Methods of polymerization of homo and hetero polymers: Methods of polymerization of homo and hetero polymers M.Shravani M.Pharm 1 st year Types of polymers: Types of polymers HETERO POLYMERS: HETERO POLYMERS Slide 4: Graft copolymer Random copolymer TYPES OF POLYMERIZATION: TYPES OF POLYMERIZATION CHAIN GROWTH POLYMERIZATION Free radical Ionic Cationic Anionic Insertion Ring opening polymerization STEP GROWTH POLYMERIZATION Chain 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 polymerization: Step growth polymerization P olymerization mechanism in which bi-functional or multifunctional monomers react to form first dimers, then trimers , longer oligomers and eventually long chain polymers. Eg : polyesters, polyamides, polyurethanes. Etc Polymer+molecule with low molecular weight. Differences between step-growth polymerization and chain-growth polymerization: Differences between step-growth polymerization and chain-growth polymerization Step growth Chain growth Growth throughout matrix Rapid loss of monomer early in the reaction Similar steps repeated throughout reaction process Average molecular weight increases slowly at low conversion and high extents of reaction are required to obtain high chain length. Ends remain active (no termination) No initiator necessary Growth by addition of monomer only at one end of chain Some monomer remains even at long reaction times Different steps operate at different stages of mechanism. Molar mass of backbone chain increases rapidly at early stage and remains approximately the same throughout the polymerization Chains not active after termination Initiator required Free radical polymerization: Free radical polymerization Initiatio n: active center created. 2 steps Radicals from initiators Transfer to monomer Types of initiation : Thermal decomposition Photolysis Redox reactions Persulfate Slide 10: Propagation: Termination : Combination of two active chain ends Impurities Combination of an active chain end with an initiator radical Cationic polymerization: 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 . Cationic polymerization: Cationic polymerization Anionic polymerization: Anionic polymerization Carried out through carbanion active species. Monomer: vinyl monomers with substituents on double bond able to stabilise a –ve charge. Eg: styrene, dienes, methacrylate, vinyl pyridine, aldehydes, epoxide, episulfide cyclic siloxane, and lactones Polar monomers: eg: acrylonitrile, cyanoacrylate, propylene oxide, vinyl ketone, acrolein, vinyl sulfone, vinyl sulfoxide, vinylsilane andisocyanate. . Slide 14: 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: 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: Ring opening polymerization Initiation: Ring cleavage Propagation:Attachment of cyclic monomers. Termination examples PA 6: Polycaprolactame from caprolactam PCL : Polycaprolactone from caprolactone Polyethylene oxide from ethylene oxide Polypropylene oxide from propylene oxide Polymerization techniques: Polymerization techniques Bulk polymerization Solution polymerization Suspension polymerization Emulsion polymerization Bulk polymerization: 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 Quiescent bulk polymerization Eg : phenol- formaldehyde condensation Stirred bulk polymerization Eg : nylon 66. Slide 21: Advantages Disadvantages The system is simple and requires thermal insulation. The polymer is obtained pure. Large castings may be prepared directly molecular weight distribution can be easily changed with the use of a chain transfer agent. Heat transfer and mixing become difficult as the viscosity of reaction mass increases. H ighly exothermic . The polymerization is obtained with a broad molecular weight distribution due to the high viscosity and lack of good heat transfer. Very low molecular weights are obtained . Solution polymerization: Solution polymerization Monomer dissolved in solvent, formed polymer stays dissolved. Depending on concentration of monomer the solution does not increase in viscosity. Advantages Disadvantages * Product sometimes * Contamination directly usable with solvent * Controlled heat * Chain transfer to release solvent * Recycling solvent Applications Acrylic coating, fibrespinning, film casting Suspension polymerization: Suspension polymerization Liquid or dissolved monomer suspended in liquid phase. Suspending agent- PVA, methyl cellulose. Initiator Particle size 10-500µm. Emulsion polymerization: Emulsion polymerization Water Monomer Surfactant Examples: Synthetic rubber-styrene-butadiene (SBR ), Polybutadiene , Polychloroprene . Plastics-PVC, polystyrene, Acrylonitrile-butadiene-styrene terpolymer ( ABS). Dispersions - polyvinyl acetate, polyvinyl acetate copolymers, latexacrylic paint, Styrene-butadiene, VAE Slide 27: Advantages Disadvantages High molecular weight polymers fast polymerization rates. allows removal of heat from the system. viscosity remains close to that of water and is not dependent on molecular weight. The final product can be used as such ,does not need to be altered or processed Surfactants and polymerization adjuvants -difficult to remove For dry (isolated) polymers, water removal is an energy-intensive process Designed to operate at high conversion of monomer to polymer. This can result in significant chain transfer to polymer. Can not be used for condensation, ionic or Ziegler-Natta polymerization.