polymerization

Dictionary: po·lym·er·i·za·tion (pə-lĭm'ər-ĭ-zā'shən, pŏl'ə-mər-) pronunciation

The bonding of two or more monomers to form a polymer.
A chemical process that effects this bonding.

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Sci-Tech Encyclopedia: Polymerization

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The linking of small molecules (monomers) to make larger molecules. Polymerization requires that each small molecule have at least two reaction points or functional groups. There are two distinct major types of polymerization processes, condensation polymerization, in which the chain growth is accompanied by elimination of small molecules such as H₂O or CH₃OH, and addition polymerization, in which the polymer is formed without the loss of other materials. There are many variants and subclasses of polymerization reactions.

An example of the condensation process is the reaction (1)
1

of ε-aminocaproic acid in the presence of a catalyst to form the polyamide, nylon-6. The repeating structural unit is equivalent to the starting material minus H and OH, the elements of water. The molecules formed are linear because the total functionality of the reaction system (functional groups per molecule) is always two. However, if a trifunctional material, such as a tricarboxylic acid, were added to the nylon-6,6 polymerizing mixture, a branched polymeric structure would result, because two of the carboxylic groups would participate in one polymer chain, and the third carboxylic group would start the growth of another. Under appropriate conditions, these chains can become bridges between linear chains and the polymer becomes cross-linked. The arrangements of the chains are shown in Fig. 1.

Polymer chains. (<i>a</i>) Linear polymer chain. (<i>b</i>) <ailnk tname= Branched polymer chain. (c) Cross-linked polymer chain.">
Polymer chains. (a) Linear polymer chain. (b) Branched polymer chain. (c) Cross-linked polymer chain.

An example of addition polymerization is reaction (2).
2

The structure of the repeating unit is the difunctional monomeric unit, or “mer.” In the presence of catalysts or initiators, the monomer yields a polymer by the joining together of n mers. If n is a small number, 2–10, the products are dimers, trimers, tetramers, or oligomers, and the materials are usually gases, liquids, oils, or brittle solids. In most solid polymers, n has values ranging from a few score to several hundred thousand, and the corresponding molecular weights range from a few thousand to several million. The end groups of this example of addition polymers are shown to be fragments of the initiator.

If only one monomer is polymerized, the product is called a homopolymer. The polymerization of a mixture of two monomers of suitable reactivity leads to the formation of a copolymer, a polymer in which the two types of mer units have entered the chain in a more or less random fashion. If chains of one homopolymer are chemically joined to chains of another, the product is called a block or graft copolymer:

Isotactic and syndiotactic (stereoregular) polymers are formed in the presence of complex catalysts, or by changing polymerization conditions, for example, by lowering the temperature. The groups attached to the chain in a stereoregular polymer are in a spatially ordered arrangement. The configuration of these ordered polymers and the disordered, atactic form is shown in Fig. 2. The regular structures of the isotactic and syndiotactic forms make them often capable of crystallization. The crystalline melting points of isotactic polymers are often substantially higher than the softening points of the atactic product.

Spatially oriented polymers. (a) Atactic (random; dlldl or lddld, and so on). (b) Syndiotactic (alternating; dldl, and so on). (c) Isotactic (right- or left-handed; dddd, or llll, and so on).

In Fig. 2 each carbon atom to which a phenyl group is attached is asymmetrically substituted. For illustration, the heavily marked bonds are assumed to project up from the paper, and the dotted bonds down. Thus in a fully syndiotactic polymer, asymmetric carbons alternate in their left- or right-handedness (alternating d, l configurations), while in an isotactic polymer, successive carbons have the same steric configuration (d or l).

Among the several kinds of polymerization catalysis, free-radical initiation has been most thoroughly studied and is most widely employed. Atactic polymers are readily formed by free-radical polymerization, at moderate temperatures, of vinyl and diene monomers and some of their derivatives. See also Catalysis; Free radical.

Some polymerizations can be initiated by materials, often called ionic catalysts, that contain highly polar reactive sites or complexes. The term heterogeneous catalyst is often applicable to these materials because many of the catalyst systems are insoluble in monomers and other solvents. These polymerizations are usually carried out in solution from which the polymer can be obtained by evaporation of the solvent or by precipitation on the addition of a nonsolvent. A distinguishing feature of complex catalysts is the ability of some representatives of each type to initiate stereoregular polymerization at ordinary temperatures or to cause the formation of polymers which can be crystallized. See also Chemical dynamics; Heterogeneous catalysis; Inhibitor (chemistry); Inorganic polymer; Organic reaction mechanism; Plastics processing; Polymer.

Dental Dictionary: polymerization

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(pol′i-mər′i-zā′shən)
n

The chaining together of similar molecules to form a compound of high molecular weight.

Britannica Concise Encyclopedia: polymerization

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Any process in which monomers combine chemically to produce a polymer. The monomer molecules — which in the polymer usually number from at least 100 to many thousands — may or may not all be the same. In nature, enzymes carry out polymerization under ordinary conditions to form proteins, nucleic acids, and carbohydrate polymers; in industry, the reaction is usually done with a catalyst, often under high pressure or heat. In addition polymerization, monomers are added successively to the reactive ends of a growing polymer molecule, similar to adding links to a chain; during the reactions, no by-products are formed. In condensation polymerization, growth of the polymer advances stepwise — monomers having reactive functional groups combine into larger molecules with their own functional groups; each reaction splits off a small molecule, often water, as a by-product.

For more information on polymerization, visit Britannica.com.

Architecture: polymerization

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A chemical reaction in which the molecular weight of the molecules formed is a multiple of that of the original substances.

Veterinary Dictionary: polymerization

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The combining of several simpler compounds to form a polymer.

Wikipedia: Polymerization

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An example of alkene polymerization, in which each Styrene monomer unit's double bond reforms as a single bond with another styrene monomer and forms polystyrene.

In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains.^[1]^[2]^[3] There are many forms of polymerization and different systems exist to categorize them.

1 Introduction
2 Step-growth
3 Chain-growth
4 See also
5 References
6 External links

Introduction

Homopolymers $A + A + A... \rightarrow AAA ...$ Co-polymers $A + B + A... \rightarrow ABA ...$

In chemical compounds, polymerization occurs via a variety of reaction mechanisms that vary in complexity due to functional groups present in reacting compounds^[4] and their inherent steric effects explained by VSEPR Theory. In more straightforward polymerization, alkenes, which are relatively stable due to $σ$ bonding 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 and polyvinyl chloride (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 (>-80^oC) 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,^[4] or 4 monomer-unit compounds. Further compounds either being referred to as oligomers^[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 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 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 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 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.

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. 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, polypropylene, and polyvinyl chloride (PVC). A special case of chain-growth polymerization leads to living polymerization.

In the radical polymerization of ethylene, 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 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

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, especially with respect to polymer branching.

Other forms of chain growth polymerization include cationic addition polymerization and anionic addition 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.

References

^ Introduction to Polymers 1987 R.J. Young Chapman & Hall ISBN 0-412-22170-5
^ 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/
^ Clayden, J., Greeves, N. et al. (2000). "Organic chemistry" Oxford
^ ^a ^b ^c ^d ^e ^f ^g Clayden, J., Greeves, N. et al. (2000), p1450-1466
^ J.M.G. Cowie "Polymers: Chemistry and Physics of Modern Materials (Chapman and Hall, 2d ed. 1991) p.4

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