PHD
  1. Inorganic chemistry  1.1. Coordination chemistry  1.1.1. Ligand field theory  1.1.2. Crystal field theory  1.1.3. Molecular Orbital Theory for Coordination Compounds  1.1.4. Stability of Coordination Compounds  1.1.5. Electronic spectra of coordination compounds  1.1.6. Isomerism in Coordination Compounds  1.1.7. Kinetics and mechanism of coordination reactions  1.2. Organometallic Chemistry  1.2.1. Metal Carbonyls  1.2.2. Metal alkyl and aryl  1.2.3. Fischer and Schrock carbonaceans  1.2.4. Oxidative Addition and Reductive Elimination  1.2.5. Metal Hydrides  1.2.6. Catalysis by organometallic compounds  1.2.7. Metallocenes and Sandwich Complexes  1.2.8. CH activation  1.3. Solid state chemistry  1.3.1. Crystal structures and lattices  1.3.2. Band theory and electrical properties  1.3.3. Defects in the crystal  1.3.4. Synthesis of solid state materials  1.3.5. Magnetic and optical properties of solids  1.3.6. Superconductivity  1.3.7. Solid state ionics  1.4. Bio-inorganic chemistry  1.4.1. Metalloproteins and enzymes  1.4.2. Metal ion transport and storage  1.4.3. The role of metals in medicine  1.4.4. Bioinspired catalysis  1.4.5. Metal–DNA interactions  1.4.6. Metal Complexes in Medical Science  1.5. Lanthanides and Actinides  1.5.1. Electronic configuration and oxidation states in lanthanides and actinides  1.5.2. Spectral and Magnetic Properties in Lanthanides and Actinides  1.5.3. Separation and Extraction of Lanthanides and Actinides  1.5.4. Coordination chemistry of f-block elements  1.6. Main Group Chemistry  1.6.1. Chemistry of boron compounds  1.6.2. Chemistry of Silicon and Germanium Compounds  1.6.3. Chemistry of Phosphorus and Sulfur Compounds  1.6.4. Organosilicon chemistry  1.6.5. Noble Gas Chemistry
  2. Organic chemistry  2.1. Reaction mechanism  2.1.1. Nucleophilic substitution reactions  2.1.2. Electrophilic addition and substitution reactions  2.1.3. Elimination reactions  2.1.4. Rearrangement reactions  2.1.5. Radical reactions  2.1.6. Pericyclic Reactions  2.1.7. Photochemical reactions  2.2. Stereoscopic  2.2.1. Chirality and optical activity  2.2.2. Structural analysis  2.2.3. Stereoelectronic effects  2.2.4. Asymmetric synthesis  2.2.5. Dynamic Stereochemistry  2.3. Organometallic Chemistry in Organic Synthesis  2.3.1. Organolithium and organomagnesium reagents  2.3.2. Transition metal catalyzed coupling reactions  2.3.3. Palladium-catalyzed reactions  2.3.4. Olefin Metathesis  2.3.5. Gold and silver catalysis  2.4. Natural products chemistry  2.4.1. Alkaloids and terpenoids  2.4.2. Steroids and Flavonoids  2.4.3. Total synthesis of natural products  2.4.4. Biosynthesis of natural products  2.5. Supramolecular Chemistry  2.5.1. Host-Guest Chemistry  2.5.2. Self-assembly and molecular recognition  2.5.3. Supramolecular Catalysis  2.5.4. Molecular Machines
  3. Physical Chemistry  3.1. Thermodynamics  3.1.1. Laws of Thermodynamics  3.1.2. Chemical Potential and Equilibrium  3.1.3. Statistical thermodynamics  3.1.4. Non-equilibrium thermodynamics  3.2. Quantum Chemistry  3.2.1. Schrödinger equation and its applications  3.2.2. Molecular orbital theory  3.2.3. Density functional theory  3.2.4. Post-Hartree–Fock methods  3.3. Chemical kinetics  3.3.1. Reaction rate theory  3.3.2. Mechanism and rate laws  3.3.3. Catalysis and enzyme kinetics  3.3.4. Reaction dynamics  3.4. Spectroscopy and molecular structure  3.4.1. Infrared Spectroscopy  3.4.2. UV-visible spectroscopy  3.4.3. Nuclear Magnetic Resonance Spectroscopy  3.4.4. Mass Spectrometry  3.4.5. Raman Spectroscopy  3.5. Surface and Colloid Chemistry  3.5.1. Absorption and Catalysis  3.5.2. Surfactants and micelles  3.5.3. Colloidal Stability  3.5.4. Nanomaterials and Interfaces
  4. Analytical chemistry  4.1. Chromatography  4.1.1. Gas Chromatography  4.1.2. High Performance Liquid Chromatography  4.1.3. Thin Layer Chromatography  4.1.4. Ion Chromatography  4.2. Electroanalytical techniques  4.2.1. Voltammetry and Polarography  4.2.2. Conductometry and potentiometry  4.2.3. Colometry  4.3. Spectroscopic Methods  4.3.1. Atomic absorption spectroscopy  4.3.2. Fluorescence Spectroscopy  4.3.3. X-ray diffraction  4.3.4. Electron paramagnetic resonance spectroscopy  4.4. Chemometrics  4.4.1. Data processing and statistical methods  4.4.2. Multidisciplinary analysis  4.4.3. Machine Learning in Analytical Chemistry
  5. Theoretical and Computational Chemistry  5.1. Molecular dynamics simulation  5.2. Quantum chemistry methods  5.3. Computational drug design  5.4. Machine Learning in Chemistry  5.5. Ab initio molecular dynamics
  6. Biophysics and Medicinal Chemistry  6.1. Drug discovery and design  6.2. Enzyme kinetics and inhibition  6.3. Chemical biology approach  6.4. Protein-ligand interactions  6.5. Bioorthogonal chemistry
  7. Materials chemistry  7.1. Polymer chemistry  7.1.1. Polymerization mechanism  7.1.2. Functional Polymers  7.1.3. Biodegradable Polymer  7.1.4. Conducting and semiconducting polymers  7.2. Nano Chemistry  7.2.1. Nanoparticles and nanostructures  7.2.2. Carbon-based nanomaterials  7.2.3. Quantum dots  7.3. Energy and Environmental Chemistry  7.3.1. Battery and Fuel Cell Chemistry  7.3.2. Green chemistry and sustainable materials  7.3.3. Photocatalysis

PHD → Materials chemistry → Polymer chemistry ↓


Polymerization mechanism


Polymerization is the process by which small molecules, known as monomers, combine to form a larger molecule, called a polymer. This process is fundamental to the field of polymer chemistry, an important area of study within materials science and chemistry. Polymers are found in a vast range of materials, both natural and synthetic, and their properties can be defined by their chemical structures resulting from the polymerization process. To understand how polymers are formed, it is necessary to study the different mechanisms of polymerization, which are broadly classified into chain-growth polymerization and step-growth polymerization.

Chain-growth polymerization

Chain-growth polymerization, also known as addition polymerization, involves the addition of monomers to an active site on a growing polymer chain. This type of polymerization can be further classified into free-radical polymerization, cationic polymerization, anionic polymerization, and coordination polymerization. Each of these types involves different initiating mechanisms and conditions for the propagation and termination of the polymer chain.

Free-radical polymerization

This is the most common type of chain-growth polymerization, widely used to produce various polymers such as polyvinyl chloride (PVC), polystyrene, and polyethylene. Free-radical polymerization involves three main steps: initiation, propagation, and termination.

Initiation: The initiation step begins with the formation of a free radical. A free radical is an atom or molecule that has an unpaired electron, making it highly reactive. Free radicals can be generated using chemical initiators such as heat, light, or benzoyl peroxide. The free radical reacts with a monomer, forming a reactive species with a free-radical center.

R• + CH2=CH2 → R-CH2-CH2•

Propagation: The radical center on the newly activated monomer attacks another monomer, continuing the chain reaction by adding more monomer to the growing chain. This process is repeated many times and forms a long polymer chain.

R-CH2-CH2• + n CH2=CH2 → R-(CH2-CH2)n-CH2-CH2•

Termination: The polymerization process terminates when the free radicals combine. Termination can occur through combination or disproportionation. In combination, two radical ends join together, while in disproportionation, the hydrogen atoms are transferred to the other chain, resulting in the formation of a double bond.

R-(CH2-CH2)n-CH2-CH2• + •CH2-CH2-R' → R-(CH2-CH2)n-CH2-CH2-CH2-CH2-R'

Cationic polymerization

Cationic polymerization is initiated by an electrophile, such as a Lewis acid, and is used primarily for the polymerization of alkenes that can stabilize the positive charge through resonance, such as isobutylene.

Initiation: Initially an electrophile forms a cation from the monomer.

BF3 + CH3OCH3 → [BF3OCH3]+ [BF3OCH3]+ + CH2=C(CH3)2 → CH3C+(CH3)CH2(CH3) + CH3OCH2BF3

Expansion: The cation attaches to another monomer, and extends down the chain.

CH3C+(CH3)CH2(CH3) + CH2=C(CH3)2 → CH3C(CH3)2CH2C+(CH3)CH3

Termination: Termination may occur by the release of a proton from the active chain end, formation of a double bond, or reaction with a nucleophile.

Anionic polymerization

Anionic polymerization is initiated by a nucleophile, such as an alkali metal compound (e.g., lithium or sodium naphthalide), and is effective for monomers containing electron-withdrawing groups, such as styrene or acrylonitrile.

Initiation: An anion attacks the monomer, forming a carbanion.

CH2=CH-CN + n-BuLi → n-Bu—CH2—C−(Li+)—CN

Propagation: The carbanion propagates down the chain by reacting with another monomer.

n-Bu—CH2—C−(Li+)—CN + CH2=CH-CN → n-Bu—(CH2—CH-CN)x-CH2—CH-C−(Li+)—CN

Termination: Anionic polymerization can proceed without termination unless a suitable proton source is introduced.

Step-growth polymerization

Step-growth polymerization involves the reaction of bi-functional or multi-functional monomers in such a way that the polymer chain grows step-by-step. This type of polymerization is typical for making polyesters, polyamides, and polyurethanes.

Mechanism: The main feature of step-growth polymerization is that any two molecules with interacting functional groups can react to form a dimer, trimer, or long oligomer, gradually increasing the size of the polymer.

For example, in the case of polyester manufacturing, a diacid (e.g., terephthalic acid) reacts with a diol (e.g., ethylene glycol) to form an ester linkage, releasing a water molecule as a byproduct:

HOOC-C6H4-COOH + HO-CH2CH2-OH → HOOC-C6H4-COO-CH2CH2-OH + H2O

Polymerization examples: For polyamides, an example would be the formation of nylon-6,6 via the reaction of hexamethylene diamine with adipic acid.

H2N-(CH2)6-NH2 + HOOC-(CH2)4-COOH → [NH-(CH2)6-NH-CO-(CH2)4-CO-OH]n + (2n-1)H2O

Catalysis and polymerization

Catalysts can significantly affect the polymerization mechanism. For example, Ziegler-Natta catalysts are used to produce polymers such as polypropylene, providing stereochemical control during polymerization.

Coordination polymerization

Coordination polymerization involves a coordination complex between a metal and a monomer. A classical example is the Ziegler-Natta polymerization, where titanium catalysts help polymerize 1-alkenes such as ethylene and propylene. This process allows control over the stereochemistry and molecular weight of the polymer produced.

Basic mechanism:

Alkene monomers coordinate to the metal center, followed by entering into a metal-carbon bond. This step is repeated several times, resulting in the formation of a polymer chain.

RCH=CH2 → RCH-CH2-TiCl3 --coordination--> Ti-CH-CH2-R

Comparison between chain-growth and step-growth polymerization

Although both chain-growth and step-growth polymerization lead to polymer formation, there are clear differences in their mechanism and kinetic behavior.

  • Kinetics: Chain-growth polymerization is a rapid process, where the polymer chain grows rapidly after initiation, whereas in step-growth polymerization, the reaction proceeds over time through the formation of oligomers and ultimately higher molecular weight polymers.
  • Molecular weight: In chain-growth, high molecular weight polymers are formed at very low conversion of monomers, whereas in step-growth, high molecular weight polymers are obtained only at very high conversion of monomers.
  • Monomer units: Chain-growth polymerization typically involves a single type of monomer, while step-growth polymerization involves two or more different monomers.
  • Types of polymers: Chain-growth is suitable for addition polymers such as polyethylene, while step-growth is more suitable for condensation polymers such as nylon and polyester.

Understanding these mechanisms helps in designing polymers with specific properties suitable for various industrial applications, leading to innovation in areas such as plastics, textiles and biomaterials.

The wide variety in polymer structures and properties arises from the variety of polymerization mechanisms discussed, making it one of the most fascinating topics within materials chemistry. Knowledge of polymerization not only enhances the development of new materials but also enables the optimization of existing materials for wider applications.


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Polymerization mechanism

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