure controller to perform reactions under controlled pressure and to measure the gas consumption (in milliliters or moles) and with a manual pressure controller to manually control the pressure in the reactor up to 200 bar.
The stirrer used in the reactor was designed to ensure significant mass transfer of gas even when the level of liquid in the reactor was high.
Figure 3 shows the online curves of five signals (eight are possible), namely the temperature of the reactor, Tr, the temperature of the jacket, Tj , the pressure of the reactor, the pressure of the reservoir, and the temperature of the reservoir. The figure shows that the reactor pressure remains constant throughout the entire reaction. The reaction is stopped by closing the gas supply to the reactor and releasing the pressure in the reactor.
Results and Discussion
Synthesis
The difference between Tr and Tj , i.e. Tr – Tj , provides preliminary information on the heat flow produced by the reaction. This heat flow can be looked on as a “rate meter” [6]. It allows the user to obtain a qualitative overview of the reaction kinetics involved.
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Summary
The MultiMax RB04-50 Reactor Box (four 50-mL reactors) was used to investigate the polymerization of ethylene and the copolymerization of ethylene and propylene. After polymerization, DSC analyses were performed to determine the degree of crystallinity and the melting behavior of the products. The application of reaction calorimetry showed that the rate of polymerization and the type of polymer chain are strongly influenced by the pressure of the monomer. The temperature difference used as indicator for heat flow shows that concentration of the reagent occurs, i.e. the reaction is not dosing controlled. Different amounts of ethylene and propylene can be used in the same reservoir. This allows the user to set different ethylene and propylene rations. Furthermore, the software allows the compressibility factor of the gas mixture to be taken into account. The DSC evaluations showed that high density polyethylene was synthesized.
Polymerization of Ethylene and Propylene: Synthesis and Analysis from One Company | Thermal Analysis Application No. UC 273 | Application published in METTLER TOLEDO Thermal Analysis UserCom 27