Step-growth polymerization
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Step-growth polymerization refers to a type of
polymerization In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many f ...
mechanism in which bi-functional or multifunctional monomers react to form first dimers, then trimers, longer
oligomer In chemistry and biochemistry, an oligomer () is a molecule that consists of a few repeating units which could be derived, actually or conceptually, from smaller molecules, monomers.Quote: ''Oligomer molecule: A molecule of intermediate relati ...
s and eventually long chain
polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
s. Many naturally occurring and some synthetic polymers are produced by step-growth polymerization, e.g.
polyester Polyester is a category of polymers that contain the ester functional group in every repeat unit of their main chain. As a specific material, it most commonly refers to a type called polyethylene terephthalate (PET). Polyesters include natura ...
s,
polyamide A polyamide is a polymer with repeating units linked by amide bonds. Polyamides occur both naturally and artificially. Examples of naturally occurring polyamides are proteins, such as wool and silk. Artificially made polyamides can be made th ...
s, polyurethanes, etc. Due to the nature of the polymerization mechanism, a high extent of reaction is required to achieve high molecular weight. The easiest way to visualize the mechanism of a step-growth polymerization is a group of people reaching out to hold their hands to form a human chain—each person has two hands (= reactive sites). There also is the possibility to have more than two reactive sites on a monomer: In this case branched polymers production take place. IUPAC deprecates the term step-growth polymerization and recommends use of the terms polyaddition, when the propagation steps are addition reactions and no molecules are evolved during these steps, and polycondensation when the propagation steps are condensation reactions and molecules are evolved during these steps.


Historical aspects

Most natural polymers being employed at early stage of human society are of condensation type. The synthesis of first truly synthetic polymeric material,
bakelite Polyoxybenzylmethylenglycolanhydride, better known as Bakelite ( ), is a thermosetting phenol formaldehyde resin, formed from a condensation reaction of phenol with formaldehyde. The first plastic made from synthetic components, it was developed ...
, was announced by Leo Baekeland in 1907, through a typical step-growth polymerization fashion of
phenol Phenol (also called carbolic acid) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile. The molecule consists of a phenyl group () bonded to a hydroxy group (). Mildly acidic, it r ...
and
formaldehyde Formaldehyde ( , ) ( systematic name methanal) is a naturally occurring organic compound with the formula and structure . The pure compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde (refer to section ...
. The pioneer of synthetic polymer science, Wallace Carothers, developed a new means of making polyesters through step-growth polymerization in 1930s as a research group leader at
DuPont DuPont de Nemours, Inc., commonly shortened to DuPont, is an American multinational chemical company first formed in 1802 by French-American chemist and industrialist Éleuthère Irénée du Pont de Nemours. The company played a major role in ...
. It was the first reaction designed and carried out with the specific purpose of creating high molecular weight polymer molecules, as well as the first polymerization reaction whose results had been predicted by scientific theory. Carothers developed a series of mathematic equations to describe the behavior of step-growth polymerization systems which are still known as the Carothers equations today. Collaborating with Paul Flory, a physical chemist, they developed theories that describe more mathematical aspects of step-growth polymerization including kinetics, stoichiometry, and molecular weight distribution etc. Carothers is also well known for his invention of
Nylon Nylon is a generic designation for a family of synthetic polymers composed of polyamides ( repeating units linked by amide links).The polyamides may be aliphatic or semi-aromatic. Nylon is a silk-like thermoplastic, generally made from pet ...
.


Condensation polymerization

"Step growth polymerization" and condensation polymerization are two different concepts, not always identical. In fact polyurethane polymerizes with addition polymerization (because its polymerization produces no small molecules), but its reaction mechanism corresponds to a step-growth polymerization. The distinction between "addition polymerization" and "condensation polymerization" was introduced by Wallace Carothers in 1929, and refers to the type of products, respectively: * a polymer only (addition) * a polymer and a molecule with a low molecular weight (condensation) The distinction between "step-growth polymerization" and "chain-growth polymerization" was introduced by Paul Flory in 1953, and refers to the reaction mechanisms, respectively: * by functional groups (step-growth polymerization) * by free-radical or ion (chain-growth polymerization)


Differences from chain-growth polymerization

This technique is usually compared with chain-growth polymerization to show its characteristics.


Classes of step-growth polymers

Classes of step-growth polymers are: *
Polyester Polyester is a category of polymers that contain the ester functional group in every repeat unit of their main chain. As a specific material, it most commonly refers to a type called polyethylene terephthalate (PET). Polyesters include natura ...
has high
glass transition The glass–liquid transition, or glass transition, is the gradual and reversible transition in amorphous materials (or in amorphous regions within semicrystalline materials) from a hard and relatively brittle "glassy" state into a viscous or ru ...
temperature Tg and high
melting point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depen ...
Tm, good mechanical properties to about 175 °C, good resistance to solvent and chemicals. It can exist as fibers and films. The former is used in garments, felts, tire cords, etc. The latter appears in magnetic recording tape and high grade films. *
Polyamide A polyamide is a polymer with repeating units linked by amide bonds. Polyamides occur both naturally and artificially. Examples of naturally occurring polyamides are proteins, such as wool and silk. Artificially made polyamides can be made th ...
(
nylon Nylon is a generic designation for a family of synthetic polymers composed of polyamides ( repeating units linked by amide links).The polyamides may be aliphatic or semi-aromatic. Nylon is a silk-like thermoplastic, generally made from pet ...
) has good balance of properties: high strength, good elasticity and abrasion resistance, good toughness, favorable solvent resistance. The applications of polyamide include: rope, belting, fiber cloths, thread, substitute for metal in bearings, jackets on electrical wire. * Polyurethane can exist as elastomers with good abrasion resistance, hardness, good resistance to grease and good elasticity, as fibers with excellent rebound, as coatings with good resistance to solvent attack and abrasion and as foams with good strength, good rebound and high impact strength. *
Polyurea Polyurea is a type of elastomer that is derived from the reaction product of an isocyanate component and a synthetic resin blend component through step-growth polymerization. The isocyanate can be aromatic or aliphatic in nature. It can be m ...
shows high Tg, fair resistance to greases, oils, and solvents. It can be used in truck bed liners, bridge coating, caulk and decorative designs. * Polysiloxane, siloxane-based polymers available in a wide range of physical states—from liquids to greases,
waxes Waxes are a diverse class of organic compounds that are lipophilic, malleable solids near ambient temperatures. They include higher alkanes and lipids, typically with melting points above about 40 °C (104 °F), melting to give low ...
, resins, and rubbers. Due to perfect thermal stability (thanks to
silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ...
, Si) uses of this material include antifoam and release agents, gaskets, seals, cable and wire insulation, hot liquids and gas conduits, etc. *
Polycarbonates Polycarbonates (PC) are a group of thermoplastic polymers containing carbonate groups in their chemical structures. Polycarbonates used in engineering are strong, tough materials, and some grades are optically transparent. They are easily work ...
are transparent, self-extinguishing materials. They possess properties like crystalline
thermoplasticity A thermoplastic, or thermosoft plastic, is any plastic polymer material that becomes pliable or moldable at a certain elevated temperature and solidifies upon cooling. Most thermoplastics have a high molecular weight. The polymer chains associate ...
, high impact strength, good thermal and oxidative stability. They can be used in machinery, auto-industry, and medical applications. For example, the
cockpit canopy A vehicle canopy is a rarely used type of door for cars. It has no official name so it is also known as an articulated canopy, bubble canopy,
of F-22 Raptor is made of high optical quality polycarbonate. * Polysulfides have outstanding oil and solvent resistance, good gas impermeability, good resistance to aging and
ozone Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the l ...
. However, it smells bad, and it shows low tensile strength as well as poor heat resistance. It can be used in gasoline hoses, gaskets and places that require solvent resistance and gas resistance. *
Polyether In organic chemistry, ethers are a class of compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula , where R and R′ represent the alkyl or aryl groups. Ethers can again be ...
shows good thermoplastic behavior, water solubility, generally good mechanical properties, moderate strength and stiffness. It is applied in sizing for cotton and synthetic fibers, stabilizers for adhesives, binders, and film formers in pharmaceuticals. * Phenol formaldehyde resin (
bakelite Polyoxybenzylmethylenglycolanhydride, better known as Bakelite ( ), is a thermosetting phenol formaldehyde resin, formed from a condensation reaction of phenol with formaldehyde. The first plastic made from synthetic components, it was developed ...
) have good heat resistance, dimensional stability as well as good resistance to most solvents. It also shows good
dielectric In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the ma ...
properties. This material is typically used in molding applications, electrical, radio, televisions and automotive parts where their good dielectric properties are of use. Some other uses include: impregnating paper, varnishes, decorative laminates for wall coverings. * Poly triazole polymers are produced from monomers which bear both an alkyne and
azide In chemistry, azide is a linear, polyatomic anion with the formula and structure . It is the conjugate base of hydrazoic acid . Organic azides are organic compounds with the formula , containing the azide functional group. The dominant appli ...
functional group. The monomer units are linked to each other by the a 1,2,3-triazole group; which is produced by the 1,3-dipolar cycloaddition, also called the
azide-alkyne Huisgen cycloaddition The azide-alkyne Huisgen cycloaddition is a 1,3-dipolar cycloaddition between an azide and a terminal or internal alkyne to give a 1,2,3-triazole. Rolf Huisgen was the first to understand the scope of this organic reaction. American chemist Karl ...
. These polymers can take on the form of a strong resin, or a gel. With oligopeptide monomers containing a terminal alkyne and terminal azide the resulting clicked peptide polymer will be biodegradable due to action of endopeptidases on the oligopeptide unit.


Branched polymers

A monomer with functionality of 3 or more will introduce branching in a polymer and will ultimately form a cross-linked macrostructure or network even at low fractional conversion. The point at which a tree-like topology transits to a network is known as the
gel point In polymer chemistry, the gel point is an abrupt change in the viscosity of a solution containing polymerizable components. At the gel point, a solution undergoes gelation as reflected in a loss in fluidity. Gelation is characteristic of polymeri ...
because it is signalled by an abrupt change in
viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the int ...
. One of the earliest so-called thermosets is known as
bakelite Polyoxybenzylmethylenglycolanhydride, better known as Bakelite ( ), is a thermosetting phenol formaldehyde resin, formed from a condensation reaction of phenol with formaldehyde. The first plastic made from synthetic components, it was developed ...
. It is not always water that is released in step-growth polymerization: in acyclic diene metathesis or ADMET dienes polymerize with loss of ethene.


Kinetics

The kinetics and rates of step-growth polymerization can be described using a polyesterification mechanism. The simple esterification is an acid-catalyzed process in which protonation of the acid is followed by interaction with the alcohol to produce an ester and water. However, there are a few assumptions needed with this kinetic model. The first assumption is water (or any other condensation product) is efficiently removed. Secondly, the functional group reactivities are independent of chain length. Finally, it is assumed that each step only involves one alcohol and one acid. :\frac=1+(n-1)kt ce This is a general rate law degree of polymerization for polyesterification where n= reaction order.


Self-catalyzed Polyesterification

If no acid catalyst is added, the reaction will still proceed because the acid can act as its own catalyst. The rate of condensation at any time t can then be derived from the rate of disappearance of -COOH groups and :rate=\frac=k ce2 ce/math> The second-order ce/math> term arises from its use as a catalyst, and k is the rate constant. For a system with equivalent quantities of acid and glycol, the functional group concentration can be written simply as :rate=\frac=k ce3 After integration and substitution from Carothers equation, the final form is the following :\frac=2kt ce2+1=X^2_n For a self-catalyzed system, the number average degree of polymerization (Xn) grows proportionally with \sqrt.


External catalyzed Polyesterification

The uncatalyzed reaction is rather slow, and a high Xn is not readily attained. In the presence of a catalyst, there is an acceleration of the rate, and the kinetic expression is altered to :\frac=k cece/math> which is kinetically first order in each functional group. Hence, :\frac=k ce2 and integration gives finally :\frac=1+ cet=X_n For an externally catalyzed system, the number average degree of polymerization grows proportionally with t\,.


Molecular weight distribution in linear polymerization

The product of a polymerization is a mixture of polymer molecules of different molecular weights. For theoretical and practical reasons it is of interest to discuss the distribution of molecular weights in a polymerization. The molecular weight distribution (MWD) had been derived by Flory by a statistical approach based on the concept of equal reactivity of functional groups.


Probability

Step-growth polymerization is a random process so we can use statistics to calculate the probability of finding a chain with x-structural units ("x-mer") as a function of time or conversion. : + \mathitBB -> AA-(BB-AA)_-BB :\mathitAB -> A-(B-A)_-B Probability that an 'A' functional group has reacted :p^ \, Probability of finding an 'A' unreacted :(1-p) \, Combining the above two equations leads to. :P_x=(1-p)p^ \, Where Px is the probability of finding a chain that is x-units long and has an unreacted 'A'. As x increases the probability decreases.


Number fraction distribution

The number fraction distribution is the fraction of x-mers in any system and equals the probability of finding it in solution. :\frac=(1-p)p^ \, Where N is the total number of polymer molecules present in the reaction.


Weight fraction distribution

The weight fraction distribution is the fraction of x-mers in a system and the probability of finding them in terms of mass fraction. :\frac=\frac=\frac=x\frac\frac Notes: :* ''Mo'' is the molar mass of the repeat unit, :* ''No'' is the initial number of monomer molecules, :* and ''N'' is the number of unreacted functional groups Substituting from the Carothers equation :X_n=\frac=\frac We can now obtain: :\frac=x(1-p)^2p^ \,


PDI

The polydispersity index (PDI), is a measure of the distribution of molecular mass in a given polymer sample. :PDI=\frac However, for step-growth polymerization the Carothers equation can be used to substitute and rearrange this formula into the following. :PDI=1+p \, Therefore, in step-growth when p=1, then the PDI=2.


Molecular weight control in linear polymerization


Need for stoichiometric control

There are two important aspects with regard to the control of molecular weight in polymerization. In the synthesis of polymers, one is usually interested in obtaining a product of very specific molecular weight, since the properties of the polymer will usually be highly dependent on molecular weight. Molecular weights higher or lower than the desired weight are equally undesirable. Since the degree of polymerization is a function of reaction time, the desired molecular weight can be obtained by quenching the reaction at the appropriate time. However, the polymer obtained in this manner is unstable in that it leads to changes in molecular weight because the ends of the polymer molecule contain functional groups that can react further with each other. This situation is avoided by adjusting the concentrations of the two monomers so that they are slightly nonstoichiometric. One of the reactants is present in slight excess. The polymerization then proceeds to a point at which one reactant is completely used up and all the chain ends possess the same functional group of the group that is in excess. Further polymerization is not possible, and the polymer is stable to subsequent molecular weight changes. Another method of achieving the desired molecular weight is by addition of a small amount of monofunctional monomer, a monomer with only one functional group. The monofunctional monomer, often referred to as a chain stopper, controls and limits the polymerization of bifunctional monomers because the growing polymer yields chain ends devoid of functional groups and therefore incapable of further reaction.


Quantitative aspects

To properly control the polymer molecular weight, the stoichiometric imbalance of the bifunctional monomer or the monofunctional monomer must be precisely adjusted. If the nonstoichiometric imbalance is too large, the polymer molecular weight will be too low. It is important to understand the quantitative effect of the stoichiometric imbalance of reactants on the molecular weight. Also, this is necessary in order to know the quantitative effect of any reactive impurities that may be present in the reaction mixture either initially or that are formed by undesirable side reactions. Impurities with A or B functional groups may drastically lower the polymer molecular weight unless their presence is quantitatively taken into account. More usefully, a precisely controlled stoichiometric imbalance of the reactants in the mixture can provide the desired result. For example, an excess of diamine over an acid chloride would eventually produce a polyamide with two amine end groups incapable of further growth when the acid chloride was totally consumed. This can be expressed in an extension of the Carothers equation as, :X_n=\frac where r is the ratio of the number of molecules of the reactants. :r=\frac were NBB is the molecule in excess. The equation above can also be used for a monofunctional additive which is the following, :r=\frac where NB is the number of monofunction molecules added. The coefficient of 2 in front of NB is require since one B molecule has the same quantitative effect as one excess B-B molecule.


Multi-chain Polymerization

A monomer with functionality 3 has 3 functional groups which participate in the polymerization. This will introduce branching in a polymer and may ultimately form a cross-linked macrostructure. The point at which this three-dimensional 3D network is formed is known as the
gel point In polymer chemistry, the gel point is an abrupt change in the viscosity of a solution containing polymerizable components. At the gel point, a solution undergoes gelation as reflected in a loss in fluidity. Gelation is characteristic of polymeri ...
, signaled by an abrupt change in viscosity. A more general functionality factor fav is defined for multi-chain polymerization, as the average number of functional groups present per monomer unit. For a system containing N0 molecules initially and equivalent numbers of two function groups A and B, the total number of functional groups is N0fav. :f_ = \frac And the modified Carothers equation is :x_ = \frac, where p equals to \frac


Advances in step-growth polymers

The driving force in designing new polymers is the prospect of replacing other materials of construction, especially metals, by using lightweight and heat-resistant polymers. The advantages of lightweight polymers include: high strength, solvent and chemical resistance, contributing to a variety of potential uses, such as electrical and engine parts on automotive and aircraft components, coatings on cookware, coating and circuit boards for electronic and microelectronic devices, etc. Polymer chains based on aromatic rings are desirable due to high bond strengths and rigid polymer chains. High molecular weight and crosslinking are desirable for the same reason. Strong dipole-dipole, hydrogen bond interactions and
crystallinity Crystallinity refers to the degree of structural order in a solid. In a crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic stru ...
also improve heat resistance. To obtain desired mechanical strength, sufficiently high molecular weights are necessary, however, decreased solubility is a problem. One approach to solve this problem is to introduce of some flexibilizing linkages, such as isopropylidene, C=O, and into the rigid polymer chain by using an appropriate monomer or comonomer. Another approach involves the synthesis of reactive telechelic oligomers containing functional end groups capable of reacting with each other, polymerization of the oligomer gives higher molecular weight, referred to as chain extension.


Aromatic polyether

The oxidative coupling polymerization of many 2,6-disubstituted phenols using a catalytic complex of a cuprous salt and
amine In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituent ...
form aromatic polyethers, commercially referred to as poly(p-phenylene oxide) or PPO. Neat PPO has little commercial uses due to its high melt viscosity. Its available products are blends of PPO with high-impact
polystyrene Polystyrene (PS) is a synthetic polymer made from monomers of the Aromatic hydrocarbon, aromatic hydrocarbon styrene. Polystyrene can be solid or foamed. General-purpose polystyrene is clear, hard, and brittle. It is an inexpensive resin pe ...
(HIPS).


Polyethersulfone

Polyethersulfone (PES) is also referred to as polyetherketone,
polysulfone Polysulfones are a family of high performance thermoplastics. These polymers are known for their toughness and stability at high temperatures. Technically used polysulfones contain an aryl- SO2-aryl subunit. Due to the high cost of raw materi ...
. It is synthesized by nucleophilic aromatic substitution between aromatic dihalides and bisphenolate salts. Polyethersulfones are partially crystalline, highly resistant to a wide range of aqueous and organic environment. They are rated for continuous service at temperatures of 240-280 °C. The polyketones are finding applications in areas like automotive, aerospace, electrical-electronic cable insulation.


Aromatic polysulfides

Poly(p-phenylene sulfide) (PPS) is synthesized by the reaction of
sodium sulfide Sodium sulfide is a chemical compound with the formula Na2 S, or more commonly its hydrate Na2S·9 H2O. Both the anhydrous and the hydrated salts in pure crystalline form are colorless solids, although technical grades of sodium sulfide are gene ...
with p-dichlorobenzene in a polar solvent such as 1-methyl-2-pyrrolidinone (NMP). It is inherently flame-resistant and stable toward organic and aqueous conditions; however, it is somewhat susceptible to oxidants. Applications of PPS include automotive, microwave oven component, coating for cookware when blend with fluorocarbon polymers and protective coatings for valves, pipes, electromotive cells, etc.


Aromatic polyimide

Aromatic polyimides are synthesized by the reaction of dianhydrides with diamines, for example, pyromellitic anhydride with ''p''-phenylenediamine. It can also be accomplished using diisocyanates in place of diamines. Solubility considerations sometimes suggest use of the half acid-half ester of the dianhydride, instead of the dianhydride itself. Polymerization is accomplished by a two-stage process due to the insolubility of polyimides. The first stage forms a soluble and fusible high-molecular-weight poly(amic acid) in a polar aprotic solvent such as NMP or
N,N-dimethylacetamide Dimethylacetamide (DMAc or DMA) is the organic compound with the formula CH3C(O)N(CH3)2. This colorless, water-miscible, high-boiling liquid is commonly used as a polar solvent in organic synthesis. DMA is miscible with most other solvents, al ...
. The poly(amic aicd) can then be processed into the desired physical form of the final polymer product (e.g., film, fiber, laminate, coating) which is insoluble and infusible.


Telechelic oligomer approach

Telechelic A telechelic polymer or oligomer is a prepolymer capable of entering into further polymerization or other reactions through its reactive end-groups. It can be used for example to synthesize block copolymers. By definition, a telechelic polymer is a ...
oligomer In chemistry and biochemistry, an oligomer () is a molecule that consists of a few repeating units which could be derived, actually or conceptually, from smaller molecules, monomers.Quote: ''Oligomer molecule: A molecule of intermediate relati ...
approach applies the usual polymerization manner except that one includes a monofunctional reactant to stop reaction at the oligomer stage, generally in the 50-3000 molecular weight. The monofunctional reactant not only limits polymerization but end-caps the oligomer with functional groups capable of subsequent reaction to achieve curing of the oligomer. Functional groups like alkyne, norbornene, maleimide,
nitrite The nitrite ion has the chemical formula . Nitrite (mostly sodium nitrite) is widely used throughout chemical and pharmaceutical industries. The nitrite anion is a pervasive intermediate in the nitrogen cycle in nature. The name nitrite also ...
, and cyanate have been used for this purpose. Maleimide and norbornene end-capped oligomers can be cured by heating. Alkyne, nitrile, and cyanate end-capped oligomers can undergo
cyclotrimerization In organic chemistry, an alkyne trimerisation is a +2+2nbsp; cycloaddition reaction in which three alkyne units () react to form a benzene ring. The reaction requires a metal catalyst. The process is of historic interest as well as being applica ...
yielding aromatic structures.


See also

* Conducting polymer *
Fire-safe polymers Fire-safe polymers are polymers that are resistant to degradation at high temperatures. There is need for fire-resistant polymers in the construction of small, enclosed spaces such as skyscrapers, boats, and airplane cabins.Zhang, H. ''Fire-Safe P ...
* Liquid crystal polymer * Random graph theory of gelation * Thermosetting plastic


References


External links


Crosslinking
{{DEFAULTSORT:Step-Growth Polymerization Polymerization reactions