Polymers

Kinetics of copolymerisation and Composition of copolymer. We must know that Where r₁ and r₂ are the reactivity ratio's for the given pair of monomers. Where the above equation is called Copolymerisation equation and it gives the copolymerisation composition without using radical concentration. Types of copolymerisation ( kinetics significance) Let us take the reactivity ratio's product of monomers i.e Product of reactivity ratios. Depending on whether this product is less, greater or equal to unity copolymers are divided into three types. 1)Ideal copolymerisation Here the product of reactivity ratios is equal to unity, i.e r₁r₂=1 2) Alternating copolymerisation Here the product of reactivity ratios is equal to zero, i.e r₁r₂=0. 3)Block copolymerisation Here the product of reactivity ratios is greater than 1, i.e r₁r₂>1.

Polyenes Polyenes are polyunsaturated organic compounds that contain atleast three alternating double and single carbon-carbon bond. For example polyacetylene, β - carotene. Example of polyene; polyacetylene. These carbon - carbon double bond undergoes an interaction known as conjugation which lead to unusual optical properties of polyenes. Some polyenes are bright in colour which is unusual in hydrocarbons. β-carotene is responsible for the orange colour of carrots. Polyenes are more reactive than simple alkenes and they also have high electrical conductivity. Dienes are related to polyenes in which there is two alternating double and single carbon-carbon bonds. Copolymerisation Check out kinetics of copolymerisation also . The polymers containing two or more different monomers are called  Copolymers  and  the process of formation of Copolymers is called copolymerisation. Nylon 6,6 is a copolymer. Let A and B be two monomers. The differen...

Polymerisation process The polymerisation process is classified into homogeneous and heterogeneous polymerisation. In homogeneous polymerisation as name suggest the reactants i.e monomer, initiator, solvents and polymer obtained are miscible and compatible. In heterogeneous polymerisation the monomers, catalyst and polymers obtained are immiscible. Homogeneous polymerisation comprises of bulk/mass polymerisation and solution polymerisation. Heterogeneous polymerisation comprises of bulk, solution, suspension, emulsion, gas phase, precipitation and interfacial polymerisation. Homogeneous polymerisation 1) Bulk/mass polymerisation In case of homogeneous bulk polymerisation the monomer is miscible in polymer obtained. Here the radical initiator is dissolve in pure liquid monomer in the absence of any solvent. The reaction is initiated by exposing the mixture to light or by heating . As reaction proceeds solution mixture becomes more viscous and a polymer is obtained...

Ring opening Polymerisation Ring opening Polymerisation have some features similar with addition and condensation polymerisation mechanism, they show at least one important different features from each of these polymerisation. In ring opening polymerisation the small molecules does not split up, while in condensation polymers it does. This Polymerisation is different from olefins polymerisation , in the sense that the driving force for this polymerisation is not derived from the loss of unsaturation of monomers. This polymerisation generally gives linear compounds. These have generally same composition as that of the monomers. Not all Cyclic compounds undergo ring opening polymerisation. The general ring opening polymerisation is given by equation below. Ring opening polymerisation. Where X is heteroatom or a group. Some common example of ring opening polymerisation reaction. Examples of ring opening polymerisation.

Anionic polymerisation   (Addition polymerisation) Treatment of cold THF (Tetrahydrofuran) solution of styrene with 0.001 equivalent of n butyl Lithium causes an immediate polymerisation. This polymerisation is an example of anionic polymerisation. Chain termination occurs when reacted with carbon dioxide or water or chain transfer seldomly occurs. Only the monomers having anion stabilizing groups such as phenyl, carbonyl and cyano groups act as good substrates for anionic polymerisation. The species initiating anionic polymerisation are alkali metals, alkali amides, alkyl lithiums and other electron sources. Anionic polymerisation of styrene.

Cationic polymerisation   (Addition polymerisation) The polymerisation of isobutylene (2- methyl propene) by traces of strong acid is an example of cationic polymerisation. The chain growth ceases when the terminal carbocation reacts with a nucleophile or losses a proton to give terminal alkenes. Monomers bearing cation stabilizing group such as alkyl shows this type of polymerisation. This process generally occurs at low temperature in methylene chloride solution, where strong acid such as lewis acid containing traces of water acts as intiating reagents. At low temperature there is no chain transfer reaction, thus linear ( not branched) polymers are formed. Mechanism Cationic polymerisation of isobutylene.

Free radical polymerisation   (Addition polymerisation) Alkenes or dienes and their derivatives are polymerized in the presence of catalyst ( free radical generating initiator) like benzoyl peroxide, acetyl peroxide and tert- butyl peroxide etc.  For example the polymerisation of ethene to polyethylene is carried out by exposing to light/ heating a mixture of ethene with benzoyl peroxide. This process generally occur through three steps namely chain initiation, chain propagation and chain termination. Mechanism 1)Initiation step In this step phenyl radical is formed from benzoyl peroxide. The phenyl radical produced is reacted with ethene to give a new and large free radical. Free radical polymerisation of ethene; initiation step. 2)Chain propagation step The new and large radical formed in initiation step is reacted with another ethene to give a new and larger radical. This sequence of reaction proceed forward. Free radical polymerisation of ethene...

Addition (chain reaction) polymerisation In addition polymerisation, the monomers which are unsaturated compounds eg;alkenes, alkadienes and their derivatives combine to form a polymer. Addition polymerisation lead to increase in chain length through formation of free radical or ionic species. Thus it is of type free radical, anionic, cationic and coordination on the basis of reactive centre. But the addition polymerisation with free radical mechanism is most common. The characteristics of chain polymerisation is that only monomer react with the reactive centre to give polymers. Monomer doesn't react with monomer. The different sized species such as dimer, trimer, tetramer,n - mer does not react with each other. Mechanism In chain growth polymerisation an intiator is used to produce an initiator species (R*) with a reactive centre. The reactive centre may be a cation, anion or a free radical. Each monomer added to the reactive centre regenerate a reactive species. Then ...

Carbonyl addition elimination mechanism This is a step growth polymerisation mechanism. This method is important for the preparation of condensation polymers. Here addition and elimination at the carbonyl double bond of carboxylic acids or it's derivative occurs. Step growth polymerisation mechanism. The species in the bracket is at metastable equilibrium and it can either return to it's original state by eliminating Y or it can proceed to its final state by eliminating X. Example of this reaction is interfacial condensation. Interfacial condensation The reaction of acid halide with a glycol or a diamine can be proceed rapidly to high molecular weight polymers, if carried at the interface between two liquids, each containing one of the reactants.  Typically an aqueous phase containing glycol or diamine and an acid acceptor is layered at room temperature over the organic phase containing acid chloride. The polymer formed at the interface can be pulled off as a...

Polycondensation This type of polymerisation generally involves repetitive condensation between two bifunctional monomers. Polycondensation results in loss of simple molecules like water, alcohol etc, and lead to formation of high molecular weight polymers. Here the product formed is also bifunctional species and the sequence of condensation goes on. Each step forms a distinct bifunctional species and each species is independent of each other, thus this process is also called step growth polymerisation. The formation of PET/ Terylene/ Dacron  occurs by this type of polymerisation. Formation of PET Polymerisation mechanism Step growth process proceeds via stepwise reaction between the funtional group of reactants. The size of polymer increases at a relatively slow pace in condensation polymerisation. One proceeds from monomer to dimer, trimer, tetramer, pentamer and so on until eventually a large sized polymer is formed. Polycondensation.

Polysaccharides Saccharides are called carbohydrates. Relatively complex saccharides or carbohydrates are called Polysaccharides. Polysaccharides are polymeric structures in which the repeating units are monosaccharides or dissacharides. Monosaccharides are simple sugar example glucose, fructose, galactose etc. Monosaccharides. Two monosaccharides undergoes condensation reaction with the elimination of simple molecules like water to give dissacharides.  Example Dissacharide. The monosaccharides in Polysaccharides are linked via glycosidic bond in which the oxygen atom is bridge between two carbon rings. A Polysaccharide may be homo-polysaccharide in which all the monosaccharides are identical or hetero- Polysaccharides in which the monosaccharides are different. Polysaccharides may also be linear or branched. Polysaccharide is a source of energy and food, it also provide support to tissues and cells, it sends cellular messages etc. (Check out Nuclei...

Proteins Proteins are linear (not branched or not ring) polymers of amino acids. The twenty genetically encoded amino acids are the molecules which share the centre core:  α   carbon atoms which is bonded to N terminus amino group i.e NH₂ , to a C terminus carboxylic acid group i.e COOH, to a hydrogen atom and a side chain of amino acid group also called the R group as shown in fig. The R group determines the identity of amino acid. Amino acid. In an aqueous solution at physiological pH about 6.8 the amino group exist in protonated form i.e NH₃⁺ and the carboxylic acid group exist in deprotonated form i.e COO⁻, thus forming zwitter ion as shown in fig. Zwitter ion. Most of proteins are made up of amino acid of L- isomers but there are also some proteins made with amino acid of D-isomers. The amino acids in proteins are linked via peptide bonds which is a type of amide bond as shown in fig. Peptide bond. The proteins with less than 20 amino acids ar...

Nucleic acid These are biologically important polymers which are present in all living cells. They direct the synthesis of proteins and are responsible for transfer of genetic information i.e hereditary characteristics. The repeating unit in nucleic acids are called nucleotides. Thus the nucleic acid are basically polynucleotide. Each nucleotide is made up of three components. 1) A nitrogen containing heterocycle base which is of two types namely purines and pyrimidines. a) The two purines are Adenine and Guanine. Adenine. Guanine. b) The three pyrimidines are Cytosine, thymine and uracil. Uracil. Thymine. Cytosine. The two purines i.e Adenine and Guanine and pyrimidine Cytosine is present in both DNA and RNA. But uracil is present in RNA instead of thymine present in DNA. 2) A pentose sugar i.e either ribose or deoxyribose. D-ribose. D-deoxyribose. 3) A phosphate group This group act as linkage between nucleic acid. Phosphori...

Gel permeation chromatography (GPC) Gel permeation chromatography is also called size exclusion chromatography (SEC). In gel permeation chromatography, the solvent is allowed to form two phases, namely stationary phase and mobile phase, in the column packed with microporous gel particle, such that the stationary phase is made up of the part of solvent which is inside the gel while the moving phase is made up of the flowing part of solvent which is outside the gel particle, during the process.     The gel used are hard and incompressible polymer. The gel commonly used are microporous glass bead and microporous polystyrene gel.     A known amount of polymer in known volume of solvent is injected in the solvent stream flowing down the gel packed column. The entry of large size polymer molecules into the gel pores are mostly restricted or completely hindered due to small size of pores and they flow outside the gel beads, thus spend less time in gel and are el...

Determination of molecular weight by Light scattering method. Due to big size macromolecules in solution, turbidity is produced. The light rays get scattered, when pass through macromolecule solution due to turbidity. The turbidity T is given by Where I₀ is the intensity of incident light. Iₜ is the intensity of transmitted light after passing through a solution of length l.        In case of proteins or higher polymers the turbidity is small and is determine from the intensity of light scattered at 90° to the beam. This can be done by using a simple photometer as shown in figure below. Turbidity increase with increase in concentration as well as molecular weight. The Turbidity is related to molecular weight by the equation given by Debye. Hc/T=1/M+2Bc Where B is second virial coefficient and H is a constant. If we plot a graph between Hc/T vs c we get a straight line with intercept 1/M. Thus This curve was plotted by Zimm, so is called Zimm's Cu...

Determination of molecular weight by osmotic pressure method. Osmotic pressure method is also called membrane osmometry. This method is widely used to determine the number average molecular weight of polymers. This method is based on the phenomenon of osmosis. If a pure solvent is separated from a solution through a semipermeable membrane, due to concentration (chemical potential) difference between the solvent and solution, the solvent will flow into the solution through semipermeable membrane. The pressure applied on the solution to completely stop the flow of solvent into it through semipermeable membrane is called osmotic pressure. The theory of osmotic pressure also applies to a solution of polymers. Ordinary solution obeys Van't Hoff equation i.e π=cRT/M Where π is osmotic pressure, c  is concentration in mass per unit volume, R is gas constant, T is temperature and M is molar mass. The polymer solution are non ideal. Taking into account their deviation and us...

Determination of molecular weight of polymers by viscosity method. It is a simple method for determining the molecular weight of polymers. Addition of polymers in the solvent increases the viscosity of the solvent, due to introduction of inhomogeneities by the polymers. If η₀ is the viscosity of the solvent and η is the viscosity of the solution at the same temperature, then the relative viscosity is given by Relative viscosity. Specific viscosity:  It is defined as the relative increase in viscosity and is given as Specific viscosity. Reduced viscosity: It is defined as the relative increase in viscosity per unit concentration (C) of polymer and is given as Reduced viscosity. Intrinsic viscosity: The reduced viscosity is dependent on concentration (C). If a graph is plotted between  reduced viscosity vs concentration then the extraplotation value when C=0 is called intrinsic viscosity. Intrinsic viscosity. Graph of reduced vi...

Molecular weight of polymers The molecular weight of simple molecules are fixed. For example molecular weight of ethylene is 28 gm. The molecular weight of polymers are not fixed. For example in Polyethylene Since the molecular weight of polymers are not fixed they are expressed in terms of average molecular weight. The average molecular weight is expressed as 1) Number average molecular weight. 2) Weight average molecular weight. 3) Z- average molecular weight. 4) Viscosity average molecular weight. 1) Number average molecular weight. Let n₁ be the number of molecules having molecular weight M₁, n₂ be the number of molecules having molecular weight M₂, and so on. The weight of fraction n₁ i.e W₁= n₁M₁ The weight of fraction n₂  i.e W₂ = n₂ M₂ .................................................. Therefore number average molecular weight = Weight of all molecules/ Number of molecules        = n₁M₁ + n₂ M₂+........./ n₁+n₂+.....