In polymer chemistry Polymer chemistry or macromolecular chemistry is a multidisciplinary science that deals with the chemical synthesis and chemical properties of polymers or macromolecules. According to IUPAC recommendations, macromolecules refer to the individual molecular chains and are the domain of chemistry. Polymers describe the bulk properties of polymer, polymerization is a process of reacting monomer A monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer or cluster[citation needed]. The most common natural monomer is glucose, which is linked by glycosidic bonds into polymers such as cellulose and starch, and is over 76% of the weight of all plant matter molecules A molecule is defined as an electrically neutral group of at least two atoms in a definite arrangement held together by very strong chemical bonds. Molecules are distinguished from polyatomic ions in this strict sense. In organic chemistry and biochemistry, the term molecule is used less strictly and also is applied to charged organic molecules together in a chemical reaction A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, often coming about only after the input of some type of energy, viz. heat, light or electricity. Classically, chemical reactions encompass to form three-dimensional networks or polymer A polymer is a large molecule composed of repeating structural units typically connected by covalent chemical bonds. While polymer in popular usage suggests plastic, the term actually refers to a large class of natural and synthetic materials with a wide variety of properties chains.[1][2][3] There are many forms of polymerization and different systems exist to categorize them.

Contents

Introduction

Homopolymers
Co-polymers

In chemical compounds, polymerization occurs via a variety of reaction mechanisms that vary in complexity due to functional groups In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction regardless of the size of the molecule it is a part of. However, its relative reactivity can be present in reacting compounds[4] and their inherent steric effects Steric effects arise from the fact that each atom within a molecule occupies a certain amount of space. If atoms are brought too close together, there is an associated cost in energy due to overlapping electron clouds , and this may affect the molecule's preferred shape (conformation) and reactivity explained by VSEPR Theory Valence shell electron pair repulsion theory is a model in chemistry used to predict the shape of individual molecules based upon the extent of electron-pair electrostatic repulsion. It is also named Gillespie-Nyholm theory after its two main developers. The acronym "VSEPR" is sometimes pronounced "vesper" for ease of. In more straightforward polymerization, alkenes In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond. The simplest acyclic alkenes, with only one double bond and no other functional groups, form an homologous series of hydrocarbons with the general formula CnH2n, which are relatively stable due to σ bonding A chemical bond is an interaction between atoms or molecules and allows the formation of polyatomic chemical compounds. A chemical bond is the attraction caused by the electromagnetic force between opposing charges, either between electrons and nuclei, or as the result of a dipole attraction. The strength of bonds varies considerably; there are & between carbon atoms form polymers through relatively simple radical reactions; in contrast, more complex reactions such as those that involve substitution at the carbonyl group require more complex synthesis due to the way in which reacting molecules polymerize.[4]

As alkenes can be formed in somewhat straightforward reaction mechanisms, they form useful compounds such as polyethylene Polyethylene or polythene (IUPAC name polyethene or poly) is the most widely used plastic, with an annual production of approximately 80 million metric tons. Its primary use is within packaging (notably the plastic shopping bag) and polyvinyl chloride Polyvinyl chloride, (IUPAC Poly) commonly abbreviated PVC, is a thermoplastic polymer. It is a vinyl polymer constructed of repeating vinyl groups (ethenyls) having one of their hydrogens replaced with a chloride group (PVC) when undergoing radical reactions,[4] which are produced in high tonnages each year[4] due to their usefulness in manufacturing processes of commercial products, such as piping, insulation and packaging. Polymers such as PVC are generally referred to as "homopolymers" as they consist of repeated long chains or structures of the same monomer unit, whereas polymers that consist of more than one molecule are referred to as "co-polymers".[5]

Other monomer units, such as formaldehyde hydrates or simple aldehydes, are able to polymerize themselves at quite low temperatures (>-80oC) to form trimers;[4] molecules consisting of 3 monomer units which can cyclize to form ring cyclic structures, or undergo further reactions to form tetramers A tetramer is a protein with four subunits . There are homo-tetramers (all subunits are identical) such as glutathione S-transferase or single-strand binding protein, dimers of hetero-dimers such as hemoglobin (a dimer of an alpha/beta dimer), and hetero-tetramers, where each subunit is different,[4] or 4 monomer-unit compounds. Further compounds either being referred to as oligomers In chemistry, an oligomer consists of a few monomer units , in contrast to a polymer that, at least in principle, consists of an unlimited number of monomers. Dimers, trimers and tetramers are oligomers. Many oils are oligomeric, such as liquid paraffin. Plasticizers are oligomeric esters widely used to soften thermoplastics such as PVC. They may[4] in smaller molecules. Generally, because formaldehyde is an exceptionally reactive electrophile it allows nucleophillic addition of hemiacetal intermediates, which are generally short lived and relatively unstable "mid stage" compounds which react with other molecules present to form more stable polymeric compounds.

Polymerization that is not sufficiently moderated and proceeds at a fast rate can be very hazardous. This phenomenon is known as Hazardous polymerization and can cause fires and explosions.

Step-growth

Main article: Step-growth polymerization Step-growth polymerization refers to a type of polymerization mechanism in which bi-functional or multifunctional monomers react to form first dimers, then trimers, longer oligomers and eventually long chain polymers. Many naturally occurring and some synthetic polymers are produced by step-growth polymerization, e.g. polyesters, polyamides,

Step-growth polymers are defined as polymers formed by the stepwise reaction between functional groups of monomers. Most step-growth polymers are also classified as condensation polymers, but not all step-growth polymers (like polyurethanes A polyurethane is any polymer consisting of a chain of organic units joined by urethane (carbamate) links. Polyurethane polymers are formed through step-growth polymerization by reacting a monomer containing at least two isocyanate functional groups with another monomer containing at least two hydroxyl (alcohol) groups in the presence of a formed from isocyanate Isocyanate is the functional group of atoms –N=C=O , not to be confused with the cyanate functional group which is arranged as –O–C≡N. Any organic compound which contains an isocyanate group may also be referred to in brief as an isocyanate. An isocyanate may have more than one isocyanate group. An isocyanate that has two isocyanate groups and alcohol bifunctional monomers) release condensates. Step-growth polymers increase in molecular weight at a very slow rate at lower conversions and reach moderately high molecular weights only at very high conversion (i.e. >95%).

To alleviate inconsistencies in these naming methods, adjusted definitions for condensation and addition polymers have been developed. A condensation polymer is defined as a polymer that involves elimination An elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either a one or two-step mechanism . Either the unsaturation of the molecule increases or the valence of an atom in the molecule decreases by two, a process known as reductive elimination of small molecules during its synthesis, or contains functional groups as part of its backbone chain In organic chemistry, the backbone chain of a polymer is the series of covalently bonded atoms that together create the continuous chain of the molecule, or its repeat unit In polymer chemistry, a structural unit is a building block of a polymer chain, and related to the repeat unit. It is the result of a monomer which has been polymerized into a long chain. When different monomer are polymerized, a copolymer is formed. It is a routine way of developing new properties for new materials does not contain all the atoms present in the hypothetical monomer to which it can be degraded.

Chain-growth

Main article: Chain-growth polymerization

Chain-growth polymerization (or addition polymerization) involves the linking together of molecules incorporating double or triple chemical bonds A chemical bond is an attraction between atoms or molecules and allows the formation of chemical compounds, which contain two or more atoms. A chemical bond is the attraction caused by the electromagnetic force between opposing charges, either between electrons and nuclei, or as the result of a dipole attraction. The strength of bonds varies. These unsaturated monomers (the identical molecules that make up the polymers) have extra internal bonds that are able to break and link up with other monomers to form the repeating chain. Chain-growth polymerization is involved in the manufacture of polymers such as polyethylene Polyethylene or polythene (IUPAC name polyethene or poly) is the most widely used plastic, with an annual production of approximately 80 million metric tons. Its primary use is within packaging (notably the plastic shopping bag), polypropylene Polypropylene , also known as polypropene, is a thermoplastic polymer, made by the chemical industry and used in a wide variety of applications, including packaging, textiles (e.g. ropes, thermal underwear and carpets), stationery, plastic parts and reusable containers of various types, laboratory equipment, loudspeakers, automotive components,, and polyvinyl chloride Polyvinyl chloride, (IUPAC Poly) commonly abbreviated PVC, is a thermoplastic polymer. It is a vinyl polymer constructed of repeating vinyl groups (ethenyls) having one of their hydrogens replaced with a chloride group (PVC). A special case of chain-growth polymerization leads to living polymerization In polymer chemistry, living polymerization is a form of addition polymerization where the ability of a growing polymer chain to terminate has been removed . This can be accomplished in a variety of ways. Chain termination and chain transfer reactions are absent and the rate of chain initiation is also much larger than the rate of chain.

In the radical polymerization Radical polymerization is a type of polymerization in which the reactive center of a polymer chain consists of a radical of ethylene Ethylene is a gaseous organic compound with the formula C2H4. It is the simplest alkene (older name: olefin from its oil-forming property). Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and also has a role in biology as a hormone. Ethylene is the most, its pi bond is broken, and the two electrons rearrange to create a new propagating center like the one that attacked it. The form this propagating center takes depends on the specific type of addition mechanism. There are several mechanisms through which this can be initiated. The free radical In chemistry, radicals are atoms, molecules, or ions with unpaired electrons on an open shell configuration. The unpaired electrons cause them to be highly chemically reactive. Radicals play an important role in combustion, atmospheric chemistry, polymerization, plasma chemistry, biochemistry, and many other chemical processes, including human mechanism was one of the first methods to be used. Free radicals are very reactive atoms or molecules that have unpaired electrons. Taking the polymerization of ethylene as an example, the free radical mechanism can be divided in to three stages: chain initiation, chain propagation, and chain termination.

Polymerization of ethylene Ethylene is a gaseous organic compound with the formula C2H4. It is the simplest alkene (older name: olefin from its oil-forming property). Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and also has a role in biology as a hormone. Ethylene is the most

Free radical addition polymerization of ethylene must take place at high temperatures and pressures, approximately 300°C and 2000 atm. While most other free radical polymerizations do not require such extreme temperatures and pressures, they do tend to lack control. One effect of this lack of control is a high degree of branching. Also, as termination occurs randomly, when two chains collide, it is impossible to control the length of individual chains. A newer method of polymerization similar to free radical, but allowing more control involves the Ziegler-Natta catalyst A Ziegler-Natta catalyst is a catalyst used in the production of polymers of 1-alkenes . Ziegler-Natta catalysts are typically based on titanium compounds and organometallic aluminium compounds, such as the undefined methylaluminoxane (MAO) or well defined triethylaluminium, (C2H5)3Al, especially with respect to polymer branching In polymer chemistry, branching occurs by the replacement of a substituent, e.g, a hydrogen atom, on a monomer subunit, by another covalently bonded chain of that polymer; or, in the case of a graft copolymer, by a chain of another type. In crosslinking rubber by vulcanization, short sulfur branches link polyisoprene chains into a multiply-.

Other forms of chain growth polymerization include cationic addition polymerization and anionic addition polymerization An anionic addition polymerization of vinyl monomers is an addition polymerization initiated by a strong base and anion, such as an alkali amide, or an organometallic compound, such as n-butyllithium. While not used to a large extent in industry yet due to stringent reaction conditions such as lack of water and oxygen, these methods provide ways to polymerize some monomers that cannot be polymerized by free radical methods such as polypropylene Polypropylene , also known as polypropene, is a thermoplastic polymer, made by the chemical industry and used in a wide variety of applications, including packaging, textiles (e.g. ropes, thermal underwear and carpets), stationery, plastic parts and reusable containers of various types, laboratory equipment, loudspeakers, automotive components,. Cationic and anionic mechanisms are also more ideally suited for living polymerizations In polymer chemistry, living polymerization is a form of addition polymerization where the ability of a growing polymer chain to terminate has been removed . This can be accomplished in a variety of ways. Chain termination and chain transfer reactions are absent and the rate of chain initiation is also much larger than the rate of chain, although free radical living polymerizations have also been developed.

See also

References

  1. ^ Introduction to Polymers 1987 R.J. Young Chapman & Hall ISBN 0-412-22170-5
  2. ^ International Union of Pure and Applied Chemistry, et al. (2000) "IUPAC Gold Book" Retrieved on 11 May 2007 from "IUPAC Gold Book" on http://goldbook.iupac.org/
  3. ^ Clayden, J., Greeves, N. et al. (2000). "Organic chemistry" Oxford
  4. ^ a b c d e f g Clayden, J., Greeves, N. et al. (2000), p1450-1466
  5. ^ J.M.G. Cowie "Polymers: Chemistry and Physics of Modern Materials (Chapman and Hall, 2d ed. 1991) p.4

External links

Categories: Polymerization reactions

Personal tools
Namespaces
">
Variants
Views
">
Actions
Search">
No Line on the Horizon is the twelfth studio album by the rock band U2. Released on 27 February 2009, it was the group's first album since How to Dismantle an Atomic Bomb , marking the longest gap between studio albums of U2's career. Work on the record began in 2006 with producer Rick Rubin, but most of the material from those sessions was
Navigation
Interaction
Toolbox
Print/export
Languages

 

The above information uses material from Wikipedia and is licensed under the GNU Free Documentation License The purpose of this License is to make a manual, textbook, or other functional and useful document "free" in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a.
Some facts may not have been fully verified for accuracy. [Disclaimers Wikipedia is an online open-content collaborative encyclopedia, that is, a voluntary association of individuals and groups working to develop a common resource of human knowledge. The structure of the project allows anyone with an Internet connection to alter its content. Please be advised that nothing found here has necessarily been reviewed by]
This page was last archived by our server on Fri Jul 30 00:21:33 2010. [ refresh local cache ]
Displaying this page or its contents does not use any Wikimedia Foundation's resources.
The owners of this site proudly support the Wikimedia Foundation.

[Hide]▼
[Hide]▲