Macromolecules

Macromolecules

Macromolecules are high polymeric compound formed by a linkage of monomeric unit in a repeatative manner.In fact,macromolecules are giant polymers and their molecular weight is very high as compared to normal polymer.Polybutadiene,polythene,rubber,polysaccharides,etc. are the examples of macromolecules.Macromolecules can be synthesized naturally as well as artificially.Macromolecules dissolve in particular solvent to give a solution that containsparticles of colloidal dimension.

Determination of molecular weight of macromolecules or polymers

i)Osmotic Pressure Method

The Vant Hoff Equation for a dilute solution is

πV=nRT

or,π=\(\frac{n}{V} \)RT

or,π=\(\frac{w}{M V}\)RT [n=\(\frac{w}{M}\)]

or,π=\(\frac{c}{M}\)RT [c=concentration in gm/litre]

or,\(\frac{π}{c}\)=\(\frac{RT}{M}\)............i)

where Π=osmotic pressure in atmosphere

c=concentration of solution in gm/l

R=gas constant

T=absolute temperature

M=molecular weight

This equation is of the form y=mx.So,if we plot a graph between \(\frac{π}{c}\) versus T,we gwt a straight line passing through the origin and having slope \(\frac{R}{M}\).From slope,we can determine the value of molecular weight of macromoecules or polymers.

ii)Viscosity Measurement Method

When a solute is added to the pure solvent,viscosity of the solution increases.In the same manner,addition of the polymer to the solvent increases its viscosity.The relative viscosity of a solution is denoted by a symbol η_r and calculated by a relation.

η_r=\(\frac{η}{η_0}\)..........i)

where η=viscosity of the solution

η_o=viscosity of pure solvent

The specific viscosity is denoted by a symbol 'η_sp' and calculated by a relation η_sp=η_r-1........ii)

From equations i) and ii),the intrinsic viscosity is defined as,

[η]=\(\frac{η_sp}{c}\)...........iii) [c=concentration of solution]

The intrinsic viscosity is related with the molecular weight of a polymer by a relation

[η]=KM^a........iv)

where K and a are constants

M=molecular weight

iii)Light Scattering Method

The determination of a molecular weight of a polymer in a solution by absorbing the intensity of scattered light depends on the following relation.

\(\frac{HC}{τ}\)=\(\frac{1}{M'}\)+A'C+B'C²+.................i)

where c=concentration of the solution

A',B',........are constant quantities

Τ=\(\frac{1}{l}\)ln\(\{I₀}{I}\) is the turbidity of the solution

l=length of the solution

I_o=intensity of the incident light

I=intensity of the transmitted light

The quantity H=\(\frac{32π^2η_0^2(\(\frac{dn}{dc}^2\)}{3Nλ_0^4}\)is a constant quantity.

where,

η₀=refractive index of solvent

N=Avogadro's number

\(\frac{dn}{dc}\)=variation of refractive index with concentration

λ₀=wavelength of light

When a quantity \(\frac{HC}{Τ}\) is plotted against 'C' we get a straight line having a slope A and intercept \(\frac{1}{M'}\)..From intercept,we can determine the value of M'.

Polymerization

POlymers or macromolecules are long chain,high molecular weight compounds obtained by combination of a large number of simple molecules.The simple molecules taking part in the formation of polymer are called monomers and the process of polymer formation is known as polymerization.

The monomers are the repeating units of polymers.On the basis of type of these monomers taking part in the polymer formation,polymers are categorized into:i)Homopolymer

ii)Copolymer

i)Homopolymer

Polymers which are obtained by the addition of only one type of monomer units are called as homopolymers.For example:polythene,polyvinyl chloride,etc.

ii)Copolymer

The polymers which contain more than one type of monomer in their molecules are called as copolymers.The monomers may be arranged in a definite structural pattern or in a random manner.For example:nylon-6,6,bakelite,etc.

Classification of polymer

i)Addition or chain polymer

ii)Step-growth(condensation polymer)

iii)Thermoplastics and Thermosetting polymer

Addition or chain polymer

The polymer in which a large number of monomers are added together to form a high molecular weight product are called addition or chain polymer and the process of polymer formation is known as addition or chain polymer and the process of polymer formation is known as chain polymerization.Here,a series of rxns exist,each of which consume a reactive particles and produce another similar particles.Thus,each individual reaction depends upon the previous one.In other words,in addition polymerization,the monomers simply add to form a polymer without the elimination of small molecules.The reactive particles can be free radical,cation or anion.Example:

nCH₂=CH₂→—(CH_2—CH_2—)_n

ii)Step growth(condensation) polymerization

The polymers formed by the condensation between the monomer molecules are called as condensation polymers.The condensation may lead in the elimination of smaller molecules like water,alcohol,aammonia,etc.Different atoms or functional groups interact during the chemical reaction.Presence of double or triple bond in the monomers is not essential in the condensation reaction.The process of formation of this polymer is known as condensation polymerization.Example:

HOOC–(CH₂)₄–COOH + H₂N–(CH₂)_6–NH₂→H₂O + HOOC–(CH₂)₄–C–NH–(CH₂)₆–NH₂

adipic acid hexamethylene diammine nylon-6,6

iii)Thermoplastic and thermosetting polymer:

on the basis of action of heat,the polymers cab be classified as thermoplastic and thermosetting plastic.

Thermoplastic:

The polymers which become soft on heating and can be moulded and remoulded into different desired shape are called thermoplastic.In thermoplastics,the intermolecular forces are weak and do not posseses a cross links between different chains.Example:polythene,PVC,etc.

Thermosetting plastic:

The plastics which do not soften on heating and cannot be moulded and remoulded into new shape are called as thermosetting plastics.They possesses strong intermolecular force and possesses a cross links between molecular chains.Example:bakelite,glyptal resins.

Classification of copolymers

Copolymers can be classified as:

i)Black copolymer:

In black copolymer,section made up of one monomer alternate with section of another.Example:

∼M₁M₁M₁M₁≈ M₂M₂M₂M₂≈ M₁M₁M₁M₁— M₂M₂M₂M₂∼

ii)Graft copolymer

Here,chain branches occur specially where branches are different to main chain.

∼M₁M₁M₁M₁M₁M₁M₁M₁M₁∼

↓ ↓

M₂ M₂

M₂ M₂

M₂ M₂

M₂ M₂

M₂ M₂

_{↓ ↓}

iii)Random copolymer

Here,repeatative units are dispersed irregularly along the linear chain.Example:

∼M₁M₁M₂M₂M₁M₂M₁M₂∼

iv)Alternating copolymer:

Here,the monomer units are arranged in the alternate position.Example:

∼M₁M₂M₁M₂M₁M₂∼

References

Paula, P.Atkins & J.de. Elements of Physical Chemistry. New Delhi: Oxford University Press, 2009.