Polymers.pdf

POLYMERS

POLYMERS:

 Polymers are compounds of very high molecular masses formed by the combination of a large number of simple molecules through chemical

bonds.

Ethane (monomer) n(CH2=CH2)

-(-CH₂-CH₂-)_n-

Polyethane (polymer)

Due to their large size they are also sometimes called macromolecules.

Small molecules which combine with each other to form polymer molecules are known as monomers.

Monomer _Polymer

HOCH₂CH₂OH

HO CO₂H

CH₂CH₂

CH₂CH₂O

CH₂CH₂O

O C

O

CH₂ CH₂

CH₂ CHCl _CH

2CH2

Cl

Polymers may be classified on the basis of structure:

Linear chain polymers:

These are polymers in which monomeric units are linked together to form linear chains.

They possess high densities, high tensile strength and high melting point.

 E.g., polyethylene, nylons and polyesters.

Branched chain polymers:

These include polymers in which the monomer units are joined to form long chains with side chains or branches of different lengths.

They have low tensile strength and low melting point.

Crosslinked polymer:

These are polymers in which monomer units are crosslinked together to form a three dimensional network.

These are hard, brittle and rigid.

E.g., melamine, bakelite etc.

(a) linear (b) branch

Polymers may be classified on the basis of origin:

Biopolymers/Natural polymers:

The polymers obtained from nature are called natural polymer (biopolymers).

E.g., starch, cellulose, protein, natural rubber etc.

Synthetic polymers:

These include polymers that have been synthesized from low molecular mass starting materials.

They are also classified on the basis of their functional characteristics (molecular forces) and end use applications, into three groups namely,

(i) PLASTICS:

Plastic can be broadly classified as:

Thermoplastics:

These have either linear or branched structure.

They can be amorphous or semi crystalline materials.

Neighbouring polymeric chains are held together by weak van der waals’ forces.

On heating, they soften very readily but on cooling they stiffen again.

 They can be remoulded, reshaped and reused. Hence, they can be recycled.

Thermosetting plastics:

They have three dimensional, cross-linked networked structure.

Neighbouring polymeric chains in thermosets are held together by crosslinks (strong covalent bonds).

 Heating does not soften them, since softening would require breaking of covalent bonds.

APPLICATIONS:

(ii) FIBERS:

These are polymers which have strong inter-molecular forces (dipole-dipole forces or hydrogen bonding) between the chains and exhibit high tensile strength and sharp melting point.

Various intermolecular forces hold the linear chains together resulting in their regular alignment.

The different types of fibres may include:

Natural fibres: Natural fibres are of either plant origin (e.g., cotton and jute) or animal origin (e.g., silk and wool).

(iii) ELASTOMERS:

These polymers have rubber-like or elastic properties capable of undergoing reversible deformation and elongation.

The polymeric chains in elastomers are held together by weak

intermolecular forces (besides occasional crosslinks) so that the original conformation is recovered easily on being deformed.

E.g., natural rubber, synthetic rubbers, etc.

(A) is an unstressed polymer; (B) is the same polymer under stress. When the stress is

Classification on the basis of tacticity (spatial arrangement):

Isotactic polymers: Polymers in which all the asymmetric carbon atoms have the same (d or l) configuration.

Atactic polymers: Polymers having random sequences of d- or l- configurations are termed as atactic polymers.

Classification on the basis of mode of synthesis:

Addition polymers (Chain growth) :

A polymer formed by direct addition of monomers without the elimination of any by product molecules is called addition polymers.

The monomers are unsaturated. The polymers bear the same empirical formula as their monomers.

E.g., polyethene and polypropylene, Polyvinyl chloride, Teflon (CF₂-CF₂) etc.

(i) Free radical

(ii) Ionic polymerization

Free Radical Polymerization

•

Usually, many low molecular weight alkenes undergo

rapid polymerization reactions when treated with small

amounts of a radical initiator.

200 °C

2000 atm

O

peroxides

polyethylene

H

C CH

CH

CH

CH

CH

CH

CH

CH

•

..

RO

_..

H

C

CH

CH

H

C

CH

CH

•

..

H

C

CH

CH

•

..

RO

:

CH

CH

H

C

H

C

CH

CH

H

C

CH

CH

•

..

H

C

CH

CH

H

C

CH

CH

•

..

RO

:

CH

CH

H

C

H

C

CH

CH

H

C

CH

CH

•

H

C

CH

CH

..

H

C

CH

CH

H

C

CH

CH

•

H

C

CH

CH

..

RO

:

CH

CH

H

C

Likewise...

H

₂

C=CHCl

polyvinyl chloride

H

₂

C=CHC

₆

H

₅

polystyrene

Ionic Polymerization

•

Whereas free radical polymerization is non-specific, the type of

ionic polymerization procedure and catalysts depend on the

nature of the substituent (R) on the vinyl monomer

•

Anionic

initiation, requires the R group to be

electron

withdrawing

in order to promote the formation of a stable

carbanion (ie, -M and -I effects help stabilise the negative

charge).

•

Cationic

initiation is therefore usually limited to the

polymerization of monomers where the R group is

electron-donating

.This helps stabilise the delocation of the positive

Anionic Polymerization

•

Involves the polymerization of monomers that have strong

electron-withdrawing groups, eg, acrylonitrile, vinyl chloride,

methyl methacrylate, styrene etc.

Cationic Polymerization

(iii) Termination

Termination of cationic polymerization reactions are less

well-defined than in free-radical processes. Two

Types of Addition Polymerizations

Ph

Anionic

C₃H₇ Li C4H9

Ph Li+

Ph n

C₄H₉

Ph Ph Li+

n

Ph

Radical

PhCO₂• Ph

n

Ph

Cationic

Cl₃Al OH₂

H

Ph

Ph n

H

Phn Ph

HOAlCl₃

PhCO₂

Ph

PhCO₂

Condensation polymers:

Polymers formed by the condensation of two or more than two monomers with the elimination of simple molecules like water, ammonia, hydrogen chloride, alcohol etc., are called condensation polymers.

Each monomer generally possess two functional groups.

Condensation or Step-growth Polymerization

Step-polymers are made by allowing difunctional monomers with complemen tary functi onal groups to react w ith one another

Condensation between two molecules

C C

O O

OCH₂CH₂O

n O O OMe MeO O H OH + Poly(ethylene terephthalate) terephthalic acid ethylene glycol

PET

This is an example of a poly(ester)

The reaction is a transesterification _{Recyclable plastic}

Step-growth Polymerization These are poly(amides) s t o c k i n g s , r o p e , t i r e s , c a r p e t f i b r e – bristles of toothbrishes,

260-280 °C 250 psi

- H₂O

MW = 10,000, m.pt. 250 °C, fibres stretched (to increase strength) to 4 times their length

Ziegler-Natta Addition Polymerization or Coordination

polymerization

R

Cl R

Cl₃Ti R Cl AlR₂

Cl₃Ti R Cl₃Ti R

R Cl₃Ti

R Cl₃Ti

Cl₃Ti

R

Cl₃Ti

R

Cl₃Ti

R TiCl₄ / AlR₃

1-4 atm, rt

n

TiCl₄ + AlR₃ Cl₃Ti

AlR₂ +

s-complex

Ziegler-Natta Chain (Addition) Polymerization

Termination reaction

Cl₃Ti

H H

Ti Cl

Cl Cl H

Ziegler-Natta Catalysts

A typical Ziegler-Natta catalyst is a

combination of TiCl

and (CH

CH

)

AlCl, or

TiCl

and (CH

CH

)

Al.

Mechanism of Coordination Polymerization

Al

(CH

CH

)

+

Ti

Cl

_ClAl

_(CH

_CH

₎

+

Mechanism of Coordination Polymerization

H

C CH

Al

(CH

CH

)

+

Ti

Cl

_ClAl

_(CH

_CH

₎

+

CH

CH

Ti

Cl

CH

CH

Ti

Cl

+

CH

CH

Ti

Cl

Mechanism of Coordination Polymerization

CH

CH

Ti

Cl

Mechanism of Coordination Polymerization

CH

CH

Ti

Cl

H

C CH

Ti

Cl

Mechanism of Coordination Polymerization

H

C CH

Ti

Cl

CH

CH

CH

CH

Ti

Cl

CH

CH

CH

CH

Mechanism of Coordination Polymerization

Ti

Cl

CH

CH

CH

CH

H

C CH

Ti

Cl

Mechanism of Coordination Polymerization

Ti

Cl

CH

CH

CH

CH

CH

CH

H

C CH

Monomer Polymer

CO₂H HO₂C

HO OH

O O

HO O HC2 HC O2

n Terephthalic acid Ethylene glycol Poly(ethylene terephthalate H Ester

Dacron (polyester)

It is used in fibers for clothing, containers for liquids and foods,

thermoforming for manufacturing, and in combination with glass fiber for

2. Bakelite

Properties and Uses of

bakelite

•

Moldings are smooth, retain their shape and are resistant

to heat, scratches, and destructive solvents. It is also

resistant to electricity, and has low conductivity. It is not

flexible.

•

Phenolic resin products may swell slightly under

conditions of extreme humidity or perpetual dampness.

•

When rubbed or burnt, Bakelite has a distinctive, acrid,

sickly-sweet or fishy odor.

•

Used in making Jewellery boxes, lamps, desk sets,

3. Nylon-6,6

Cl Cl

O O

4 H2N 4 NH2

Adipoyl chloride 1,6-Diaminohexane

Cl N

H NH

H

O O

4 4

NaOH

HO N

H NH

H

O O

4 4

n

6 carbon

diacid 6 carbondiamine Nylon-6,6

Nylon 66 has high mechanical strength, rigidity, good stability under heat and/or chemical resistance.

4. Nylon 6

Properties and Uses

Nylon 6 fibres are tough, possessing high tensile strength, as well as elasticity and lustre.

They are wrinkle-proof and highly resistant to abrasion and chemicals such as acids and alkalis.

The fibres can absorb up to 2.4% of water, although this lowers tensile strength.

It is widely used for gears, fittings, and bearings, in automotive industry for under-the-hood parts, and as a material for power tools housings. Nylon 6 is used as thread in bristles for toothbrushes, surgical sutures, and strings for acoustic and classical musical instruments, including guitars, sitars, violins, violas, and cellos.

5. Poly(methyl methacrylate) or Acrylic

glass or Plexi glass

PMMA is a strong and lightweight material.

PMMA swells and dissolves in many organic solvents; It has poor resistance to many chemicals.

Uses

For rear-lights and instrument clusters for vehicles, appliances and lenses for glasses.

6. Polytetrafluoroethylene(Teflon)

PTFE is hydrophobic

PTFE has one of the lowest coefficients of friction against any solid.

PTFE is used as a non-stick coating for pans and other cookware

7. Polystyrene

Or Styrene

Polystyrene is clear, hard, and brittle

8. Polyvinyl chloride (PVC)

PROPERTIES

PVC is a white, brittle solid. It is insoluble in alcohol but slightly soluble in tetrahydrofuran

USES

PVC comes in two basic forms: rigid (sometimes abbreviated as RPVC) and flexible or placitized

RPVC is used in construction for pipes, doors and windows. For making bottles, other non-food packaging, and cards (such as bank or membership cards). It can be made softer and more flexible by the addition of plasticizers

(phthalates).

CONDUCTING POLYMERS:

A polymer which can conduct electricity is termed as conducting polymers. e.g., Polyaniline (used in rechargeable batteries), polypyrrole etc.

Two types of conducting polymer may be distinguished:

Ionically conducting polymers or solid polymer electrolytes/Extrinsically conducting polymers:

These may be defined as solid ionic conductors formed by the dissolution of inorganic salts in suitable polymer solution and evaporating the solvent. They owe their conductivity to the externally added ingredients.

The polymer suitable for solid polymer electrolyte should have a) atoms with electron donor capability to form coordinate bonds with cations of simple

These are of following two types:

Conductive element filled polymers:

This type includes polymers that act as a binder to hold the conducting element (such as carbon black, metallic fibers, metallic oxides, etc.)

together in the solid entity.

Blending conducting polymers:

These polymers are obtained by blending a conventional polymer with a conducting polymer.

Such polymers possess better physical, chemical, electrical and mechanical properties and they can be easily processed.

Intrinsic electronically conducting polymers: the polymers which are conducting by virtue of their own structure

These possess molecular structure with an extensive system of conjugated double bonds and π electrons.

 The conjugated system has low ionization potential and high electron affinity and hence it is easy to add or remove electrons to create an excess charge by the use of electron donors (reducing agents) or acceptors

(oxidizing agents).

These polymers have several advantages such as light weight, flexibility, ultra-thin film formation capability, high energy density and ease of

They may be further classified into:

Conducting polymers having conjugated π electrons in the

backbone:

Such polymers contain π electrons in the backbone which is responsible for the extensive conductivity.

 They exhibit electrical conductivity only after thermal or photolytic exposure.

The order of conductivity (10-10 S cm-1of these polymers restricts their applicability.

They may be classified as:

I) p-doped polymers: It is obtained by subjecting conducting polymers (having conjugated π electrons) to oxidation by treating with Lewis acid (A) or iodine vapour or iodine in CCl₄.

E.g., (CH)_x + A (CH)_x+ _A- _{(Oxidation process)}

(CH)_x + 2FeCl₃ (CH)_x+FeCl₄- _{+ FeCl} 2

2 (CH)_x + 3I₂ 2 (CH)_x+I₃-







Doped conducting polymers:

II) n-doped polymer:

It is obtained by It is obtained by subjecting conducting polymers (having

conjugated π electrons) to reduction by treating with Lewis base (B) like sodium naphthalide.

(CH)x + B (CH)x - _B+

(CH)_x + Na+_(C

10H8)- Na+ (CH)x- + C10H8

Such polymers (e.g., Emeraldine salt) have conductivity (103 S cm-1) comparable to that of metals.



APPLICATIONS OF CONDUCTING POLYMERS:

In rechargeable batteries: These batteries are small in size (button type), long lasting and can produce current density upto 50 mA/cm2. Moreover, these rechargeable batteries have ecological advantage as they do not involve heavy metals so they do not appear to have any serious toxicological problems

In Analytical sensors: Conducting polymers are also used for making sensors for pH, O₂, NO_x, NH₃ and glucose.

In electrochromic displays and optical filters: Ionically conducting

polymers can absorb visible light to give coloured products so can be useful for electrochromic displays and optcal filters (windows with adjustable

transparency). Thus, conducting polymers can be used as elcetrochromic materials (i.e., the materials which change colour reversibly during the electrochemical processes of charge and discharge).

BIODEGRADABLE POLYMERS:

Polymers which are readily decomposed by microorganisms (fungi or bacteria) via enzymatic activity are known as biodegradable polymers.

Types of biodegradable polymers:

Natural biodegradable polymers: Natural rubber, collagen, lignin, poly (gamma-glutamic acid) are some of the examples of natural biodegradable polymers.

Acetal Hemiacetal Ether Nitrile Phosphonate Polycyanocrylate O H₂ + C O H H R' OH O C O

H

H

R R' R OH +

O C C

C C C OH OH OH

OH

OH C OH C

C C OH OH OH

OH

H2O ₊

C==O H

H2O

R C O C R'

H H

H H

O H₂

R C OH H H

R' C OH H H

+

R C R C _N H

R C R C _O H

N H₂

R C R C _O H

O H

O

H₂ H₂O

RO P OR' O

OR''

O

H P OH O

OR''

O H₂

+ +

R OH HO R'

R C C C C R' CN

C

OR''

CN H

H _{O C}

OR''' O H H O H₂

R C C C CN C OR'' H H _O H H

APPLICATIONS OF BIODEGRADABLE POLYMERS:

Poly (β-hydroxy butyrate) or PHB: PHB is used in the manufacture of

shampoo bottles.

β-hydroxy butyrate-β-hydroxy valerate or HB-HV copolymers: The HB-HV copolymers are suitable as matrices for controlled release of drugs due to their favorable biocompatibility and biodegradation properties.

Poly (lactic acid) or PLA: As PLA breaks down in the environment back to lactic acid, which can be metabolized; it has found commercial use in medical applications such as sutures, drug-delivery systems and wound clips. It is also used in some agricultural applications, such as timed-release coatings for

LIMITATIONS OF BIODEGRADABLE POLYMERS:

Biodegradable polymers are not suitable for recycling, especially in the case of commingled plastics.