Welcome everyone to this first class of NPTEL
course Principles of Polymer Synthesis.
In this particular course I am going to cover
as the title suggests the basic principles
of the synthesis of polymers.
So, lot of chemistry aspects will be covered,
and of course considering the constraints
of the course for example, 20 hours certain
special applications also will be covered.
And so, most of the important aspects will
be discussed as far as the principles of the
synthesis are concerned; the way I have structured
this course is like this.
In this first class I am going to give you
an idea about the historical development of
polymer science, how it came about, everything
just did not appear out of blue.
So, it took decades of honest efforts from
scores of scientists in order to establish
what we now know as polymer chemistry and
what we now take for granted as the structures
of polymers.
So, basically the main focus of this particular
course will be on the structure-property relationship.
What are the structures and what are the kinds
of properties that could be generated from
these kinds of structures, the kind of predictions
that we could make out of them.
And not only that; as the title again as the
title of the course suggests it is how do
you synthesize this polymers?
The general principles of some of the most
important polymerization processes will be
discussed.
So, we will now start with the first class,
the topic of this class is historical development
of polymer science.
So, what are polymers these are long chain
molecules and identical or nearly identical
units held together by covalent bonds.
So, basically we have many units hundreds
of units or may be thousands of units, which
are termed as monomers as you know now these
are termed as monomers and these are all connected
through covalent bonds.
For traditional polymer materials, there are
materials which are supra molecular polymers
where all these units are connected through
you know supramolecular associations reversible
bonds or pi pi stacking interactions you could
have hydrogen bonding interactions so on and
so forth which can be broken and reformed
reversibly.
We are not in general concerned with those
kinds of polymers, more traditional polymers
covalently bound, monomer units, strung together
to form polymers.
Those are the polymers we are going to discuss.
So, this is the general definition that I
have actually framed here identical or nearly
identical units that are held together by
covalent bonds.
Now, here I have actually classified the polymers
based on their sources, like you know we could
have natural polymers which could be based
on say polysaccharides lot of sugar units,
covalently bound, they could be based on natural
polymers they could be proteins, based on
amino acids or they could be nucleic acid
natural rubber so on and so forth or based
on the source we could prepare the polymers
in a completely synthetic, you know in the
laboratory we can make polymers, which are
not natural synthetic polymers it could be
polyethylene polystyrene nylons etcetera.
Some of this you probably already know as
far as the names of these polymers are concerned.
Or it could be semi synthetic polymers, like
natural polymers we modify them with different
groups functional groups say for example,
cellulose nitrate acetate etcetera.
Now by no means this is the only way to classify
polymers.
There are different ways to classify polymers
to which we will come slowly, but the reason
I am talking about polymers to start with
I mean I have not yet talking about historical
development, I have started with polymers
themselves some examples of them, this is
just to illustrate what are the recent things
that we talk about in polymers, means what
we know about polymers.
And then, we will try to impress upon you
the fact that some of these things which we
know all now as very simple things like the
covalent bonds and all between small monomeric
units which you have come to accept as the
facts where not very easy to establish even
100 years back we will come to that slowly.
Now, this slides about the importance of polymer
industry as the title suggest for this particular
slide.
Now more than half of the total output of
organic chemical industry, if you look at
organic chemistry the industry, it is contributed
by the polymer industry more than half of
the total output only a few polymers actually
account for this.
So you can understand; what are the importance
what is the importance of polymers.
Here are some data, forty one million metric
tons of thermoplastic resins produced in the
USA in the year of 2000 only and there is
polypropylene high density polyethylene, linear
low density polyethylene, out of say for example,
more than half of the total output of organic
chemical industry, for what we say as the
polymer industry.
In this around 40 percent already is contributed
by some of these polymers and as you can see
these polymerization are affected with transition
metal catalysts.
Then combine the output of 40 percent is provided
by LDPE, low density polyethylene, poly vinyl
chloride, polystyrene, poly vinyl alcohol,
PMMA which is poly methyl methacrylate so
on and so forth and these polymers are produced
by free radical pathway or you can say some
of these polymers basically are produced by
chain polymerizations we will talk about that
in detail as we go on.
And rest of that which is a combined output
of 18 percent polyurethanes polyamides polycarbonates
PET, PBT so on and so forth, those are from
step polymerization.
Here as you can see that this kind of distribution
that I have made.
So, different synthetic procedure are used
for preparing some of these different polymers,
which are most commonly used and other less
widely used polymers are there acrylics cellulosics,
silicones, polysulfones, etcetera and then
there are also speciality polymers like you
know polyether ether ketone, which is termed
as PEEK polyether ether ketone or poly benzimidazoles
those kind of things.
Here what I would like to tell you also is
that you know this this is ok, but the kind
of things that we are going to discuss in
the course, are going to be something like
this.
So, these are the 
classifications we could made of different
kinds of polymers say we have what you call
as commodity polymers, we have engineering
polymers, and we have high performance polymers.
So, in commodity polymers, under the class
of commodity polymers, we could talk about
low density polyethylene, high density polyethylene,
then polypropylene, polyvinyl chloride, polystyrene
etcetera.
General household applications 
these polymers are used for general household
applications and these are not you know mechanically
very robust polymers and the temperatures
that they can withstand are also limited say
100 to 120 degree Celsius approximately.
Now and some of the applications for example,
they are used for packaging they can be used
for clothing materials or they can be used
for say you know plates making plates or cups
medical trays so on and so forth.
Now, engineering polymers they have more robust
mechanical properties and they basically can
with stand higher temperature, say in the
range of 200 to 250 degree Celsius again,
they are like approximate temperatures and
they basically have better load bearing capacity
if you compare with commodity polymers.
The production cost is generally higher and
some of the polymers that will come in this
category are polycarbonates or epoxies, acrylonitrile
butadiene styrene co polymers, which is ABS
or polyethylene terephthalate which is PET
or polybutylene, polyoxymethylene so on and
so forth.
And lastly there are high performance polymers
which can also be called speciality polymers
speciality polymers, those could be like PEEK,
polyether ether ketone or polyimides or polysulfones,
they can with stand even higher temperature,
they have very good mechanical properties
generally and they are very costly to make.
So, only in case of special applications so
these are the 3 separate classes of polymers,
that we are going to discuss later on as far
as the synthesis of these polymers are concerned
as far as the structure property relationship
of this polymers are concerned.
So, after this kind of background after giving
you this kind of background now it is the
time to talk about some historical development,
because as you can see the polymer industry
has come a long way we can actually prepare
whatever structures we want we can actually
generate many properties engineer, many different
properties it may be requiring more cost depending
on the class of applications that we envisage,
but we can do that and we can characterize
the structures also very well those kind of
things we are not available even 100 years
back.
So, I am going to give you a little bit of
peak into that kind of history that is how
I have designed this particular class.
So, I will go back to that.
So, in this slide I have listed a number of
experiments that could be considered as the
earliest significant polymer experiments.
For example, in 1800 and 5 it was observed
that natural rubber when it is stretched it
generates some heat and when it is under stretched
condition; that means, it is loaded with a
with a load, then it will contract when you
are heating this material.
At that point of time it was believed that
an absorption of calorific fluid occurs, when
you are heating a loaded sample an absorption
of calorific fluid occurs and that helps the
material to contract much in the same manner
as for example, when you put water into the
ropes they will contract.
And the next event that I wanted to mention
was in 1826 when faraday analysed a an aqueous
suspension of rubber latex.
So, he was correctly able to show that this
phase contains proteins and he was able to
comment on the elemental composition of rubber
also and not only that amazingly he did an
experiment, where he heated this rubber with
sulphur and he observed the generation of
hydrogen sulphide gas, but this is something
that he explained away as you know it is a
way to reduce the amount of hydrogen in a
material when you heat it with sulphur.
So, this was one of the greatest opportunities
missed you know now that this is nothing,
but vulcanization of rubber where you can
put sulphur into a generally tacky natural
rubber material and then it has cross linking
and that renders it elastomeric; that means,
it can be stretched to a higher length you
remove your force and it comes back to it
is original configuration.
And afterwards in 1839 it was shown that when
you distil styrene there is a residue lift,
which gains thickness with time and it was
believed that it is nothing, but styrene oxide.
And later on it was also confirmed that in
absence some oxygen also this particular observation
was there.
That means, it was not styrene oxide, but
it was some compound of styrene.
They fail to realise that it was actually
a polymer of styrene.
Okay, So, with these kind of introduction
now I would like to tell you something else,
how this particular field developed in a way
that lot of other experiments were done these
were some of the main experiments lot of other
experiments where done lot of other polymers
where made, in the 19th century which where
you know commercially viable, but they did
not realise that they where polymeric materials.
So, I am going to give you an idea of what
occurred around that point of time.
So, around 1839 Charles Goodyear he actually
described the vulcanization of rubber vulcanization
of rubber.
So, that was the birth of rubber industry.
Now, something I want to mention here that
up to late 19th century up to late 19th century
whatever polymers where made they were actually
modification of natural polymers.
So, chemically modified natural polymers all
of them were like that.
An example could be the nitration of cellulose,
cellulose is a natural polymer.
Now this nitration was described way back
in 1833 and afterwards in 1847 this was controlled
in a controlled fashion this nitration was
done and then you know smokeless gunpowder
was developed, which was used as explosive.
And later on it was found that if you add
camphor to this to nitrocellulose, if you
add camphor this camphor has a plasticizing
effect on nitrocellulose and that renders
the nitrocellulose melt easily processible.
And these kind of materials where later used
for the films, for the cinema and you know
these were very highly flammable.
So, it was not a coincidence that there were
many incidences of fire that were reported
from the cinema halls.
And these class of materials this nitrocellulose
plus camphor this class of materials are known
as celluloid you may have heard about this
particular term.
So, celluloid films were invoke at that point
of time.
Now, first major discovery as far as the synthetic
polymers were concerned was in 1900 and 7
I have a slide for this after that was by
Baekland, where he discovered a way to control
the condensation of phenol and formaldehyde
and he generated the first fully synthetic
polymer, first fully synthetic polymer.
Which was known as Bakelite, now this Bakelite
that he had generated this he commercialised
this under the trade name Bakelite and this
material was an excellent electrical insulator.
So many different uses where envisages for
this particular material.
So, say this is the slide which describes
actually the material.
So, it is a network formed from phenol and
formaldehyde and you know from starting from
telephone to radio many of these materials
where made from Bakelite.
So, this was the first synthetic fully synthetic
polymer that was made, but what about the
name polymer, if you look at this particular
name this name was coined long time back in
1833 by Berzelius.
So, at that point of time what he talked about
is polymers are compounds of the same chemical
composition they exhibit different properties.
So, those are not in a traditional sense of
way the polymers that we know now.
So, they included isomers they included homologs
they included polymorphs.
So, those were not the now real polymers that
we talk about now.
The real evolution happened when, Harman Staudinger
entered that field.
So, in the early twentieth century all these
properties of polymers like they are highly
viscous, like they have low diffusion coefficients
or some of them are very difficult to crystallize
they were all explained away by saying that
this could be you know you can explain this
by the formation of Colloidal aggregates,
small molecules could associate and they can
form aggregates and these aggregates can explain
everything.
And this is something that all of this changed
when Hermann Staudinger a German chemist he
came into the picture in 1920’s he theorised
the existence of very long chains, that are
held together by covalent bonds with molecular
weight hitting hundreds of thousands.
So, this is something this particular concept
this is something that mate with lot of resistance
at that particular point.
Now I will give you an example of how you
know the structure of natural rubber was basically
explained without considering that it was
a polymer.
So, if you consider say for example, natural
rubber.
In 1900 and 5 it was described as a material
that consists of this molecule cyclooctadiene
dimethyl cyclooctadiene.
So, several of these molecules can come together
several of this molecules in this fashion
and they can have this kind of interaction
through partial valence and this material.
So, you know this kind of association of small
molecules not necessarily a polymer and this
could explain the properties of rubber that
was what was claimed, but what Staudinger
showed, basically is like this you have a
double bond here.
So, if you hydrogenate this double bond and
if this particular theory is true then you
can actually get small molecules, but that
was not the case now we know of course.
So, this is rubber 
we have polymers of Cis isoprene this is what
we know.
So, what he did is that a hydrogenation of
the double bond and when you do a hydrogenation
we did not get any volatile compound or any
cyclic product and this material has similar
property as natural rubber.
So, what he concluded is that the material
that is produced after hydrogenation of this
natural rubber has very similar properties
and they are also macromolecular in nature.
So, I am using this term macromolecule here
this is the term that was introduced in 1920’s
by Staudinger himself to explain the properties
of these materials.
Now, some of these things actually encountered
lot of resistance and rightly so, because
at that point the understanding of structures
where very much limited.
I will go to the next slide and I will explain
to you, a few comments.
So, Heinrich Wieland who got Nobel Prize in
90’s 1927, he told Staudinger a friendly
advised that abandon your idea of large molecules
organic molecules with molecular weights exceeding
5000 do not exist, what Wieland believed is
that at that the rubber that you have if you
it is basically a an impure material.
If you try to purify this material and you
can purify it properly then you will get crystallize
materials with small molecules.
So, he advised Staudinger that large molecules
do not exist, in a public debate at a conference
in Dusseldorf in Germany September 1926 one
of the chemists actually mentioned that zoologies
will be shocked, if they found that in Africa
somewhere an elephant was found which is 1500
feet long and 300 feet height.
We are also shocked the same way the idea
of polymers like this long chain with all
these molecular units connected together by
covalent bond is pure nonsense.
But later on of course, you know after all
these resistance as you know Staudinger showed
through hydrogenation that of course, this
cyclooctadiene theory is not is not right
for rubber.
So, after high hydrogenation what you get
is the same compound and one of the things
the crystallographers believed also at that
time is that the polymer molecules are.
So, big they cannot fit into a small unit
cell, but it was shown through crystallography
the Staudinger himself showed this for polyoxymethelenes,
that part of the molecule can actually be
accommodating in unit cell and still it can
be a polymer.
So, it is not a problem full molecule need
not be in a unit cell and so crystallography
aided actually Staudinger’s hypothesis a
lot.
An in 1930’s in between 1930 to 1940 Carothers
made nylon the first synthetic fibre hundreds
of molecular units where joined together through
covalent bonds, it was unambiguously proved
and also there where industrial developments
of polystyrene PVC polyvinyl chloride polyolefins
polymethyl methacrylate.
And that also calumniated into Ziegler Natta
polymerization which was discovered in 1953,
you know for high density unbranched polyethylene
and isotactic polypropylene with specific
stereo regularity those things were also discovered
and the Nobel Prize was awarded in 1963.
So, ultimately all of this things tell us
that Staudinger himself and some of the other
chemists at that point of time, they had to
undergo lot of struggles and I would like
to conclude this class by quoting from the
quoting from the Nobel committees announcement,
for Staudinger in 1953 actually he got Nobel
Prize, for his enormous contribution to the
development of polymer chemistry.
So, professor A Fletcher, a member of the
Nobel committee he actually summarised his
achievements, by these words it is no secret
that for a long time many colleagues rejected
your views and this was understandable perhaps
in the world of high polymers almost everything
was new and untried longstanding established
concept had to be revised or new once created.
So, the development of macromolecular science
does not present a picture of peace full idle.
So, I hope that I have given you a glimpse
of how the polymer chemistry was developed,
how for long time people had been actually
working on polymers without actually understanding
the structure of it, they were thinking it
was composed of small units, but they could
be held by non-covalent forces like colloidal
aggregates.
And later on Staudinger came and in front
of lot of criticism and lot of resistance
he established his theory and it was actually
ultimately shown, that polymers with long
chains can be created.
And the final example that I would like to
give is of Emil Fischer in the early 1900’s
he was a pioneer of protein chemistry.
So, he had made a long polypeptide I guess
17 to 18 amino acids join together to a tedious
synthesis.
So, that itself should have shown to him and
others that polymers are indeed possible,
but even he was sceptical of these kind of
ideas and he thought that molecules exceeding
molecular weight of 4000 could not exist.
Nowadays we know hundred hundreds and hundreds
and thousands of Dalton of molecular weight
of polymers are available.
With this, I would like to conclude this particular
lecture.
And, what I would do in the next lecture is
to go into the classification of polymers
and slowly try to talk about how you can determine
the molecular weight of these polymers.
Thank you.
