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Unit 6 POLYMERIZATION CATALYSIS

Упражнение 1. Прочитайте текст вслух.

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 methylaluminoxane.

In many applications Ziegler-Natta polymerization is succeeded by metallocene catalysis polymerization. Polymerizations catalysed by metallocenes occur via the Cossee-Arlman mechanism.

Упражнение 2. Подберите из текста английские эквиваленты к следующим словосочетаниям и выучите их наизусть.

Упражнение 3. Подберите русские эквиваленты к следующим словосочетаниям и выучите их наизусть.

Упражнение 4. Передайте на английском языке основное содержание текста упражнения 1.

Упражнение 5. Передайте устно на английском языке основное содержание текста.

Открытое Циглером и Натта явление стереоспецифического катализа позволило синтезировать высокомолекулярные полимеры моно- и диолефинов, в том числе все четыре стереорегулярных полибутадиена. Данный тип полимеризации обеспечивает возможность получения полимеров заданной регулярной структуры и молекулярно-массового распределения.

Ионно-координационная полимеризация — каталитический процесс образования макромолекул, в котором стадии разрыва л-связи предшествует возникновение координационного комплекса между мономером и катализатором. Структура мономера и тип катализатора оказывают основное воздействие как на процесс комплексообразования, так и на стереорегулярность полимера.

Из катализаторов Циглера-Натта в производстве полимеров обычно используют комплексы алюминийалкилов и галогенпроизводные титана и ванадия. Катализаторами могут являться также п-аллильные комплексы переходных металлов и оксидно-металлические катализаторы.

Упражнение 6. Переведите с листа на русский язык следующий отрывок:

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.

Упражнение 7. Переведите на слух по предложению следующий отрывок.

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 an 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 when the alcohol attacks the carbonyl carbon. This enables ester synthesis and hydrolysis. The reaction is an equilibrium between the ester and its cleavage to carboxylic acid and alcohol.

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.

Упражнение 8. Переведите письменно на русский язык следующие определения:

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.

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 react 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.

Упражнение 9. Переведите письменно на английский язык следующие предложения.

3. Каталитические яды — посторонние примеси в реакционной смеси, приводящие к частичной или полной потере активности катализатора.

4. Катализаторы Циглера-Натта позволили получать полиэтилен и полипропилен с чрезвычайно высокой молекулярной массой, малоразветвленной структурой и высокой степенью кристалличности.

Упражнение 10. Переведите на слух в быстром темпе следующие словосочетания

энергия активации — Cossee-Arlman mechanism — polymerization technique — отрицательный катализ — polymerizations — гетерогенный — promoter — уступать место — влиять на скорость реакции — metallocene catalysis — coordination polymerization — технология полимеризации — activation energy — biocatalysts — альтернативный путь реакции — chemically unchanged — полимеризация на металлоцене — co-catalyst — organometallic active center — химически неизменный — скорость химической реакции — быть чрезвычайно важным для — ускорять реакцию — transition state — замедлять реакцию — центр роста — Ziegler-Natta catalysts — inhibitor

Упражнение 11. Ответьте на вопросы по содержанию текстов данного урока.

1. Give a definition of catalysis.

2. What is the term for biocatalysts?

3. Describe the essential stages of heterogeneous catalysis.

4. Why do catalyzed reactions go faster?

5. Why do we call Zigler-Natta catalysts stereospecific?

Упражнение 12. Передайте основное содержание текстов урока на русском и английском языках.