Addition Polymerization Mechanism

Mechanism of Addition Polymerization

• Active center in the initiation step can be radical, anionic, or cationic
• Termination step results in a dead chain

Ionic Polymerization

• Can be anionic and Cationic
• Involves a carbocation or carbanion active center

Cationic Polymerization

• Uses monomers with electron-donating/releasing groups, such as isobutylene, butadiene, vinyl ethers, styrene, and methyl styrene
• Lewis acids can be used for initiation
• Termination happens when no monomer exists in the system

Anionic Polymerization

• Uses monomers with electron-withdrawing groups like -COOH, -CHO, -NO2, -COOR, -CN etc. Examples include acrylic acid, acrylonitrile, acrylamide, and nitroethylene
• Catalysts used are of the electron-donating type, such as alkali metals, alkoxides, sodamide (in liquid ammonia), and sodium carbonate
• If monomer depletes, another monomer is added to make block copolymers
• H₂O Water, ammonia and alcohol are used for termination of the polymerization

Free Radical Polymerization

• Needs high temperature to decompose the initiator and start the polymerization
• Potassium persulfate requires 60-65 °C
• Produces uncontrolled molecular weight with high polydispersity index
• Results in dead chains

Controlled Radical Polymerization

• Requires low activation energy
• Can be performed at low temperatures
• Produces controlled molecular weight with a narrow dispersity index
• Results in living chains with end functionality

Atom Transfer Radical Polymerization (ATRP)

• Depends on the transition metal complex, alkyl halide initiator type and solvent
• Can be mediated by metals like Ti, Mo, Re, Fe, Ru, Os, Rh, Co, Ni, Pd, and Cu
• Cu complexes are most efficient catalysts for a variety of monomers
• 4 rules determine catalyst reactivity: distance between N atoms, ligand topology, N-ligand nature, steric bulk around the metal center

Alkyl Halide Reactivity (Initiators)

• Affected by carbon substitution linked to the halide
• Influenced by the transferable atom (halogen) and attached groups

Coordination Polymerization

• Polymer synthesis where the monomer and active center are coordinated
• Monomer and active center bind to the catalyst
• Monomer is incorporated into the polymer chain
• Degree of stereochemical control depends on catalyst type and reaction conditons

Catalyst Influence

• Polymer structure and stereochemistry depend on different Catalysts and reaction conditions like temperature, pressure, etc.

Tacticity of Polymers

• Isotactic: pendant groups are on the same side of the hydrocarbon backbone
• Syndiotactic: exhibits a regular structure with alternating side groups

Atactic Polymer

• Pendant groups are organized randomly around the backbone

Comparison of Stereoisomers

• Atactic polymers are soft, rubbery, amorphous, and relatively weak with flexibility
• Syndiotactic exhibit better impact strength than isotactic
• Isotactic polymers have the highest melting point, greatest crystallinity, and superior mechanical properties
• Isotactic polymers use Ziegler-Natta catalysts based on titanium tetrachloride (TiCl4) and triethylaluminum (Al(C2H5)3)

Historical Milestones

• 1953: Ziegler discovered ethylene polymerization using triethylaluminum or diethylaluminum chloride with titanium tetrachloride yielding high molecular weight polymers
• 1954: Natta found stereochemical control in polypropylene polymerization and catalysts impacts on the polymer structure
• 1963: Ziegler and Natta won the Nobel Prize for their work and Catalysts were named Ziegler-Natta catalysts

Ziegler-Natta Catalysts Components

• Transition Metal Compound (Group IV-VIII) include halides, subhalides, oxyhalides, alkoxides, β-diketonates, cyclopentadienyl dihalides, etc
• Base Metal Alkyl or Hydride (Group I-IV): alkylates the transition metal compound to form the active catalyst

Ziegler-Natta Catalysts Function

• Transition metal compound acts as a precatalyst
• Base metal alkyl activates a precatalyst to create the catalyst

Ziegler-Natta Catalysts Mechanism

• Monomer coordinates to a transition metal in the active site generally titanium/vanadium
• Transition metal precatalyst is alkylated by a base metal alkyl cocatalyst
• Alkylation creates a transition metal-carbon bond and then Coordinated monomer inserts into the polarized transition metal-carbon bond,

Cossee and Arlman Mechanism Overview

• A four-membered ring transition state is formed
• Migrating the alkyl (polymer) to one end of the olefin
• π-bonding between titanium and olefin weakens the labile Ti-C bond

Metallocene Catalysts

• Comprised of a Group IV metallocene like zirconocene/titanocene and methylaluminoxane (MAO)
• Chiral metallocenes are required for isotactic polypropylene propagation

Syndiotactic Propagation Catalysts

• Catalyst should have Cs symmetry with bent sandwich structure
• Syndiotacticity is believed to originate from active site isomerization with each monomer addition

Ring-Opening Polymerization (ROP)

• Cyclic compounds with heteroatoms/unsaturation undergo ROP

Ring-Opening Polymerization Mechanisms

• Radical
• Cationic
• Anionic

Condensation Polymerization Characteristics

• Step-growth process
• Monomers have more than 2 functional groups to react with each other
• Results in a small molecule being eliminated, like water, ROH, or NH3.

Types of Polycondensation

• Homopolycondensation results in Nylon-6
• Copolycondensation results in poly(ethylene terephthalate) or Terelene

Polyamide

• Reaction of carboxylic acid and an amine during preparation from diamines and dicarboxylic acids, 2 water molecules results
• Production of Nylon 66

Polyesters

• Reaction of a carboxylic acid and alcohol

Mono Functional Compound Addition

• Mono functional compound added will block functional groups and will consequently interrupt polycondensation.
• It will cause added excess of the functional groups of the other type,

Polycondensation Reactions

• Groups are normally the reactants taken in excess and can lead to reduced MW
• More than two functional groups (Polyfunctional) are needed for Polycondensation reactions

Excess Functional Groups result

• Unreacted groups will remain in excess
• Excess equal to that of the multifunctional compound
• This imbalance and property change leads to lower polymerization degree

Functionality in Polycondensation Reactions

• Bifunctional systems give linear macromolecules
• There must more than 2 functional groupsPoly condensation reactions

Polymer Synthesis includes

• Addition Polymerization
• Uncontrolled Free Radical Polymerization - Ionic Polymerization
• Living Radical Controlled Polymerization
• Coordination Polymerization
• Ring Opening Polymerization
• Condensation Polymerization

Polymerization Methods

• Bulk Polymerization involves no solution or suspension
• Polymerization in solution: polymerization rate is lower, requires additional purification
• Emulsion Polymerization uses Monomer, Initiator and Water for control
• Suspension Polymerization emulsion eliminates emulsifier and impurities
• Interfacial Polycondensation technique uses reaction carried out at the interface between two liquid phases

Free vs Controlled Radical Polymerization

• Controlled Radical Polymerization Requires low activation energy and can be performed at low temperature
• Uncontrolled (Free) Radical Polymerization Requires high temperature is for decomposition.

Homogenous Polymerization

• Bulk
• Solution

Heterogenous Polymerization

• Emulsion
• Suspension
• Interfacial

Homogeneous Systems

• Start with a single phase

Mass/ Bulk Polymerization

• Includes a monomer
• Includes an initiator
• Monomer as a solvent for the monomer and the resulting polymer

Mass/ Bulk Polymerization Advantages

• No waste/Disposal needed
• High Molecular weights pure/High reaction rates achieved

Mass/ Bulk Polymerization Disadvantages

• Heat/agitation is difficult and sometime impossible.
• Polymerization reactions are strongly exothermic

Application of bulk polymerization

• Used for unusual shapes/small sizes are and small objects are made with it

Solution Polymerization

• Solvent is added to avoid temp problems
• Solvent act as a diluent

Solution Polymerization

• The lower rate of polymerization
• Residue required purification

Heterogeneous systems

• System always has two phases (dispersed and dispersing)

Emulsion Polymerization

• Emulsion includes soaps, alkyls, long salts and sulfonic acids

Suspension Stabilization Polymerization

• Add stabilizer as inorganic/organic compound to prevent in non- solvent (water) reaction

Suspension Polymerization Advantages

• Easy/ free process

Suspension Polymerization Disadvantages

• Beads
• Industrial importance