Polymerization Techniques Bulk polymerization Solution polymerization Suspension polymerization

1. Polymerization Techniques • Bulk polymerization • Solution polymerization • Suspension polymerization • Emulsion polymerization

2. Monomers may be polymerized by the following methods • (1) polymerization in homogeneous systems • (2) polymerization in heterogeneous systems • Polymerization in Homogeneous systems: • The homogeneous polymerization techniques involve pure monomer or homogeneous solutions of monomer and polymer in a solvent. • These techniques can be divided into two methods: • (i) the bulk and • (ii) the solution polymerizations

3. 1. Bulk Polymerization: In the reactor:- • Liquid monomer • Initiator • Inhibitors • Chain transfer agents Homogeneous : polymer remains dissolved in monomers. Ex. PMMA Heterogeneous : aka. Precipitation polymerization polymer is insoluble in its monomers. Ex. Polyacrylonitrile, PVC Problem : heat transfer not good Make objects with a desirable shape by polymerization in a mold.

4. Model of Batch Polymerization Monomer

5. Pros & Cons of Bulk Polymerization Advantage Disadvantage • Obtain purest possible polymer • Conveniently cast to shape • Obtain highest polymer yield per • reactor volume • Difficult to control • Reaction has to be run slowly • Cannot get high rate and high • MW at the same time • Difficult to remove last traces • of unreacted monomer

6. 2. Solution Polymerization Solution Polymerization: Monomer dissolved into inert- solvent / inhibitor • Monomer • Initiator • CTA • Inert solvent Solvent helps controlling heat transfer from reaction. Use for : • Thermosetting condensation polymer (stop before gel point) • Ionic polymerization • Ziegler-Natta solution process

7. Model of Solution Polymerization Monomer I I Initiator I I I Solvent

8. The effect of solvent solubility on the molecular weight of polyurethane produced by solution method Viscosity of polymer  MWpolymer  High viscosity = high molecular weight !

9. Pros & Cons of Solution Polymerization Advantage Disadvantage solvent • Rate [M]  reduce rate, • chain length xn • Solvent waste • Need solvent separation & • recovery • Have traces of solvent, monomer • Lower yield • Solvent may not be really inert • (May interfere w/ rxn.-act as CTA) • Reduces the tendency toward • autoacceleration • Increases heat capacity/heat- • transfer • Reduces viscosity • Minimize runaway reaction

10. Polymerization in heterogeneous systems • Polymerization occurs in disperse phase as large particles in water or occasionally in another non-solvent (suspension polymerisation), or dispersed as fine particles • The last-named process is usually known as emulsion polymerisation

11. Suspending agent (Hydrophobic) (Hydrophilic) monomer + Water Water Initiator 3. Suspension Polymerization: Monomer into water, suspending agents (Ex.Ionic detergent, barium sulfate) Model of suspension polymerization - Ex. Polyvinyl alcohol - Beads of polymer ( 10-1000 m)

12. Monomer (hydrophobic) Initiator (dissolved inmonomer) Monomer phase Chain-transfer agent (dissolved inmonomer) Water – suspending medium Protective Colloid Suspending agent Insoluble inorganic salt Typical Composition:

13. Pros & Cons of Suspension Polymerization • Disadvantage • Low yield / reactor volume • Traces of suspending agent on particle surfaces • Cannot run continuously • Cannot be used for • -condensation polymers • -ionic or Ziegler-Natta polymerization Advantage 1. Easy heat removal and control 2. Obtain polymer in a directly useful from

14. Emulsion Polymerization: Use emulsifier / soap monomer Initiator Water Soap (Hydrophilic) 4. Emulsion Polymerization • Reaction occurs in water phase until polymer gets very • hydrophobic and then dissolve back in the monomer region. Ex.Latex - very very small particle stable in solution - particle size << 1 m - can generate very high MW. polymer

15. Emulsion Polymerization (cont.): • Typical ingredient • 100 part (by wt.) monomer (water insoluble) • 180 part water • 2-5 parts acid soap • 0.1-0.5 part water-soluble initiator • 0-1 part CTA (monomer soluble)

16. Steps in Emulsion Polymeriztion Water-soluble initiator Polymer born in water Monomer swollen micelle Polymer moves to micelle • growing polymer particle • Monomers inside the micelle decrease Unreacted monomers in other micelles and in droplets diffuse through water to the growing particles Reaction terminates when 2nd radical gets in reaction starts again for the 2nd chain when 3rd particle gets in.

17. Ref: S.L. Rosen, John Wiley & Sons 1993

18. Pros & Cons of some polymerization techniques

19. GLASS TRANSITION TEMPERATURE(Tg) Polymers are generally amorphous in nature -- chains are coiled, interwined and entangled giving a random arrangement.

21. Behaviour of polymers on heating is different from that of crystalline solids. A solid polymer will be hard, stiff and glossy as the chains occupy definite positions in the solid and virtually all the chain movements are frozen. When temperature increases the localised units (chain segments) within a long chain molecule starts moving. This movement is called segmental motion. If the temperature increases further the whole chain starts moving. This type of movement is called molecular motion.

22. Definition: The temperature at which a polymer transforms from stiff, hard, glossy state to rubbery state is called glass transition temperature (Tg). The temperature at which the polymer melts and starts flowing is called flow temperature. (Tf or Tm). • SIGNIFICANCE OF Tg • It is used as a measure for evaluating the flexibility of a polymer and the type of response the polymeric material would exhibit to mechanical stress. • Tg is very useful in choosing the right processing temperature for fabrication (molding, calendering and extrusion) • Tg is very useful in determining the coefficient of thermal expansion, heat resistant refractive index, electrical property etc.

23. FACTORS AFFECTING GLASS TRANSITION TEMPERATURE: Any structural features or externally imposed conditions that influence chain mobility will also affect the value of Tg. Some of these structural factors include chain flexibility; stiffness, including steric hindrance, polarity, or interchain attractive forces; geometric factors; copolymerization; molecular weight, branching; cross-linking; and crystallinity.

24. 1. Chain Flexibility • a free rotation along carbon chain of the polymer the polymer is flexible • Linear polymer chains containing only single bonds have high degree of freedom for rotation and are more flexible. • More the flexibility more will be the segmental mobility and hence lower will be the Tg.

25. 2. Presence of side groups: (Bulky groups) Presence of side groups restricts the free rotation or movement of polymer chain the polymer becomes more rigid and the Tg increases.

26. 3. Inter molecular forces: Presence of intermolecular forces in the polymer chain due to polar groups, dipoles etc exerts strong force of attraction like hydrogen bonding with neighbouring chains. This restricts the segmental mobility and hence increases the Tg. Eg: Tg of polypropylene is -18̊ C where as taht of nylon-6,6 is 57̊ C. This is because nylon-6,6 contains polar amide bonds, there will be hydrogen bonding as shown below.

27. 4. Molecular mass: • Tg increases with increase in the molecular mass up to 20,000. • Molecular mass beyond 20,000 will have negligible effect on Tg. Under high molecular mass, long polymeric chains coil and entangle with one another. • This restricts free mobility of the chains and thus increases Tg. • Under low molecular mass, more number of small chains will have number of loose ends. This leads to segmental motion at the loose end resulting in lower Tg.

28. 5. Crystallinity: Close and orderly arrangement of chains leads to crystallinity in a polymer. When chains are close (as in crystalline solid) intermolecular force of attraction (Vaander Walls force) will be more. So more energy is required to initiate segmental motion which leads to high Tg. (Higher crystallinity is expected if the chains are linear without branching). 6. Stereoregularity of the polymer: Isotactic polymer is more symmetric than syndiotactic, which is more symmetric than atactic polymer. When the structure is symmetric, chains can come closer which leads to increase in crystallinity and thereby increasing Tg of the polymer. Thus, Tg of these polymers is in the order of isotactic˃syndiotactic˃atactic polymer.

30. THERMO PLASTIC POLYMERS: • Some polymers soften on heating and can be converted into any shape that they can retain on cooling. • The process of heating, reshaping and retaining the same on cooling can be repeated several times. • Such polymers, that soften on heating and stiffen on cooling, are termed ‘thermoplastics’. These are the linear or slightly branched long chain molecules capable of repeatedly softening on heating and hardening on cooling. • These polymers possess intermolecular forces of attraction intermediate between elastomers and fibres. • Polyethylene, PVC, nylon and sealing wax are examples of thermoplastic polymers.

31. Poly carbonate: Polycarbonates are the polyesters of unstable carboxylic acids. They contain [-O-CO-O-] linkage through the chain. Polycarbonates are prepared by condensation of (i) Posgene and sodium salt of Bis-phenol-A (ii) Diphenyl carbonate and Bis-phenol-A • Properties: • It has high tensile strength and impact resistance. • It is white transparent thermoplastic polymer. • Highly soluble in alkalies and acids • Got very high melting point. • Applications: • Used in the manufacture of many useful articles such as telephone parts, unbreackable glazing appliances, etc due to its high tensile strength. • Used in the manufacture of safety goggles, automobile lenses, DVD, CD etc due to its transparent nature.

33. Acrylonitrile butadiene styrene (ABS): monomers used in the synthesis of ABS are, ACRYLONITRILE, BUTADIENE AND STYRENE. • Properties: • Very strong having very high impact resistance • High thermal and chemical resistance. • Provides glossy surface. • Got wide range of operating temperature (-40̊ C to +90̊ C). • Applications:pipe systems, musical instruments (recorders, plastic clarinets, and piano movements), golf club heads (because of its good shock absorbance), automotive trim components, automotive bumper bars, medical devices for blood access, enclosures for electrical and electronic assemblies, protective headgear, whitewater canoes, buffer edging for furniture and joinery panels, luggage and protective carrying cases, small kitchen appliances, and toys,

35. Preparation x,y,z depends on blending percentages of starting materials

36. THERMOSETTING POLYMERS: • Undergo some chemical change on heating and convert themselves into an infusible mass. • They are like the yolk of egg, which on heating sets into a mass, and, once set, cannot be reshaped. Such polymers, that become infusible and insoluble mass on heating, are called ‘thermosetting” polymers. • These polymers are cross linked or heavily branched molecules, which on heating undergo extensive cross linking in moulds and again become infusible. • These cannot be reused. • Some common examples are epoxy resins, bakelite, urea-formaldelyde resins, etc.

38. Epoxy resins: (Araldite) These are the polymers contain epoxy group on both the ends. The most common epoxy resin is obtained by condensation reaction of epichlorohydrin and Bis-phenol-A in the presence of NaOH as catalyst.

39. They are used as laminating materials for electrical equipments. • Epoxy resins find a large number of uses due to their remarkable chemical • resistance and good adhesion. • Epoxy resins are excellent structural adhesives for many surfaces like glass, • metals, wood etc· • They are used as surface coatings for skid-resistant surfaces for highways • Mould made from epoxy resins are employed for production of components of • aircrafts and automobiles Epoxy resins are applied over cotton, rayon and • bleached fabrics to impart crease resistance and shrinkage control • They are one of the principal constituents of fiber -reinforced plastics

40. Properties: • Epoxy resins possess good electrical insulating • properties. • Epoxy resins are resistant to water, acids, alkalis and • various organic solvents. • They offer very good wearing and abrasion resistance. • The polar nature of the resins makes them excellent adhesives. • Cross linked polymers have high toughness and heat resistant • properties. • Applications: • It is used as an adhesive to bind wood, glass, concrete, ceramic, metallic • and leather materials. • Epoxy resins are applied over cotton, rayon and bleached fabrics to • impart grease resistance and shrinkage control. • Used in manufacture of skid resistant industrial floorings and highway • surfacing. • Used as laminating materials for electrical equipments. • Moulds which are made from epoxy resins are used in the production of • aircraft and automobile components.

42. Curing of epoxy resins Epoxy resins are converted in to three dimensional cross linked polymers by reaction with curing agents such as polyamides, polyamines, dicarboxylic acids, acid anhydrides, etc. The process is called curing of epoxy resins. On curing epoxy resins show outstanding properties of toughfness, chemical inertness, flexibility and strong adhesion.

43. Phenol Formaldehyde resins: condensation between phenol and formaldehyde Aqueous solution of formaldehyde with phenol is heated to 70-75 C. The monomers like monomethylol phenol, dimethylol phenol, trimethylol phenol are formed. The nature of the reaction depends on the ratio of phenol to formaldehyde in the solution.

44. Case (i) If the phenol to formaldehyde ratio is is greater than one (P/F˃1), the monomethylol phenol is expected in higher percentage. monomethylol phenol undergoes polymerisation in presence of acidic catalyst forms a polymer called Novolac.

45. Case (ii) If the phenol to formaldehyde ratio is less than one (P/F<1), the di and trimethylol phenol are expected in higher percentage. This di and trimethylol phenols undergo polymerisation in presence of base catalyst forms a polymer called Resol. Novolac or resol undergo curing when mixed with hexamine (Hexamethylenetetramine). Hexamine helps in the cross linking of the chains. The highly cross linked product is called Bakelite.

46. Novolac or resol resins when mixed with drying oil are used • as varnishes. • Both novolac and resol are linear polymers therefore • thermoplastic in nature. • Resol is characterised by ether linkage where as novolac is • characterised by methylene linkage. • The curing agent, hexamine acts as a source of • formaldehyde when it reacts with water. • Properties of phenol-Formaldehyde resins: • 1. Bakelite is resistant to water, acids, and organic solvents. • 2. Due to the presence of phenolic –OH group it is attacked by alkalies. • 3. Good electrical insulator and got excellent adhesive properties. • 4. Rigid, hard and scratch resistant thermosetting polymer.

47. Applications: 1. Used in the manufacture of heater handles, TV and radio cabinets, because of it high temperature resistance. 2. As Bakelite is good insulator, it is used for making electric insulator parts liks switches, plugs, and switch boards. 3. For making brake linings, abrasive wheels and sand papers. 4. To seal electrical bulbs to their caps. 5. For making decorative laminates and wall papers. 6. In the production of ion exchange resins used in the purification of water.

48. BIODEGRADABLE POLYMERS: Deterioration in properties is due to a phenomenon called ‘polymer degradation’, which is characterised by an uncontrolled change in the molecular weight or constitution of the polymer. Conventionally, the degradation is a reduction in the molecular weight of the polymer. “Biodegradable polymers are the polymers which will degrade by the action of naturally occurring microorganisms like bacteria, fungi or sunlight”.

49. Requirements of Biodegradable polymers: • Biodegradable polymers should have hydrolysable linkages like esters, amides or ether. • They should be hygroscopic in nature. • The product formed after degradation should act as compost.

50. The biodegradable polymers can be classified according to their chemical composition, origin and synthesis method, processing method, economic importance, application,