2.3 Free radical polymerization

Free radical polymerization is a key industrial process for making everyday materials like plastics and rubbers. It involves creating reactive radical species that build long polymer chains through a series of steps: initiation, propagation, and termination.

The process begins with initiators generating radicals, which then attack monomer molecules. This continues rapidly, forming polymer chains until radicals combine or transfer. Understanding each step's kinetics is crucial for controlling the final polymer's properties.

Fundamentals of free radical polymerization

• Free radical polymerization forms the backbone of many industrial polymer synthesis processes
• Involves the creation and propagation of reactive radical species to form long polymer chains
• Crucial in producing a wide range of everyday materials (plastics, rubbers, adhesives)

Mechanism of free radical polymerization

Top images from around the web for Mechanism of free radical polymerization

• 

  Introduction to polymer chemistry - Wikiversity View original

  Is this image relevant?

• 

  Organic chemistry 22: Radicals - alkene halogenation, polymerization View original

  Is this image relevant?

• 

  Introduction to polymer chemistry - Wikiversity View original

  Is this image relevant?

• 

  Organic chemistry 22: Radicals - alkene halogenation, polymerization View original

  Is this image relevant?

1 of 2

Top images from around the web for Mechanism of free radical polymerization

• 

  Introduction to polymer chemistry - Wikiversity View original

  Is this image relevant?

• 

  Organic chemistry 22: Radicals - alkene halogenation, polymerization View original

  Is this image relevant?

• 

  Introduction to polymer chemistry - Wikiversity View original

  Is this image relevant?

• 

  Organic chemistry 22: Radicals - alkene halogenation, polymerization View original

  Is this image relevant?

1 of 2

• Begins with the formation of free radicals from initiator molecules
• Radicals attack carbon-carbon double bonds in monomers, creating new radical centers
• Process continues, rapidly building up long polymer chains
• Terminates when two radicals react with each other or through chain transfer

Key steps: initiation, propagation, termination

• Initiation creates initial radical species through decomposition of initiator molecules
• Propagation involves repeated addition of monomer units to growing polymer chain
• Termination occurs when radicals combine or disproportionate, stopping chain growth
• Each step has distinct kinetics and influences final polymer properties

Chain transfer reactions

• Involve transfer of radical activity from growing chain to another molecule
• Can occur with solvent, monomer, polymer, or deliberately added chain transfer agents
• Impacts molecular weight distribution and branching of final polymer
• Useful for controlling polymer architecture and introducing functional end groups

Initiators and initiation

• Initiators play a crucial role in starting the polymerization process
• Choice of initiator affects reaction rate, molecular weight, and polymer end groups
• Understanding initiation kinetics essential for controlling polymerization outcomes

Types of free radical initiators

• Peroxides decompose to form oxygen-centered radicals (benzoyl peroxide)
• Azo compounds generate carbon-centered radicals (AIBN)
• Redox systems produce radicals through electron transfer reactions
• Photoinitiators create radicals upon exposure to specific wavelengths of light

Thermal vs photochemical initiation

• Thermal initiation uses heat to break chemical bonds in initiator molecules
• Photochemical initiation employs light energy to generate radicals
• Thermal methods offer broader applicability but less precise control
• Photoinitiation allows for spatial and temporal control of polymerization

Initiation efficiency and rate

• Initiation efficiency measures the fraction of generated radicals that start polymer chains
• Affected by solvent cage effects and initiator structure
• Rate of initiation influences overall polymerization kinetics and molecular weight
• Can be controlled through initiator concentration and decomposition rate

Propagation and kinetics

• Propagation drives the growth of polymer chains through monomer addition
• Kinetics of propagation determine the rate of polymerization and molecular weight
• Understanding propagation mechanisms crucial for optimizing reaction conditions

Propagation rate constants

• Measure the speed of monomer addition to growing polymer chains
• Vary depending on monomer structure and reaction conditions
• Typically high for free radical polymerization, leading to rapid chain growth
• Can be determined experimentally through pulsed laser polymerization techniques

Chain growth vs step growth

• Free radical polymerization follows chain growth mechanism
• High molecular weight polymers form early in the reaction
• Contrasts with step growth where molecular weight increases gradually
• Results in different molecular weight distributions and polymerization kinetics

Kinetics of free radical polymerization

• Overall rate depends on initiation, propagation, and termination rates
• Steady-state approximation often used to simplify kinetic analysis
• Rate equation: $R_p = k_p[M][P•]$, where $k_p$ is propagation rate constant
• Molecular weight related to ratio of propagation to termination rates

Termination mechanisms

• Termination ends the growth of polymer chains
• Influences final molecular weight distribution and polymer structure
• Understanding termination crucial for controlling polymerization outcomes

Combination vs disproportionation

• Combination joins two growing chains, doubling molecular weight
• Disproportionation transfers hydrogen, creating saturated and unsaturated end groups
• Relative importance depends on monomer structure and reaction conditions
• Affects polymer end group functionality and molecular weight distribution

Chain transfer to monomer

• Growing radical abstracts hydrogen from monomer molecule
• Creates new radical on monomer, initiating a new polymer chain
• Reduces average molecular weight of final polymer
• More prevalent in monomers with easily abstractable hydrogens (styrene)

Chain transfer to solvent

• Radical abstracts hydrogen from solvent molecule
• Terminates growing chain and creates new radical on solvent
• Can significantly impact molecular weight in solution polymerization
• Used deliberately with chain transfer agents to control polymer properties

Molecular weight control

• Controlling molecular weight crucial for tailoring polymer properties
• Affects mechanical strength, processability, and end-use performance
• Various techniques available to manipulate molecular weight during polymerization

Kinetic chain length

• Average number of monomer units added before termination occurs
• Related to the ratio of propagation rate to termination rate
• Influences average molecular weight of the polymer
• Can be controlled through initiator concentration and monomer feed rate

Mayo equation

• Relates inverse of degree of polymerization to chain transfer constants
• Allows prediction of molecular weight based on reaction conditions
• $\frac{1}{\overline{DP_n}} = \frac{1}{\overline{DP_{n,0}}} + C_S\frac{[S]}{[M]}$
• Useful for designing polymerizations with specific molecular weight targets

Gel effect and autoacceleration

• Occurs in later stages of polymerization as viscosity increases
• Reduces mobility of large polymer chains, slowing termination rate
• Results in sudden increase in polymerization rate and molecular weight
• Can lead to runaway reactions and heat management challenges in industrial settings

Copolymerization in free radical systems

• Involves polymerization of two or more different monomers
• Allows creation of polymers with tailored properties
• Crucial for developing materials with specific performance characteristics

Reactivity ratios

• Describe relative tendencies of monomers to react with growing chain ends
• Determined by the chemical structure of monomers and radical species
• Influence composition and sequence distribution in copolymers
• Can be measured experimentally and used to predict copolymer composition

Composition drift

• Change in copolymer composition as polymerization progresses
• Occurs when monomers have different reactivity ratios
• Can result in heterogeneous product with varying properties
• Managed through controlled monomer feeding techniques (starved-feed)

Block vs random copolymers

• Random copolymers have statistically distributed monomer units
• Block copolymers consist of distinct segments of each monomer type
• Free radical polymerization typically produces random copolymers
• Block copolymers require special techniques (living polymerization, RAFT)

Industrial applications

• Free radical polymerization widely used in industrial polymer production
• Versatility and robustness make it suitable for various polymerization methods
• Choice of method depends on desired product properties and process considerations

Bulk vs solution polymerization

• Bulk polymerization conducted without solvent, maximizes polymer yield
• Solution polymerization uses solvent to control viscosity and heat transfer
• Bulk method prone to gel effect and heat management issues
• Solution method allows better control but requires solvent removal

Emulsion polymerization

• Monomers dispersed as droplets in water with surfactants
• Polymerization occurs in micelles, producing polymer latex
• Allows for high molecular weight polymers with fast reaction rates
• Widely used for producing paints, adhesives, and synthetic rubber

Suspension polymerization

• Monomer droplets suspended in water with stabilizers
• Each droplet acts as a mini bulk polymerization reactor
• Produces polymer beads suitable for further processing
• Commonly used for producing polystyrene beads and ion exchange resins

Advantages and limitations

• Free radical polymerization offers both benefits and challenges
• Understanding these factors crucial for choosing appropriate polymerization method

High reaction rates

• Rapid chain growth allows for high productivity in industrial settings
• Enables production of high molecular weight polymers in short times
• Facilitates continuous production processes
• Can lead to heat management challenges in large-scale reactions

Tolerance to impurities

• Less sensitive to trace amounts of water or oxygen compared to ionic polymerizations
• Allows for use of technical grade monomers and solvents
• Reduces costs associated with purification of raw materials
• Enables polymerization in aqueous systems (emulsion, suspension)

Limited control over polydispersity

• Simultaneous initiation, propagation, and termination lead to broad molecular weight distributions
• Difficult to produce polymers with very narrow polydispersity indices
• Can result in inconsistent material properties in some applications
• Addressed through development of controlled radical polymerization techniques

Modern developments

• Recent advances aim to overcome limitations of conventional free radical polymerization
• Focus on improving control over molecular weight, architecture, and functionality
• Enable synthesis of complex polymer structures for advanced applications

Living free radical polymerization

• Minimizes termination reactions to maintain active chain ends
• Allows for controlled growth and narrow molecular weight distributions
• Enables synthesis of block copolymers and other advanced architectures
• Includes techniques like RAFT, ATRP, and NMP

RAFT polymerization

• Reversible Addition-Fragmentation chain Transfer
• Uses chain transfer agents to control radical concentration
• Produces polymers with predetermined molecular weights and low dispersity
• Versatile technique applicable to wide range of monomers and conditions

ATRP and NMP techniques

• Atom Transfer Radical Polymerization (ATRP) uses transition metal catalysts
• Nitroxide-Mediated Polymerization (NMP) employs stable nitroxide radicals
• Both provide control over polymer structure and composition
• Enable synthesis of well-defined block copolymers and functional polymers

Characterization methods

• Accurate characterization crucial for understanding polymer properties
• Various techniques provide information on molecular weight, structure, and thermal behavior
• Essential for quality control and structure-property relationship studies

Gel permeation chromatography

• Separates polymer molecules based on hydrodynamic volume
• Provides information on molecular weight distribution and polydispersity
• Requires calibration with known standards for accurate results
• Widely used for routine polymer characterization in industry and research

NMR spectroscopy

• Nuclear Magnetic Resonance reveals polymer chemical structure
• Provides information on monomer sequence and end group analysis
• Useful for determining copolymer composition and tacticity
• Can be used to study polymer dynamics and interactions in solution

Thermal analysis techniques

• Differential Scanning Calorimetry (DSC) measures thermal transitions
• Thermogravimetric Analysis (TGA) assesses thermal stability and composition
• Dynamic Mechanical Analysis (DMA) probes viscoelastic properties
• Essential for understanding polymer behavior under different temperature conditions