POLYMERS 3/PH/AM
Lecture 2: Meet the Molecules
The Centipede was happy quite,
Until the Toad, in fun,
Said “Pray, which leg comes after which?”
This raised her doubts to such a pitch,
She fell exhausted in a ditch,
Not knowing how to run.

(1) THE MACROMOLECULE AS A COLLECTION OF ATOMS

(2) THE MACROMOLECULE AS A PHYSICAL OBJECT

(3) HOW CAN WE OBSERVE MACROMOLECULES?

(1) THE MACROMOLECULE AS A COLLECTION OF ATOMS
Configuration and Conformation.
Configuration refers to different ways of sticking the same atoms together. You cannot
change the configuration of a molecule without breaking and remaking bonds.
Conformation refers to different ways of bending the same molecule, without breaking
bonds.

Configuration (1)
This is a Glucose Molecule.
In order to switch between the two
configurations, it is necessary to break
bonds at the right-hand carbon atom.
Starch and Cellulose
These are two polymers of glucose,
differing only in being joined by (cellulose) or -(starch) glycoside
bonds.
These small chemical differences,
however, make the polymers very
different in practice. Cellulose is a
strong insoluble crystalline structural
material in plants, while starch is
soluble, meltable (in baking) material
which is easily digested and sued by
plants as a food store. Cows, etc.,
need bacteria to break down cellulose
in their food..

Configuration (2)
The most important configurational difference – tacticity.
These are molecular models of polyvinyl chloride
(PVC). If one stretches out the main chain:
isotactic PVC has all the chlorine atoms coming
of the same side
Syndiotactic PVC has the chlorine atoms
coming out on alternate sides.
This is a linear (unbranched) Polyethylene chain, in two views.
Isotactic Polypropylene has methyl groups coming off every second chain carbon
atom, all on the same side.
Syndiotactic Polypropylene with methyl groups alternating
Atactic Polypropylene has methyl groups coming off randomly.
These three polypropylenes are different polymers.

Conformation (1)
So far, we have drawn all our polymer chains stretched out straight in a zig-zag. Is this a
realistic picture?
The left hand picture: the all-trans conformation of a polyethylene chain: this is the
minimum energy conformation. This would be the equilibrium conformation at Absolute
Zero. At T>0 deviations from the minimum-energy conformation are possible due to
thermal motion. According to Boltzmann’s law the probability of realization of the
conformation with the excess energy U over the minimum energy conformation is
Since there are so vastly many more conformations like the one on the right, one may ask:
Are straight chains ever found?
In fact, long straight all-trans segments are commonly found in crystals. This is because
crystallization releases energy to the environment, and the increase in multiplicity of states
for the rest of the universe more than offsets the reduced multiplicity of states for the
polymer chain.

Conformation (2)
Polypropylene – 3-fold helix
The helix represents the folding up
of one of the chain patterns below (I
can’t tell you which!). The folding
moves the methyl group apart so
they do not interfere with each other.
 Amino Acids – Polypeptides and Proteins
For the structure of these, I would recommend going to books. You can also check out the
following two websites.
http://www.bact.wisc.edu/MicrotextBook/BacterialStructure/Proteins.html
stingray.bio.cmu.edu/~web/bc1/pstruc/pstruc.htm

Conformation (3)
Here are an ethane molecule and a butane
molecule. The latter can act as a simple model
for the chains of polyethylene and all vinyl
polymers.
The central bond of the butane molecule is
taken as fixed, but the carbon atoms at each
end of this bond are capable of rotation. In
butane the groups are methyl, but they serve as
models for chains extending onwards.
The black blobs represent methyl groups
(butane) or chain extensions (PE), while the
white blobs represent hydrogen atoms.
As one group rotates relative to the other, it
goes between staggered (,120,240) and
eclipsed (60,180,300) conformations.
When eclipsed the hydrogen atoms and the
big groups all interfere, this being
unfavourable energetically. When
staggered the interference is much less,
especially in the trans conformation.
This shows the energy associated with
different rotations of the butane molecule
(solid) and ethane molecule (dotted). The
end on views are staggered.
The eclipsed conformations are quite
unlikely at room temperature, but since
at room temperature, RT ~ 2.4 kJ.mole-1
there will be considerable a population in
the gauche states.

(2) THE MACROMOLECULE AS A PHYSICAL OBJECT
The next overheads are the lecture notes to:
Introduction to polymer science by Prof. A.R.Khokhlov, (Moscow State University)
Lecture 2
http://polly.phys.msu.su/education/courses/polymer-intro/lecture2.pdf

(3) HOW CAN WE OBSERVE MACROMOLECULES?
Sometimes we can actually see them under the electron microscope. DNA is a good
example of this.
Scattering Methods
Light Scattering
Neutron Scattering
Atomic Nucleus
Scattering Length (fm)
1H
- 3.741
2D
+ 6.671
C
+ 6.646
N
+ 9.362
O
+ 5.803
Direct Measurement
Size exclusion chromatography.
MALDI Mass Spectroscopy.