Unit I: Polymers · Straight, un-branched chains can pack together more closely than highly branched chains, giving polymers that have higher density, are more crystalline and therefore

Unit I: Polymers:

Introduction:

Polymers are materials made of long, repeating chains of monomers. These chains of monomers are also called

macromolecules. Most polymer chains have a string of carbon atoms as a backbone. A single macromolecule can

consist of hundreds of thousands of monomers. The monomers, into a chain are held together by covalent

bonds. A process of reacting monomer molecules together in a chemical reaction to form polymer chains or

three-dimensional networks is called polymerization. A polymer is composed of many simple molecules that are

repeating structural units called monomers. A single polymer molecule may consist of hundreds to a million

monomers and may have a linear, branched or network structure.

The term polymer is often used to describe plastics, which are synthetic polymers. However, natural polymers

also exist. Rubber and wood, for example, are natural polymers that consist of a simple hydrocarbon, isoprene.

Proteins are natural polymers made up of amino acids and nucleic acids (DNA and RNA) are polymers of

nucleotides.

Polymers are abundant in nature, found in all living systems and materials such as wood, paper, leather, natural

fibers have found extensive use. While natural polymers retain their intrinsic importance, today synthetic

materials are mostly used. They have unique properties, depending on the type of molecules being bonded and

how they are bonded. Some polymers bend and stretch, like rubber and polyester. Others are hard and tough, like

epoxies and glass. The physical properties of a polymer such as its strength and flexibility depend on:

(i) Chain Length:

The strength of polymers depends on the chain length. In general, longer the chains stronger will be the polymer.

(ii) Side Groups:

Polar side groups (including those that lead to hydrogen bonding) give stronger attraction between polymer

chains, making the polymer stronger.

(iii) Branching:

Straight, un-branched chains can pack together more closely than highly branched chains, giving polymers that

have higher density, are more crystalline and therefore stronger.

(iv) Cross-linking:

If polymer chains are linked together extensively by covalent bonds, the polymer is harder and more difficult to

melt.

History Polymeric Materials:

Polymeric materials have been used from prehistoric times. The term polymer was coined in 1833 by Jöns Jakob

Berzelius. The first man-made polymer, formed by chemical modification of natural materials, was produced in

the second half of the nineteenth century. Friedrich Ludersdorf and Nathaneil Haward discovered that adding

sulfur to raw natural rubber (poly-isoprene) helped prevent the material from becoming sticky. In 1844 Charles

Goodyear received a U.S. patent for vulcanizing natural rubber with sulfur and heat. Thomas Hancock had

received a patent for the same process in the UK the year before. This process strengthened natural rubber and

prevented it from melting with heat without losing flexibility. This made practical products such as waterproofed

articles possible. Vulcanized rubber represents the first commercially successful product of polymer research.

In 1884 Hilaire de Chardonnet started the first artificial fiber plant as a substitute for silk, but it was very

flammable. In 1907 Leo Baekeland invented the first synthetic plastic, a thermosetting phenol-formaldehyde resin

called Bakelite. Fully synthetic polymers were developed in the twentieth century, most in the period 1950-1970s

(so-called plastics of modern society). Henri, along with Christian Schönbein and others, developed derivatives of

the natural polymer cellulose, producing new semi-synthetic materials, such as celluloid and cellulose acetate.

Classification of Polymers:

1. On the Basis of Thermal Response:

On the basis of thermal response, polymers can be classified into two groups:

(a) Thermoplastic Polymers:

Thermoplastic polymers can be re-melted back into a liquid

plasticized by heating and then solidified by cooling

properties. Think of thermoplastics as butter which can be melted and cooled multiple times to form various

shapes. When you heat thermoplastics

thermoplastic chains are held together by

speaking, thermoplastic chains are clumped together like a ball of tangled yarn. These are 100% recyclable.

Examples of such polymers are polyolefins, nylons, linear polyesters and polyethers, PVC, s

Most linear and slightly branched polymers are thermoplastic.

Major thermoplastics are produced by

are held together by relatively weak intermolecular forces

when exposed to heat and then returns to its original condition when cooled.

wide range of applications because they can be formed and reformed in so many shapes.

Some examples are food packaging, insulation, automobile bumpers and credit cards.

(b) Thermosetting Polymers or Thermosets

Thermosetting polymers or thermosets

Therefore they cannot be reshaped by heating

permanent solid state because they undergo certain chemical changes on heating and

convert themselves into an infusible mass

process involves chemical reaction resulting

linking (permanent chemical bonds) between polymer chains

the chains and leads to a rigid material.

rubbers, polyester resins, silicon resins, polyurethanes

and are used in automobiles and construction

boat hulls and glues, insulation parts, car parts, etc

2. On the Basis of Structure:

Bases on the structure, polymers are classified as:

(a) Linear Polymers:

Linear polymers are simplest polymer i

units are linked together to form long straight chains.

These are closely packed and hence have high densities, high melting point and high tensile strength.

nylon, polyethene, etc. When formed, these line

be very strong and hard to break through.

1. On the Basis of Thermal Response:

On the basis of thermal response, polymers can be classified into two groups:

melted back into a liquid. Therefore they can be repeatedly softened

by heating and then solidified by cooling on application of thermal energy, without much change in

as butter which can be melted and cooled multiple times to form various

thermoplastics, the molecules do not chemically bond with each other. Instead,

thermoplastic chains are held together by Vander Waal forces, weak attractions between molecules. Visually


Examples of such polymers are polyolefins, nylons, linear polyesters and polyethers, PVC, s

polymers are thermoplastic.

chain polymerization. Molecules in such polymers

intermolecular forces so that the material softens

when exposed to heat and then returns to its original condition when cooled. They have a


ng, insulation, automobile bumpers and credit cards.

or Thermosets:

thermosets cannot be repeatedly softened by heating.

Therefore they cannot be reshaped by heating. These polymers always remain in a

because they undergo certain chemical changes on heating and

convert themselves into an infusible mass (i.e., solidifies or sets irreversibly). The setting

resulting in three-dimensional networked by cross-

between polymer chains which restricts the motion of

the chains and leads to a rigid material. E.g., phenolic resins, urea epoxy resins, diene

polyester resins, silicon resins, polyurethanes, etc. These are strong and durable

in automobiles and construction and also are used to make toys, varnishes,

insulation parts, car parts, etc.

structure, polymers are classified as:

are simplest polymer in which monomeric

units are linked together to form long straight chains.

These are closely packed and hence have high densities, high melting point and high tensile strength.

When formed, these linear polymers can create strands of fibers or form a mesh that can

be very strong and hard to break through.

Linear Chain Branched Chain

. Therefore they can be repeatedly softened or

on application of thermal energy, without much change in

as butter which can be melted and cooled multiple times to form various

, the molecules do not chemically bond with each other. Instead,

aal forces, weak attractions between molecules. Visually


Examples of such polymers are polyolefins, nylons, linear polyesters and polyethers, PVC, sealing wax, etc.

so that the material softens

They have a


repeatedly softened by heating.

always remain in a

because they undergo certain chemical changes on heating and

setting

restricts the motion of

epoxy resins, diene

trong and durable

also are used to make toys, varnishes,

These are closely packed and hence have high densities, high melting point and high tensile strength. Examples:

ar polymers can create strands of fibers or form a mesh that can

Thermoplastics

Thermosets

Branched Chain Cross-linked Chain

(b) Branched Chain Polymers:

In these polymers, monomers are linked together to form a main chain and side chains of different lengths arises

from the main chain thus forming branches. These polymers are irregularly packed and have low tensile strength,

low density and low melting points. Examples: amylopectin, glycogen, low density polyethene, etc.

(c) Cross Linked Polymers or Network Polymers:

In these polymers, the monomer units are linked together to form a three dimensional network. The monomers

are formed from bi-functional and tri-functional monomers and contain strong covalent bonds between various

linear polymer chains. These polymers are hard, rigid and brittle in nature. Examples: Melamine, Bakelite, etc.

3. On the Basis of Polymerization Mechanism:

Polymerization mechanism is the sequence of elementary chemical reactions by which polymerization (process

of converting monomer molecules into a polymer) proceeds. Based on polymerization mechanism, polymers can

be classified into two groups:

(a) Addition or Chain-Growth Polymerization:

Addition polymerization is the type of polymerization in which the similar or different monomers repeatedly

added to form a polymer (called addition polymer) without the co-generation of other products. When we keep

adding monomers to obtain a large chain, such a process is also called as the chain growth polymerization. The

monomers are unsaturated compounds. For example: alkenes, alkynes, alkadienes and their derivatives. E.g.,

(vinyl chloride) (polyvinyl chloride)

The mode of polymerization reaction in this case is governed by: free radicals or ion species. There are four types

of addition polymerizations which are: Free adical polymerization, Cationic polymerization, Anionic

polymerization and Coordination polymerization.

(i) Free Radical Polymerization Reaction:

A polymerization where a free radical is formed causing a chain reaction. For example: Polymerisation of vinyl

chloride to give polyvinyl chloride:

325-350 K/13 atm

CH2=CHCl [-CH2-CHCl-]n

(vinyl chloride) (polyvinyl chloride)

Mechanism:

Initiation Step:

It can be initiated by using a radical initiator, such as peroxide or certain azo compounds. On heating the initiator

molecule decomposes into a free radical (I.). Some common initiators are presented below:

Propagation Step:

Free radical (I.) combines with the monomer and forms a new radical for the chain propagation. The formed

radical reacts with another monomer and generate a new radical which propagates the chain. The chain

propagation process determines the length of a polymer chain.

Termination Step:

The macro-free radicals are deactivated by recombination of free radicals. There are three ways to terminate the

propagating chain either by coupling (with another growing free radical) or by disproportionation reaction or by

reaction with an impurity (such as oxygen) or by solvent.

The coupling is caused as a result of reaction of the growing free radical chain with the other growing free radical.

The disproportionation is caused by the acceptance of one hydrogen free radical by one free radical from the

other which is converted to an alkene or alkene derivative.

(ii) Cationic Polymerization Reaction: It is a polymerization where a cation intermediate is is generated by the addition of a Lewis acid such as BF3, AlCl3,

etc. with the monomer typically alkene causing a chain reaction. It results in forming a long chain of repeating

monomers. The cation formed in the initiation step reacts with another monomer and generate a new cation, this

process is repeated until chain termination occurs.

Chain propagation can be affected by three ways either by loss of a proton or by addition of a nucleophile or by

reacting with the solvent molecule.

(iii) Anionic Polymerization Reaction

Anionic polymerization occurs by the addition of a nucleophile

new anions, this process occurs only when the nucleophilicity of the initiator nucleophiles are strong enough to

attack the electron rich olefins. Similarly, if electron withdrawing substituents are attach

increases the rate of addition. Here the chain propagation step was terminated by reaction of generated

nucleophiles with impurity or solvent molecules.

(iv) Coordination Polymerization Reaction

This method was invented by two scientists Ziegler and Natta who won a Nobel Prize for their work. They

developed a catalyst which let us control the free radical polymerization. It produces a polymer which has more

density and strength.

(b) Condensation or Step-Growth P

This type of reaction involves the repetitive condensation between two bi

some simple molecules such as water and alcohol due to the condensation mechanism. Condensation mechanism

refers to combining of smaller molecules to obtain larger molecules.

Reaction:

Anionic polymerization occurs by the addition of a nucleophile initiator with the monomer alkene and generates a


attack the electron rich olefins. Similarly, if electron withdrawing substituents are attach


nucleophiles with impurity or solvent molecules.

Reaction:

two scientists Ziegler and Natta who won a Nobel Prize for their work. They


Polymerization:

This type of reaction involves the repetitive condensation between two bi-functional monomers. There is a loss of


ules to obtain larger molecules. Examples: Nylon 66, Dacron, etc.

Terylene

initiator with the monomer alkene and generates a


attack the electron rich olefins. Similarly, if electron withdrawing substituents are attached to the olefin bond


two scientists Ziegler and Natta who won a Nobel Prize for their work. They


functional monomers. There is a loss of


Nylon 66, Dacron, etc.

Terylene

In these types of polymerization reaction, the product that is formed after every step is again a

species and so these species undergo a sequence of condensation process. At each step we have distinct

functional species and every species is independent of the other species and so we call this process as the step

growth polymerization. Example: ethylene glycol and terephthalic acid.

types of polymerization reaction, how the polymerization of monomers take place and the addition and

condensation mechanism.

Polymer Structure:

The properties of polymers are strongly influen

the sequence of monomer units in the case of copolymers, the stereochem

geometric isomerization in the case of diene

(a) Copolymers:

Polymers are composed of many simple molecules that are repeating

bonds hold the atoms in the polymer

of polymer chains together to form the polymeric material.

different types of monomer species. The

polymers, ter-polymers and quarter-polymers, respectively.

There are many commercially relevant copolymers. Some examples include

styrene (ABS), styrene/butadiene co-polymer

(SIS) and ethylene-vinyl acetate (formed by

polyester family (formed by step-growth polymerization

the glass transition temperature (Tg), which is i

be classified based on how these units a

copolymers, random or statistical copolymers

(i) Block Copolymers:

Block copolymers comprise two or more

homopolymer subunits may require an intermediate non

copolymers with two or three distinct blocks are called

Technically, a block is a portion of a macromole

feature which is not present in the adjacent portions.

copolymer might be ~A-A-A-A-A-A-A-B-B

Homopolymer

Block copolymers are made up of blocks of different

poly(methyl methacrylate) or PS-b-PMMA (where b = block) is usually made by first polymerizing

subsequently polymerizing methyl methacrylate

polymer is a diblock copolymer because

multiblocks, etc. can also be made.

(ii) Alternating Copolymers:

Alternating copolymers have regular alternating A and B units and is

B-)n-. Molar ratio of each monomer in

of styrene maleic anhydride copolymer, most chains

chains ending in maleic anhydride add a styrene unit. This leads to a predominantly alternating structure.

In these types of polymerization reaction, the product that is formed after every step is again a



ethylene glycol and terephthalic acid. So far we have seen that there are two


trongly influenced by the chain structure, the overall chemical composition and

the sequence of monomer units in the case of copolymers, the stereochemistry or tacticity of the chain

geometric isomerization in the case of diene-type polymers for which several synthesis routes may be possible.

composed of many simple molecules that are repeating structural units called monomers. Covalent

polymer molecules together and secondary bonds th

chains together to form the polymeric material. Copolymers are obtained by

e copolymers obtained from two, three and four monomers are called

polymers, respectively.

There are many commercially relevant copolymers. Some examples include: acrylonitrile butadiene

polymer (SBR), nitrile rubber, styrene-acrylonitrile, styrene

formed by chain-growth polymerization) and nylon 66

growth polymerization). Copolymerization is particularly useful in tuning

), which is important in the operating conditions of polymers.

be classified based on how these units are arranged along the chain, as: block copolymers,

statistical copolymers and graft copolymers.

comprise two or more homo-polymer subunits linked by covalent bonds. The union of the

subunits may require an intermediate non-repeating subunit, known as a

with two or three distinct blocks are called diblock copolymers and triblock copolymers

Technically, a block is a portion of a macromolecule, comprising many constitutional units, that has at least one

feature which is not present in the adjacent portions. A possible sequence of repeat units A and B in a triblock

B-B-B-B-B-B-A-A-A-A-A~.

Block copolymer

Block copolymers are made up of blocks of different polymerized monomers. For example, polystyrene

PMMA (where b = block) is usually made by first polymerizing

methyl methacrylate (MMA) from the reactive end of the polysty

because it contains two different chemical blocks. Triblocks, tetrablocks,

gular alternating A and B units and is described by formula:

ar ratio of each monomer in polymer is normally close to one. E.g., in free-radical copolymerization

copolymer, most chains ending in styrene add a maleic anhydride unit


Alternating copolymer

In these types of polymerization reaction, the product that is formed after every step is again a bi-functional



So far we have seen that there are two


the overall chemical composition and

istry or tacticity of the chain and the

for which several synthesis routes may be possible.

units called monomers. Covalent

molecules together and secondary bonds then hold groups

obtained by polymerization of

three and four monomers are called bi-

acrylonitrile butadiene

, styrene-isoprene-styrene

nylon 66, and the co-

is particularly useful in tuning

mportant in the operating conditions of polymers. Copolymers can

block copolymers, alternating

subunits linked by covalent bonds. The union of the

repeating subunit, known as a junction block. Block

triblock copolymers, respectively.

cule, comprising many constitutional units, that has at least one

A possible sequence of repeat units A and B in a triblock

. For example, polystyrene-b-

PMMA (where b = block) is usually made by first polymerizing styrene and then

(MMA) from the reactive end of the polystyrene chains. This

it contains two different chemical blocks. Triblocks, tetrablocks,

y formula: -A-B-A-B-A-B- or -(-A-

radical copolymerization

ene add a maleic anhydride unit and almost all


A step-growth copolymer -(-A-A-B-B-)n-

is in principle a perfectly alternating copolymer of these two monomers, but is usually considered as

a homopolymer of the dimeric repeat unit A

(CH2)6-NH-, formed from a dicarboxylic acid

(iii) Random or Statistical Copolymers

Statistical copolymers are the simplest type of copolymer

copolymerized. In such copolymers the sequence of monomer residues follows a statistical rule.

commonly used to modify and/or improve the mechanical and physical properties of many polymers.

Ran

Examples of commercially relevant random copolymers include

copolymers and resins from styrene-acrylic or

(iv) Graft Copolymers:

Graft copolymers consist of a main polymer chain or backbone (A) covalently bonded to one or more side chains

(B). These are a special type of branched copolymer in which the side chains are structurally distinct from the

main chain. The individual chains of a graft copolymer may be homopolymers or copolymers. For

example, polystyrene chains may be grafted onto

C=C double bond per repeat unit. The polybutadiene is dissolved in styrene, which is then subjected to

radical polymerization. The growing chains can add across the double bonds of rubber molecules forming

polystyrene branches. The graft copolymer is formed in a mixture with un

molecules.

As with block copolymers, the quasi-composite

polystyrene chains grafted onto polybutadiene

is much less brittle than ordinary polystyrene. The product is called

(b) Tacticity (Configuration) of Polymers

Tacticity (relating to arrangement or order

a macromolecule. It is particularly significant in

unit with a substituent R on one side of the polymer backbone is followed by th

substituent on the same side as the previous one, the other side as the previous one or positioned randomly with

respect to the previous one. In a hydrocarbon macromolecule with all carbon atoms making up the backbone in

a tetrahedral molecular geometry, the zigzag backbone is in the paper plane with the substituents either sticking

out of the paper or retreating into

Natta. Monotactic macromolecules have one stereoisomeric atom per repeat unit,

macromolecules have more than one stereoisomeric atom per unit.

The practical significance of tacticity rests on the effects on the physical properties of the

of the macromolecular structure influences the degree to which it has rigid,

flexible, amorphous long range disorder.

geometric arrangement of the functional (side) groups.

polymer (stereoregular) and Atactic polymer (

(i) Isotactic Polymers:

In isotactic polymers all the substituents are located on the same side of the polymer chain (macromolecular

backbone). They consist of 100% meso diads.

formed by the condensation of two bifunctional monomers A

perfectly alternating copolymer of these two monomers, but is usually considered as

of the dimeric repeat unit A-A-B-B. An example is nylon 66 with repeat unit

dicarboxylic acid monomer and a diamine monomer.

(iii) Random or Statistical Copolymers:

he simplest type of copolymers where two or more co-monomers are simultaneously

the sequence of monomer residues follows a statistical rule.


Random copolymer

Examples of commercially relevant random copolymers include: rubbers made from styrene

acrylic or methacrylic acid derivatives.

of a main polymer chain or backbone (A) covalently bonded to one or more side chains

are a special type of branched copolymer in which the side chains are structurally distinct from the

ain. The individual chains of a graft copolymer may be homopolymers or copolymers. For

chains may be grafted onto polybutadiene, a synthetic rubber which retains one reactive

. The polybutadiene is dissolved in styrene, which is then subjected to

. The growing chains can add across the double bonds of rubber molecules forming

polystyrene branches. The graft copolymer is formed in a mixture with un-grafted polystyrene chains and rubber

Graft copolymer

composite product has properties of both components

polybutadiene, the rubbery chains absorb energy when the substance is hit, s

is much less brittle than ordinary polystyrene. The product is called high-impact polystyrene

(Configuration) of Polymers:

ting to arrangement or order) is the relative stereochemistry of adjacent

is particularly significant in vinyl polymers of the type -H2C-CH(R)-

R on one side of the polymer backbone is followed by the next repeating unit with the



, the zigzag backbone is in the paper plane with the substituents either sticking

the paper. This projection is called the Natta projection

ecules have one stereoisomeric atom per repeat unit,

have more than one stereoisomeric atom per unit.

rests on the effects on the physical properties of the polymer

of the macromolecular structure influences the degree to which it has rigid, crystalline

long range disorder. There are three different types of polymers depending upon the relative

of the functional (side) groups. These are: Isotactic polymer (stereoregular),

Atactic polymer (stereo irregular).

all the substituents are located on the same side of the polymer chain (macromolecular

of 100% meso diads. and orient in very closely dense fashion. This permits effective

monomers A–A and B–B

perfectly alternating copolymer of these two monomers, but is usually considered as

with repeat unit -OC-(CH2)4-CO-NH-

monomers are simultaneously

the sequence of monomer residues follows a statistical rule. This technique is


made from styrene-butadiene

of a main polymer chain or backbone (A) covalently bonded to one or more side chains

are a special type of branched copolymer in which the side chains are structurally distinct from the

ain. The individual chains of a graft copolymer may be homopolymers or copolymers. For

which retains one reactive

. The polybutadiene is dissolved in styrene, which is then subjected to free-

. The growing chains can add across the double bonds of rubber molecules forming

grafted polystyrene chains and rubber

properties of both components. In the example of

the rubbery chains absorb energy when the substance is hit, so it

impact polystyrene or HIPS.

of adjacent chiral centers within

where each repeating

e next repeating unit with the



, the zigzag backbone is in the paper plane with the substituents either sticking

Natta projection after Giulio

ecules have one stereoisomeric atom per repeat unit, ditactic to n-tactic

polymer. The regularity

crystalline long range order or

There are three different types of polymers depending upon the relative

(stereoregular), Syndiotactic

all the substituents are located on the same side of the polymer chain (macromolecular

orient in very closely dense fashion. This permits effective

formation of inter-polymer forces of attraction to give a cohesive polymer system and thus a useful fiber. These

are usually semi-crystalline and often form a helix configuration. Example: Polypropylene formed by Ziegler–Natta

catalysis. polypropylene and pure acrylonitrile.

Isotactic poly(vinyl chloride)

(ii) Syndiotactic Polymers:

In syndiotactic polymers or syntactic macromolecules the substituents have regular on alternate positions along

the polymeric chain. This regular polymer structure allows close enough alignment or orientation to form enough

inter-polymer forces of attraction, giving a cohesive enough system to form a useful fiber. They consist 100% of

racemo diads. Syndiotactic polystyrene, made by metallocene catalysis polymerization, is crystalline with

a melting point of 161 °C. Example: The polymers of cellulose and some chloro-fibers, Gutta percha, etc.

Syndiotactic polystyrene

(iii) Atactic Polymers:

In atactic polymers the substituents are placed randomly along the chain. The percentage of meso diads is

between 1 and 99%. These are formed by free-radical mechanisms such as polyvinyl chloride. Due to their random

nature these are usually amorphous. In hemi-isotactic polymers every other repeat unit has a random

substituent. These are technologically very important. A good example is polystyrene (PS). If a special catalyst is

used in its synthesis it is possible to obtain the syndiotactic version of this polymer, but most industrial

polystyrene produced are atactic.

Atactic polypropylene

Effect of Tacticity on the Properties of Polymers:

The tacticity of a polymer chain can have a major influence on its properties. Isotactic and syndiotactic polymers

are considered as stereospecific or stereoregular while atactic polymers are viewed as random or stereo–

irregular. The overall molecular symmetry and crystallinity are in the order isotactic > syndiotactic >> atactic.

Isotactic polymers are generally characterized by high rigidity substance, high melting temperature (Tm) and high

mechanical properties with relatively high resistance to solvents and chemicals and excellent resistance to

mechanical stress. Atactic polymers being more disordered cannot crystallize. For example atactic polypropylene

is a soft, rubbery material with no commercial value.

(iii) Geometric Isomerism:

When there are unsaturated sites along a polymer chain, several different isomeric forms are possible. The 1,3-

butadiene can be polymerized to give 1,2-poly(1,3-butadiene) or either of two geometric isomers of 1,4-poly(1,3-

butadiene).

The numbers preceding the poly prefix designate the first and last carbon atoms of the backbone repeating unit.

1,2-poly(1,3-butadiene) has a vinyl-type structure, where the substituent group (ethene) contains an unsaturated

site. Therefore, this geometric isomer can be atactic, syndiotactic or isotactic. In the case of the commercially

more important 1,4-poly(1,3-butadiene), all four carbons in the repeating unit lie along the chain. Carbons 1 and 4

can lie either on the same side of the central double bond (i.e., cis-configuration) or on the opposite side

(i.e., trans-configuration). The structure of polybutadiene used in SBR rubber (i.e., a copolymer of styrene and

butadiene) is principally the trans-1,4 isomer with some cis-1,4- and 1,2-poly(1,3-butadiene) content.

(iv) Nomenclature of Polymers:

(i) Naming of Vinyl Polymers:

When the monomer name consists of one word vinyl then the polymers are designated by attaching the

prefix poly to the monomer name (e.g., polystyrene, polyethylene and polypropylene). However, when the

monomer name consists of more than one word or is preceded by a letter or number, the monomer name is

enclosed by parentheses preceded by the prefix poly. For example, the polymer obtained from the polymerization

of 4-chlorostyrene is poly(4-chlorostyrene) and that from vinyl acetate is poly(vinyl acetate). Tacticity may be

noted by prefixing the letter i (isotactic) or s (syndiotactic) before poly as in i-polystyrene. Geometric and

structural isomers may be indicated by using the appropriate prefixes, cis or trans and 1,2- or 1,4-, before poly, as

in trans-1,4-poly(1,3-butadiene).

(ii) Naming of Non-vinyl Polymers:

Non-vinyl polymers such as condensation polymers usually named according to the initial monomer or the

functional group of the repeating unit. For example, the most important commercial nylon, commonly called

nylon-6,6 (66 or 6/6), is more descriptively called poly(hexamethylene adipamide) denoting the polyamidation of

hexamethylenediamine (alternatively called 1,6-hexane diamine) with adipic acid. Similarly, the aliphatic nylon

obtained by the polyamidation of hexamethylenediamine with a 10-carbon dicarboxylic acid, sebacic acid, is

nylon-6,10 or poly(hexamethylene sebacamide).

In some cases, common names are used almost exclusively in place of the more chemically correct nomenclature.

For example, the polycondensation of phosgene and bisphenol-A—the common name for 2,2-bis(4-

hydroxyphenyl)propane produces the thermoplastic, polycarbonate. Often, the prefix bisphenol-A is placed

before polycarbonate to distinguish it from other polycarbonates that can be polymerized by using bisphenol

monomers other than bis-phenol-A, such as tetramethylbisphenol-A.

The IUPAC name for polystyrene is poly(1-phenylethylene) and that for polytetrafluoroethylene (tradename,

Teflon) is poly(difluoromethylene). The IUPAC name for the polycarbonate of bisphenol-A mentioned earlier is

poly(oxycarbonyloxy-1,4-phenyleneisopropylidene-1,4-phenylene).

A very useful set of two-, three and four-letter abbreviations for the names of many common thermoplastics,

thermosets, fibers, elastomers and additives have been developed. Sometimes, abbreviations for the same

polymer may vary, but there is widespread agreement on the abbreviations for a large number of important

polymers. These abbreviations are convenient and widely used. Examples: PS abbreviation for polystyrene, PVC

for poly(vinyl chloride), PMMA for poly(methyl methacrylate), PTFE for polytetrafluoroethylene and PC for bis-

phenol-A polycarbonate.

Following IUPAC recommendations, copolymers are named by incorporating an italicized connective term

between the names of monomers contained within parentheses or brackets or between two or more polymer

names. The connective term designates the type of copolymer as indicated for six important classes of

copolymers in following Table.

Scheme for Naming Copolymers

Type Connective Example Type Connective Example

Unspecified -co- Poly[styrene-co-(methyl

methacrylate)]

Alternating -alt- Poly[styrene-alt-(maleic

anhydride)]

Statistical -stat- Poly(styrene-stat-butadiene) Block -block- Polystyrene-block-polybutadiene

Random -ran- Poly[ethylene-ran-(vinylacetate)] Graft -graft- Polybutadiene-graft-polystyrene

Molecular Weight of Polymers:

Introduction:

Polymers are long chain molecules produced by linking small repeat units (monomers) together. There are many

ways to link different types of monomer to form polymers. They exhibit very different physical properties

compared to the monomers, dependent on the length of the polymer chains. The presence of small amounts of

very long or very short chains can have drastic effects on properties of the material. Therefore knowledge of the

molecular weight of polymers is very important because the physical properties of polymer molecules are

affected by their molecular weight. The interrelation between molecular weight and strength for a typical polymer

is given as:

Unlike simpler pure compounds, most polymers (mainly synthetic) are not composed of identical molecules. The

HDPE molecules, for example, are all long carbon chains, but the lengths may vary by thousands of monomer

units. Because of this, polymer molecular weights are usually given as averages. Molecular weight of a polymer is

defined as sum of the atomic weight of each of the atoms in the molecules, which is present in the polymer. Two

experimentally determined values are common:

(i) The number average molecular weight (Mn) (ii) The weight average molecular weight (Mw)

(i) Number Average of Molecular Weight (Mn):

Number average molecular weight is measured to determine number of molecules in given sample of the

polymer. It is the ratio of total weight (W) of all the molecules with the total number of polymer molecules (N)

present in a polymer sample. It is calculated as:

Numberaveragemolecularweight�� =�� !�� "��#$��%

��&$"'�(�!�� "��#$��%=

)

*+ =

*+,�,

*+,

Where, W is the total mass, N is the the total no. of molecules, Ni is the no. of molecules of mass Mi. If N1, N2,

N3,…… etc. be the molecules with molecular masses M1, M2, M3,……etc. respectively, then:

Total mass of N1 molecules = N1M1



ΣNiMi = N1M1 + N2M2 + N3M3 + ......

ΣNi = N1 + N2 + N3 + ......

Therefore,

�� = +-�-.+/�/.+0�0.⋯

+-.+/.+0.⋯

Characteristics of Number Average Molecular Weight:

• It is determined by measurement of colligative properties.

• It is good index of physical properties such as impact and tensile strength but not for other properties such

as flow.

(ii) Weight Average of Molecular Weight (Mw):

Weight average of molecular weight (Mw) is measured to determine the mass of polymer molecule. It is the ratio

of product of the total weight fraction (Wi) and mass of all the molecules (Mi) with the total weight fraction of

polymer molecules (Wi) present in a polymer sample. It is calculated as:

Weightaveragemolecularweight��)� =�� 3!(�#��&�!�� "��#$��%4"��#$��("�%%�!�� "��#$��%

��&$"'�(�!�� "��#$��%

�) =*),�,

*), =

*�+,�,�5�,

*+,�, =

*+,�,/

*+,�,

If N1, N2, N3,……etc. be the molecules with molecular masses M1, M2, M3,……etc. respectively, then:

�) =+-�-

/ .+/�//.+0�0

/.⋯…

+-�-.+/�/.+0�0.⋯

Characteristics of Weight Average Molecular Weight:

• It is determined by measurement of light scattering and ultra-centrifugation techniques which measure

molecular size.

• Mw is always greater than Mn (except in mono-disperse system in which all molecules have identical

molecular mass).

Chemical Structures and Thermal

Transitions of Polymers:

Semi-crystalline polymers have both

amorphous and crystalline regions. The

amorphous regions can be either in the glassy

or rubbery state. Individual chains in

amorphous region are randomly coiled and

interwined with no molecular order or

structure. Commercial grade (atactic)

polystyrene and poly(methylmethacrylate) is

an examples of amorphous in solid state.

At a low temperature the amorphous regions of a polymer are in the glassy

state. In this state the molecules are frozen on place. They may be able to

vibrate slightly, but do not have any segmental motion in which portions of

the molecule wiggles around. When the amorphous regions of a polymer

are in the glassy state, it generally will be hard, rigid and brittle. The glassy

state is similar to a super cooled liquid where the molecular motion is in the

frozen state. This glassy state is also analogous to a crystalline solid with

molecular disorder as a liquid.

When the polymer is heated, the polymer chains are able to wiggle (to move up) around each other. This state is

called the rubbery state. At this state the polymer becomes soft and flexible similar to rubber. The temperature at

which the glassy state changes to rubbery state is called the glass transition temperature (Tg). The glass

transition is a property of only the amorphous portion of a semi-crystalline solid. The crystalline portion remains

crystalline during the glass transition, which is not the same as melting. For example the chewing gum at body

temperature is soft and pliable, which is characteristic of an amorphous solid in the rubbery state. If you put a cold

drink in your mouth or hold an ice cube on the gum, it becomes hard and rigid. The Tg of the gum is somewhere

between 0oC and 37

oC.

Below Tg, long range cooperative motions of individual chains cannot occur. However short-range motion

involving several contiguous groups along the chains backbone or substituent group is possible. Such motions are

called secondary-relaxation processes and can occur at temperatures as low as 70K. By comparison, Tg vary from

150K for polymers with very flexible chains such as polydmethylsiloxane [-Si(CH3)2-o-] so well over 600K for those

with highly rigid aromatic backbones such as the high-modulus fiber, poly[2,2’-(m-phenylene)-5,5’-

bibenzimidazole], PBI, with a Tg in the range from 700-773K.

Thermodynamic transitions are classified as being first or second-order. In a first-order transition there is a

transfer of heat between system and surroundings and the system undergoes an abrupt volume change. In a

second-order transition, there is no transfer of heat, but the heat capacity does change. The volume changes to

accommodate the increased motion of the wiggling chains, but it does not change discontinuously.

Comparison between Glass Transition and Melting

S. No. Glass Transition Melting

1.

2.

3.

4.

Property of the amorphous region

Below Tg, disordered amorphous solid with immobile molecules

Above Tg, disordered amorphous solid in which portions of

molecules can wiggle around

A second order transition

Property of the crystalline region

Below Tg, ordered crystalline solid

Above Tm, disordered melt

A first-order transition

Problems for Practice:

1. Calculate the Mn and MW for a polymer sample comprising of 5 moles of polymer molecules having

molecular weight of 45000 g/mol, 10 moles of polymer molecules having molecular weight of 55000 g/mol

and 2.5 moles of polymer molecules having molecular weight of 38000 g/mol.

2. In a polymer sample comprising of 5 moles of polymer molecules has molecular weight of 40000 g/mol and

15 moles of polymer molecules having molecular weight of 30 000 g/mol. Calculate the Mn and MW for a

polymer sample.

3. Calculate the MW for a polymer sample comprising of 9 moles of polymer molecules having molecular

weight of 30000 g/mol and 5 moles of polymer molecules having molecular weight of 50000 g/mol.

4. Calculate Mn and MW of an equimolecular (i.e. equimolar) mixture of samples of molecular masses of

50000 and 100000.

Ans:

5. How are polymers classified on the basis of polymerization process?

Ans: On the basis of mode of polymerisation, polymers are classified into the following groups:

Addition polymers or chain growth polymers: 1)A polymerformed by direct addition of repeated monomers

without the elimination of byproduct molecules is called addition polymer.

6. What is polymer Tacticity?

Tacticity (from Greek τακτικός taktikos "of or relating to arrangement or order") is the relative

stereochemistry of adjacent chiral centers within a macromolecule. The practical significance

of tacticity rests on the effects on the physical properties of the polymer.

7. What is the difference between Atactic isotactic and syndiotactic polymers?

Ans: Isotactic and syndiotactic polymers provide long-range order, which leads to higher crystallinity in

the polymer chain. Polypropylene is a great example of howtacticity has a dramatic effect on the physical

properties of the polymer. Atactic polypropylene has little order in the polymer backbone and is

amorphous.

8. What are syndiotactic polymers?

Ans: In chemistry of industrial polymers: Organometallic catalysis. Isotactic and syndiotactic polymers are

referred to as stereoregular—that is, polymers having an ordered arrangement of pendant groups along the

chain. A polymer with a random orientation of groups is said to be atactic.

9. Are all polymers amorphous?

Ans: The structure of a polymer is defined in terms of crystallinity. ... A well-ordered polymer is considered

crystalline. The opposite is an amorphous polymer. Molecular arrangements Polymers – the materials often

referred to as plastics, elastomers or rubber – are made up of long chains of molecules.

10. Why do polymers have average molecular weight?

Ans: Unlike simpler pure compounds, most polymers are not composed of identical molecules. ... Since

larger molecules in a sample weigh more than smaller molecules, the weight average Mw is necessarily

skewed to higher values.

11. What is addition polymerization reaction?

Ans: An addition polymer is a polymer that forms by simple linking of monomers without the co-generation

of other products. Addition polymerization differs from condensation polymerization, which does co-

generate a product, usually water. ...Addition polymers are formed by the addition of some simple

monomer units repeatedly.

12. How does addition polymerisation differ from Condensation polymerization types of polymerisation?

Ans: These are mostly radical based polymerization, for those monomers with double bonds.

Condensation polymerization, as a contrast, normally involves the generation of small molecule products,

like water. ... Meanwhile, water is generated. It looks like two molecules "condense" with each other to

form this polymer.

13. Why is this polymerization reaction known as addition polymerization?

Ans: Addition polymerization occurs by a chain reaction in which one carbon-carbon double bond adds to

another. Monomers continue to react with the end of the growing polymer chain in an addition

polymerization reaction until the reactive intermediate is destroyed in a termination reaction.

14. What is the mechanism of polymerization?

Ans: A polymerization mechanism is the sequence of elementary chemical reactions by which

polymerization (the process of converting monomer molecules into a polymer) proceeds.

15. What is average molecular weight of polymer?

Ans: It is important to be able to characterize the polymer structure. Determining the weight-average

molecular weight or the number-average molecular weight is a part of any polymer characterization. As an

example, a polyvinyl chloride molecule may have a molecular weight of 35,000 amu (or 35,000 g/mol).

16. What is number average molecular weight?

Ans: The number average molecular weight is not too difficult to understand. It is just the total weight of

all the polymer molecules in a sample, divided by the total number of polymer molecules in a sample.

17. Why isotactic and syndiotactic polymers are referred to as stereoregular while atactic stereoirregular?

18. What is number average molecular weight? How is it measured?

Documents

Unit I: Polymers · Straight, un-branched chains can pack together more closely than highly branched chains, giving polymers that have higher density, are more crystalline and therefore