What are polymers (Что такое полимеры). Учебное пособие
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1.What are the main stages of polymer processing?
2.What is the typical machinery for polymer processing?
3.How do we call the stage when additives are introduced into a polymer melt?
4.Describe a unit for production of polymer films.
5.Can we just take a polyethylene out of the reactor and make a shampoo bottle?
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Unit 6
POLYMERIZATION CATALYSIS
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A catalyst is a substance which alters the rate of a chemical reaction but is chemically unchanged at the end of the reaction. Most catalysts make chemical reactions go faster (“positive catalysts” or “promoters”). However, sometimes we want a chemical reaction to go more slowly. So we choose a “negative catalyst” (an “inhibitor”).
Catalysts work by providing an alternative mechanism of reaction involving a different transition state and lower activation energy. Catalysts can be either heterogeneous or homogeneous. Biocatalysts are often seen as a separate group.
Catalysis is of paramount importance in the polymer industry. The production of most industrially important polymers involves catalysis. For example, coordination polymerization is a form of catalyzed polymerization in which monomer adds to a growing polymer macromolecule through an organometallic active center. The development of this polymerization technique started in the 1950s with heterogeneous Ziegler-Natta catalysts based on titanium tetrachloride and an aluminum co-catalyst such as methyl-aluminoxane.
In many applications Ziegler-Natta polymerization is succeeded by metallocene catalysis polymerization. Polymerizations catalyzed by metallocenes occur via the Cossee-Arlman mechanism.
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Catalysts can be divided into two main types - heterogeneous and homogeneous. In a homogeneous reaction, the catalyst is in the same phase as the reactants. Heterogeneous catalysis involves the use of a catalyst in a different phase from the reactants. Typical examples involve a solid catalyst with the reactants as either liquids or gases.
Most examples of heterogeneous catalysis go through the same stages. One or more of the reactants are adsorbed onto the surface of the catalyst at active sites. Then there is some sort of interaction between the surface of the catalyst and the reactant molecules which makes them more reactive. The reaction happens. The product molecules are desorbed.
A good catalyst needs to adsorb the reactant molecules strongly enough for them to react, but not so strongly that the product molecules stick more or less permanently to the surface. Metals like platinum and nickel make good catalysts because they adsorb strongly enough to hold and activate the reactants, but not so strongly that the products can’t break away.
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Acid catalysis is mainly used for organic chemical reactions. This is a typical example of a homogeneous catalysis reaction. There are many possible chemical compounds that can act as sources for the protons to be transferred in an acid catalysis system. A compound such as sulfuric acid can be used. Usually this is done to create a more likely leaving group, such as converting an OH group into an H2O+ group.
With carbonyl compounds such as esters, synthesis and hydrolysis go through a tetrahedral transition state, where the central carbon has oxygen, an alcohol group, and the original alkyl group. Strong acids protonate the carbonyl, which makes the oxygen positively charged, so that it can easily receive the double bond electrons. This enables ester synthesis and hydrolysis. The reaction is the equilibrium between the ester and its cleavage to carboxylic acid and alcohol:
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On the contrary, strong bases deprotonate the attacking alcohol or amine which also promotes the reaction. However, bases also deprotonate the acid, which is irreversible. Therefore, in a strongly basic, aqueous environment, esters only hydrolyze.
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1.Typically 1 gram of a commercial catalyst is capable of producing 10 kg of polymer. This figure may be higher or lower depending on the exact formulation of catalyst, and on the polymerization conditions used.
2.Catalytic, rather than stoichiometric reactions are preferred in environmentally friendly green chemistry due to the reduced amount of waste generated.
3.The titanium catalyst compounds are deposited on pores in the support and these very porous particles expose many active sites to the monomer.
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4.In nature enzymes are catalysts in the metabolic pathway.
5.Metallocenes as olefin polymerization catalysts are a class of organometallic complexes with a metal sandwiched between organic ligands such as cyclopentadienyl.
6.A single chemical reaction is said to have undergone autocatalysis, or be autocatalytic, if the reaction product is itself the catalyst for that reaction.
7.One common type of fuel cell electrocatalyst is based upon tiny nanoparticles of platinum which adorn slightly larger carbon particles.
8.Phase boundary catalysis is a type of heterogeneous catalytic system which facilitates the chemical reaction of a particular chemical component in immiscible phase reacting on a catalytic active site located at phase boundary.
9.A simple model for heterogeneous catalysis involves the catalyst providing a surface on which the reactants (or substrates) temporarily become adsorbed.
10.The Ziegler-Natta catalysts represented a major breakthrough in polymerization chemistry because they produce a variety of commercially important polymers and can be highly stereoselective.
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1.Give a definition of catalysis.
2.What does the term «biocatalyst» mean?
3.Describe the essential stages of heterogeneous catalysis.
4.Why do catalyzed reactions go faster?
5.Why do we call Zigler-Natta catalysts stereospecific?
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Unit 7
POLYMER LIQUID CRYSTALS
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Liquid crystals are substances that may flow like a liquid, but have the molecules in the liquid arranged and/or oriented in a crystal-like way. In the 1960s, a French theoretical physicist, Pierre-Gilles de Gennes turned his interest to liquid crystals and soon found fascinating analogies between liquid crystals and superconductors as well as magnetic materials. His work would later be rewarded with the Nobel Prize in Physics in 1991. Today, liquid crystals are found in every home, for example in thermometers and computer screens.
There are many different types of liquid crystal (LC) phases, which can be distinguished based on their different optical properties (such as birefringence). When viewed under a microscope using a polarized light source, different liquid crystal phases will appear to have a distinct texture. Each “patch” in the texture corresponds to a domain where the LC molecules are oriented in a different direction. Within a domain, however, the molecules are well ordered.
Liquid crystals can be divided into thermotropic and lyotropic LCs. Thermotropic LCs exhibit a phase transition into the LC phase as temperature is changed, whereas lyotropic LCs exhibit phase transitions as a function of concentration of the mesogen in a solvent (typically water) as well as temperature.
Liquid crystals find wide use in liquid crystal displays, which rely on the optical properties of certain liquid crystalline substances in the presence or absence of an electric field.
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