Polymerization is a process of reacting monomer molecules together in a chemical reaction to forinear chains or a three-dimensional network of polymer chains ^[1]. There are many forms of polymerization and different systems exist to categorize them. In chemistry, a monomer (from Greek mono one and meros part) is a small molecule that may become chemically bonded to other monomers to form a polymer. ... In chemistry, a molecule is an aggregate of at least two atoms in a definite arrangement held together by special forces. ... Polymer is a generic term used to describe a very long molecule consisting of structural units and repeating units connected by covalent chemical bonds. ...

The main categories are

• Addition polymerization
• Condensation polymerization

Terminology in the literature to represent these types of polymerizations is not standard: there are multiple terms for the same type. For example, "step polymerization" and "step-growth polymerization" refer to the same type of polymerization. Addition polymerization, also called polyaddition or chain growth polymerization, is a polymerization technique where monomer molecules add on to a growing polymer chain one at a time. ... Step-growth polymerization is a polymerization process that involves a chemical reaction between multifunctional monomer molecules. ...

Overview

Addition polymerization involves the linking together of molecules incorporating double or triple chemical bonds. These unsaturated monomers (the identical molecules which make up the polymers) have extra internal bonds which are able to break and link up with other monomers to form the repeating chain. Addition polymerization is involved in the manufacture of polymers such as polyethene, polypropylene and polyvinyl chloride (PVC). A special case of addition polymerization leads to living polymerization. A chemical bond is the physical phenomenon of chemical substances being held together by attraction of atoms to each other through sharing, as well as exchanging, of electrons or electrostatic forces. ... Polyethylene or polyethene is one of the simplest and most inexpensive polymers. ... Polypropylene lid of a Tic Tacs box, with a living hinge and the resin identification code under its flap Polypropylene or polypropene (PP) is a thermoplastic polymer, used in a wide variety of applications, including food packaging, textiles, laboratory equipment, automotive components, and polymer banknotes. ... Polyvinyl chloride, commonly abbreviated PVC, is a widely-used plastic. ... In polymer chemistry, living polymerization is a special form of addition polymerization where the ability of a growing polymer chain to terminate has been removed . ...

Condensation polymerization occurs when monomers bond together through condensation reactions. Typically these reactions can be achieved through reacting molecules incorporating alcohol, amine or carboxylic acid (or other carboxyl derivative) functional groups. When an amine reacts with a carboxylic acid an amide or peptide bond is formed, with the release of water (hence condensation polymerization.) This is the process through which amino acids link up to form proteins, as well as how kevlar is formed. A condensation reaction (also known as a dehydration reaction or dehydration synthesis when water is lost) is a chemical reaction in which two molecules or moieties react and become covalently bonded to one another by the concurrent loss of a small molecule, often water, methanol, or a type of hydrogen... In chemistry, an alcohol is any organic compound in which a hydroxyl group (-OH) is bound to a carbon atom of an alkyl or substituted alkyl group. ... Ammonia Amines are organic compounds and a type of functional group that contain nitrogen as the key atom. ... Structure of a carboxylic acid The 3D structure of the carboxyl group A space-filling model of the carboxyl group Carboxylic acids are organic acids characterized by the presence of a carboxyl group, which has the formula -C(=O)-OH, usually written as -COOH. In general, the salts and anions... Amide functional group In chemistry, an amide is one of two kinds of compound. ... A peptide bond is a chemical bond formed between two molecules when the carboxyl group of one molecule reacts with the amino group of the other molecule, releasing a molecule of water (H2O). ... An amino acid residue is what is left of an amino acid once a molecule of water has been lost (an H+ from the nitrogenous side and an OH- from the carboxylic side) in the formation of a peptide bond. ... Kevlar is the DuPont Companys brand name for material made out of synthetic fiber of poly-paraphenylene terephthalamide which is constructed of para-aramid fibers that the company claims is five times stronger than the same weight of steel, while being lightweight, flexible and comfortable. ...

The chain growth-step growth system categorizes polymers based on their mechanism. While most polymers will fall into their similar category from the addition-condensation method of categorization, there are a few exceptions.

Chain growth polymers are defined as polymers formed by the reaction of monomer with a reactive center. These polymers grow to high molecular weight at a very fast rate. It is important to note that the overall conversion rates between chain and step growth polymers are similar, but that high molecular weight polymers are formed in addition reactions much more quickly than with step polymerizations. In chemistry, a monomer (from Greek mono one and meros part) is a small molecule that may become chemically bonded to other monomers to form a polymer. ... In chemistry, a reactive center is a particular location, usually an atom, within a chemical compound that is the likely center of a reaction in which the chemical is involved. ...

Step growth polymers are defined as polymers formed by the stepwise reaction between functional groups of monomer. Most step growth polymers are also classified as condensation polymers, but not all step growth polymers (like polyurethanes formed from isocyanate and alcohol bifunctional monomers) release condensates. Step growth polymers increase in molecular weight at a very slow rate at lower conversions and only reach moderately high molecular weights 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 of small molecules during its synthesis, or contains functional groups as part of its backbone chain, or its repeat unit does not contain all the atoms present in the hypothetical monomer to which it can be degraded. 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. ...

Addition polymerization involves the breaking of double or triple bonds, which are used to link monomers into chains. In the polymerization of ethene (fig. 1), its pi bond is broken and these 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 mechanism was one of the first methods to be used. Free radicals are very reactive atoms or molecules which have unpaired electrons. Taking the polymerization of ethene as an example, the free radical mechanism can be divided in to three stages: initiation, propagation and termination. In chemistry free radicals are uncharged atomic or molecular species with unpaired electrons or an otherwise open shell configuration. ...

Initiation is the creation of free radicals necessary for propagation. The radicals can be created from radical initiators, such as organic peroxide molecules, molecules containing an O-O single bond, by reacting oxygen with ethene. The products formed are unstable and easily break down into two radicals. In an ethene monomer, one electron pair is held securely between the two carbons in a sigma bond. The other is more loosely held in a pi bond. The free radical uses one electron from the pi bond to form a more stable bond with the carbon atom. The other electron returns to the second carbon atom, turning the whole molecule in to another radical. Image File history File links File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... In chemistry, radical initiators are substances that can produce radical species under mild conditions and promote radical reactions. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... General Name, Symbol, Number oxygen, O, 8 Chemical series Nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless Atomic mass 15. ... Ethylene or ethene is the simplest alkene hydrocarbon, consisting of two carbon atoms and four hydrogens. ...

Propagation is the rapid reaction of this radicalised ethene molecule with another ethene monomer, and the subsequent repetition to create the repeating chain.

Termination occurs when a radical reacts in a way that prevents further propagation. The most common method of termination is by coupling where two radical species react with each other forming a single molecule. Another, less common method of termination is disproportionation where two radicals meet, but instead of coupling, they exchange a proton, which gives two terminated chains, one saturated and the other with a terminal double bond.

Free radical addition polymerization of ethylene must take place at high temperatures and pressures, approximately 300°C and 2000 At. 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 reagent used in the production of unbranched, stereoregular vinyl polymers. ...

The problem of branching occurs during propagation, when a chain curls back on itself and breaks - leaving irregular chains sprouting from the main carbon backbone. Branching makes the polymers less dense and results in low tensile strength and melting points. Developed by Karl Ziegler and Giulio Natta in the 1950s, Ziegler-Natta catalysts (triethylaluminium in the presence of a metal(IV) chloride) largely solved this problem. Instead of a free radical reaction, the initial ethene monomer inserts between the aluminium atom and one of the ethyl groups in the catalyst. The polymer is then able to grow out from the aluminium atom and results in almost totally unbranched chains. With the new catalysts, the tacticity of the polypropene chain, the alignment of alkyl groups, was also able to be controlled. Different metal chlorides allowed the selective production of each form i.e., syndiotactic, isotactic and atactic polymer chains could be selectively created. Tacticity (from Greek taktikos: of or relating to arrangement or order) is the relative stereochemistry of adjacent chiral centers within a macromolecule. ...

However there were further complications to be solved. If the Ziegler-Natta catalyst was poisoned or damaged then the chain stopped growing. Also, Ziegler-Natta monomers have to be small, and it was still impossible to control the molecular mass of the polymer chains. Again new catalysts, the metallocenes, were developed to tackle these problems. Due to their structure they have less premature chain termination and branching. In chemistry, and in particular, in organometallic chemistry, a metallocene is a compound consisting of an aromatic organic ligand bound to a metal. ...

Other forms of addition polymerization include cationic and anionic polymerization. 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. Cationic and anionic mechanisms are also more ideally suited for living polymerizations, although free radical living polymerizations have also been developed.

History

Polymerization dates back to the beginning of DNA based life, as both DNA and proteins can be considered polymers. The first 'synthetic' polymers of the 19th century were actually formed by modifying natural polymers. For example nitrocellulose was manufactured by reacting cellulose with nitric acid. The first genuinely man-made polymer, the phenol-formaldehyde resin bakelite, was synthesized in 1872. However, research into polymers and polymerization only really accelerated in the 1930s after the serendipitous discovery of polyethene by the chemical company ICI. The general structure of a section of DNA Deoxyribonucleic acid (DNA) is a nucleic acid —usually in the form of a double helix— that contains the genetic instructions specifying the biological development of all cellular forms of life, and most viruses. ... A representation of the 3D structure of myoglobin, showing coloured alpha helices. ... Nitrocellulose (Cellulose nitrate, guncotton) is a highly flammable compound formed by nitrating cellulose (e. ... Cellulose (C6H10O5)n is a long-chain polymeric polysaccharide carbohydrate, of beta-glucose. ... R-phrases , S-phrases , , , , Flash point not applicable RTECS number QU5775000 Supplementary data page Structure and properties n, εr, etc. ... Bakelite is a brand named material based on the thermosetting phenol formaldehyde resin polyoxybenzylmethylenglycolanhydride, developed in 1907-1909 by Dr. Leo Baekeland. ... Polyethylene or polyethene is one of the simplest and most inexpensive polymers. ... Imperial Chemical Industries (ICI) is a British chemical company, based in London. ...

See also

• Plasma polymerization
• Zieglar-Natta catalyst
• Metallocene

Plasma polymerization uses plasma sources to generate a gas discharge that provides energy to activate or fragment gaseous or liquid monomer, often containing a vinyl group, in order to initiate polymerization. ... A Ziegler-Natta catalyst is a catalyst used in the production of unbranched, stereoregular polyalkene polymers. ... In chemistry, and in particular, in organometallic chemistry, a metallocene is a compound consisting of an aromatic organic ligand bound to a metal. ...

References

• ^[1] Introduction to Polymers 1987 R.J. Young Chapman & Hall ISBN 0-412-22170-5