EDUCATION HOLE PRESENTS
ENGINEERING CHEMISTRY
Unit-II
Polymers ....................................................................................................................
Polymers
Polymers are a large class of materials consisting of many small molecules (called monomers)
that can be linked t...
3. Synthetic polymers
The polymers which are prepared in the laboratory are called synthetic polymers. These are also
call...
to cross-linking. The term vulcanization is often used now to describe the cross-linking of all
elastomers.
Much of the ru...
3000 or more. Synthetic polymers have been developed that posess desirable characteristics,
such as a high softening point...
following diagram. The first two equations illustrate the initiation process, and the last two
equations are examples of c...
The relative importance of these terminations varies with the nature of the monomer undergoing
polymerization. For acrylon...
above) serve as initiating reagents. At low temperatures, chain transfer reactions are rare in such
polymerizations, so th...
A natural resinous polymer called lignin has a cross-linked structure similar to bakelite. Lignin is
the amorphous matrix ...
Synthetic fibers
Fibers represent a very important application of polymeric materials, including many examples
from the ca...
decomposition. Such biodegradable materials can be composted, broken down and returned to
the earth as useful nutrients. H...
was reported in connection with the structure of a metallocene compound known as ferrocene
(see Figure 2).
Figure 1. Anion...
Possibly the earliest biomedical application of an organometallic compound was the discovery,
by Paul Ehrlich, of the orga...
Grignard Reagents
One of the most commonly used classes of organometallic compounds is the organomagnesium
halides, or Gri...
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  1. 1. EDUCATION HOLE PRESENTS ENGINEERING CHEMISTRY Unit-II
  2. 2. Polymers ................................................................................................................................. 3 Classification....................................................................................................................................3 Classification based on source of availability........................................................................................................3 1. Naturally occurring polymers...........................................................................................................................3 2. Semi synthetic polymers...................................................................................................................................3 3. Synthetic polymers ...........................................................................................................................................4 Classification based on Mode of polymerization...............................................................................4 1. Photopolymers and Copolymers .................................................................................................................4 2. Addition and condensation polymers...............................................................................................................4 Application............................................................................................................................................................4 Elastomers........................................................................................................................................................4 Plastics.............................................................................................................................................5 Fibers ....................................................................................................................................................................5 Chain and Step growth polymerization.................................................................................... 6 Chain growth polymerization ...........................................................................................................6 1. Radical Chain-Growth Polymerization .............................................................................................................6 2. Cationic Chain-Growth Polymerization............................................................................................................8 Step growth polymerization.............................................................................................................9 Thermoplastic vs Thermosetting resins ............................................................................................9 Elastomers............................................................................................................................. 10 Synthetic fibers .............................................................................................................................. 11 Conducting Polymers ..................................................................................................................... 11 Biodegradable polymers ................................................................................................................ 11 General methods of synthesis of organometallic compound.............................................................................12 Applications in polymerization and catalysis .................................................................................. 14 Grignard Reagents.......................................................................................................................... 15 Carbines ..............................................................................................................................................................15
  3. 3. Polymers Polymers are a large class of materials consisting of many small molecules (called monomers) that can be linked together to form long chains, thus they are known as macromolecules. The picture at the top of the page is a short section of such a chain. A typical polymer may include tens of thousands of monomers. Because of their large size, polymers are classified as macromolecules. Humans have taken advantage of the versatility of polymers for centuries in the form of oils, tars, resins, and gums. However, it was not until the industrial revolution that the modern polymer industry began to develop. In the late 1830s, Charles Goodyear succeeded in producing a useful form of natural rubber through a process known as "vulcanization." Some 40 years later, Celluloid (a hard plastic formed from nitrocellulose) was successfully commercialized. Despite these advances, progress in polymer science was slow until the 1930s, when materials such as vinyl, neoprene, polystyrene, and nylon were developed. The introduction of these revolutionary materials began an explosion in polymer research that is still going on today. Unmatched in the diversity of their properties, polymers such as cotton, wool, rubber, Teflon(tm), and all plastics are used in nearly every industry. Natural and synthetic polymers can be produced with a wide range of stiffness, strength, heat resistance, density, and even price. With continued research into the science and applications of polymers, they are playing an ever increasing role in society. The following sections provide an introduction to the science of macromolecules. Classification Classification based on source of availability 1. Naturally occurring polymers These occur in plants and animals and are very essential for life. These include starch cellulose, proteins, nucleic acids and natural rubber. Starch is a polymer of glucose, cellulose is also a polymer of glucose, proteins are polymers of amino acids. Natural rubber consists of repeat units of isoprene (2-Methyl-1,3-Butadiene). 2. Semi synthetic polymers These are mostly derived from naturally occurring polymers by chemical modifications. Cellulose on acetylation with acetic anhydride in presence of sulphuric acid forms cellulose diacetate used in making threads and materials like films, glasses etc. Vulcanised rubber has much improved properties and is used in making tyres etc. Gun-cotton, which is cellulose nitrate, is used in making explosives.
  4. 4. 3. Synthetic polymers The polymers which are prepared in the laboratory are called synthetic polymers. These are also called man made polymers . These include fibres, plastics and synthetic rubbers and find diverse uses as clothing, electric fittings, eye lenses, substitute for wood and metals. Classification based on Mode of polymerization 1. Photopolymers and Copolymers Polymers when made by polymerisation of a single monomeric chemical species are known as homopolymers. Polythene formed from ethylene is a homopolymer. When the polymers are synthesised by polymerisation of two or more than two different monomers, they are called copolymers. When styrene and butadiene are polymerised, it gives the copolymer called styrene-butadiene rubber. 2. Addition and condensation polymers • Addition polymers A polymer formed by the direct repeated addition of monomer is called addition polymerisation. In this type of polymers, the monomers are unsaturated compounds and are generally derivatives of ethane. • Condensation polymers Condensation polymerisation involves a series of condensation reactions generally involving two different monomers. Each monomer normally contains two functional groups. During these reactions, there is a loss of small molecules usually water. Some of the condensation polymers are ; nylon, terelene, alkyd resins, bakelite. The formation of nylon-66 is shown. Application Elastomers Rubber is the most important of all elastomers. Natural rubber is a polymer whose repeating unit is isoprene. This material, obtained from the bark of the rubber tree, has been used by humans for many centuries. It was not until 1823, however, that rubber became the valuable material we know today. In that year, Charles Goodyear succeeded in "vulcanizing" natural rubber by heating it with sulfur. In this process, sulfur chain fragments attack the polymer chains and lead
  5. 5. to cross-linking. The term vulcanization is often used now to describe the cross-linking of all elastomers. Much of the rubber used in the United States today is a synthetic variety called styrene-butadiene rubber (SBR). Initial attempts to produce synthetic rubber revolved around isoprene because of its presence in natural rubber. Researchers eventually found success using butadiene and styrene with sodium metal as the initiator. This rubber was called Buna-S -- "Bu" from butadiene, "na" from the symbol for sodium, and "S" from styrene. During World War II, hundreds of thousands of tons of synthetic rubber were produced in government controlled factories. After the war, private industry took over and changed the name to styrene-butadiene rubber. Today, the United States consumes on the order of a million tons of SBR each year. Natural and other synthetic rubber materials are quite important. Plastics Americans consume approximately 60 billion pounds of plastics each year. The two main types of plastics are thermoplastics and thermosets. Thermoplastics soften on heating and harden on cooling while thermosets, on heating, flow and cross-link to form rigid material which does not soften on future heating. Thermoplastics account for the majority of commercial usage. Among the most important and versatile of the hundreds of commercial plastics is polyethylene. Polyethylene is used in a wide variety of applications because, based on its structure, it can be produced in many different forms. The first type to be commercially exploited was called low density polyethylene (LDPE) or branched polyethylene. This polymer is characterized by a large degree of branching, forcing the molecules to be packed rather loosely forming a low density material. LDPE is soft and pliable and has applications ranging from plastic bags, containers, textiles, and electrical insulation, to coatings for packaging materials. Another form of polyethylene differing from LDPE only in structure is high density polyethylene (HDPE) or linear polyethylene. This form demonstrates little or no branching, enabling the molecules to be tightly packed. HDPE is much more rigid than branched polyethylene and is used in applications where rigidity is important. Major uses of HDPE are plastic tubing, bottles, and bottle caps. Fibers Fibers represent a very important application of polymeric materials, including many examples from the categories of plastics and elastomers. Natural fibers such as cotton, wool, and silk have been used by humans for many centuries. In 1885, artificial silk was patented and launched the modern fiber industry. Man-made fibers include materials such as nylon, polyester, rayon, and acrylic. The combinations of strength, weight, and durability have made these materials very important in modern industry. Generally speaking, fibers are at least 100 times longer than they are wide. Typical natural and artificial fibers can have axial ratios (ratio of length to diameter) of
  6. 6. 3000 or more. Synthetic polymers have been developed that posess desirable characteristics, such as a high softening point to allow for ironing, high tensile strength, adequate stiffness, and desirable fabric qualities. These polymers are then formed into fibers with various characteristics. Nylon (a generic term for polyamides) was developed in the 1930's and used for parachutes in World War II. This synthetic fiber, known for its strength, elasticity, toughness, and resistance to abrasion, has commercial applications including clothing and carpeting. Nylon has special properties which distinguish it from other materials. One such property is the elasticity. Nylon is very elastic, however after elastic limit has been exceeded the material will not return to its original shape. Like other synthetic fibers, Nylon has a large electrical resistance. This is the cause for the build-up of static charges in some articles of clothing and carpets. From textiles to bullet-proof vests, fibers have become very important in modern life. As the technology of fiber processing expands, new generations of strong and light weight materials will be produced. Chain and Step growth polymerization Chain growth polymerization 1. Radical Chain-Growth Polymerization Virtually all of the monomers described above are subject to radical polymerization. Since this can be initiated by traces of oxygen or other minor impurities, pure samples of these compounds are often "stabilized" by small amounts of radical inhibitors to avoid unwanted reaction. When radical polymerization is desired, it must be started by using a radical initiator, such as a peroxide or certain azo compounds. The formulas of some common initiators, and equations showing the formation of radical species from these initiators are presented below. By using small amounts of initiators, a wide variety of monomers can be polymerized. One example of this radical polymerization is the conversion of styrene to polystyrene, shown in the
  7. 7. following diagram. The first two equations illustrate the initiation process, and the last two equations are examples of chain propagation. Each monomer unit adds to the growing chain in a manner that generates the most stable radical. Since carbon radicals are stabilized by substituents of many kinds, the preference for head-to-tail regioselectivity in most addition polymerizations is understandable. Because radicals are tolerant of many functional groups and solvents (including water), radical polymerizations are widely used in the chemical industry. In principle, once started a radical polymerization might be expected to continue unchecked, producing a few extremely long chain polymers. In practice, larger numbers of moderately sized chains are formed, indicating that chain-terminating reactions must be taking place. The most common termination processes are Radical Combination and Disproportionate. These reactions are illustrated by the following equations. The growing polymer chains are colored blue and red, and the hydrogen atom transferred in disproportionate is colored green. Note that in both types of termination two reactive radical sites are removed by simultaneous conversion to stable product(s). Since the concentration of radical species in a polymerization reaction is small relative to other reactants (e.g. monomers, solvents and terminated chains), the rate at which these radical-radical termination reactions occurs is very small, and most growing chains achieve moderate length before termination.
  8. 8. The relative importance of these terminations varies with the nature of the monomer undergoing polymerization. For acrylonitrile and styrene combination is the major process. However, methyl methacrylate and vinyl acetate are terminated chiefly by disproportionation. Another reaction that diverts radical chain-growth polymerizations from producing linear macromolecules is called chain transfer. As the name implies, this reaction moves a carbon radical from one location to another by an intermolecular or intermolecular hydrogen atom transfer (colored green). These possibilities are demonstrated by the following equations Chain transfer reactions are especially prevalent in the high pressure radical polymerization of ethylene, which is the method used to make LDPE (low density polyethylene). The 1º-radical at the end of a growing chain is converted to a more stable 2º-radical by hydrogen atom transfer. Further polymerization at the new radical site generates a side chain radical, and this may in turn lead to creation of other side chains by chain transfer reactions. As a result, the morphology of LDPE is an amorphous network of highly branched macromolecules. 2. Cationic Chain-Growth Polymerization Polymerization of isobutylene (2-methylpropene) by traces of strong acids is an example of cationic polymerization. The polyisobutylene product is a soft rubbery solid, Tg = _ 70º C, which is used for inner tubes. This process is similar to radical polymerization, as demonstrated by the following equations. Chain growth ceases when the terminal carbocation combines with a nucleophile or loses a proton, giving a terminal alkene (as shown here). Monomers bearing cation stabilizing groups, such as alkyl, phenyl or vinyl can be polymerized by cationic processes. These are normally initiated at low temperature in methylene chloride solution. Strong acids, such as HClO4 , or Lewis acids containing traces of water (as shown
  9. 9. above) serve as initiating reagents. At low temperatures, chain transfer reactions are rare in such polymerizations, so the resulting polymers are cleanly linear (unbranched). Step growth polymerization A large number of important and useful polymeric materials are not formed by chain-growth processes involving reactive species such as radicals, but proceed instead by conventional functional group transformations of poly functional reactants. These polymerizations often (but not always) occur with loss of a small byproduct, such as water, and generally (but not always) combine two different components in an alternating structure. The polyester Dacron and the polyamide Nylon 66, shown here, are two examples of synthetic condensation polymers, also known as step-growth polymers. In contrast to chain-growth polymers, most of which grow by carbon-carbon bond formation, step-growth polymers generally grow by carbon-heteroatom bond formation (C-O & C-N in Dacron & Nylon respectively). Although polymers of this kind might be considered to be alternating copolymers, the repeating monomeric unit is usually defined as a combined moiety. Thermoplastic vs Thermosetting resins Most of the polymers described above are classified as thermoplastic. This reflects the fact that above Tg they may be shaped or pressed into molds, spun or cast from melts or dissolved in suitable solvents for later fashioning. Because of their high melting point and poor solubility in most solvents, Kevlar and Nomex proved to be a challenge, but this was eventually solved. Another group of polymers, characterized by a high degree of cross-linking, resist deformation and solution once their final morphology is achieved. Such polymers are usually prepared in molds that yield the desired object. Because these polymers, once formed, cannot be reshaped by heating, they are called thermosets .Partial formulas for four of these will be shown below by clicking the appropriate button. The initial display is of Bakelite, one of the first completely synthetic plastics to see commercial use (circa 1910).
  10. 10. A natural resinous polymer called lignin has a cross-linked structure similar to bakelite. Lignin is the amorphous matrix in which the cellulose fibers of wood are oriented. Wood is a natural composite material, nature's equivalent of fiberglass and carbon fiber composites. A partial structure for lignin is shown here Elastomers Rubber is the most important of all elastomers. Natural rubber is a polymer whose repeating unit is isoprene. This material, obtained from the bark of the rubber tree, has been used by humans for many centuries. It was not until 1823, however, that rubber became the valuable material we know today. In that year, Charles Goodyear succeeded in "vulcanizing" natural rubber by heating it with sulfur. In this process, sulfur chain fragments attack the polymer chains and lead to cross-linking. The term vulcanization is often used now to describe the cross-linking of all elastomers. Much of the rubber used in the United States today is a synthetic variety called styrene-butadiene rubber (SBR). Initial attempts to produce synthetic rubber revolved around isoprene because of its presence in natural rubber. Researchers eventually found success using butadiene and styrene with sodium metal as the initiator. This rubber was called Buna-S -- "Bu" from butadiene, "na" from the symbol for sodium, and "S" from styrene. During World War II, hundreds of thousands of tons of synthetic rubber were produced in government controlled factories. After the war, private industry took over and changed the name to styrene-butadiene rubber. Today, the United States consumes on the order of a million tons of SBR each year. Natural and other synthetic rubber materials are quite important.
  11. 11. Synthetic fibers Fibers represent a very important application of polymeric materials, including many examples from the categories of plastics and elastomers. Natural fibers such as cotton, wool, and silk have been used by humans for many centuries. In 1885, artificial silk was patented and launched the modern fiber industry. Man-made fibers include materials such as nylon, polyester, rayon, and acrylic. The combination of strength, weight, and durability has made these materials very important in modern industry. Generally speaking, fibers are at least 100 times longer than they are wide. Typical natural and artificial fibers can have axial ratios (ratio of length to diameter) of 3000 or more. Synthetic polymers have been developed that posess desirable characteristics, such as a high softening point to allow for ironing, high tensile strength, adequate stiffness, and desirable fabric qualities. These polymers are then formed into fibers with various characteristics. Nylon (a generic term for polyamides) was developed in the 1930's and used for parachutes in World War II. This synthetic fiber, known for its strength, elasticity, toughness, and resistance to abrasion, has commercial applications including clothing and carpeting. Nylon has special properties which distinguish it from other materials. One such property is the elasticity. Nylon is very elastic, however after elastic limit has been exceeded the material will not return to its original shape. Like other synthetic fibers, Nylon has a large electrical resistance. This is the cause for the build-up of static charges in some articles of clothing and carpets. Conducting Polymers A large number of important and useful polymeric materials are not formed by chain-growth processes involving reactive species such as radicals, but proceed instead by conventional functional group transformations of poly functional reactants. These polymerizations often (but not always) occur with loss of a small byproduct, such as water, and generally (but not always) combine two different components in an alternating structure. The polyester Dacron and the polyamide Nylon 66, shown here, are two examples of synthetic condensation polymers, also known as step-growth polymers. In contrast to chain-growth polymers, most of which grow by carbon-carbon bond formation, step-growth polymers generally grow by carbon-heteroatom bond formation (C-O & C-N in Dacron & Nylon respectively). Although polymers of this kind might be considered to be alternating copolymers, the repeating monomeric unit is usually defined as a combined moiety. Biodegradable polymers Plastics derived from natural materials, such as cellulose, starch and hydroxycarboxylic acids are more easily decomposed when exposed to oxygen, water, soil organisms and sunlight than are most petroleum based polymers. The glycoside linkages in polysaccharides and the ester groups in polyesters represent points of attack by the enzymes of microorganisms that facilitate their
  12. 12. decomposition. Such biodegradable materials can be composted, broken down and returned to the earth as useful nutrients. However, it is important to recognize that proper composting is necessary. Placing such materials in a landfill results in a slower anaerobic decomposition, which produces methane, a greenhouse gas. Derivatives of cellulose, such as cellulose acetate, have long served for the manufacture of films and fibers. The most useful acetate material is the diacetate, in which two thirds of the cellulose hydroxyl groups have been esterified. Acetate fibers loose strength when wet, and acetate clothing must be dry cleaned. The other major polysaccharide, starch, is less robust than cellulose, but in pelletized form it is now replacing polystyrene as a packing material. The two natural polyesters that are finding increasing use as replacements for petroleum based plastics are polylactide (PLA) and polyhydroxyalkanoates (PHA), the latter most commonly as copolymers with polyhydroxybutyrate (PHB). Structures for the these polymers and their monomer precursors are shown below. General methods of synthesis of organometallic compound (Grignard Reagent) Organometallic compounds have at least one carbon to metal bond, according to most definitions. This bond can be either a direct carbon to metal bond ( σ bond or sigma bond) or a metal complex bond ( π bond or pi bond). Compounds containing metal to hydrogen bonds as well as some compounds containing nonmetallic ( metalloid ) elements bonded to carbon are sometimes included in this class of compounds. Some common properties of organometallic compounds are relatively low melting points, insolubility in water, solubility in ether and related solvents, toxicity, oxidizability, and high reactivity. An example of an organometallic compound of importance years ago is tetraethyllead (Et 4 4Pb) which is an antiknock agent for gasoline. It is presently banned from use in the United States. The first metal complex identified as an organometallic compound was a salt, K(C 2 H 4 )PtCl 3 , obtained from reaction of ethylene with platinum (II) chloride by William Zeise in 1825. It was not until much later (1951–1952) that the correct structure of Zeise's compound (see Figure 1)
  13. 13. was reported in connection with the structure of a metallocene compound known as ferrocene (see Figure 2). Figure 1. Anion of Zeise's compound Figure 2. Ferrocene Preparation of ferrocene was reported at about the same time by two research groups, and a sandwich structure was proposed, based on ferrocene's physical properties (Kauffman, pp. 185– 186). The sandwich structure was confirmed by x-ray diffraction studies. Since then, other metallocenes composed of other metals and other carbon ring molecules. Possibly the first scientist to synthesize an organometallic compound was Edward Frankland, who prepared diethylzinc by reaction of ethyl iodide with zinc metal in 1849 (Thayer 1969b, pp. 764–765). 2 CH 3 CH 2 I + 2 Zn → CH 3 CH 2 ZnCH 2 CH 3 + ZnI 2 In organometallic compounds, most p-electrons of transition metals conform to an empirical rule called the 18-electron rule. This rule assumes that the metal atom accepts from its ligands the number of electrons needed in order for it to attain the electronic configuration of the next noble gas . It assumes that the valence shells of the metal atom will contain 18 electrons. Thus, the sum of the number of d electrons plus the number of electrons supplied by the ligands will be 18. Ferrocene, for example, has 6 d electrons from Fe(II), plus 2 × 6 electrons from the two 5- membered rings, for a total of 18. (There are exceptions to this rule, however.)
  14. 14. Possibly the earliest biomedical application of an organometallic compound was the discovery, by Paul Ehrlich, of the organoarsenical Salvarsan, the first antisyphilitic agent. Salvarsan and other organoarsenicals are sometimes listed as organometallics even though arsenic is not a true metal. Vitamin B 12 is an organocobalt complex essential to the diet of human beings. Absence of or deficiency of B 12 in the diet (or a body's inability to absorb it) is the cause of pernicious anemia. Applications in polymerization and catalysis Organometallic compounds are very useful as catalysts or reagents in the synthesis of organic compounds, such as pharmaceutical products. One of the major advantages of organometallic compounds, as compared with organic or inorganic compounds, is their high reactivity. Reactions that cannot be carried out with the usual types of organic reagents can sometimes be easily carried out using one of a wide variety of available organometallics. A second advantage is the high reaction selectivity that is often achieved via the use of organometallic catalysts. For example, ordinary free-radical polymerization of ethylene yields a waxy low-density polyethylene, but use of a special organometallic catalyst produces a more ordered linear polyethylene with a higher density, a higher melting point , and a greater strength. A third advantage is that many in this wide range of compounds are stable, and many of these have found uses as medicinal and pesticides. A fourth advantage is the case of recovery of pure metals. Isolation of a pure sample of an organ metallic compound containing a desired metal can be readily accomplished, and the pure metal can then be easily obtained from the compound. (This is generally done via preparation of a pure metal carbonyl, such as Fe[CO] 5 or Ni[CO] 4 , followed by thermal decomposition.) Other commonly used organometallic compounds are organ lithium, organizing, and organocuprates (sometimes called Gilman reagents). The name "ferrocene" was coined by one of Harvard University professor R. B. Woodward's postdoctoral students, Mark Whiting. The entire class of transitional metal dicyclopentadienyl compounds quickly became known as "metallocenes" and this has since been expanded for compounds [(H 5 ‒C 5 H 5 ) 2M] in general. G. Wilkinson and Woodward published their results on ferrocene in 1952. Figure 4. Uranocene
  15. 15. Grignard Reagents One of the most commonly used classes of organometallic compounds is the organomagnesium halides, or Grignard reagents (generally RMgX or ArMgX, where R and Ar are alkyl and aryl groups, respectively, and X is a halogen atom), used extensively in synthetic organic chemistry. Organ magnesium halides were discovered by Philippe Barbier in 1899 and subsequently developed by Victor Grignard. They are usually prepared by reaction of magnesium metal with alkyl or aryl halides. Other commonly used organ metallic compounds are the organ lithium and organizing compounds. Carbines Carbines are the electrons of free carbines that have two spin states, singlet and triplet. The electrons are paired as a sp 2 lone pair in the singlet (:CH 2 ); there is one electron in each of the sp 2 and p orbital’s in the triplet (·CH 2 ). Carbines are generally unstable in the free state, but are stable when bonded to metal atoms. Metal-carbine complexes have the general structure L n M=CXY, where L n M is the metal fragment with n legends, and X and Y are alkyl groups, aryl groups, hydrogen atoms, or heteroatom’s (O, N, S, or halogens). The first carbine complex [(CO) 5 W = CPh(OMe)] was reported by E. O. Fischer and A. Maasbol in 1964 (Donitz, Or gel, and Rich, pp. 373–375). In 1974 Richard R. Schrock prepared compounds in which the substituent’s attached to carbon were hydrogen atoms or alkyl groups; these complexes are known as Schrock- type carbine complexes. The two types of carbine complexes differ in their reactivity’s. Fischer- type complexes tend to undergo attack at carbon atoms by nucleophiles (negatively charged species) and are electrophonic (electron-attracting). Schrock-type complexes undergo attack at carbon atoms by electrophones and are considered to be nucleophilic species.

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