S Ramakrishnan discusses condensation polymerization, also known as step-growth polymerization. He describes how polymers form through reactions between small molecules that link together, like LEGO blocks fitting together. An example given is the reaction between a dicarboxylic acid and a diol to form a polyester polymer. Ramakrishnan defines important terms like functionality, fractional conversion, and degree of polymerization. He derives the Carothers equation, which relates the fractional conversion (extent of reaction) to the average length of the polymer chains formed (degree of polymerization).
Photogeneration of Gelatinous Networks from Pre-existing PolymersGregory Carroll
In this manuscript we report the crosslinking
of pre-existing macromolecules in solution through the use
of photoactive benzophenone chromophores. We show that
a bifunctional crosslinker composed of two benzophenone
chromophores as well as a single benzophenone chromophore
crosslink poly (butadiene) and poly (ethylene oxide)
in solution to form insoluble gels when irradiated with UV
light. The molecular weight between crosslinks of the photogenerated
gels was compared for the two crosslinkers, for an
equivalent amount of benzophenone chromophores in each
solution, by measuring the swelling ratio of the gels formed.
Gels formed from the bifunctional benzophenone crosslinker
were shown to contain more than twice as many
crosslinks compared to gels formed from the crosslinker
composed of a single benzophenone chromophore. EPR
measurements of a nitroxide derivative absorbed into the
gels further supported a higher crosslink density for the
gels formed from the bifunctional benzophenone crosslinker.
This document discusses covalent bonding and molecular structures. It defines covalent bonds as bonds formed by shared electron pairs between atoms. It explains that molecules are groups of atoms held together by covalent bonds in a specific ratio and shape. The document discusses drawing Lewis dot structures and molecular diagrams to represent molecules and the bonding between their atoms. It provides examples of drawing the Lewis dot structure for carbon tetrachloride and matching molecular diagrams to chemical formulas.
This document discusses covalent bonding and molecular structures. It defines covalent bonds as bonds formed by the sharing of electron pairs between atoms. It explains that molecules are groups of atoms held together by covalent bonds, and that their structures can be represented through chemical formulas, structural diagrams, and Lewis dot diagrams. It provides examples of how to determine the elements and numbers of each from a chemical formula, and how to draw Lewis dot diagrams of molecules by matching atoms to reach full valence shells.
This document provides an introduction to molecular orbital theory (MOT). It defines MOT as occurring when individual atomic orbitals combine to form molecular orbitals as electrons are associated with multiple nuclei when atoms bond. The key points covered are:
1) MOT explains questions not answered by valence bond theory, such as why H2 does not exist and why O2 is paramagnetic.
2) Bonding orbitals have lower energy than atomic orbitals and result from constructive interference, while antibonding orbitals have higher energy from destructive interference.
3) MOT is used to explain the bonding in molecules such as H2, O2, CO, and NO.
The document summarizes key concepts in polymer science including: 1) atomic structure and bonding properties that enable polymer formation, 2) the types of bonds that hold polymers together (e.g. covalent, secondary), 3) how functional groups allow monomers to link up into polymer chains, 4) the differences between thermoplastics and thermosets, and 5) ways multiple monomers can be combined in copolymers.
The fluid mosaic model proposes that cell membranes are two-dimensional solutions of oriented globular proteins and lipids. The model views membranes as a fluid structure, where proteins and lipids can move laterally within the plane of the membrane. This fluid structure is stabilized by noncovalent interactions between hydrophobic and hydrophilic regions of lipids and proteins. The fluid mosaic model provides a framework for understanding the molecular organization and functions of cell membranes.
The document discusses chemical bonding and molecular structures. It begins by defining a chemical bond as the force that binds two atoms together within a molecule. It then discusses the different types of bonds ranked by decreasing bond strength - ionic, covalent, coordinate, hydrogen, and Van der Waals. Ionic bonds form through the transfer of electrons from metals to nonmetals. Covalent bonds form through the sharing of electron pairs between atoms. The document also discusses bond parameters such as bond length, bond order, bond energy, bond angle, and dipole moment. It introduces concepts such as Lewis structures, formal charge, resonance structures, and hybridization. It concludes with an overview of valence bond theory and molecular orbital theory.
The document discusses molecular orbital theory (MOT), an approach to bonding in which orbitals encompass the entire molecule rather than being localized between atoms. MOT was put forward by Hund and Mulliken and later modified by Jones and Coulson. It addresses some of the drawbacks of valence bond theory, including explaining the paramagnetic nature of O2. MOT uses the linear combination of atomic orbitals (LCAO) approach and Hund's rule to determine molecular orbital configurations and energies.
Photogeneration of Gelatinous Networks from Pre-existing PolymersGregory Carroll
In this manuscript we report the crosslinking
of pre-existing macromolecules in solution through the use
of photoactive benzophenone chromophores. We show that
a bifunctional crosslinker composed of two benzophenone
chromophores as well as a single benzophenone chromophore
crosslink poly (butadiene) and poly (ethylene oxide)
in solution to form insoluble gels when irradiated with UV
light. The molecular weight between crosslinks of the photogenerated
gels was compared for the two crosslinkers, for an
equivalent amount of benzophenone chromophores in each
solution, by measuring the swelling ratio of the gels formed.
Gels formed from the bifunctional benzophenone crosslinker
were shown to contain more than twice as many
crosslinks compared to gels formed from the crosslinker
composed of a single benzophenone chromophore. EPR
measurements of a nitroxide derivative absorbed into the
gels further supported a higher crosslink density for the
gels formed from the bifunctional benzophenone crosslinker.
This document discusses covalent bonding and molecular structures. It defines covalent bonds as bonds formed by shared electron pairs between atoms. It explains that molecules are groups of atoms held together by covalent bonds in a specific ratio and shape. The document discusses drawing Lewis dot structures and molecular diagrams to represent molecules and the bonding between their atoms. It provides examples of drawing the Lewis dot structure for carbon tetrachloride and matching molecular diagrams to chemical formulas.
This document discusses covalent bonding and molecular structures. It defines covalent bonds as bonds formed by the sharing of electron pairs between atoms. It explains that molecules are groups of atoms held together by covalent bonds, and that their structures can be represented through chemical formulas, structural diagrams, and Lewis dot diagrams. It provides examples of how to determine the elements and numbers of each from a chemical formula, and how to draw Lewis dot diagrams of molecules by matching atoms to reach full valence shells.
This document provides an introduction to molecular orbital theory (MOT). It defines MOT as occurring when individual atomic orbitals combine to form molecular orbitals as electrons are associated with multiple nuclei when atoms bond. The key points covered are:
1) MOT explains questions not answered by valence bond theory, such as why H2 does not exist and why O2 is paramagnetic.
2) Bonding orbitals have lower energy than atomic orbitals and result from constructive interference, while antibonding orbitals have higher energy from destructive interference.
3) MOT is used to explain the bonding in molecules such as H2, O2, CO, and NO.
The document summarizes key concepts in polymer science including: 1) atomic structure and bonding properties that enable polymer formation, 2) the types of bonds that hold polymers together (e.g. covalent, secondary), 3) how functional groups allow monomers to link up into polymer chains, 4) the differences between thermoplastics and thermosets, and 5) ways multiple monomers can be combined in copolymers.
The fluid mosaic model proposes that cell membranes are two-dimensional solutions of oriented globular proteins and lipids. The model views membranes as a fluid structure, where proteins and lipids can move laterally within the plane of the membrane. This fluid structure is stabilized by noncovalent interactions between hydrophobic and hydrophilic regions of lipids and proteins. The fluid mosaic model provides a framework for understanding the molecular organization and functions of cell membranes.
The document discusses chemical bonding and molecular structures. It begins by defining a chemical bond as the force that binds two atoms together within a molecule. It then discusses the different types of bonds ranked by decreasing bond strength - ionic, covalent, coordinate, hydrogen, and Van der Waals. Ionic bonds form through the transfer of electrons from metals to nonmetals. Covalent bonds form through the sharing of electron pairs between atoms. The document also discusses bond parameters such as bond length, bond order, bond energy, bond angle, and dipole moment. It introduces concepts such as Lewis structures, formal charge, resonance structures, and hybridization. It concludes with an overview of valence bond theory and molecular orbital theory.
The document discusses molecular orbital theory (MOT), an approach to bonding in which orbitals encompass the entire molecule rather than being localized between atoms. MOT was put forward by Hund and Mulliken and later modified by Jones and Coulson. It addresses some of the drawbacks of valence bond theory, including explaining the paramagnetic nature of O2. MOT uses the linear combination of atomic orbitals (LCAO) approach and Hund's rule to determine molecular orbital configurations and energies.
Experiment 1 Molecular Models Modeling the shape of small organic.docxnealwaters20034
Experiment 1: Molecular Models Modeling the shape of small organic molecules
Previously we have considered molecules and ions for which one chemical formula corresponded to one chemical compound only. Not all chemical compounds are like that. For example, consider the formula C2H6O. It turns out that there is more than one compound with that chemical formula:
Ethanol Dimethyl ether
These two molecules have completely different chemical and physical properties. They are called structural isomers. They have the same chemical formula with different bonding between atoms. Another example would be the compounds that correspond to butane, with the chemical formula C4H10. There are two structural isomers of butane.
1. Explain why the two structures above are NOT considered structural isomers.
2. Construct two structural isomers of C4H10. Draw them below using expanded structural or line formulas. When you are finished, compare them with the results of other students.
Geometric Isomerism:
An example of a different kind of isomerism occurs when the molecules have the same bonding between the atoms but their arrangement in space is different. We say that these compounds are geometric isomers. A classic example involves molecules that contain double bonds.
Circle the structure named cis-2-butene. The double bond between the carbon atoms does not allow the free rotation of the methyl (CH3) groups with respect to one another, preventing the interconversion between the trans and cis isomers. Geometric isomers have different physical properties but almost identical chemical properties
3. What do you think is the meaning of the prefix “cis-” vs “trans-”?
Here’s another example of geometric isomers.
Construct cyclopentane, C5H10, which does not contain any double bonds.
(the blue lines show these atoms are on the other side of the ring)
Replace one of the hydrogens with chlorine to obtain trans-1,3dichloropcyclopentane (as in the drawing below). Build the trans-isomer of this molecule (based on what you learned above) and draw your structure in the empty box.
There is no free rotation around the C-C bonds that connect the carbons where the chlorine atoms are bound because of the rigidity of the cyclopentane molecule. Therefore, there is no interconversion between the cis and trans forms.
Thus, cis- and trans- prefixes refer to geometrical isomers!
We have briefly introduced the concepts of structural and geometric isomers. There is yet a third type of isomerism that we will leave out of this discussion: it is the so-called optical isomerism that will be covered in the organic chemistry courses.
Follow-up Questions
1. In the first few pages you learned about structural isomers and geometric isomers. Define these terms below:
Structural isomer
___________________________________________________________________________________________________.
This document discusses polymers, including their classification into thermoplastics, thermosetting polymers, and elastomers. It describes two main ways polymers are created - addition polymerization and condensation polymerization. Addition polymerization builds polymer chains by adding monomers together without creating a byproduct, while condensation polymerization condenses out a small molecule like water as a byproduct. The document provides examples of calculating the degree of polymerization and amount of initiator required for polyethylene production. It examines the effect of temperature on thermoplastics and their mechanical properties.
This course provides an in-depth understanding of three-dimensional macromolecular structure and the relationship between the conformation of proteins and nucleic acids and their biological functions. Students will learn to visualize and analyze macromolecular structures using molecular graphics software and assess the structural basis of biological activity. The course covers topics related to multi-molecular assemblies, catalytic machines, and membrane proteins. Students will be assessed through a final exam and computer graphics exercises completed in a lab notebook.
Atoms and molecules class 9 important questions science chapter 3 q &aRanjani Deepak
The document provides important questions and answers from the science chapter "Atoms and Molecules" for Class 9. It includes 17 questions covering topics like definition of cation, Dalton's atomic theory, chemical formulae, molar mass, moles, and the law of constant proportions. The questions aim to help students score more marks in exams.
This document summarizes information about delocalization in chemical bonding. It begins with definitions of delocalized and localized electrons. It then discusses different types of delocalization including in isolated, conjugated, and cumulated systems. Examples are provided of delocalization in benzene and isolated and cumulated systems. Effects of delocalization include increased stability and acidity as well as larger dipole moments. The document concludes with sections on resonance, how it occurs, and rules for drawing resonance contributors.
Here are the key points about polar and nonpolar covalent bonds:
- Nonpolar covalent bonds form between atoms with similar electronegativity. Electrons are equally shared. Examples include H2, Cl2, etc.
- Polar covalent bonds form between atoms with different electronegativity. Electrons are unequally shared, resulting in partial positive and negative charges on the atoms. Examples include HCl, H2O.
- Polar molecules are attracted to each other due to their partial charges. They are soluble in polar solvents like water. Nonpolar molecules are not attracted and are soluble in nonpolar solvents like hexane.
- Most bonds have some ionic character based on electrone
The document is a lab activity on molecular polarity using a PhET simulation. Students investigate how electronegativity affects bond polarity and molecular polarity when atoms bond. They describe different bond types such as nonpolar covalent, polar covalent, and ionic based on the electronegativity values of the atoms. Students also explore how molecular polarity arises from bond dipoles combining through vector addition and how molecular polarity determines if molecules will dissolve in polar solvents like water.
Molecular orbital theory provides an approach to calculate molecular orbitals through a variational method. This involves taking linear combinations of atomic orbitals to form molecular orbitals. Electrons occupy these molecular orbitals according to certain rules. The molecular orbital theory can explain properties such as why some molecules are paramagnetic that valence bond theory cannot. Calculating molecular orbitals variationally involves using trial wave functions in the Schrodinger equation to find the lowest possible energy state.
Question 5b) In Question 5(a) you have written the stepwise.docxmakdul
Question 5:
b) In Question 5(a) you have written the stepwise equilibrium reactions that must occur in order to produce the final complex, [Fe(acac)3]. All three equilibria can be occurring in solution to differing extents depending on what species are present and how Le Châtelier’s Principle might affect the reaction position. The three test tubes in Diagram 2 (steps 3, 4 and 5 in the PROCEDURE) show varying extents of reaction. Of the four complexes present in the reaction scheme, which solvent would you expect each one to be soluble in: polar water or non-polar dichloromethane? (First answer is provided.)
Question 6:
If the colour intensity of the layers changes position on going from test tube 2 to test tube 3, what does this suggest about the complexes that are present and, therefore, what has happened to the equilibrium position?
Foundations of Chemistry Laboratory Manual EQUILIBRIUM and LE CHÂTELIER’S PRINCIPLE
1
EXPERIMENT 4F
Equilibrium and
Le Châtelier’s Principle
(This experiment is done in pairs. Note: you may wish to divide
part 1 and 2 between partners.)
Useful background reading (this is not compulsory but may be helpful):
Tro, 4th and 5th Edition: Sections 15.3, 15.7, 15.8, 14.9 (Intro only) – Questions 1 and 2
Sections 12.1 and 12.6 – Question 3
What is the relevance of this prac…?
The prac brings together several concepts that underpin many areas of chemistry study. You
will undertake your first laboratory synthesis in which you make a compound (much like
cooking but you don’t get to lick the bowl!).
You will then analyse, using Le Châtelier’s Principle, how the reaction conditions may be
optimised in order to maximise the amount of product you obtain. Le Châtelier’s Principle can
be used to predict outcomes on a small scale such as your reaction vessel, on a miniscule scale
such as in cells and on a planetary scale such as in Earth’s atmosphere.
Finally, you will examine how the charge of a species determines what solvents it can be
dissolved in. The type of possible intermolecular forces present between the solute and solvent
will dictate solubility and this is investigated during this practical. Intermolecular forces are
incredibly important and we take them for granted all the time. They are responsible for oxygen
being a gas at room temperature so we can breathe it in and water being a liquid at room
temperature so we can drink it.
Learning objectives (remember these are different to the scientific objectives):
On completion of this practical, you should have:
Become familiar with the class of chemical compounds called “co-ordination complexes”
Understand that a co-ordination complex consists of a metal cation at the centre
surrounded by ligands
Recall the concept of equilibrium from lectures and consider how it relates to this
practical
A BIG Question
What is life?
Life is dependent on many things working
together in concert to give a ...
Here are the key points about polar and nonpolar covalent bonds:
- Nonpolar covalent bonds form between atoms with similar electronegativity. Electrons are shared equally. Examples include H2, Cl2, etc.
- Polar covalent bonds form between atoms with different electronegativity. Electrons are pulled slightly closer to the more electronegative atom. This creates partial positive and negative charges called dipoles.
- Molecules with polar bonds are polar molecules. They are attracted to electric fields due to the separation of charge in the dipoles. Water is a common example.
- Molecules with nonpolar covalent bonds have equal charge distribution and are nonpolar molecules. They
Ionic bonding occurs through the transfer of electrons between atoms to form ions, resulting in electrostatic attraction. Covalent bonding involves the sharing of electrons between atoms to form molecules. The shape of covalently bonded molecules can be predicted using VSEPR theory. Hydrogen bonding and van der Waals forces are weaker intermolecular forces that influence properties like boiling points. Bonding types exist on a continuum between purely ionic and purely covalent.
Introduction
History
Definition
Types of H bond
Hydrogen bond in water
Bifurcated and over - Coordinated hydrogen bond in water
Hydrogen bonds in DNA and proteins
Hydrogen bonds in polymers
Systematic hydrogen bond
Importance of hydrogen bond
Conclusion
References
1. The document discusses intermolecular forces and how they affect the properties of different substances. It provides explanations for questions related to boiling points, freezing points, vapor pressure, and conductivity.
2. The explanations show how stronger intermolecular forces allow substances to have higher boiling points and freezing points. They also discuss how molecular structure impacts a substance's ability to form dipoles or participate in hydrogen bonding.
3. The questions cover a range of topics including the exceptions to boiling point trends, factors that determine vapor pressure and freezing point, and how molecular structure enables conductivity in certain solids like graphite.
This document provides an outline for a lesson on covalent bonding and molecular structure. It includes 5 lessons: 1) a review of topic 4, 2) more shapes of molecules up to 5-6 electron domains, 3) deciding the best resonance structure using formal charge, 4) a case study of ozone looking at resonance, polarity and formal charge, and 5) hybridization of orbitals in covalent bonding focusing on carbon. Interactive simulations and videos are embedded to illustrate concepts like Lewis structures, molecular shapes, resonance structures, and hybridization. Practice problems are included throughout to check understanding.
Organic Chemistry deals with carbon compounds and their structures and reactions. It originally focused on living substances but now includes synthetic compounds. Carbon forms the backbone of organic molecules due to its ability to form chains and complex structures through covalent bonding, including single, double and triple bonds. The hybridization of atomic orbitals, such as sp3 hybridization, allows carbon to form four bonds to other atoms and explains the shapes of organic molecules like methane. Understanding bonding concepts such as ionic and covalent bonds is essential to organic chemistry.
Molecular orbital theory can be used to explain the magnetic properties of diatomic oxygen (O2) that valence bond theory cannot. Molecular orbital theory treats bonding by considering orbitals that extend over the entire molecule formed from linear combinations of atomic orbitals. For O2, the molecular orbitals formed result in two unpaired electrons in antibonding orbitals, explaining O2's paramagnetism contrary to the prediction of diamagnetism by valence bond theory. The molecular orbital approach involves combining atomic orbitals to form molecular orbitals, arranging the molecular orbitals by energy, and filling the orbitals with electrons based on available energy levels.
The document summarizes key concepts from several sections of a chemistry textbook chapter on covalent bonding. It discusses the formation of single, double, and triple covalent bonds; molecular structures and Lewis dot structures; molecular shapes determined by VSEPR theory; hybridization; and electronegativity and its role in determining bond polarity and molecular polarity. Examples are provided to illustrate concepts like naming binary compounds and acids, exceptions to the octet rule, and properties of polar and nonpolar covalent compounds.
1. The document discusses different theories of chemical bonding including single, double and triple bonds formed by the sharing of electron pairs between atoms.
2. It describes Lewis structures, valence shell electron pair repulsion theory (VSEPR) and hybridization which are used to determine the shape and geometry of molecules.
3. The limitations of octet rule, valence bond theory, and an introduction to molecular orbital theory are provided. The document provides examples to illustrate concepts of bonding theories.
Investigation Of The Thermal Decomposition Of Copper...Alexis Naranjo
This molecular dynamics simulation examines the indentation response of an aluminum-amorphous silicon core-shell nanostructure. The study investigates the deformation behavior of the amorphous silicon shell and aluminum core under spherical indentation. It also explores how the density of the amorphous silicon, indenter radius size, and core/shell ratio size affect the structural deformation of the nanostructure. The simulation aims to provide insights into optimizing the properties of core-shell nanostructures for applications.
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Experiment 1 Molecular Models Modeling the shape of small organic.docxnealwaters20034
Experiment 1: Molecular Models Modeling the shape of small organic molecules
Previously we have considered molecules and ions for which one chemical formula corresponded to one chemical compound only. Not all chemical compounds are like that. For example, consider the formula C2H6O. It turns out that there is more than one compound with that chemical formula:
Ethanol Dimethyl ether
These two molecules have completely different chemical and physical properties. They are called structural isomers. They have the same chemical formula with different bonding between atoms. Another example would be the compounds that correspond to butane, with the chemical formula C4H10. There are two structural isomers of butane.
1. Explain why the two structures above are NOT considered structural isomers.
2. Construct two structural isomers of C4H10. Draw them below using expanded structural or line formulas. When you are finished, compare them with the results of other students.
Geometric Isomerism:
An example of a different kind of isomerism occurs when the molecules have the same bonding between the atoms but their arrangement in space is different. We say that these compounds are geometric isomers. A classic example involves molecules that contain double bonds.
Circle the structure named cis-2-butene. The double bond between the carbon atoms does not allow the free rotation of the methyl (CH3) groups with respect to one another, preventing the interconversion between the trans and cis isomers. Geometric isomers have different physical properties but almost identical chemical properties
3. What do you think is the meaning of the prefix “cis-” vs “trans-”?
Here’s another example of geometric isomers.
Construct cyclopentane, C5H10, which does not contain any double bonds.
(the blue lines show these atoms are on the other side of the ring)
Replace one of the hydrogens with chlorine to obtain trans-1,3dichloropcyclopentane (as in the drawing below). Build the trans-isomer of this molecule (based on what you learned above) and draw your structure in the empty box.
There is no free rotation around the C-C bonds that connect the carbons where the chlorine atoms are bound because of the rigidity of the cyclopentane molecule. Therefore, there is no interconversion between the cis and trans forms.
Thus, cis- and trans- prefixes refer to geometrical isomers!
We have briefly introduced the concepts of structural and geometric isomers. There is yet a third type of isomerism that we will leave out of this discussion: it is the so-called optical isomerism that will be covered in the organic chemistry courses.
Follow-up Questions
1. In the first few pages you learned about structural isomers and geometric isomers. Define these terms below:
Structural isomer
___________________________________________________________________________________________________.
This document discusses polymers, including their classification into thermoplastics, thermosetting polymers, and elastomers. It describes two main ways polymers are created - addition polymerization and condensation polymerization. Addition polymerization builds polymer chains by adding monomers together without creating a byproduct, while condensation polymerization condenses out a small molecule like water as a byproduct. The document provides examples of calculating the degree of polymerization and amount of initiator required for polyethylene production. It examines the effect of temperature on thermoplastics and their mechanical properties.
This course provides an in-depth understanding of three-dimensional macromolecular structure and the relationship between the conformation of proteins and nucleic acids and their biological functions. Students will learn to visualize and analyze macromolecular structures using molecular graphics software and assess the structural basis of biological activity. The course covers topics related to multi-molecular assemblies, catalytic machines, and membrane proteins. Students will be assessed through a final exam and computer graphics exercises completed in a lab notebook.
Atoms and molecules class 9 important questions science chapter 3 q &aRanjani Deepak
The document provides important questions and answers from the science chapter "Atoms and Molecules" for Class 9. It includes 17 questions covering topics like definition of cation, Dalton's atomic theory, chemical formulae, molar mass, moles, and the law of constant proportions. The questions aim to help students score more marks in exams.
This document summarizes information about delocalization in chemical bonding. It begins with definitions of delocalized and localized electrons. It then discusses different types of delocalization including in isolated, conjugated, and cumulated systems. Examples are provided of delocalization in benzene and isolated and cumulated systems. Effects of delocalization include increased stability and acidity as well as larger dipole moments. The document concludes with sections on resonance, how it occurs, and rules for drawing resonance contributors.
Here are the key points about polar and nonpolar covalent bonds:
- Nonpolar covalent bonds form between atoms with similar electronegativity. Electrons are equally shared. Examples include H2, Cl2, etc.
- Polar covalent bonds form between atoms with different electronegativity. Electrons are unequally shared, resulting in partial positive and negative charges on the atoms. Examples include HCl, H2O.
- Polar molecules are attracted to each other due to their partial charges. They are soluble in polar solvents like water. Nonpolar molecules are not attracted and are soluble in nonpolar solvents like hexane.
- Most bonds have some ionic character based on electrone
The document is a lab activity on molecular polarity using a PhET simulation. Students investigate how electronegativity affects bond polarity and molecular polarity when atoms bond. They describe different bond types such as nonpolar covalent, polar covalent, and ionic based on the electronegativity values of the atoms. Students also explore how molecular polarity arises from bond dipoles combining through vector addition and how molecular polarity determines if molecules will dissolve in polar solvents like water.
Molecular orbital theory provides an approach to calculate molecular orbitals through a variational method. This involves taking linear combinations of atomic orbitals to form molecular orbitals. Electrons occupy these molecular orbitals according to certain rules. The molecular orbital theory can explain properties such as why some molecules are paramagnetic that valence bond theory cannot. Calculating molecular orbitals variationally involves using trial wave functions in the Schrodinger equation to find the lowest possible energy state.
Question 5b) In Question 5(a) you have written the stepwise.docxmakdul
Question 5:
b) In Question 5(a) you have written the stepwise equilibrium reactions that must occur in order to produce the final complex, [Fe(acac)3]. All three equilibria can be occurring in solution to differing extents depending on what species are present and how Le Châtelier’s Principle might affect the reaction position. The three test tubes in Diagram 2 (steps 3, 4 and 5 in the PROCEDURE) show varying extents of reaction. Of the four complexes present in the reaction scheme, which solvent would you expect each one to be soluble in: polar water or non-polar dichloromethane? (First answer is provided.)
Question 6:
If the colour intensity of the layers changes position on going from test tube 2 to test tube 3, what does this suggest about the complexes that are present and, therefore, what has happened to the equilibrium position?
Foundations of Chemistry Laboratory Manual EQUILIBRIUM and LE CHÂTELIER’S PRINCIPLE
1
EXPERIMENT 4F
Equilibrium and
Le Châtelier’s Principle
(This experiment is done in pairs. Note: you may wish to divide
part 1 and 2 between partners.)
Useful background reading (this is not compulsory but may be helpful):
Tro, 4th and 5th Edition: Sections 15.3, 15.7, 15.8, 14.9 (Intro only) – Questions 1 and 2
Sections 12.1 and 12.6 – Question 3
What is the relevance of this prac…?
The prac brings together several concepts that underpin many areas of chemistry study. You
will undertake your first laboratory synthesis in which you make a compound (much like
cooking but you don’t get to lick the bowl!).
You will then analyse, using Le Châtelier’s Principle, how the reaction conditions may be
optimised in order to maximise the amount of product you obtain. Le Châtelier’s Principle can
be used to predict outcomes on a small scale such as your reaction vessel, on a miniscule scale
such as in cells and on a planetary scale such as in Earth’s atmosphere.
Finally, you will examine how the charge of a species determines what solvents it can be
dissolved in. The type of possible intermolecular forces present between the solute and solvent
will dictate solubility and this is investigated during this practical. Intermolecular forces are
incredibly important and we take them for granted all the time. They are responsible for oxygen
being a gas at room temperature so we can breathe it in and water being a liquid at room
temperature so we can drink it.
Learning objectives (remember these are different to the scientific objectives):
On completion of this practical, you should have:
Become familiar with the class of chemical compounds called “co-ordination complexes”
Understand that a co-ordination complex consists of a metal cation at the centre
surrounded by ligands
Recall the concept of equilibrium from lectures and consider how it relates to this
practical
A BIG Question
What is life?
Life is dependent on many things working
together in concert to give a ...
Here are the key points about polar and nonpolar covalent bonds:
- Nonpolar covalent bonds form between atoms with similar electronegativity. Electrons are shared equally. Examples include H2, Cl2, etc.
- Polar covalent bonds form between atoms with different electronegativity. Electrons are pulled slightly closer to the more electronegative atom. This creates partial positive and negative charges called dipoles.
- Molecules with polar bonds are polar molecules. They are attracted to electric fields due to the separation of charge in the dipoles. Water is a common example.
- Molecules with nonpolar covalent bonds have equal charge distribution and are nonpolar molecules. They
Ionic bonding occurs through the transfer of electrons between atoms to form ions, resulting in electrostatic attraction. Covalent bonding involves the sharing of electrons between atoms to form molecules. The shape of covalently bonded molecules can be predicted using VSEPR theory. Hydrogen bonding and van der Waals forces are weaker intermolecular forces that influence properties like boiling points. Bonding types exist on a continuum between purely ionic and purely covalent.
Introduction
History
Definition
Types of H bond
Hydrogen bond in water
Bifurcated and over - Coordinated hydrogen bond in water
Hydrogen bonds in DNA and proteins
Hydrogen bonds in polymers
Systematic hydrogen bond
Importance of hydrogen bond
Conclusion
References
1. The document discusses intermolecular forces and how they affect the properties of different substances. It provides explanations for questions related to boiling points, freezing points, vapor pressure, and conductivity.
2. The explanations show how stronger intermolecular forces allow substances to have higher boiling points and freezing points. They also discuss how molecular structure impacts a substance's ability to form dipoles or participate in hydrogen bonding.
3. The questions cover a range of topics including the exceptions to boiling point trends, factors that determine vapor pressure and freezing point, and how molecular structure enables conductivity in certain solids like graphite.
This document provides an outline for a lesson on covalent bonding and molecular structure. It includes 5 lessons: 1) a review of topic 4, 2) more shapes of molecules up to 5-6 electron domains, 3) deciding the best resonance structure using formal charge, 4) a case study of ozone looking at resonance, polarity and formal charge, and 5) hybridization of orbitals in covalent bonding focusing on carbon. Interactive simulations and videos are embedded to illustrate concepts like Lewis structures, molecular shapes, resonance structures, and hybridization. Practice problems are included throughout to check understanding.
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RESONANCE April 2017.pdf
1. GENERAL ARTICLE
Condensation Polymerization
S Ramakrishnan
S Ramakrishnan is a
professor in the Department
of Inorganic and Physical
Chemistry at the Indian
Institute of Science. His
research group works
towards exploring new
strategies to develop polymers
that can self-segregate, fold,
and organize into sub-10 nm
size microphase separated
morphologies; in addition,
they work on hyperbranched
polymers and 2D polymers.
The very idea that large polymer molecules can indeed ex-
ist was hotly debated during the early part of the 20th cen-
tury. As highlighted by Sivaram in his articles on Carothers
and Flory, Staudinger’s macromolecular hypothesis was fi-
nally accepted, and the study of polymers gained momentum
because of the remarkable efforts of the these two individuals
who laid down the foundations concerning the processes that
led to the formation of large polymer molecules, and to those
that led to an understanding of many of their extraordinary
physical properties. Condensation polymerizations, as the
name suggests, utilizes bond-forming reactions that generate
a small molecule condensate, which often needs to be contin-
uously removed to facilitate the formation of the polymer. In
this article, I shall describe some of the essential principles of
condensation polymerizations or more appropriately called
step-growth polymerizations; and I will also describe some
interesting extensions that lead to the formation of polymer
networks and highly branched polymers.
All of us learn about organic chemical reactions in high school
and college; we are taught about many types of chemical reac-
tions, like addition, eliminations, hydrolysis, oxidation, esterifi-
cation, etc. Of these reactions, some of them, like the reaction
between a carboxylic acid and an alcohol, results in the linking of
two different molecular fragments; in this case, the acid and alco-
hol fragments are linked via an ester linkage. There are numerous
other chemical reactions that result in such linking of two molec-
ular fragments. All such reactions, in principle, could be poten- Keywords
Polymers, polycondensation,
polyester, chain-growth, step-
growth, degree of polymerization,
gel-point, polymer networks,
hyperbranched polymers.
tially useful to prepare long-chain polymer molecules. However,
only a few among these are actually useful to prepare polymers.
In this article, I will explain what it takes for a chemical reaction,
to find a place in this unique league of reactions.
RESONANCE | April 2017 355
2. GENERAL ARTICLE
How are Polymers Really Formed?
At first, let us begin by treating molecules as LEGO-type building
blocks with certain strict rules for linking them; a ball can readily
fit with a socket, whereas, neither two balls nor two sockets can
stick together (Figure 1).
It is evident that blocks carrying a ball and a socket would string
together to form a long chain. Similarly, if we begin with blocks
carrying two sockets and those carrying two balls, these too would
form long chains. We also recognize that there should be nothing
that prevents the ball at one end of a chain linking up with the
socket at the other to form a ring! This would, of course, very
much depend on the flexibility of the chain formed; clearly, stiff
rod-like chains will find it more difficult to form rings. The pro-
cess of linking a large number of small molecules, akin to the
building blocks, is essentially the process of polycondensation or
step-growth polymerization. Before we leave this LEGO-style
discussion, I would leave you with two alternate scenarios; one is
to use building blocks bearing two sockets and two balls, as de-
picted in the figure, and the other is to use blocks that contain two
balls but only one socket! What would be the possible outcome
in these two cases? Try this out on your own; we will return to
such possibilities later in the article.
Figure 1. Ball and socket
depiction of the polyconden-
sation process. Either a
single building block bear-
ing a ball and a socket
(AB) or two different build-
ing blocks, each bearing two
balls (BB) or two sockets
(AA), can link to form lin-
ear chains. Rings could also
be formed during such pro-
cesses by the ball at one
end of the chain linking
with the socket at the other.
Two other types of build-
ing blocks are also shown,
which would lead to other
interesting structures, which
will be discussed later.
356 RESONANCE | April 2017
3. GENERAL ARTICLE
Figure 2. Example of a
polycondensation process
between a dicarboxylic
acid and a diol to form a
polyester.
Now, Some Real Chemistry!
Having described a simple conceptual framework for the forma-
tion of polymers starting from suitable building blocks, let us look
at some real cases. First, we examine the simplest case of the re-
action of a dicarboxylic acid, like adipic acid, and a diol, like
1,6-hexane diol (Figure 2). Each of these molecules would be
considered to have a functionality of 2, implying that each carry
two reaction sites and therefore both are bifunctional. Under acid-
catalysed conditions, this should lead to the formation of a long
chain polyester by a process termed as polycondensation – ‘poly’
implying that several such events occur, and ‘condensation’ im-
plies that there is a condensate that is formed. The condensate
formed in this case, as you would have guessed, is H2O. Such re-
actions would be classified as AA + BB type condensation, anal-
ogous to connecting blocks with two balls and those with two
sockets1 1 Remember: we start with
a very large number of such
molecules, and many reactions
occur simultaneously and ran-
domly between any carboxylic
acid and alcohol group.
. Carothers posed some fairly simple questions about
such polycondensations [1]:
a) How does the average length of the chain vary with the number
of ball-socket connections that have been made?
b) What if some of the blocks are broken and contain only one ball
instead of two; what would be the effect of having such blocks on
the average length of the chain?
c) What if a few blocks have three balls instead of two?
Before we take this up, let us define a few terms. Fractional con-
version, p is a term that describes the extent to which the reaction
RESONANCE | April 2017 357
4. GENERAL ARTICLE
has occurred, which in the above example would be the fraction
of carboxylic acid groups that has reacted to form an ester. In the
building block analogy, p would be the fraction of balls that have
been fixed to sockets. In the case of polyester formation, one way
to estimate p would be to determine the residual carboxylic acid
concentration (or acid number) by titration – this is often done in
the industry in order to follow the progress of such polyconden-
sation processes. An alternate approach would be to measure the
amount of the condensate, H2O, formed; this can be done by col-
lecting the water formed, often by removing it as an azeotrope2
2Azeotrope is a mixture of two
or more liquids that exhibit ei-
ther a maximum or minimum
in vapour pressure as a function
of composition. One implica-
tion of this, is that water can be
removed as an azeotrope with
benzene at 69.3oC (mole frac-
tion of water in the azeotrope is
∼ 0.3).
.
Now, to address the first question regarding the length of the chain
or the molecular weight of the polymer formed, we must first rec-
ognize that each time a reaction between a carboxylic acid and
an alcohol occurs, a linkage between the fragments is generated
and as a result, the total number of molecules in the reaction flask
is reduced by 1 (neglect the condensate: water). Suppose we be-
gin with NA/2 molecules of the dicarboxylic acid (AA) and NB/2
molecules of the diol (BB), then the total number of carboxylic
acid and alcohol groups in the reaction mixture would be NA and
NB, respectively; since each of the molecules have two functional
groups. Thus, at a fractional conversion p, the number of car-
boxylic acid groups (A groups) that have reacted would be pNA
(this number will be same as the number of alcohol groups that
has reacted, since the carboxylic acid can only react with the al-
cohol). Now, since each time an ester formation occurs, the total
number of molecules in the reaction mixture reduces by 1, at con-
version p, the total number of molecules remaining equals,
NA/2 + NB/2 − pNA. (1)
In the case where equal moles of AA and BB type molecules are
taken, the total number molecules at the beginning will be,
NA/2 + NB/2 = NA; as NA/2 = NB/2. (2)
Now, as depicted in Figure 2, the average number of repeat units3
3Repeat unit is the smallest
molecular segment required to
represent the polymer chain.
,
n, which is also often referred to as the ‘degree of polymeriza-
tion’ (DP), can be estimated by simply addressing the following
358 RESONANCE | April 2017
5. GENERAL ARTICLE
question: Suppose, we took 50
molecules of each
monomer to begin with,
and after some linkages
between them have
occurred, there are only
10 molecules in the
flask, what would be the
average degree of
polymerization?
Suppose, we took 50 molecules of each monomer to
begin with, and after some linkages between them have occurred,
there are only 10 molecules in the flask, what would be the aver-
age degree of polymerization? The answer is obvious – it would
be the total number of molecules one started with, divided by
the number of molecules remaining; which is (50 +50)/10 = 10.
This means, on an average, each polymer chain would have 10
monomer units in them; in this case 5 each of the diacid and diol
fragments. This implies, as represented in Figure 2, n will be 5.
(Remember: in the case of AA + BB type condensations, DP =
2n)
Following this line of argument, the general formula for DP can
be derived from the expressions (1) and (2),
DP = (Total number of molecules taken)/
(Number of molecules left at conversion p)
= (NA/2 + NB/2)/(NA/2 + NB/2 − pNA)
= NA/(NA − pNA) = 1/(1 − p). (3)
DP = 1/(1-p) is the well-known ‘Carothers equation’ that de-
scribes the variation of the average degree of polymerization as
a function of conversion. From the arguments used to derive this
expression, it is clear that it is valid only when equal moles of AA
and BB are taken; i.e., there is stoichiometric balance. A plot of
the variation of molecular weight with conversion for step-growth
polymerization is shown in Figure A (Box 1); it is also compared
with simple chain-growth polymerizations. The Carothers equation,
which is rigorously
obeyed by most
polycondensations, is
remarkable for its
simplicity but it imposes
certain very strict
conditions for a bond
forming reaction to be
useful for the
preparation of polymers.
Only Some Condensation Reactions are Useful for Polymer
Formation
The Carothers equation, which is rigorously obeyed by most poly-
condensations, is remarkable for its simplicity but it imposes cer-
tain very strict conditions for a bond forming reaction to be useful
for the preparation of polymers. These are:
(a) Polycondensation reactions must be rapid and must proceed
to very high conversions.
RESONANCE | April 2017 359
6. GENERAL ARTICLE
Box 1. Step-Growth vs. Chain-Growth Polymerization.
Polymers can be prepared by two processes; one is step-growth and the other is chain-growth polymer-
ization. In chain-growth polymerizations, an initiator, like a free-radical, reacts with monomers that carry
a reactive double bond, such as in ethylene. The process of monomer addition is a chain-reaction. Once
initiated, many such addition reactions occur resulting in the enchainment of numerous ethylene units to
generate a long polymer chain. This process of monomer addition to the reactive chain end continues
till an event that terminates the chain-growth occurs. The molecular weight development as a function of
conversion for such a process is shown in the plot. Typically, free-radical chain polymerizations generate
high molecular weight polymers rapidly by enchaining a very large number (100 to 1000) of monomers
every second. Each growing polymer radical has a fairly short life time (< 1 minute), and hence fairly
high molecular weight polymers are formed right from the beginning of polymerization as seen in the plot.
As conversion (here, p would be the fraction of monomer transformed to polymer) increases, additional
polymer chains are formed; all growing chains, on an average, have the same life time, and therefore the
average molecular weight does not vary much with conversion, p. This is the crucial difference between
step-growth and chain-growth polymerizations, as evident from the plot depicting the variation of molecular
weight versus p. Majority of commercial polymers are in fact prepared by chain-growth polymerizations
– a few important examples are polyethylene, polypropylene, polyvinyl chloride (PVC), polystyrene (sty-
rofoam), etc. Just polyethylene and polypropylene account for over 50 % of the total plastics produced
worldwide!
Figure A. Plot showing the variation of molecular weight as a function of fractional conversion (p) for
simple step-growth and chain-growth polymerizations. As discussed, high molecular weight is formed
only at very high conversions in the case of step- growth polymerizations [curve depicts the variation: DP
= 1/(1-p)]. However, for simple chain polymerizations, high molecular weight polymer is formed even at
very low conversions. With increasing time, more monomers are converted to polymer without affecting
the overall average molecular weight of the polymer.
(b) There must not be any side-reactions; meaning alternate re-
action pathways must not occur.
360 RESONANCE | April 2017
7. GENERAL ARTICLE
(c) Polycondensations need to be carried out using very high pu-
rity monomers.
Because of these strict constraints, only few organic reactions
have been used commercially to prepare polymers via polycon-
densation or step-growth processes. Examples of these are poly-
esters, polyamides, polyimides, polyurethanes, polycarbonates,
and a few others. The functional group
reactivity is assumed to
be the same in all the
species, namely
monomer, dimer, trimer,
etc., and therefore, one
assumes a statistically
random probability for
the occurrence of
reaction between the
different species during
the polycondensation.
Step-growth polymerization is a more inclu-
sive term to describe such processes because not all processes
that follow such growth kinetics need to be based on a condensa-
tion reaction; for instance, the preparation of polyurethanes from
a di-isocyanate and a diol proceeds without the formation of a
condensate (see Box 2).
A high yielding reaction in the context of simple organic reac-
tions would be those that go to over 90% conversions. However,
a quick look at the Carothers equation, would immediately re-
veal that at a conversion of 90% (i.e., p = 0.9) the DP of the
RESONANCE | April 2017 361
8. GENERAL ARTICLE
polymer would be only 10! This represents a very low molecu-
lar weight polymer. In the case of poly(hexamethylene adipate),
this would give a molecular weight of 228 × 5 = 1140 (Remem-
ber: DP = 10, implies n = 5); 228 is the repeat unit (C12H20O4)
formula weight. Even at a conversion of 99%, the DP would be
100, which means the polymer molecular weight would be 11400
only! Driving organic reactions to conversions higher than 99%,
would need special efforts and reaction conditions. This makes
it amply evident as to why the first condition of rapid reaction
rates and high conversions is crucial for a polycondensation re-
action to be of commercial value. One might add that commer-
cial polymers, like polyethylene terephthalate (PET), nylons, etc.,
typically have molecular weights ranging from 10,000 to 50,000.
This clearly means that the conversions have to be driven to >
99%. It is important to recognize here that the formalism devel-
oped is based on one very vital assumption – the reactivity of each
of the functional group (A or B) in the monomer is independent
of the other. In other words, the reactivity of the functionality
in the monomer is not affected by whether the other functional
group has already reacted or not. Thus, as the reaction proceeds,
the functional group reactivity is assumed to be the same in all
the species, namely monomer, dimer, trimer, etc., and therefore,
one assumes a statistically random probability for the occurrence
of reaction between the different species.
In order to understand the importance of monomer purity, first, let
us examine what would happen if there is a stoichiometric imbal-
ance – meaning that the number of moles of AA and BB are not
equal. In an extreme case, if you were asked the question – what
would you expect will happen if 1 mole of adipic acid was reacted
with 2 moles of hexane diol? You would apply some intuitive
reasoning, and probably figure out that you would get a product
where both the acid groups of the diacid would be esterified to
give you a trimer. While this is broadly correct, in practice, one
would get a mixture of the trimer and some oligomers; the latter
is a term used for very low molecular weight polymers.
For a more formal understanding, let us designate the extent of
362 RESONANCE | April 2017
9. GENERAL ARTICLE
stoichiometric imbalance by a term r, which is the ratio of the
number of moles of the two monomers. Hence r = NB/NA, where
NB < NA, implies that 0 < r < 1.
Based on the earlier argument (3),
DP = (NA/2 + rNA/2)/(NA/2 + rNA/2 − pNA)
(here: NB is replaced by rNA)
= (1 + r)/(1 + r − 2rp) (4)
This modified expression reveals the important consequence of
stoichiometric imbalance. Note that this expression would re-
duce to the original one, when r = 1. Let us now examine the
case where there is 1 mole% imbalance, which means that r =
0.99. From the above expression, one can quickly estimate that
the DP at a conversion of 99%, would be 2.01/0.0498 ≈ 40. Re-
call that at 99% conversion, if the stoichiometry were balanced (r
= 1), then the expected DP is 100! Thus, even just 1 mole% im-
balance between the amounts of the two monomers would reduce
the molecular weight by a factor of more than two!
One of the possible origins of stoichiometric imbalance is im-
purity of the monomers. Any impurity, even those that do not
participate in any reaction (say, due to incomplete drying of the
monomer), would cause a stoichiometric imbalance and lead to a
substantial lowering of the polymer molecular weight. The same
argument can be readily extended to the presence of a mono-
functional impurity, i.e., the presence of a broken LEGO block
with only one ball/socket. The physical properties
of polymers are strongly
influenced by their
molecular weight, and
failure to achieve high
molecular weight is a
severe detriment to
achieving optimum
physical properties such
as, melt viscosity, tensile
strength, etc.
Since, a monofunctional impurity
would necessarily become a chain-end, it puts a lower limit on
the number of chains that would remain even at 100% conver-
sion, and consequently leads to a similar lowering of the molecu-
lar weight (as was done earlier, one can show that DP = (1 + r)/(1
+ r – p), where r is the mole-fraction of monofunctional impu-
rity; again if r = 0, the original formula is recovered). Thus, the
presence of either a monofunctional impurity or stoichiometric
imbalance leads to a dramatic decrease in the molecular weight
of the polymer formed. Since the physical properties of poly-
RESONANCE | April 2017 363
10. GENERAL ARTICLE
mers are strongly influenced by their
One interesting feature
that you will observe in
the case of
hyperbranched structures
is that the
polycondensation
process will always lead
to a polymer that
contains a single socket
(A group) and numerous
balls (B groups).
molecular weight, failure
to achieve high molecular weight is a severe detriment to achiev-
ing optimum physical properties such as, melt viscosity, tensile
strength, etc. Similarly, side reactions could often lead to either
the lowering of molecular weight due to loss of functionality or to
the formation of cross-linked products, if side reactions provide
alternate pathways for bond-formation.
Let us now return to the other question I posed earlier, concerning
the nature of the products that would be formed when more com-
plex building blocks, carrying more than two functional groups
each, are allowed to react with each other (Figure 1). Specifi-
cally, what would happen when either AB2 or A2B2 type building
blocks are used. If you begin linking the blocks together, you
will realize that AB2 type monomers will grow to form highly
branched tree-like structures that are called hyperbranched poly-
mers, as depicted in Figure 3. These polymers are readily solu-
ble in solvents but typically exhibit very poor mechanical proper-
ties, when compared to their linear analogues because of the ab-
sence of a very important topological feature, namely ‘chain en-
tanglements’. Long linear polymer chains are typically entangled
in melt, which is reminiscent of an entangled bundle of thread
that we often encounter, and could become a rather frustrating
task to disentangle. Bulk polymers possess such chain entangle-
ments that in a sense become physical (mechanical) linkages be-
tween the different chains, and consequently are responsible for
their mechanical properties. Because of their highly branched na-
ture (and short chain segments between branching points), hyper-
branched polymers do not entangle and hence exhibit very poor
mechanical properties. However, one interesting feature that you
will observe in the case of hyperbranched structures is that the
polycondensation process will always lead to a polymer that con-
tains a single socket (A group) and numerous balls (B groups). In
fact, the number of balls is expected be DP + 1, where DP (degree
of polymerization) is the number of AB2 molecules that are linked
together to form the chain (of course, in the absence of intra-
molecular cyclization). Flory, [2] in fact was the first to postulate
364 RESONANCE | April 2017
11. GENERAL ARTICLE
that AB2 type monomers would give highly branched yet soluble
polymers and derived the statistical formulation to describe the
development of molecular weight, and molecular weight distri-
bution as a function of conversion p. However, it was only after
about 40 years that the first study of such hyperbranched poly-
mers was reported [3]. Some of the unique characteristics of hy-
perbranched polymers that have made them a subject of intense
study during the past two decades are:
a) They exhibit very low melt and solution viscosity because they
are very compact structures (occupy far less volume than their
linear analogues) and,
b) They possess a large number of peripheral functional groups
(balls decorate the periphery of these structures) that can be suit-
ably transformed to modulate their solubility characteristics.
For instance, you can transform a fairly hydrophobic4 4A dislike for water.
polymer
into a water soluble one by suitably transforming the peripheral
groups alone [3]!
Now, in the case of the A2B2 type of molecules, the result is very
different; as shown in Figure 3, a random network structure will
be formed. Polymers that are formed by such a process are called
cross-linked polymers, and cannot be dissolved in any solvent nor
can they be melted, unlike simple linear polymers. Using very
similar arguments as presented earlier (under some assumptions
for simplification), the evolution of molecular Polymerizing mixtures
can readily be
transformed to
cross-linked gels either
due to inadvertent
presence of
polyfunctional
impurities or even if
there are side reactions
that provide alternate
pathways for bond
formation.
weight or DP with
conversion p, can be shown to vary as [4]: DP = 2/(2-p fav); where
fav is the average functionality of the monomers.
Here again, if one gives fav a value of 2, the original Carothers
equation is retrieved. This expression reveals something very in-
teresting – when p = 2/ fav, the denominator goes to zero and the
DP will become infinity! This point is called the ‘gel-point’. It
implies that at this conversion, all the molecules in the reaction
mixture have become connected to each other and the entire mass
has become a single giant molecule! (Remember: at 100 % con-
version (p = 1), even in the simple case, the DP will become
RESONANCE | April 2017 365
12. GENERAL ARTICLE
Figure 3. Structures
formed by polyconden-
sation of polyfunctional
monomers. In the case
of AB2 self-condensation,
high molecular weight
hyperbranched polymers
would be formed. As they
grow in size, these would
adopt a compact pseudo-
spherical structure that
would carry a large number
of balls (B-type functional
groups) on their molecular
periphery. On the other
hand, A2B2 type monomers
would lead to an insolu-
ble, highly cross-linked
product. Topologically
cross-linked structures
would contain several ring
subunits because of the
presence of a large number
of both A and B type
functionality that could
react intra-molecularly,
whereas a hyperbranched
polymer could have at the
most a single ring because
a growing chain, at any
time, can have only a single
A-type functional group!
infinite. This is a trivial case, since 100 % conversion is impossi-
ble to achieve!). The average functionality fav is calculated read-
ily by estimating the total moles of functional groups in all the
monomers together and dividing it by the total number of moles
of the monomer(s). Cross-linked polymers can also be prepared
by adding a small amount of a polyfunctional monomer, such as
B3 or A3, to simple AA + BB type polycondensations. Using the
formula, one can estimate the conversion at which a polyconden-
sation will lead to gelation or cross-linking. For instance, if the
average functionality fav = 2.1, then at p =2/2.1 ∼ 95%, the sys-
tem will gel. Such conversions are readily attained, and so poly-
merizing mixtures can readily be transformed to cross-linked gels
either due to inadvertent presence of polyfunctional impurities or
even if there are side reactions that provide alternate pathways
for bond formation (which will lead to an apparent increase in
fav). Once gelation happens, it is obvious why such polymers can
neither be dissolved nor melted. Commonly, such cross-linked
polymers are also referred to as thermosetting polymers. Here,
366 RESONANCE | April 2017
13. GENERAL ARTICLE
Proteins, DNA and RNA
– molecules of life, are
all long-chain molecules,
which are sequence
regulated with different
types of repeat units
arranged in a specific
sequence along the
backbone.
often low molecular weight precursors are allowed to react fur-
ther to form such cross-linked polymers in the required shape or
size – ‘Bakelite’ made from phenol and formaldehyde is an ex-
ample of such a cross-linked polymer. In the case of Bakelite,
phenol can be viewed as having a functionality of at least 3 (or
5, if the meta sites also undergo electrophilic substitution), while
formaldehyde has a functionality of 2. To understand this fur-
ther, you may refer to the polymerization process leading to the
formation of phenol-formaldehyde resins [4].
Just over 100 years ago, a majority of the scientific community
did not believe that long chain-like molecules, namely polymers,
really exist. It was only by first demonstrating their preparation in
a controlled fashion by Carothers and Staudinger [5] using sim-
ple, well-known organic reactions that it was possible to convince
the naysayers. Today, we recognize that polymers are essential to
life itself. Proteins, DNA and RNA, considered to be molecules
of life, are all long-chain molecules. Although, they are very dif-
ferent from synthetic polymers in the sense that they are sequence
regulated; meaning that they contain different types of monomers
(or repeat units) arranged along a backbone in a specific sequence,
which is crucial to their extraordinary functions. One might add
that all these biopolymers are also prepared by a condensation
or step-growth process often starting from individual monomers.
But instead of allowing them to form via a random process, they
are enchained by carefully selecting the specific monomer one at
a time in a pseudo chain-growth process. The secret of Nature
is that it uses a third component – often a template, to effect the
polycondensation by first binding, and then adding one monomer
at a time to the growing polymer chain. Therefore, Nature is not
burdened by random statistical probabilities, which a poor syn-
thetic chemist has to contend with!
Acknowledgement
I would like to thank Ms Saheli Chakraborty for help with some
of the figures.
RESONANCE | April 2017 367
14. GENERAL ARTICLE
Suggested Reading
[1] W H Carothers, Chem. Rev., Vol.8, p.353, 1931.
[2] P J Flory, J. Am. Chem. Soc., Vol.74, p.2718, 1952.
Address for Correspondence
S Ramakrishnan
Department of Inorganic and
Physical Chemistry
Indian Institute of Science
Bangalore 560 012, India.
Email:
raman@ipc.iisc.ernet.in
[3] Y H Kim and O W Webster, J Am Chem. Soc., Vol.112, p.4592, 1990.
[4] Principles of Polymerization by George Odian, 4th Edition, Wiley-Interscience,
2004.
[5] H Staudinger, Trans. Faraday Soc., Vol.32, No.97, 1936, This entire issue of the
Transactions of the Faraday Society carries a remarkable collection of articles,
which is an outcome of the Discussions of the Faraday Society on The Phenom-
ena of Polymerization and Condensations.
368 RESONANCE | April 2017