Aadrsh kumar tiwari bbau

Stereo-Chemistry of Macromolecules: configurations & Conformations of Macromolecules

1. BBAU LUCKNOW Aadrsh Kumar Tiwari Roll no. - 131021 A Central University Power Point Presentation on Stereochemistry of Macromolecule

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). For an isotactic triad is (Pm) A heterotactic triad is 2Pm(1-Pm) A syndiotactic triad is (1-Pm) 2 2

7. The tactic configuration in a polymer molecule can be depicted as: 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: 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.

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

19. 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.

20. 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 , 13 C , 15 N , 17 O, and 19 F.

21. 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 Vinyl, R 1 CH=CH 2 Trans-R 1 CH=CH 2 Vinylidene , R 1 R 2 C=CH 2 R 1 R 2 C=CHR 3 Cis-R 1 CH=HR 2 10.1 and 11.0 10.4 11.3 12.0 14.2(varriable)

22. 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.

23. 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.

24. 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:

25. 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 19 F nuclei.

26. 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 method:-

27. 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: If sample is a single crystal , gives the information about all of its possible orientations a time. If sample is powder of very tiny crystals and the minute powdered particles are randomly oriented,

28. 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:

29. Polarized-light microscopy Phase-contrast microscopy 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 polymer may be damage several times within few seconds to a

30. 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.

31. 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.

32. 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.

33. 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.

34. -20 100 200 300 4 8 12 16 20 Specific heat, J/g °C Temperature, °C Fig: curve of specific heat as a function of increasing temp. for quenched PET

35. 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.

36. 20 40 60 80 120 100 240 160 140 220 200 180 260 Endothermic dT exothermic Fig: DTA curve for amorphus PET Temperature, °C

37. 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.

38. Temperature, °C % of wt.remaining 20 40 60 80 100 120 400 200 600 800 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 char above 800°C.

39. 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.

40. 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.

41. 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 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 .

42. 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.

43. 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.

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