TYPES OF POLYMERIZATION• CHAIN GROWTH POLYMERIZATION• Free radical• Ionic• Cationic• Anionic• Insertion• Ring opening polymerization• STEP GROWTH POLYMERIZATION
Chain growth polymerization• Addition polymerization• All the atoms in monomer is used to produce a polymer.• Steps in chain reaction:• initiation• propagation• termination
Step growth polymerizationPolymerization mechanism in which bi-functional ormultifunctional monomers react to form first dimers,then trimers, longer oligomers and eventually longchain polymers.•Eg: polyesters, polyamides, polyurethanes. Etc•Polymer+molecule with low molecular weight.
Differences between step-growth polymerization and chain-growth polymerization Step growth Chain growth• Growth throughout matrix • Growth by addition of monomer only at one end of chain• Rapid loss of monomer early in the reaction • Some monomer remains even at long reaction times• Similar steps repeated • Different steps operate at throughout reaction process different stages of mechanism.• Average molecular weight • Molar mass of backbone chain increases slowly at low increases rapidly at early stage conversion and high extents and remains approximately the of reaction are required to same throughout the obtain high chain length. polymerization• Ends remain active (no • Chains not active after termination) termination• No initiator necessary • Initiator required
Free radical polymerization• Initiation: active center created.• 2 steps• Radicals from initiators• Transfer to monomer• Types of initiation:• Thermal decomposition• Photolysis• Redox reactions• Persulfate
• Propagation:• Termination :• Combination of two active chain ends• Impurities• Combination of an active chain end with an initiator radical
Cationic polymerization• Cationic initiator binds & transfers charge to monomer.• Reactive monomer reacts with other monomer to form a polymer.• Active site: carboniumion , oxonium, sulfonium or phosphonium ion• Monomers: alkoxy. phenyl, vinyl, 1,1-dialkyl-substituted alkene monomers.• Initiator: provide electrophile eg: bronsted acids(acetic acid,HCL), Lewis acids+electron donor.• Application :polyisobutylene.
Anionic polymerization• Carried out through carbanion active species.• Monomer: vinyl monomers with substituents on double bond able to stabilise a –ve charge. o Eg: styrene, dienes, methacrylate, vinyl pyridine, aldehydes, epoxide, episulfide cyclic siloxane, and lactones• Polar monomers: o eg: acrylonitrile, cyanoacrylate, propylene oxide, vinyl ketone, acrolein, vinyl sulfone, vinyl sulfoxide, vinylsilane andisocyanate.• .
• Solvents- polar solvents decrease stability.• initiation : electron transfer, strong acids.• Propagation: very fast,low temp, heat is released.• Termination: quenching, water, alcohol, chain transfer.• Application :polydiene synthetic rubbers, solution styrene/butadiene rubbers (SBR), and styrenic thermoplastic elastomers
Insertion polymerization• Coordination polymerization• Monomer adds to growing macromolecule through an organometallic active center.• Ziegler natta catalysts- titanium tetrachloride+aluminium cocatalyst.• Mechanism;
Ring opening polymerization• Initiation: Ring cleavage• Propagation:Attachment of cyclic monomers.• Termination• examples o PA 6: Polycaprolactame from caprolactam o PCL : Polycaprolactone from caprolactone o Polyethylene oxide from ethylene oxide o Polypropylene oxide from propylene oxide
Bulk polymerization• Mass or block polymerization: Polymerization of the undiluted monomer.• carried out by adding a soluble initiator to pure monomer into liquid state.• Viscosity increases dramatically during conversion• 2 types o Quiescent bulk polymerization o Eg: phenol- formaldehyde condensation o Stirred bulk polymerization o Eg: nylon 66.
Advantages Disadvantages• The system is simple and • Heat transfer and mixing requires thermal insulation. become difficult as the• The polymer is obtained pure. viscosity of reaction mass increases.• Large castings may be • Highly exothermic. prepared directly molecular weight distribution can be • The polymerization is easily changed with the use obtained with a of a chain transfer agent. broad molecular weight distribution due to the high viscosity and lack of good heat transfer. • Very low molecular weights are obtained.
Solution polymerizationMonomer dissolved in solvent, formed polymer stays dissolved.Depending on concentration of monomer the solution does notincrease in viscosity.Advantages Disadvantages* Product sometimes * Contamination directly usable with solvent* Controlled heat * Chain transfer to release solvent * Recycling solventApplicationsAcrylic coating, fibrespinning, film casting
Suspension polymerization• Liquid or dissolved monomer suspended in liquid phase.• Suspending agent- PVA, methyl cellulose.• Initiator• Particle size 10-500µm.
Advantages Disadvantages • Surfactants and• High molecular polymerization adjuvants - weight polymers difficult to remove• fast polymerization rates. • For dry (isolated) polymers,• allows removal of heat from water removal is an energy- the system. intensive process• viscosity remains close to • Designed to operate at high that of water and is not conversion of monomer to dependent on molecular polymer. This can result in weight. significant chain transfer to• The final product can be used polymer. as such ,does not need to be • Can not be used for altered or processed condensation, ionic or Ziegler-Natta polymerization.