Stereo-Chemistry of Macromolecules: configurations & Conformations of Macromolecules

1. Aadrsh Kumar Tiwari
Roll no. - 131021
A Central University

2. 1.Configuration of Polymer Chains
2.Tacticity in Polymers-Monotactic and
Ditactic Polymers
3.Diads
4.Stereoregular Polymers
5.Experimental and Spectroscopic Methods
for the Determination of configuration,
conformation of single macromolecule
Content

3. Configuration & Conformation of Macromolecule
•The geometrical structure of a macromolecule of polymer
depends upon the spatial arrangement of the monomeric
units with respect to each-other.
• The spatial arrangement of monomeric units in a
polymer chain can be discussed by the terms,
configuration & conformation.
Configuration :- A configuration is an arrangement fixed by
chemical bonding adjacent monomeric units and between atoms
of individual monomeric units.
•The configuration remains unchanged as long as the chemical
bonds are not reformed.
•A polymer chain can not shift from one configuration to another
without breaking /reforming the chemical bonds.
Example- configuration are arrangement arounds asymmetric C-
atoms, several stereoregular arrangements, head-head, tail-tail &
head-tail arrangements in vinyl polymers.

4. Conformation:-A conformation is an arrangement
resulting from the rotation of chain segments or
adjacent monomeric units around the single bonds.
•This type of rotation does not consist of any breaking
or reforming of the chemical bonds.
•Polymer chains configuration depends on applied
stress, thermal energy and solvent interaction.
Example- conformations of polymer chains
include transe versus gauche arrangements of
consective C-C single bonds and helical
arrangements found in some polymer crystal
structure.

5. Tacticity
The term tacticity is derived from a Greek word
‘tactikos’, means arrangement or order.
Tacticity is the relative stereochemistry of adjacent
chiral centres within a macromolecules.
Diads
Two adjacent structural units in a polymer molecule
consistute a diad.
If, the diad consists two adjacent identically oriented
units called meso diad (meso compound) as mm.
If, thediad consitsts of units oriented in opposite called
a racemo diad (racemic compound) as mr.

6. Tacticity Measurements
•Tacticity can be measured by proton or carbon-13
NMR .
•This technique enables a quantitative assignment of
degree of tacticity by integrating the peak area of
known diad (rr, mm, rm) triad (rrr, rrm, rmr, rmm,
mmm)* and higher order polymer subunit frequency
(ppm).
•Bernoullian or Markovian analysis of these peak
areas then can be used to calculate the tacticity of the
polymer.
Triad composition can be calculated from probability
of the finding meso diads(Pm).
1. For an isotactic triad is (Pm)
2
2

7. The tactic configuration in a polymer molecule can be
depicted as:
1.Atactic Configuration: In atactic (A= non or
random, tactic=arrangement ) configuration, the
substituents in macromolecule are placed randomly
along the chain.
Due to random nature, atactic polymer are usually
amorphus.

8. 2. Stereoregular Isotactic Configuration
In isotactic (iso = same, tactic = arrangement)
configuration , all the substituent groups R lie above or
below the plane of the main chain of polymer.
Isotactic polymers are usually semi-crystalline and often
form a helix configuration.
3. Syndiotactic Configuration: In syndiotactic (syn =
alternate or opposite, tactic = arrangement)
configuration, the substituent groups R lie alternatively
above and below the plane along the chain of polymer
molecule.

9. Head-to-head, Tail-to-tail configuration:
•In vinyl polymers the complete configuration can be
further described by definding polymer head/tail
configuration.
•In a regular macromolecule all the monomer units are
normally linked in a head to tail configuration so that all
ß-substituents are separated by three carbon atoms.
•In head to head configuration the separation is only by
2 carbon atoms and the separation with tail to tail
configuration is by 4 carbon atoms.

10. Configurations involving a C=C bond :
Polymers 1,3-diene contain one residual double bond
per repeat unit after polymerization.
These polymers can consists of sequences with several
different configurations.
Monosubstituted butadiene (e.g. isoprene), the following
structures are possible:

11. Stereoregular
configuration
•Stereoregular configuration is found in stereoregular
polymers; where each monomer segment is in a regular
configuration.
•It provides a definite structural regularity to the polymer
molecule as a whole.
•This structural regularity of a polymer molecule is
defined as optical and geometrical isomerism.
Optical Isomerism:
The polymers, which are capable of rotating the plane of
polarized light known as optically active compounds.
While in simple low-molecular-weight-compounds,
optical activity is associated with the presence of
asymmetric carbon atoms, this is not true in polymers.
It was found that every second chain carbon atom in
vinyl polymer is asymmetric in nature.

12. Let, us take the example of poly-ethylene polymer
molecule.
Its planar zig-zag structure:
If one of the hydrogen atoms in all ethylene units of
polyethylene molecule is substituted by a substituted by
a R( R may be CH3,Cl or CN), then the structural
formula of this polymer would as follows:

13. •In above polymer chain, every alternate carbon atom
can be cosidered to be asymmetric.
•This means that alternate carbon atom carries four
different substituents, H, R, and two polymer chain
segments.
•The regularity in which the successive asymmetric
carbon sites, exhibit their d or l form gives rise to three
different types of isomeric structure in the polymer
molecule i.e. as follows:
1. Isotactic configuration of polyethylene molecule:

14. 2. Syndiotactic configuration of polyethylene molecule:
3. Atactic configuration of polyethylene molecule:
•These are three types of the polymers have the same chemical
structure, but provide entirely different properties because of their
different configuration and the geometrical structure.

15. Geometrical isomerism:
•In optical isomerism, two carbon atoms are attached
with (C-C) single bond, while in geometrical isomerism
carbon-carbon bond is double, i.e. C=C.
•Geometrical isomerism is exhibited due to different
arrangement and configurations of subunits groups
found on the C=C.
•For example, let
1,3-butadiene molecule , it has two double bonds in its
structure as:

16. •The resultant polymer consists of a double bond in each
repeat unit.
•Each of these double bonds provides, a site for a steric
isomerism.
•The two possible isomerisms are:
H
1,2-vinyl configuration
•A third configuration, called 1,2-vinyl is also possible
as:

17. •During the polymerization of 1,3-butadiene, if all the
repeat units take cis-configuration, then 100% cis - 1,3-
polybutadiene is formed due to bending back of C-C
chain segments and whole molecule looks like a spring
and shows high elongation.
•If all the repeat unit take trans-configuration during
polymerization, then the resultant polymer is 100%
trans-polybutadiene, due to the straightening out of all
the C-C chain segments, the whole molecule assume a
straight and stiffened rod-like structure.
•It exhibits low elongation.

19. Spectroscopic Methods
•Infrared spectroscopy
•Nuclear magnetic resonance spectroscopy
•Electron paramagnetic resonance
spectroscopy
•X-Ray diffraction method
•Electron microscopy
•Thermal analysis
•Physical testing

20. Infrared spectroscopy:
•The infrared frequencies in the wavelength range 1-50
m are associated with molecular vibration and vibration-
rotation spectra.
Experiment: for preparation of sample of polymer for
infrared methods ,the compression molding technique is
applied.
• The sample is dissolve either carbon-tetra-chloro-
ethylene or carbon di-sulphide because there spectrum
is usually free of intense absorption bands.
•Now prepare a thin film by micro-toming or milling,
casting is from solution and pressing a finally ground
mixture of sample with KBr to form a disc or water.

21. •The observation obtained from infrared region are in the
range of 2-15 m wavelenght.
•Therefore, it is required to supplement the observations
in far infra red region, i.e., upto 200 m.
•In some polymers , like poly tetra-fluoro-ethylene, most
of the absorption bands occur above 15 m.
Nuclear magnetic resonance spectroscopy:
NMR spectroscopy is an important tool for the
determination of micro-structure of polymers.
Experimental method:
NMR technique utilizes the property of spin of nuclei
which possess odd atomic number and mass number,
both.
Example of these atoms are the isotopes of hydrogen
,13C , 15N , 17O, and 19F.

22. If such nuclei are placed in strong magnetic field, their
energy level splits into two, with parallel and anti-parallel
spin.
In the process of transition states either absorption or
emission of energy takes place.
•The NMR spectroscopy absorption wave-length of
olefinic groups:-
Wave-length ( m)Group containing C=C
1. Vinyl, R1CH=CH2
2. Trans-R1CH=CH2
3. Vinylidene , R1R2C=CH2
4. R1R2C=CHR3
5. Cis-R1CH=HR2
10.1 and 11.0
10.4
11.3
12.0
14.2(varriable)

23. •The NMR spectroscopy is useful in the field of polymer
science in the following manner:-
1.The determination of the stereo-chemical
configurations of the polymer chains has been
achieved by NMR techniques.
• Poly (methyl-methacrylate) was the first polymer,
which was studied by Bovey in 1960.
• The statistical frequency of all possible combinations
upto four pair of units , i.e. , either the same (meso)
or opposite (racemic) configurations can be
elucidated by NMR.
2.Geometrical isomerism in polymer chains can be
success for determined by NMR methods.
3.The sequence of monomers in a copolymer has been
analysis by NMR spectroscopy.

24. •Since about 1960 nuclear magnetic resonance
(NMR) spectroscopy has become a major tool
for the study of chain configuration, sequence
distribution, and micro-structure in polymers.
•It use has evolved from early broad line studies
of the onset of molecular motion in solid
polymer, through the widely practiced solution
studies of proton NMR , to the application of the
more difficult but more powerful carbon 13 NMR
methods to both liquids and solids.
•Despite the wide spread use of NMR, a brief
summary of its origins an experimental method
is warranted.

25. Electron paramagnetic resonance
spectroscopy
(EPRS)•The electron paramagnetic resonance (EPR)
spectroscopy is useful in the detection of free radicals.
•The basic principle and operation technique of EPR is
same, while their applications are altogether different.
•Free radical consist of an unpaired electron, therefore,
can be determined by their magnetic moment.
Experimental method:
•As in NMR spectroscopy the action of a strong
magnetic field on a material containing free radicals
removes the degeneracy of their ground state energy
level.
• for low radical concentrations the new energy levels
are given by the two terms, first is:

26. Where, g= tensor relating the field direction & the
symmetry directions in the radical,
= magnetic moment of the electron spin
= magnetic permeability of a vacuum.
•The second term represents coupling of the electron
spin with the nuclear spins in the molecule.
Application:
•The EPR results have contributed in high energy, in the fields
of the characterization of the structure of free radicals.
•The structure of the radical can be interpreted by EPR
spectroscopy.
•The spectrum reveals fine structure due coupling between
the unpaired electron and adjacent 19F nuclei.

27. X-Ray diffraction method:-
•The X-ray diffraction method has become an important
tool for the study of the arrangement of atoms or
molecules through the interaction of the electron
magnetic radiation.
Experimental method:
•In the X-ray diffraction method , the x-rays are
generated by bombarding a beam of high voltage
electrons on a metal target.
•The electrons are used to move in a vacuum tube .
•The produced X-rays are allowed to pass through a
beryllium or polyester window in the tube.
•The wave-length of the generated X-rays depend on the
voltage applied and the metal target.
•The diffracted X-rays can be detected by following

28. 1.Photo-graphic film or plate:
• The specimen to be analysed by this method is
placed b/w the X-rays and a photographic film.
• The accurate measurement of angles and distances ,
and qualitative determination of the diffraction
pattern are completely found on photographic film.
2.Radiation counter method:
• Counting method possess its advantages when
correct measurement of the intensity of the diffracted
beam is required.
• The X-rays diffraction method is also affacted by the
physical state of sample:
a) If sample is a single crystal , gives the information
about all of its possible orientations a time.
b) If sample is powder of very tiny crystals and the
minute powdered particles are randomly oriented,

29. all orientations will be included with in the sample.
Applications:
•The polymer crystals consists of perfect geometrical
arrangement of atoms.
•The repeat unit of monomers in a polymer can be
determined by X-ray diffraction technique.
•Distortions in crystal structure of polymers can be
detected by X-rays diffraction method.
Microscopic methods:
The following types of microscopy is useful in the
detection of polymer structure:
1. Light microscopy: To determine the texture of a solid
opaque sample of polymer, the light microscopy has
been used.
For the purpose the common techniques are used:

30. a) Polarized-light microscopy
b) Phase-contrast microscopy
c) Interference microscopy
2. Electron microscopy: Electron microscopy has been
a powerful tool in the study of the morphology of
crystalline polymers by using an electron in place of
light.
• The usual techniques of replication ,heavy-metal
shadowing and solvent etching are generally used.
• Direct observation of thin specimen, e.g. polymer
single crystal is also possible and allows the
observation of the electron diffraction pattern of the
same specimen area, for determining the
crystallographic directions and relating them to
morphology.
• There is one disadvantages that the crystal of

31. minute.
•This problem can be solved by maintaining low temp. ,
i.e. below the room temperature.
Scanning Electron microscopy(SEM):
•In SEM a fine beam of electrons is scanned across the
opaque specimen of polymer.
•This specimen consist of a light conducting film which is
applied by evoparation.
•When electron beam hit the specimen , the secondary
electrons are emitted.
•These electrons are collected and then they produce a
single to modulate the intensity of the electron beam on
a viewing screen.

32. Thermal analysis:
•Thermal analysis of the polymers can be easily
performed by various instruments:
•The calorimetric analysis, differential thermal analysis
(DTA),thermo-gravimetric analysis (TGA), thermo-
mechanical analysis (TMA), electrical thermal
analysis(ETA), and affluent gas analysis (EGA) are
useful to study a wide variety of characteristic details of
the system to temp. , degradation, polymerization and
other chemical changes.Differential scanning calorimetry:
Experimental method:
•In contrast to earlier use of a large, expensive adiabatic
calorimeter for measurements of specific heat and
enthalpies of transition, these measurements are now
usully carried out on quite small samples in a DSC.

33. •The term is applied to two different modes of analysis,
of which the one more closely related to traditional
calorimetry is described here.
•In DSC an average temperature circuit measures and
controls the temperature of sample & reference holders
to conform to a predetermined time temperature
program.
•This temp. is plotted on one axis of an x-y recorder.
•At the same time, a temp. difference circuit compares
the temp. of the sample and reference holders &
proportions power to the heater in each holder so that
the temp. remains equal.
•When sample undergoes a thermal transition, the
power to the two heaters is adjusted to maintain their
temp. & a signal proportional to the power difference is
plotted on the second axis of the recorder.

34. •The area under the resulting curve is direct measure of
the heat of transitions.
Application:
•As a typical result specific heat temp. cause obtained
(by adiabatic calori-metry) on heating quenched
(amorphus) specimens of poly ethylene tere-phthalate
(PET).
•Each curve rises linearly with temp. at low temp.& then
rises more steeply at the glass transition , 60-80°C.
•With the onset of mobility of the molecular chains above
this transition, crystallization take place.
•As indicated by the sharp drop in the specific heat curve
, at still higher temp. 220-270°C ,the crystal melt with a
corresponding rise in the specific heat curve.

35. 0-20 100 200 300
4
8
12
16
20
Specificheat,J/g°C
Temperature, °C
Fig: curve of specific heat as a function of increasing
temp. for quenched PET

36. Differential thermal analysis:
Experimental methods: In this method of analysis, the
sample and an inert reference substance is heated at
the same rate.
The temp. difference b/w the sample & reference
substance is measured and plotted on a graph as a
function of temperature.
Application:
•A typical DTA result is the differential thermal analysis
curve for poly (ethylene tere-phthalate) .
•The lower crystalline melting range in the specimen is
attributed to impurities in the polymer.

37. 20 40 60 80 120100 240160140 220200180 260
EndothermicdTexothermic
Fig: DTA curve for amorphus PET
Temperature, °C

38. Thermo-gravimetric method
(analysis) or TGA:
•In TGA a sensitive balance is used to
follow the “weight change” of the sample
as a function of temperature.
•Typical applications include the
assessment of thermal stability and
decomposition temp. , extent of curve in
condensation polymers, composition and
some information on sequence distribution
in copolymers, & composition of filled
polymers, among many others.

39. Temperature, °C
%ofwt.remaining
20
40
60
80
100
120
0 400200
600 800
PVC
PTFE
HPPE
PMMA
PI
Fig: relative thermal stability of polymers as determined by wt.
loss on heating 5°C/min in nitrogen in TGA.
PVC first loses HCl: later the mixture of unsaturated C-C
backbone and unchanged PVC partly chars & partly degrades to
small fragments. PMMA , branched Polyethylene(HPPE), & poly-
tetra fluoro-ethylene(PTFE) degrade completely to volatile
fragments, while polyimide(PI) partially decomposes forming a

40. Thermo-mechanical analysis:
•Thermo-mechanical analysis (TMA) measure the
“mechanical response” of a polymer system as the temp.
is changed.
•Typical measurements include dilatometry , penetration
or heat deflection , torsion modulus and stress – strain
behaviour.
Physical Testing:
The important test methods for measuring the physical
properties of polymers are as follows:
Tensile strength :-this is the force, developed
when the polymer sample is elongated at
constant rate of extention.
•Tensile strength is determined by stress-strain
curve for any plastic material.

41. •The tensile strength is generally measured at rates of
strain of 1-100% per minute.
•At higher rate of strain, i.e. upto 10.6 % per minute, the
tensile strength and modulus increase several fold, while
elongation decreases.
Fatigue:
•Most of materials fail at a stress and cause rupture in a
single stress cycle when the cycle mechanical stress are
applied on them, such phenomenon is called fatigue.
•Various modes of fatigue testing are use , such as
alternating tensile , compressive stress & cycle flexural
stress.
Impact:
•Impact strength of polymers is generally measured by
tests in which a pendulum hit the specimen with a strong
striking edge.

42. •After breaking the specimen, the energy required to
break the specimen can be calculated.
•The breaking or rapture in polymer specimen may be
divided into two classes :
a)Brittle b)Ductile
a) Brittle:
• Brittle rupture occurs if the polymer behaves
classically upto the point of failure.
• The brittle point is usually determined by putting a
specimen to impact in a standardized way.
• The temp. of the test is allowed to change until that
temp. is found where half the specimen fail by brittle
rupture.
• The brittle point is almost related to the glass
transition temperature .

43. •Brittle failure is characterized by lack of distortion of the
broken parts.
b)Ductile:
In ductile rupture the specimen is permanently distorted
near the failure point.
Tear resistance:
•In packaging purpose, plastics are used in the form of
films.
•Their resistance of tearing is an important property.
•In tear strength test , a specimen is torn apart at a cut
made by a sharp blade.
•The energy is provided by a falling pendulum , and the
work done is measured by the residual energy of the
pendulum.
•The tear strength and tensile strength are closely
related properties.

44. Hardness:
•Hardness is a composite property, it contains the
concepts of resistance to penetration , scratching ,
marring etc.
•Most of the hardness tests are based on resistance to
penetration by an indicator pressed into the plastic under
a constant pressure.
Abrasion resistance:
•Abrasion means scratches.
•It usually measured by scratch test in which the polymer
is allowed for several scratches from the contact with an
abrasion wheel or a streem of falling abrasive material.
•The degree of abrasion can be measured by loss of
weight for severe damage.
•It is usually determined by loss of glass or development
of haze in transparent specimen.