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Step-growth polymerization

 

 
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Step-growth polymerization
 
 
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Step-growth polymerization is a polymerization
Polymerization

In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains....
 process that involves a chemical reaction
Chemical reaction

A chemical reaction is a process that always results in the interconversion of chemical substances. The substance or substances initially involved in a chemical reaction are called reactants....
 between multifunctional monomer
Monomer

A monomer is a small molecule that may become Chemistry chemical bonding to other monomers to form a polymer....
 molecules. In a step-growth reaction, the growing chains may react with each other to form even longer chains. This applies to chains of all lengths. Thus, a monomer or dimer may react in just the same way as a chain hundreds of monomer units long. This is in contrast to a chain-growth polymerization, where only monomers may react with growing chains (In chain-growth polymerization, two growing chains can't join together the way they can in a step-growth polymerization).

The most common class of step-growth polymerization is called condensation polymerization and the product a condensation polymer
Condensation polymer

Condensation polymers are any class of polymers formed through a condensation reaction, releasing a small molecule by-product such as water or methanol, as opposed to addition polymers which involve the reaction of unsaturated monomers....
, because the chemical reaction
Chemical reaction

A chemical reaction is a process that always results in the interconversion of chemical substances. The substance or substances initially involved in a chemical reaction are called reactants....
 by which the monomer molecules bond is often a condensation reaction
Condensation reaction

A condensation reaction is a chemical reaction in which two molecules or moieties combine to form one single molecule, together with the loss of a small molecule....
 that produces a small molecule byproduct. A multifunctional monomer is a molecule that has more than one potential reactive site by which it can form intermolecular chemical bond
Chemical bond

A chemical bond is the physical process responsible for the attractive interactions between atoms and molecules, and that which confers stability to diatomic and polyatomic chemical compounds....
s.






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Encyclopedia


Step-growth polymerization is a polymerization
Polymerization

In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains....
 process that involves a chemical reaction
Chemical reaction

A chemical reaction is a process that always results in the interconversion of chemical substances. The substance or substances initially involved in a chemical reaction are called reactants....
 between multifunctional monomer
Monomer

A monomer is a small molecule that may become Chemistry chemical bonding to other monomers to form a polymer....
 molecules. In a step-growth reaction, the growing chains may react with each other to form even longer chains. This applies to chains of all lengths. Thus, a monomer or dimer may react in just the same way as a chain hundreds of monomer units long. This is in contrast to a chain-growth polymerization, where only monomers may react with growing chains (In chain-growth polymerization, two growing chains can't join together the way they can in a step-growth polymerization).

The most common class of step-growth polymerization is called condensation polymerization and the product a condensation polymer
Condensation polymer

Condensation polymers are any class of polymers formed through a condensation reaction, releasing a small molecule by-product such as water or methanol, as opposed to addition polymers which involve the reaction of unsaturated monomers....
, because the chemical reaction
Chemical reaction

A chemical reaction is a process that always results in the interconversion of chemical substances. The substance or substances initially involved in a chemical reaction are called reactants....
 by which the monomer molecules bond is often a condensation reaction
Condensation reaction

A condensation reaction is a chemical reaction in which two molecules or moieties combine to form one single molecule, together with the loss of a small molecule....
 that produces a small molecule byproduct. A multifunctional monomer is a molecule that has more than one potential reactive site by which it can form intermolecular chemical bond
Chemical bond

A chemical bond is the physical process responsible for the attractive interactions between atoms and molecules, and that which confers stability to diatomic and polyatomic chemical compounds....
s. The easiest way to visualize a step growth polymerization is a group of people holding hands to form a human chain: each person has two hands (=reactive sites).

A pioneer in step-growth polymerisation is Wallace Carothers
Wallace Carothers

Wallace Hume Carothers was an United States chemist, inventor and the leader of organic chemistry at DuPont, credited with the invention of Nylon....
 who invented nylon
Nylon

Nylon is a generic designation for a family of synthetic polymers known generically as polyamides and first produced on February 28, 1935 by Wallace Carothers at DuPont....
, a condensation product of hexamethylene diamine and adipic acid
Adipic acid

Adipic acid is the organic compound with the chemical formula 42. From the industrial perspective, it is the most important dicarboxylic acid: About 2.5 billion kilograms of this white crystalline powder are produced annually, mainly as a precursor for the production of nylon....
. Each monomer has two functional groups (two amino groups or two carboxyl group
Carboxyl group

A carboxyl group is a set of four atoms bonded together and present in carboxylic acids, including amino acid. Usually abbreviated as either CO2H or COOH, this set of atoms constitutes a functional group....
) and so each monomer can form an amide
Amide

In chemistry, an amide is one of three kinds of compounds:* the organic chemistry functional group characterized by a carbonyl group linked to a nitrogen atom , or a compound that contains this functional group ; or...
 link with each of its neighbour.

The functionality of a monomer is the number of reactive sites. A functionality of 2 will yield a linear polymer. Examples:
  • hexamethylene diamine and adipic acid
    Adipic acid

    Adipic acid is the organic compound with the chemical formula 42. From the industrial perspective, it is the most important dicarboxylic acid: About 2.5 billion kilograms of this white crystalline powder are produced annually, mainly as a precursor for the production of nylon....
     create nylon
    Nylon

    Nylon is a generic designation for a family of synthetic polymers known generically as polyamides and first produced on February 28, 1935 by Wallace Carothers at DuPont....
  • terephthalic acid
    Terephthalic acid

    Terephthalic acid is the organic compound and one of three isomeric phthalic acids, all with chemical formula C6H42....
     and ethylene glycol
    Ethylene glycol

    Ethylene glycol is an alcohol with two -OH groups , a chemical compound widely used as an automobile antifreeze. In its pure form, it is an odorless, colorless, syrupy, sweet tasting, toxic liquid....
     create PET
    Polyethylene terephthalate

    Polyethylene tephthalate , commonly abbreviated PET, PETE, or the obsolete PETP or PET-P), is a thermoplastic polymer resin of the polyester family and is used in synthetic fibers; beverage, food and other liquid Packaging; thermoforming applications; and engineering resins often in combination with glass fiber....


In theory the polymerisation will continue to result in a single macromolecule
Macromolecule

The term macromolecule by definition implies "large molecule". In the context of biochemistry, the term may be applied to the four conventional biopolymers , as well as non-polymeric molecules with large molecular mass such as macrocycles....
. The relationship between the extent of the reaction and the average number of monomer units in a polymer chain is given by the Carothers equation. High molecular weight polymer is formed only at high degrees of conversion (extent of reaction). In practise the average length of the polymer chain is limited by such things as the purity of the reactants and the viscosity
Viscosity

Viscosity is a measure of the Drag of a fluid which is being deformed by either shear stress or extensional stress. In everyday terms , viscosity is "thickness"....
 of the medium.

A monomer with functionality 3 will introduce branching in a polymer and will ultimately form a cross-link
Cross-link

Cross-links are bonds that link one polymer chain to another. They can be covalent bonds or ionic bonds. "Polymer chains" can refer to synthetic polymers or natural polymers ....
ed macrostructure. The point at which this three-dimensional structure is formed is known as the gel point
Gel point

The Gel point in polymer chemistry is the point at which an infinite polymer network first appears. Assuming that it is possible to measure the extent of reaction, p, defined as the fraction of monomers that appear in cross-links, the gel point can be determined....
 because it is signalled by an abrupt change in viscosity
Viscosity

Viscosity is a measure of the Drag of a fluid which is being deformed by either shear stress or extensional stress. In everyday terms , viscosity is "thickness"....
. One of the earliest so-called thermosets is known as bakelite
Bakelite

Bakelite is a material based on the thermosetting plastic phenol formaldehyde resin polyoxybenzylmethylenglycolanhydride, developed in 1907?1909 by Demographics of Belgium Dr....
.

It is not always water that is released in step-growth polymerization: in acyclic diene metathesis
Acyclic diene metathesis

Acyclic diene metathesis or ADMET is a special type of olefin metathesis used to polymerize certain terminal dienes to polyenes:The new double bonds formed can be in cis isomer....
 or ADMET dienes polymerize with loss of ethylene
Ethylene

Ethylene is the chemical compound with the formula C2H4. It is the simplest alkene. Because it contains a carbon-carbon double bond, ethylene is called an unsaturated hydrocarbon or an olefin....
.